Heliothis virescens (Lepidoptera: Noctuidae): Initiation of Summer Diapause G. D. BUTLER, JR., L. T. WILSON,' AND T. J. HENNEBERRY Western Cotton Research Laboratory, Agricultural Research Service, US. Department of Agriculture, Phoenix, Arizona 85040 J. Econ. Entomol. 78: 320-324 (1985) ABSTRACT Summer diapause of the tobacco budworm, Heliothis vlrescens (F.),is a period of extended duration of the pupal stage, particularly of males, initiated by high temperature. In summer insectary studies, up to 95% of the male pupae exhibited delayed development. The number entering summer diapause was associated with high temperatures before pupation. Almost all pupae that had entered summer diapause over a period of several months emerged during the last part of September and the first part of October. In laboratory studies, high fluctuating temperatures caused the highest percentage of individ- uals to enter summer diapause. An analysis of variance showed that the effect of temperature was much greater than photoperiod, but with a significant interaction between these two factors. A daily 8-h exposure at 43°C caused an average of 95.6%of males to diapause and 58.7% of females. Summer diapause only occurred when the maximum temperature ex- ceeded 32°C. When exposed to a fluctuating temperature of 23.9 to 40.6"C for all of the larval stage, all male pupae diapaused, while only 50% of the females entered summer diapause. When exposed for just the week before pupation, from 63 to 85% of the males diapaused compared to 15 to 50% of the females. Exposure of pupae 2 days after pupation still caused 24% of males to diapause (1% females) and 3 to 4 days after pupation, 8 to 9% to diapause (1-5%of females). Averaged across all photoperiods and temperatures, 61% of the males entered summer diapause, compared to 26.7% of the females. The substantial number of inviable eggs produced by females mated to males from pupae that did not enter summer diapause when exposed to high temperatures, compared to the low number of inviable eggs produced by females mated to males from pupae that did enter summer diapause, signifies the survival advantage of a physiological condition such as summer dia- pause. DIAPAUSE occurs in the tobacco budworm, He- with the start of the rainy season (Hackett and Ziothis virescens (F.), pupal stage and is initiated Gatehouse 1982). in the fall, the major regulatory factors being pho- A similar delay in emergence of tobacco bud- toperiod, temperature, and food (Phillips and worm was first noted in the USDA insectary at Newsom 1966, Benschoter 1968, Fye and Carran- Phoenix, Ariz., during the summer of 1974. This za 1973, Fye 1979). paper presents evidence of summer diapause of Summer diapause may be defined as diapause H. virescens in insectary studies, and the effect of induced before the height of summer, which then various factors on the initiation of summer dia- is terminated and followed by reproductive, de- pause. velopmental, or feeding activities in autumn or winter (Masaki 1980). It is almost the opposite of Materials and Methods winter diapause in its relation to photoperiodic and thermal conditions. Long photoperiod and high The experiments reported here were conducted temperature tend to induce or maintain summer at Phoenix, Ariz., from 1976 to 1978 and in 1983. diapause, whereas short photoperiod and low tem- H. virescens eggs were received from the Western perature induce winter diapause. In the Sudan Ge- Cotton Research Laboratory, Phoenix, Ariz., and -Lira, after more than 2 months in diapause at low Cotton Insect Laboratory, Brownsville, Tex., rear- temperature, some H.armigera (Hubner) pupae ing facilities, or were obtained from moths col- remained in diapause for more than 2 months af- lected in the field at Phoenix. ter transfer to high temperatures and only com- Insectary. First-instar larvae were placed week- menced developing when the temperature was ly or at biweekly intervals in 22.5-m1 cups con- lowered. This may provide a mechanism to bridge taining ca. 14 ml of alfalfa meal medium modified the hot, dry season and to synchronize emergence from a diet reported by Henneberry and Kishaba (1966). The insects (ca. 500) were taken to an out- door insectary at weekly intervals during 1976 and 1977, when available. The dates of pupation and Dept. of Entomology, Univ. of California, Davis, CA 95616. of adult emergence were recorded for each indi- 320 April 1985 BUTLER ET AL.: INITIATION OF SUMMER DIAPAUSE 32 1 Table 1. Percentage of H. virescens pupae in summer c3 diapause after ea. 500 first-stage larvae were placed in an z insectary on different dates in 1976 and 1977 IO0 0 Percentage of individuals in summer diapause 0 Males 0 Week of egg I I hatching date 1976 1977 Male Female Male Female t 0 Females 8 May 4 1 9 7 15 1 4 58 14 60 0 I 22 c 3 0 69 4 29 0 1 27 4 5 June 60 5 67 2 12 90 24 15 1 19 95 83 95 37 26 83 24 55 5 3 July 42 1 89 23 10 4 0 78 7 17 6 1 57 12 W 0 24 16 5 98 19 c3 a 0 0 31 9 1 56 11 u 0 1 1 I I W ,7 '4% 35 6 8 1 > 0 2 4 6 8 IO 14 17 5 65 3 U 21 0 1 11 2 HOURS PER DAY ABOVE 43°C Fig. 1. Temperature effect on the proportion of male and female H. virescens entering summer diapause. vidual. Pupae were sexed according to Butt and Cantu (1962). Laboratory. In 1977, the effect of temperature pae and nondiapause male and female moths, and photoperiod on initiating summer diapause reared from larvae/pupae maintained under fluc- was evaluated in four cabinets at 26.7"C and raised tuating temperatures of 23.9 to 40.6"C, were to a peak of 43°C for either 2, 4, 6, or 8 h each crossed with control moths reared from larvae day during the middle of each photoperiod. Each maintained at a constant 26.7"C. Moths (three pairs cabinet contained boxes with 11-, 12-, 13-, and per container with eight replications) were held at 14-h light periods, for a total of 16 treatments. 26.7"C in oviposition containers and supplied with Each box had 300 larvae. Diapause in relation to 10% sucrose solution for food. Numbers of eggs time of pupation was estimated by experimenta- deposited were counted and numbers unhatched, tion in 1977 and 1978. Larvae reared from eggs counted after 4 days. Subsequently, all female deposited by field-collected moths were used in moths were dissected and examined for the pres- both experiments. In 1977, first-instar larvae were ence of spermatophores. Data were subjected to held at 21.4 to 38.9"C daily fluctuating tempera- an analysis of variance and the means separated tures and 12-h light period. At daily intervals, ca. according to Duncan (1955). 100 larvae were transferred to a cabinet with tem- peratures fluctuating between 16.1 and 30.6"C. In 1978, first-instar larvae were held at 21.1"C and Results and Discussion 14-h light period. At daily intervals, ca. 100 larvae Insectary. Based on daily insectary tempera- were transferred to a cabinet in which the tem- tures and development rates from Butler and perature fluctuated from 23.9 to 40.6"C with a 12-h Hamilton (1976) and Butler et al. (1979), a 99% light period. In both years, the insects were ex- confidence interval was calculated for the duration amined daily and the dates of pupation noted. Af- of the pupal stage each week and compared with ter the normal period plus 2-fold the SD for adult that observed during the summers of 1976 and emergence, the number of pupae in diapause was 1977. The percentage of males in summer dia- recorded in relation to the number of days before pause from the weeks of 8 May to 21 August var- or after pupation when the temperature change ied from 0 to 95% during 1976 and from 0 to 98% had been made. The increment of degree-days during 1977, while that of females varied from 0 (OD) > 32°C was calculated in 2-h increments ac- to 83% in 1976 and from 0 to only 37% in 1977 cumulating the difference between the tempera- (Table 1). The first week of July 1976 and 1977. ture for each 2 h and the diapause threshold (32"C), and the second week of August 1977 had some of adjusting for the 2-h time step. the highest temperatures of the summer (40- In addition to the experiments on diapause ini- 43.8"C). At these times, larvae in the 19 June and tiation in relation to temperature and photoperiod, 24 July series were pupating and during these pe- studies also were conducted in 1983 to evaluate riods the highest incidence of diapause was ob- the mating success and fecundity of H. virescens served each year. The normal duration of the pu- reared at various temperature regimes. Male H. pal period during the summer was 10 to 15 days. virescens moths from heat-induced diapause pu- Almost all pupae entering summer diapause over I 322 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 78, no. 2 Table 2. Percentage of pupae in summer diapause, from 300 larvae at each different temperature and photoperiod Photoperiod (P) h Aug (T x S)" "D > 32°C at LD 11:12 LD 12:12 LD 13:ll LD 14:lO ~ Avg (TI (T) 43°C 8 6 9 9 8 9 6 P 6 0 1.875 2 3.9 1.7 5.1 2.7 15.2 4.8 5.0 0 7.3d 2.3e 4.8a 3.125 4 37.7 1.8 53.4 3.2 70.6 18.0 55.4 0.8 54.313 6.0de 29.6b 3.379 6 77.6 40.8 92.0 45.1 85.8 42.8 92.0 30.6 86.9a 39.8~ 63.3~ 4.260 8 92.0 77.1 96.7 50.0 95.8 46.3 97.9 61.4 95.6a 58.7b 77.2d Avg across Overall Avg across temperatures total P (S) avg (P x S) 52.8b 30.3~ 61.8ab 25.2cd 66.8a 28.0~ 62.6ab 23.2d 61.0a 26.7b 43.8 Avg across sex (T x P) "D > 32°C LD 11:12 LD 12:12 LD 13:ll LD 14:lO 1 875 2.8h 3.9gh 10.0fg 2.5h 3.125 19.7ef 28.3e 44.3d 28.le 3.379 59.