TEXAS AGRICULTURAL EXPERIMENT STATION A. B. CONNER, DIRECTOR College Station, Texas BULLETIN NO. 638 AUGUST 1943 HIBERNATION OF THE BOLL WEEVIL H. J. REINHARD Division of Entomology LEE-ERfiKQY’ 50f“ , ~ “M 5",‘ ~ *’ y: 1L1?» ,. In w“ ;‘~».\:>~i!- h. ‘¢' -, *1 “ ,,\"' ~ ~ ~= q-‘< ‘ b". >"‘=’!""" "~"“~ "K" t‘ » *I--v..-.'§*§‘-.'>]. I ‘ \Y y‘ 51"": <3 A5,} (‘w 3:1‘ ¢-. --~ ._ E-k-w‘ wJI II; EQL. .0 AGRICULTURAL AND MECHANICAL COLLEGE OF TEXAS T. O. WALTON, Pmasidafnt 6W4. B41-843-4500-L180 )5 With the approach of cool weather in the fall the boll weevil instinc- tively seeks sheltered situations in which to pass the winter or hiber- c nate. The insect does not enter a period of true hibernation but remains i * more or less active during warm spells in the winter and may leave the ' selected shelter, only to reenter it when temiperatures decrease and be- come hazardous. Those individuals that pass the winter successfully constitute the only source from which infestations ma-y develop in the 5 crop the following spring. Knowledge of the winter survival of the é weevil is therefore of practical value in anticipating the development . of injurious infestations. Among the factors influencing the extent of weevil mortality during the winter, minimum temperatures are most important. From 6 to 19 per cent of the overwintering weevil population survived when minimum temperatures did not fall below 25 degrees F‘. On the other hand, during seasons when minimum temperatures ranged from 17 to 0 de- grees F., less than 2 per cent of the weevils survived. The average S111‘- vival of the boll weevil in hibernation cages at College Station for 18 seasons (fall of 1925 to spring of 1942) was 6.04 per cent. _-The mor- _ tality among the weevils caged prior to October 15 was nearly three i‘ times greater than that recorded for installations made at subsequent dates. This illustrates the importance of early destruction of the wee- vil’s food supply. 1 Emergence of‘ the weevil from hibernation begins when average mean temperatures reach 55 to 60 degrees F. After emergence is definitely under way, rainfall has an important influence on the rate at which the surviving weevils leave shelter. Frequent well distributed showers followed by warm sunshine are favorable conditions for a maximum rate i of emergence. During the 18-year period at least 50 per cent of the Sur- viving weevils emerged by May 15; but emergence may not be com- pleted until the middle of July. The heaviest emergence takes place during the latter half of May. c The percentage of weevil survival has no direct relation to the extent o1’ injury that may be produced during the season. Prevailing climatic conditions are a. limiting factor in this respect. A very light weevil sur- I vival followed by favorable weather conditions may increase rapidly and produce widespread injury to the crop, as Was experienced in 1935. ‘ On the other hand, prolonged hot, dry weather may efiectively check ‘ the activities of a heavy Weevil survival as was recorded in 1931. Since _ future weather conditions remain an unknown factor, the development a T 0t injurious weevil infestations cannot be anticipated very far in ad- vance of-their actual occurrence. CONTENTS Page Introduction f 5 Method of Conducting Hibernation Studies__- 6 Duration of Hibernation Period 8 g = Climatic Conditions During Hibernation Period . 8 Effect of Minimum Temperature on Weevil Survival ____________________ __ 9 f Relation of Shelter to Weevil Survival 9 Survival in Woods Shelter 10 Survival in Open Field 11 Relation of Food Supply to the Rate of Survival a 12 Emergence from Hibernation 14 a Climatological Factors Affecting Emergence ________________________________ __ 14 Temperature 14 Rainfall 15 Time and Rate of Emergence 15 Peak of Emergence 18 ' é Dispersal at Time of Emergence 19 - if Weevil Survival in Relation to Extent of Injury __________________________________ __ 20 “i Summary 22 fl i “rim . WFW5”iWiW‘WWWQMTFWT/§73W‘Q9Y= ~ » HIBERNATION OF THE BOLL WEEVIL by H. J. REINHARD a With the advent of decreasing temperatures in the fall, boll weevils zpegin to seek sheltered situations in which t0 pass the cold or winter riod. The movement of the insect into winter quarters is a rather 1 adual process and usually extends from early fall to the time of frost J when the “cotton plants are generally killed in the field. In seeking ‘protection the weevils may select any shelter available in the cotton *e1d or they may move to fence rows grown ‘up in grass or weeds, hay fitacks, barns, gin houses, or any other similar situations which may be fgvailable. In timbered or wooded sections of the State many weevils ‘ifind and utilize the favorable shelter afforded by Spanish moss, leaves, ‘ind other materials". It has, in fact, been repeatedly-demonstrated by ‘examinations made during the winter season that Spanish moss when lifailable is one of the most common shelters selected by the boll weevil {or protection from cold weather. The cold season, during which an insect remains inactive and does jnot feed, is called the hibernation period. Strictly speaking, there is no eriod of true hibernation _in the boll weevil’s seasonal history. It re- mains more or less active and during warm spe-lls in the winter may Ieave its shelter entirely only to reenter it when temperatures again ecrease and become hazardous. During unusually mild seasons weevils lmay continue to feed and reproduce during the winter months as long fa. green cotton squares and bolls remain available. This, however, is lnot a common occurrence in most sections of‘the State. Frosts ordi- inarily kill all cotton, which is practically the only food on which the ‘insect can survive and reproduce. g The so-called hibernation period of the boll weevil or that season ‘uring which the insect is without food or means of multiplication is 5- particular importance to cotton growers, because at this time the [weevil is‘ perhaps most vulnerable to attack by clean-up measures. Ob- lviously the weevils are producing no damage after the crop has been gharvested but it should be remembered" that those weevils which suc- Icessfully pass the winter constitute the only source of infestation to fthe new crop the following spring. Any material reduction -of the num- fiber of overwintering weevils, whether due to natural factors or to proper YAclean-up measures applied, results in a corresponding decrease in the épotential source for early damage to the new or subsequent crop. It is Knot always the case that a heavy winter survival of weevils is followed {évby severe injury or that a light survival indicates prospects for little or no damage to cotton. The prevailing climatic conditions, especially uring the early part of the growing season, seemingly determine in a 6 BULLETIN NO. 638, TEXAS AGRICULTURAL EXPERIMENT STATION very large measure the extent of damage that‘ is produced by the weevil population that survives the Winter. In the past, cotton growers per- haps have over emphasized the importance of the percentage of winter survival of the weevil as an indication of future infestations of the cur- rent crop. The extent to which the weevil may be expected to injure any crop cannot be predicted with accuracy very far in advance of actual occurrence. The rate of weevil increase is directly influenced, either favorably or unfavorably, by prevailing weather conditions as the sea- son advances. In other words, that portion of the weevil population which survives the winter successfully constitutes the source from which all subsequent weevil injury develops; knowledge of it is therefore es- sential to growers in anticipating the development of injurious infesta- tions as influenced by climatic conditions‘. Studies on boll weevil hibernation have been conducted by the Texas Agricultural Experiment Station at College Station since the fall of 1923 and the results are presented in this Bulletin. METHOD OF CONDUCTING HIBERNATION STUDIES Since the immature stages of the boll weevil are ordinarily destroyed by cold weather during the fall or winter all observations on the hiber- nation of this insect are confined to the weevil or mature insect. To determine the percentage of winter survival boll weevils were col- lected in lots of 500 each and installed in cages under open field conditions with Spanish moss provided as shelter material. The collections and installations of weevils were made at weekly intervals beginning the first week in October and continued until the occurrence of the first frost. In nature the majority of weevils seemingly move to shelter dur- ing the time indicated and the individuals installed in the hibernation cages are assumed to represent a fairly close approximation of the natural overwintering population. All the weevils were collected from green cotton in local fields and placed in the cages without food on the date of collection. The type of cage used in these studies (Figure 1) is a wood frame construction of 2 x 2 inch material and measures 4 x 4 x 4 feet. The sides and top are covered with 16-mesh wire screen. One side of the cage is provided with a door which is closely fitted to prevent the escape of any confined weevils. A few inches from the top in the cage Spanish moss is suspended from two diagonal wires as shelter for the weevils. This material was selected in these studies‘, since, as already pointed out, the weevil commonly chooses it in nature as a protective shelter against cold. The bottom of the cage is also covered with about a three-inch layer of the same material. The total amount of moss used in each cage ranges from 20 to 30 pounds and occupies in large part the space within the cage at the time the weevils are installed. After the insects were installed in the fall the cages were left undis- turbed until the following spring. It should be mentioned that in most HIBERNATION OF THE BOLL WEEVIL Figure 1. Type of cage used in "the boll weevil hibernation studies. cases the hibernation cages were situated under open field conditions, that is, fully exposed t0 sunshine, winds, and rains. Beginning March 1 during the period 1925 to 1935, inclusive, all active weevils were re- moved daily from each cage as they emerged from shelter. From 1936 to 1942, inclusive, removal of active weevils was delayed until one week after cotton first appeared in local fields; the first date when emergence ' was recorded each season usually occurred between April 1 and 15. All records were made daily at 1:00 P. M. and continued until the emer- gence was completed. Throughout the period of these studies climato- logical data were recorded for the purpose of noting the effect of these factors on winter survival and emergence from hibernation in the spring, All the weevils used were taken at random from green cotton through- out the time of normal entrance into hibernation. They may be con- sidered to represent an average cross-section of the hibernating weevil population in nature during any year included in these studies at Col- lege Station. Under natural conditions it appears quite likely that a smaller percentage of the weevils actually seek shelter prior to October 116 than is the case with the individuals which were confined in these cage tests. Many of the weevils installed early in the fall remained active and starved before temperatures low enough to induce hiberation occurred; whereas in nature weevils have free access to food until forced to seek winter shelter by the proper combination of factors. For these reasons the data presented herein may be considered conservative 8 BULLETIN NO. 638, TEXAS AGRICULTURAL‘ EXPERIMENT STATION with respect t0 the extent of winter. survival of the insect‘ under natural or field conditions. DURATION OF HIBERNATION PERIOD Entrance of the weevil into hibernation apparently begins at‘ no reg- ular or specific time in the fall. It is likely influenced to some extent by certain physiological changes within the insect as well as by the direct effect of climatic factors. It has been shown that some indi- viduals may seek winter quarters and begin hiberation as early as Sep- tember when green cotton squares and bolls may still be available for food in the field. The extent to which this occurs is seemingly limited and usually the majority of weevils do not cease to feed and multiply until about the time frosts occur. Throughout the period covered in these studies the weevils which were caged subsequent to October 15, survived in greatest numbers, indicating that this is the most favorable time for beginning entrance into winter shelter. To secure data on weevils hibernating prior to this time the cage installations for observa- tion on winter survival were begun during the first week of October, which, it is assumed, is early enough to include the first definite move- ment of the insect from the field to winter quarters. It has been pointed out that the weevil is more or less active during the warmer spells throughout the Winter. Those overwintering indi- viduals in the least protected situations are likely the first ones to be influenced by warm weather and leave their shelter, while others with more adequ-ate protection may remain wholly inactive from the time of entrance into hibernation in the fall until the following June, In other words, the duration of the hibernation period cannot be limited by any very definite dates. At College Station the weevils normally become active in numbers after March 1 and for this reason removal of the active individuals from the hiberation cages was begun on that date during the early part of this study. During seasons when the insect‘ has survived the winter in large numbers all individuals may not leave their winter quarters within the cages until some time during the early part of July and perhaps even later under field conditions‘. Apparently, the growing season may be well advanced before all surviving weevils have emerged from winter shelter and resumed normal activities. The average period of boll weevil hibernation may be considered to extend from November to May, inclusive, a period of about seven months. f The extremes of the period may possibly range from four to nine months. GLIMATIG CONDITIONS DURING HIBERNATION PERIOD Weather conditions, among other factors, during the hibernation period of the weevil have a direct influence on the percentage of sur- vival. Assuming that the protection supplied in the hibernation cages approximates the type of shelter selected by weevils under natural con- a ditions, it may be stated that mild winters with a minimum temperature HIBERNATION OF THE BOLL WEEVIL 9 a lower than 25 degrees F., are followed by at least an average emer- cemut when the temperature falls below 15 degrees F., the survival Auch less than average, depending on the severity of the cold weather. 3 winter seasons during the period covered by these studies include trasting years with respect to both minimum temperatures and rain- f ‘ For example, a minimum temperature of 0 degrees F. was recorded“ p930, while the following year was exceptionally mild with barely freez- t; temperatures prevailing at any time during the winter. Rainfall ‘ng the hibernation period of the weevil is also subject to consid- le variation, ranging from about 6 to 24 inches. An analysis of weevil survival data recorded in these cage experiments shows no 1' ' correlation between the amount of rainfall and the successful over- lering of the weevil, but there is apparently a very direct relation geen the percentage of survival and minimum temperature. Effect of Minimum Temperatures on Weevil Survival 1 f determined by cage studies during the seasons 1925 to 1942, in- ‘ve, the extent of weevil survivalvranged from less than 1 to more '9 19 per cent. In this connection it should be pointed out that i under most favorable conditions more than 80 per cent of all the ned weevils succumbed during the winter season. Obviously, not ’is mortality is the direct" result of unfavorable climatological fac- Many of the weevils which hibernate -are apparently old or worn {individuals which die naturally, regardless of prevailing weather iitions. In the random collection of weevils made for cage installa- it may be assumed that the proportion of the individuals which ‘ vigorous enough to withstand the rigors of a dormant season ap- ‘p ated that existing in natural or field populations. ‘ong the factors influencing the extent of weevil mortality during fwinter, minimum temperatures seem most important. This rela- hip is graphically illustrated in Figure 2. It will be noted that 7 lly 10 to 19 per cent of the caged hibernating weevils survived f-V931, 1934, 1937 and 1941 when the minimum temperatures for Zyears did not fall below 26 degree-s F. On the other hand, in ;,~ 1930, 1933, 1935, 1936, and 1940 with minimum temperatures l; to 0 degrees ‘F. prevailing, less than 2 per cent of the individuals _the winter successfully. The data further indicate that during k-vsurvival of about 4 to 7 per cent may be expected. Temperatures _- i these limits‘ resulted in a sub-average percentage of survival ‘the caged weevils. ~ RELATION OF SHELTER TO WEEVIL SURVIVAL ‘c. kind of shelter available to the weevil for protection during the “ant season is also an important factor that influences the extent {ch the insect may survive the winter successfully. Obviously, those rs with minimum temperatures not falling below 25 to 20 degrees 10 BULLETIN NO. 63s, TEXAS AGRICULTURAL EXPERIMENT‘ STATION r-I 03 P w > h 5 m +> $1 o O a o 0-4 0 10 2O 30 4Q Minimum Temperature Figure 2. Percentage of boll weevil survival with relation to minimum temperatures. individuals which find the most suitable shelter in the field will be least affected by minimum temperatures. For example, in wooded regions" of the State the weevil finds more favorable shelter and survives the winter in greater numbers than in those sections where such type of shelter is not available. The relative differences in weevil survival as determined by hibernation cages situated in the woods and open fields are dis- cussed in the following paragraphs. - Survival in Woods Shelter During the three fall seasons, 1923 to 1925, boll ‘weevils were in- stalled in hibernation cages situated in woods shelter to determine the rate of winter survival under these apparently favorable conditions. Prior to the installation of any weevils the bottom of each cage was covered with 12 to 18 inches of dry leaves or Johnson grass for pro- tection of the overwintering insects. The dates of installation ranged from October 2 to December 1. The mininmum temperatures recorded HlBERNATION O13‘ THE BOLL ,WEEVIL~ l1 Table 1. Survival of weevils in cages situated 1n woods shelter Date of I Shelter No. weevils No. weevils ‘ Per cent installations material installed emerged survival _ . 2—Dec. 1, 1923 ......... __ Leaves‘ 4570 1213 26.54 7v.2)—Dec. 1, 1924 ________ Leaves 61s 57 9.22 3, 1925 ................. _. Johnson grass 500 0 0.00 i=1 ..... ______________ ' 5680 m0 verage- --_i .............. ___L ‘ 11.92 jring the dormant seasons, 1923-24, 1924-25, and 1925-26 were 19, 18 jd_20 degrees F., respectively. A summary of the data secured on the '~centage of survival of the weevil in these tests is presented in Table , It will be no-ted that early fall installations, from which few weevils Jmally survive, were included in the tests. However, the average l» ival totaledvnearly 12 per cent of the 5,680 weevils confined for ob- “ation. This average rate of winter survival is approximately two es greater than that recorded in cages situated in the open field. = r Survival in Open Field Most of the important cotton producing counties of Texas are located sections where woods shelter is not generally available for protection ' 'ng the hiberation period of the weevil. For this reason most of the rnation cages were placed under open field conditions to approximate vifnearly as possible the type of shelter available to the weevil through- the prairie or untimbered sectio-ns of the State. D-uring these studies lgotal of 75,625 field-collected boll weevils were confined in separate girnation cages in lots averaging about 500 each for observation on f= winter mortality of the insect. The dates of installation ranged from 'tem_ber 29 to December 4. The materials supplied the confined wee- 1 for protection during the winter season included dry weeds, John- ‘ grass, and Spanish moss in amounts of 20 to 30 pounds per cage. "Tummary of the data accumulated on the survival of the weevil is ented in Table 2. It will be noted that the extent of survival ranged w“ less than 1 to more than 19 per cent. These variations, as has "eady been shown, are closely associated with the minimum tempera- is to which the confined weevils were exposed. The weevil survival ier open field» conditions as determined by local cage tests during -_teen seasons, averaged 6.04 per cent. In this connection it should iiipointed out that under favorable conditions large numbers of weevils fer hibernation so that the seemingly small average percentage of val recorded in the cage tests may prove quite significant‘ when giidered in terms of the number of weevils which become active at ut the time when cotton isbeginning to come up. 12 BULLETIN NO. ass, TEXAS AGRICULTURAL EXPERIMENT STATION Table 2. Survival of weevils in cages situated under open field conditions Date of Shelter No. weevils \ No. weevils Per cent installations material installed emerged survival Dec. 2-4, 1924_ .............. -_ Grass and weeds 711 29 4,08 Oct. 20—Nov. 13, 1925 ....... _- Johnson grass 930‘ 35 3.76 Oct. 1—Nov. 20, 1926 ....... -- {ggggifhnnfggsss 3984 210 5.27 Oct. 1—Dec. s, 192:7 _______ _- {ggggfghnnfggss 4500 19 .420 Oct. 13—N0v. 25, 192s ....... -- {gggflissfjggss 4500 284 6.21 Oct. 3—Nov. 16, 1929 _______ __ Spanish moss 3500 1 .03 Oct. 3—Nov. 14, 1930 _______ __ Spanish moss 5000 850 17.00 Oct. 2—Nov. 25, 1931 _______ _- Spanish moss 5000 469 9.38 Oct. 7—Nov. 11, 1932 _______ _- Spanish moss 4000 11 .27 Oct. 7—Nov. 25, 1933 _______ -- Spanish moss 4500 705 ‘ 15.67 Sept.29—Nov. 24, 1934 ....... __ Spanish moss? 4500 14 .31 Oct. 12—Nov. 23, 1935 _______ -- Spanish moss 5000 82 1.64 Oct. 3—Nov~.n 7, 1930 ....... _- Spanish moss 4500 472 10.49 Oct. 2—Nov. 13, 1937 _______ -_ Spanish moss 5000 336 6.72 Oct. 1—Nov. 12, 1938 _______ -- Spanish moss 5000 311 6.22 Oct. 7—Nov. 18, 1939 _______ -_ Spanish moss 5000 2 l .04 Oct. 5—Nov. 14, 1940 _______ __ Spanish moss 5000 961 19.22 Oct. 4—N0v. 15, 1941 ....... __ Spanish moss 5000 91 1 I 82 Total ....................... -_ 75.6% 4,882 ‘ Average- ................... -_ 6.04 RELATION OF FOOD SUPPLY TO THE RATE OF SURVIVAL Since entrance of the weevil into winter quarters under field condi- tions may extend over a period of two or three months, it is of interest to consider the relation between the time of removal of the food supply to the rate of survival. In the field most of the individuals normally seek shelter during Octo- ber and November. The data recorded on survival with respect t0 the time when the weevils are installed in cages are summarized in Table 3. Compared with later installations, the weevils confined in the hiberna- tion cages prior to October 15 survived in smallest numbers. In fact, only 2.8 per cent‘ of these individuals passed the winter successfully. Obviously, most of the weevils caged this early in the season starved before temperatures were low enough to induce entrance into hiberna- tion. The survival among the weevils placed in cages during the latter half of October was about three times greater than that recorded from HIBERNATION OF THE BOLL WEEVIL l3 Table 3. Survival of weevils in cages with relation to the time of removal from Food a A Survival of weevils when installed during biweekly periods indicated Year - of Oct. 1-15 Oct. 16-31 Nov. 1-15 Nov. 16-30 Emer- gence Number Number Number Number Number Number Number Number weevils weevils weevils weevils weevils weevils weevils weevils installed survived installed survived installed survived installed survived 1925 ___ ___ ___ ___ ___ ___ 618 57 1926 510 7 420 28 ___ ___ 1927 1,000 10 1,484 98 1,000 80 500 22 1928 1,500 4 1,000 3 1,000 4 500 6 , 1929 500 3 2,000 119 1,000 86 1,000 76 1930 1,000 0 1,000 1 1,000 0 500 0 ~ 1931 1,000 76 2,500 489' 1,500 285 ___ ___ 1932 1,000 23 2,000 144 1,500 239 500 63 - 1933 1,000 2 2,000 9 1,000 0 ___ ___ ‘ 1934 1,000 106 1,500 293 1,000 101 1,000 205 1935 1,000 4 1,000 2 1,000 1,000 6 - 1906 500 2 1,500 21 2,500 500 9 1987 1,000 50 3,000 328 500 9'4 ___ _-_ 1988 1,000 31 2,500 224 1,500 81 ___ ___ 1939 2,()00- ’ 11 2,000 90 1,000 210 ___ ___ 1940 1,000 0 2,000 1 1,500 1 500 ' 0 1941 1,000 151 3,000 664 1,000 146 ___ ___ , 1942 1,000 4 2,000 27 2,000 e0 ___ I! Total 17,000 477, 30,994 2,520 20,420 1,467 6,618 444 cent I ~.| ival 2.81 8.13 V ‘ 7.18 6.71 _stallations made during the first half of the'month. This point‘ is. uite significant with respect to effective clean-up practices in the fall. If cotton stalks are destroyed generally before October 15 a very large dirt of the weevil population succumbs to starvation. The remaining "dividuals apparently enter winter shelter in such weakened condition p, at few have sufficient vitality to withstand the minimum winter tem- ratures. To reemphasize the relation between destruction of cotton bi the fall and survival of weevils it may be stated that Whenever the food I ‘the insect is removed or destroyed prior to October 15 only 28 out of ery 1000 weevils‘ may be expected to enter shelter with sufiicient vi- lity to live through the winter season. But when their food was not moved until the latter half of October, 81 out of each 1000 overwinter- _ individuals emerged the following spring. The additional feeding riod from October 16 to 31 is vitally important to the weevil for suc- sful hibernation. It will be noted that no significant increase in the c te of winter survival ensued for each two weeks feeding period that ye weevils were allowed after November 1. These observations indi- j e that the critical pro-hibernation period of the weevil occurs locally l October, and whenever green cotton squares and bolls remain avail- ggle for food throughout this month the insect survives the hazards of ' e dormant season in maximum numbers. 14 BULLETIN NO. 638, TEXAS AGRICULTURAL EXPERIMENT‘ STATION EMERGENCE FROM ‘HIBERNATION The basic data on the number of weevils installed in hibernation cages and the number which emerged therein at bi-Weekly intervals from March to July during the seasons, 1925 to 1942, inclusive, are presented in Table 4. It will be noted that the emergence from hibernation or winter shelter is usually protracted and rather irregular from year to year. It is not affected by the presence of food and seemingly depends largely upon prevailing weather conditions, particularly temperature and rain- fall. Table 4. Installations of weevils and. the number that emerged in hibernation cages at bi-weeklly periods from 1925 to 1942, inclusive Total Number of weevils emerged during periods indicated Total Dumber number Year weevils March March ~ April April May May June June weevils installed 1-15 16-311 1-15 16-30 1-15 16-31 1~15 16—30 emerged 1925 711 4 12 11 2 __ __ __ __ 29 1926i 930 14 211 _ _ _ - - _ _ _ _ _ _ _ 35 1927 39844 61 30 57 9 16 32 5 _ _ 21 0 1928 4500 13 2 2 2 _.. __ -_ _- 19 1929 4500 5 21 l 41 29 70 91 2+1 3 284 1930 3500 _- __ __ 1 \II __ __ __ 1 1931 5000 42 64 75 127 152 162 192 - 36 850 1932 5000 13 9'6 129 83 25 94 29 l __ 469' 1933 4000 2 1 4 2 1 __ 1 __ 11 1 9S4 4500 19' 22 81 96 1S9 244 54 _ _ T05 1935 ‘4500 3 1 2 __ 1 7 __ __ 14 1936 5000 - _ 4 4 13 7 10' 30 19 _ _ S2 1937 4500' _- __ __ 179 128 150 u 15 __ 472 1938 5000 __ -_ 36 91 72 100 34 3 336 1939 5000 __ __ 67 92 82 48 22 __ 311 1940 5000 __ __ 1 1 __ __ __ __ 2' 1941 5000 _ _ _ _ _ _ 182 159 311 236 52 961* 1942 5000 I __ _- 13 9' 30 19 16 4 91 *21 of these weevils emerged July 1-15. 1986-1942‘, inclusive, emergence was recorded beginning one week after cotton first came up in local fields. Climatological Factors» Affecting Emergence Temperature: Among the factors affecting emergence of the weevil from hibernation, temperature is seemingly the most important. Little, if any, activity occurs-among the dormant weevils when temperatures range below 45 degrees F. Apparently, there is" no very definite ten- dency toward emergence from hibernation until average mean tempera- tures reach 55 to 60 degrees F. These conditions are normally approx- imated in Marc-h. As the prevailing mean temperatures rise to» 70 de- grees F. or above, the weevils leave winter shelter in increased numbers. Those individuals in the least protected situations are seemingly the first to react to the occurrence of effective temperatures, while others with more adequate protection respond more slowly. In fact, some of the caged weevils did not emerge until after average mean temperatures of 75 to 80degrees F. had prevailed for four or five weeks. It already has been noted that when the winter is relatively mild and the minimum temperature does not fall below 26 degrees F., usually HIBERNATION OF THE BOLL- WEEVIL l5 to 19 per cent of the weevils survive. 1t is interesting to compare percentages of survival following these mild winters, when the mini- m temperatures were similar, and to note the effect of the frequency Tthe occurrence of high mean temperatures during March. Table 5. Relation of high mean temperatures in Marc-h to perwnliage of weevil survival Min. temp. No. days in March . . Year of previous when mean temp. Percentage Emergence winter was above 75° F. survival 1937 27° , 6 10.49 1941 26° 3' 19.212 1938 25-“ 2A 6.72 1939 28' 16 6.22 i t ,hen mean temperatures reach 75 degrees F., activity among the Qjrwintering weevils is affected and the frequency with which such f temperatures occur indicates wasted energyand vitality at a time food of the insect is not available. Excessive activity during March ‘robably the reason for the relatively lower emergence during the _j 193s and 1939. * After emergence of the weevil from hibernating quarters is itely under way, rainfall has an important influence on the rate at ‘h the surviving individuals‘ leave shelter. During protracted dry ‘ls in April and May, even though effective temperatures prevail, gence decreases and may cease entirely until the occurrence of rain. uent well distributed light showers throughout the emergence period Vmost favorable for the maximum rate of emergence. An increase f: number of weevils leaving the Spanish moss shelter Within the fs was especially noticeable when light rains were followed by warm “l ine. The duration of the effectiveness of limited rainfall, especially 1 g warm weather, is apparent for only a short period and emergence '-= when moisture conditions fall below a certain minimum require- for the resumption of activity by the overwintering weevils. Time and Rate of Emergence jnowledge of the time ‘and rate at which any overwintering popula- fiof weevils leaves shelter to resume normal activities is of practical p‘; with respect to the status of field infestation during the early part 1F growing season. The results o-f a comparison of the time and of weevil emergence are shown in Table 6 whenlthe records were 1n March 1 and in Table 7 when the records were delaye-d until a ‘after cotton first appeared in local fields. These records are based study of the emergence from installations made in hibernation from 1925 to 1942, inclusive. It will be noted that there appears _| 16 i BULLETIN NO. 633. TEXAS AGRICULTURAL EXPERIMENT STATION Table 6. Per cent of weevil emergence in hibernation cages at bi-weekly intervals from 1925 to 1.935, inclusive Per cent emergence during periods indicated Year " - Mar. 1-15 Mar._16-31 Apr. 1-15 Apr. 16-30 May 1-15 May 16-31 June 1-15 June 16-30 1925 .56 1.69 1.55 .28 ___ ___ ___ ___ 1926 1.50 2.26 ___ ___ ___ ___ ___ ___ 1927 1.53 .75 1 43 .23 .40 .80 .13 ___ 192s .29 04 .04 .04 ___ ___ ___ ___ 1929 .11 47 91 .64 ‘L56 2.02 .53 .07 1930 ___ ___ ___ .03 ___ ___ _-- ___ 1931 84 1.28 1 50 2.54 3.04 3.24 3.84 .72 1932 .26 1.92 2 58 1.66 .50 1.88 .58 ___ 1933 .05 .02 10 .05 .02 ___ .02 ___ 1934 .42 .49 1 80 2.14 4.20 5.42 1.20 ___ 1935 .07 .02 04 ___ .02 .16 ___ __- Average .51 p .81 .91 69 .89 1.23 .57 .07 Per cent ' of total emergence 8.98 14 .26 16.02 12.15 15. 67 21.65 10. 04 1.23 to be no regularity from season to season with respect to the time over which emergence extends or to the rate at which it occurs. Except in 1930, the weevils generally became active and began to leave winter quarters during March. In five of the eighteen seasons they continued to emerge until the latter part of June and in 1941 until the middle of July. Armajority of the surviving weevils in the cages resumed activ- ity during April and May when temperatures and rainfall during these months approached the normal. In local sections of the State this period of maximum Weevil emergence extends from the time when the new crop is up generally until the plants have begun to produce squares. Table 7. Per cent of weevil emergence in hibernation cages at ‘bi-weekly , intervals from 1936 to 1942, inclusive Per cent emergence during periods indicated Year Mar. 16-31 Apr. 1-16 Apr. 16-30 May 1-15 May 16-31 June 1-16 Junel6—30‘ July 1-15 1936 .08 .26 .14 .20 .60 .36 ___ ___ 1937 ___ ___ 3.98 2.84 3.34.» .33 ___ ___ 1938 ___ .72 1.82 1.44 2.00 .68 .06 ___ 1939 ___ 1.34 1.84 1.64 .96 .44 ___ ___ 1940 ___ .02 .02 ___ ___ ___ ___ ___ 1941 ___ ___ 3.64 3.18 6.22 4.72 1.04 .42 1942 ___ .26 .18 .38 .32 .08 ___ Average .01 _ .37 1 6e 1.41 1 93 .98 .17 | .06 Per cent | of total | _ emergence; .15 5.61 25.19 21.40 29.29 14.87 _ 2.58 .91 Some of the weevils that emerge from hibernation prior to April un- doubtedly succumb, since food or green cotton plants are not generally available at that time. Also some reenter hibernation with the occur- HIBERNATION OF THE BOLL WEEVIL 17 of lower tempartures and apparently find better shelter than was _o_usly occupied and may be less affected by warm weather. return of weevils to hibernation shelter is indicated by the sim- averages of total emergence recorded as shown in Tables 8 and 9. y of the weevils that resumed activity in March and were not re- fd from the cages reentered hibernation shelter. This is indicated 4 8. Accumulative per cent of weevil emergence in hibernation cages; by bi-weekly periods and the accumulative per cent of total emergence from 1925 to 1935, inclusive Accumulative per cent of emergence prior to dates indicated April 1 April 16 May 1 May 16 June 1 June 15 | July 1 -_- ------------- -- 2.25 3.80 4.0a 4.0a e 4.08 4.08 4.08 ---------- -- 3.76 3-76 3.76 3.76 3.76 3.76 3.76 ---------- -- 2.28 3.71 3.94 4.34 5.14 5.27 5.27 - --------- -- .33 .38 .42 .42 .42 .42 .42 ---------- -- .58 1 49 2.13 3.69 5.71 6.24 6.31 ---------- -- _-- ___ .03 .03 .03 .03 .03 ---------- -- 2.12 3 62 6.16 9.20 12.44 16.28 17.00 ---------- -- 2.18 4.76 6.42 6.92 8.80 9.38 9.38 ------------- -- .07 -1T .22 .25 .25 .27 .27 ---------- _- .91 2.71 4.84 9.04 14.47 15.67 15.67 ............. -- .09 .13 .13 .16 .31 .31 .31 _ e ----------- -- 1.32 2 23 2.92 3.81 5.04 5.6-1 5.68 r ~ I ative per - tal emergence $.23 39.26 51.41 67.08 88.73 98.77 100.00 n emergence of 30 per cent during April of the years 1936 to 1942. tically the same proportion, 28 per cent, emerged for the period “i to 1935. If all emerged weevils had remained active the former rtion probably would have been much larger. Weevils that resume tyw after cotton is' up, but before the plants have begun to pro- 9. Accumulative per cent of weevil emergence in hibernation cages by bi-weekly periods and the accumulative per cent of total emergence from 1936 to 1942., inclusive Accumulative per cent of emergence prior to dates indicated April 1 April 16 May 1 May 16 June 1 June 15 July 1 July 15 I .08 .34 .48 .68 1.28 1.64 1.64 ___ 0 0 3.98 6.82 10.16 10.49 10.49 ___ 0 .72 2.54 3.98 5.98 61.66 6.72 ___ 0 1.34 3.18 4.82 5.78 6.22 6.22 _-- 0 .02 .04 .04 .04 .04 .04 ___ 0 0 3.64 6.82 13.04 17.76 18.80 19.22 0 .26 .44 1.04 1.42 1.74 1.82 --- .01 .38 2.04 3.46 5.38 - 6.36 “ 6.53 6.59 15 592 3096 5250 81.64 9651 9909 10000 18 BULLETIN NIO. 638, TEXAJS AGRICULTURAL EXPERIMENT STATION duce squares, are commonly able to subsist on the tender foliage u fruits are set. i. Throughout central Texas cotton is not normally up to a stand in i field prior to April 16, and as a rule the plants are not fruiting -.~f until the latter part of May. With these points in mind, it is of inte ; to consider the available information in terms of the emergence c pleted at specific dates during April, May, and June. The data. ~~5 sented in Tables 8 and 9 picture the status of weevil emergence at ~_ Weekly intervals throughout the period indicated. The figures show“ trend toward any regularity in the procedure of emergence. For = ample, all dormant weevils in the cages became active during M 1926 when the survival was light, but during seasons of heavy survii as in 1931 and 1934, activity was barely well under way by the end i‘ March. ‘ During the period 1925 to 1935, when weevils were removed as th became active during March, an average of nearly one-fourth of t population that overwintered successfully was taken out of the cag during the month. Similarly a little more than one-fourth was removl during April, and emergence for the two months totaled 51 per- cen Table 8. When removal of weevils was delayed until after cotton fi =_ appeared in‘ the field 50 per centof the total emergence was not coi pleted until about the middle of May, Table 9. The greatest increase ' the resumption of weevil activity, as indicated by the average accumuli tive percentage of emergence, occurs during the last two weeks of Ma or about the time when the plants are beginning to produce frui é abundantly. i It may be safely assumed that the percentage of total weevil surviva or emergence for the same periods under field conditions is‘ greater tha’ the figures indicated above, since these data are based on overwinterin populations which contained a larger proportion of early entrants int hibernation than the corresponding populations overwintering in th field. Furthermore, many individuals in the field undoubtedly find mor adequate shelter than that supplied in the cages and are therefore n0 so unfavorably affected by prevailing minimum temperatures. Should weather conditions in the spring retard normal planting o the subsequent appearance of cotton, boll weevil emergence also woul be delayed. However even under favorable weather conditions the we vils may remain in hibernation or if previously active may return t shelter with the occurrence of falling temperatures. These facts explains why it is not possible to effectively control boll weevils by delayed plant-g‘ ing of the crop. I Peak of Emergence: At College Station the weevil becomes active ingi March when temperatures for the month approach the normal. This l, movement gains momentum during April but does not seem to attainfi its maximum until the proper combination of effective climatic conditions prevails". Cage records show that the peak of emergence or the time A \ HIBERNATION OF THE‘ BOLL WEEVIL 19 ‘ng which the maximum number of weevils ‘resume activity does not _r until the latter half of May. The significance of this point should _mphasized. In other words, the majority of weevils passing the i successfully remain inactive or dormant until after the crop is advanced and fruits are usually present in abundance. The peak ieevil emergence based on the yearly average percentage of the data rded on caged weevils from 1936 to 1942, inclusive, is graphically cent t6d / \ / a / ¢\/\ r11 15 3o my '15 31 June 15 3o July 15 Average boll weevil emergence in hibernation cages from 1936 to 1942, inclusive. (A) Emergence at bi-weekly intervals; (B) Accumulative emer- gence at bi-weekly intervals. trated in Figure 3. After the peak has been attained, the emergence ines rapidly and is usually complete in south-central Texas by the pof_ June. Dispersal at Time of‘ Emergence liter resuming activity and leaving Winter quarters in the spring, weevil apparently seeks the most available source of food. This ins the heavier infestations which usually occur early in the season rose portions of cotton fields nearest to Wooded areas, fence rows, ther situations which provide favorable Winter quarters for the in- During seasons of heavy Winter survival the weevil obviously s in all directions from the center of any large- hibernating popu- on. The maximum distance which the insect can cover to reach ble food has not been determined. ‘that dispersal of hibernated weevils within such fields may proceed a_ 20 BULLETIN NO. 638, TEXAS AGRICULTURAL EXPERIMENT STATION After- reaching cotton plants that have begun to produce squares ovi wintered weevils‘ show but little tendency toward wider‘ disseminati by means of flight. Field records made in this connection indicate the average maximum distance covered by hibernated individuals duri‘ the first week of activity on freely fruiting cotton plants totaled than 100 feet from the point of entrance into the field. As the supp _ of uninfested squares became limited in the localized areas the weevil gradually moved to fresh squares by crawling from plant to plant a ‘ by short intermittant flights. During the early part of the growi __ season when the supply of squares is still limited it appears quite likel a, a more rapid pace. At any rate the weevil upon entering a field of you cotton is diligent in its search for squares and seems able to locate t p. plants which produce the first fruit. WEEVIL SURVIVAL IN RELATION TO EXTENT OF INJURY Those weevils which pass through the dormant season successfullyi constitute the source from which injurious infestations may subse; quently develop. It is likewise true that the extent to which any over-j wintering population of weevils may prove destructive by multiplication: in the crop is directly influenced by climatic conditions which prevail; during the early summer months. In any consideration of weevil survival with relation to the extent of injury produced to the crop, as measured in terms of yield per acre, it must be remembered that other factors such as the deleterious efiect of? climatic conditions on the crop or reductions of full yields per acre due .- to other insects and plant diseases cannot be readily separated or 30011-1 rately measured. However, it has been repeatedly shown that heavyig boll weevil infestations during the greater part of the fruiting period? of cotton plants are manifested by noticeable reductions in yields. Since 1 a heavy emergence of overwintered weevils constitutes a potential source: from which early and destructive infestations may develop, it is of in» terest to compare the crop yields secured during years of heavy and§ light weevil survival. l The data recorded on weevil survival, rainfall, and cotton yields in 4 Brazos County during the crop years 1928 to 1941, inclusive, are shown 3 in Table 10. These data indicate no apparent correlation between thei percentage of weevil survival and the average yield produced. When? average yields of lint have exceeded 200 pounds per acre the percentage of survival has varied from .42 in 1928 to 10.49 in 1937. On the other » hand in 1929 and 1936 when average yields were lowest or about 100 pounds per acre the survival was 6.32 and 1.64 per cent, respectively. In the four years with high yields the rainfall during the critical period of crop production (May-July) varied from 5.32 to 11.05 inches, while in the two years of low production the rainfall for May to July, inclu- sive, was more than 20 inches. It will be noted that extremes of local HIBERNATION OF THE? BOLL WEEVIL 21 Vlble 10. Average per cent of weevil survival and average Yield of lint cotton per acre in Brazos County, 1928 to 1941, inclusive . Per cent Rainfall in May, June, and July v Average yield f Year weevil lint per ' survival Total Departure Distribution acre (inches) from normal (days) (pounds) 1928 .42 11.05 -|-' .68 18 218 192) 6.32 20.91 +10.54 26 99' . 1930 .03 12.90 + 2.53 19 181 ~ 1931 17.00 4.80 — 5.64 17 196 1932 9.38 7.16 --— 3.28 161 155 »1933 .27 11.01 + .57 15 196 1934 15.67 1.311 -— 9.13 157 1935 .31 15.65 + 5.21 32 181 1936 1.64 21.34 +1011 26 107 1937 10.49 5.32 —- 5.31 15 208 1998 6.72 10.14 — .49 26 242 1939 6.22 10.16 ' -— .47 17 248 1940 .04 16.98 + 6.35 24 175 1941 19.22 14.94 + 4.31 33 143 >'l survival were recorded in 1930 and 1931 and again in 1940 and P1. Yet the average yields for the years within each of these two flps did not vary greatly. However, a comparison of the climatic con- ns that prevailed during May-July 1930 and 1931 shows the occur- le of a marked difference, especially with respect to the amount of all. The few weevils‘ that survived the minimum temperature of Tgrees F. in 1930 encountered approximately- normal temperatures ay, June, and July. The rainfall during this period totaled 12.9 ‘es (2.53 inches above normal) and was distributed over 19 days. '_= these favorable climatic conditions the small number of over- ‘red weevils multiplied rapidly and produced general field infesta- , which resulted in subsequent reductions from full yields per acre _A ghout the county. On the other hand, the mild temperatures which filed throughout the winter 1930-31 were exceptionally favorable dormant weevil population and large numbers emerged up to the lwing July. This emergence seemingly indicated prospects for severe = e to the crop. However, it did not develop. The activities of this 0H overwintered population of weevils were practically eliminated by nged spells of dry weather during May, June, and July. The rain- uring these months totaled only 4.80 inches, which was 5.64 inches normal. The continuance of dry weathe-r during the following is: checked any general deve-lopment of late weevil infestations and ff ently losses in yields attributable to weevil injury were practically ‘ible throughout the season. Further observations which are of in this connection are presented in the following paragraph. A ing the winter 1939-40, the weevil survival was one» of the “st on record. As determined by local cage experiments, only .04 ynt of the overwintering population passed the dormant period suc- ‘ally. Nevertheless, general and destructive weevil infestations ed throughout Brazos County that ‘season. Rainfall in May, June, ‘uly totaled 16.98 inches (6.35 inches" above normal) and was dis- x 22 BULLETIN NO. 638, TEXAJS AGRICULTURAL EXPERIMENT STATION tribut'ed over 24 days. These and similar conditions which occurred ' 1935 seemingly approached an optimum for the maximum rate of we multiplication. This evidence clearly illustrates that a light winter s: vival of weevils, if favored by optimum climatic conditions, may beco an important limiting factor in the production of a crop. Knowle of the percentage 0f weevil survival in the spring is useful in “Q ing the potential source from which any subsequent injury to the c; may originate. This information, when considered in connection with p prevailing weather conditions during the early part of the growing se = constitutes the only reliable index by means of which future infestati may be anticipated. Unfortunately, weather conditions beyond a limited period remain an unknown factor, and for this reason advan forecasts with reference to boll weevil injury based on survival al are generally uncertain. I SUMMARY During the fall, boll weevils instinctively seek sheltered situations,‘ which to pass the winter but the insects do not enter a period of hibernation. I They remain more or less active during warm spells __ winter and may emerge from shelter until forced to reseek protection; the recurrence of lower temperatures. The average duration of the ' called weevil hibernation period, as determined by local cage studi‘; extends from November to May. _ ’ p Under open field conditions, the average survival of the boll wee at College Station for 18 seasons (fall of 1925 to spring of 1942) p 6.04 per cent. The maximum was 19.22 per cent in 1941 and the m_ imum .03 per cent in 1930. The survival of weevils in cages‘ pro-tee“ by woods shelter for three seasons (1923-25) averaged 11.92 per Among the factors affecting survival of the weevil minimum tempe. tures are most important. Approximately 10 to 19 per cent of the _ fined weevils survived when minimum temperatures did not fall bell 26 degrees F., but less than 1 per cent of the weevils passed the win successfully when minimum temperatures reached 15 to 0 degrees Winter survival among weevils placed in hibernation cages from O » ber 1 to 15 and October 16 to 31 averaged 28 and 81 per thousa.‘ respectively. The later installations of weevils made from Novem: 1 to 15 and November 16 to 3O showed no significant increases in I: rate of survival. Activity of the weevil in hibernation begins when average mean peratures reach 55 to 60 degrees F. Such temperatures may cause s of the weevils to become active and emerge but when cold weather curs many of the weevils reenter hibernation and may find better p' tectiomagainst‘temperature changes and emerge later. This is 1 reason why delayed planting is not successful in avoiding boll wee damage. After emergence is definitely under way, rainfall has an of portant influence on the rate at which surviving weevils leave shelt ‘ HIBERNATION OF THE BOLL WEEVIL _ ’2a ‘uent well distributed showers followed by warm sunshine are most able for a maximum rate of emergence. Qe percentage .of weevil survival has no direct relation to the extent I» ury that may be produced during the season. f The prevailing conditions are the limiting factor in this respect. Frequent piers during the early part of the growing season are favorable for l injury. _ Under such conditions a light weevil survival may be fled by widespread damage to the crop as was experienced in 1935 . 940." On the other hand, prolonged hot, dry weather efiectively bed the activities of the maximum weevil survival recorded in 1931. l, future weather conditions are an unknown factor, the development urious weevil infestations cannot be anticipated very far in ad- of their actual occurrence.