Dormancy in the Strawberry Leafroller (Lepidoptera: Tortricidae) J. J. OBRYCKI, A. D. GABRIEL, C. J. ORR, ANDJ. W. BING Department of Entomology, Iowa State University, Ames, Iowa 500ll Environ. Entomol. 19(4): 932-936 (1990) ABSTRACT A 2-yr study of strawberry leafroller, Ancylis comptana (Froelich), populations in central Iowa showed that short daylengths and low temperatures maintain diapause during autumn and that this autumnal diapause ends without a specific cue by the end of December. The completion of diapause does not require chilling. The postdiapause developmental rate Downloaded from https://academic.oup.com/ee/article/19/4/932/2480444 by guest on 27 September 2021 of A. comptana is linearly related to temperatures between 14 and 30"C; adult emergence requires 154 degree-days (DD) above a lower thermal threshold (t) of 10.5°C. Field cage studies demonstrated that 50% emergence of adult A. comptana from overwintering larvae can be predicted accurately based upon the accumulation of approximately 150 DD >10°C. KEY WORDS Insecta, dormancy, Ancylis comptana, diapause INSECT DORMANCYmay be separated into two Earliglow). On each sample date, larvae were placed phases: diapause and postdiapause development individually in a Petri dish containing moistened (Tauber et al. 1986). Photoperiod and temperature filter paper and one strawberry leaflet. Leaflets were are two key environmental factors that regulate changed when they began to mold or deteriorate. diapause development in several insect species. Temperatures were regulated within ± 1°C. Following the end of diapause, insects undergo a Voucher specimens are deposited in the Iowa State period of postdiapause development, which is reg- University insect collection, Ames. ulated by a range of factors (e.g., temperature, Diapause Maintenance and Termination. Our food, moisture) (Boyne et al. 1985, Tauber et al. methods followed those of Tauber & Tauber (1973), 1986, Nechols et al. 1987). An understanding of in which the quantification of responses during diapause and postdiapause developmental rates is dormancy is based upon periodic sampling of field necessary for the accurate prediction of the sea- populations throughout the entire period of dor- sonal activity of temperate-zone insects (Tauber et mancy. The overwintering populations of A. comp- al. 1986). tana consisted of field-collected (fall 1986 and 1987) The strawberry leafroller, Ancylis comptana late-stage larvae in rolled strawberry leaves from (Froelich), an occasional pest of strawberry (Fra- Ankeny, Iowa. These individuals were maintained garia x Ananassa) (Rosales: Rosaceae) in the Mid- outdoors in aluminum screen boxes (40 by 30 by west, is an introduced lepidopterous species (Ga- 10 cm) placed at ground level in a larger screen- briel 1989). Late-stage A. comptana larvae covered cage (100 by 70 by 15 cm) and covered overwinter in rolled strawberry leaves in the leaf with straw. We removed samples from the over- litter found in strawberry plantings (Webster 1918, wintering larval populations in September, Octo- Fink 1932, Bennett 1961). Previously, we deter- ber, and November in 1986 and in September, mined the influence of temperature and strawberry October, November, December, and January dur- cultivar on preimaginal development and the ovi- ing 1987-1988. Only three monthly samples were positional responses of A. comptana females to se- taken in 1986 because of high (>80%) mortality lected strawberry cultivars (Gabriel & Obrycki of overwintering A. comptana larvae. Larvae from 1990). In this study, we present results of a com- the field were distributed equally among the con- bined laboratory and field study in which we de- stant temperatures and photoperiods (Tables 1 and termined the roles of temperature and photoperiod 2). In the first year of this study, 22 and 14°C on diapause maintenance and termination and on conditions were used; in the second year, only 22°C postdiapause development of A. comptana. was used. Larvae were checked every second day for pu- pation in the September-December samples. In- dividuals in the January samples were checked dai- Materials and Methods ly for pupation. The sex of pupae was determined, Field-collected A. comptana larvae were main- and the pupae were placed in individual gelatin tained in plastic Petri dishes (100 mm) and pro- capsules (size 00) and checked daily for adult eclo- vided with fresh greenhouse-grown strawberry leaf- sion. To determine the role of chilling (defined as lets (cultivars Selva, Honeoye, Redcoat, or exposure to low temperatures [Tauber et al. 1986]) 0046-225Xj90j0932-0936$02.00jO © 1990 Entomological Soc'iety of America August 1990 OBRYCKI ET AL.: DORMANCY IN Ancylis comptana 933 Table 1. Developmenlal responses to temperature (OC) and photoperiod by overwintering A. comptana; Ankeny, Iowa, 1986 Mean number of days ± SD to complete development Sample date No. adults emerging Photo- Larval Pupal period. L:D 22· 14· 22· 14· 22· 14· 26 Sept. 86" 16:8 28 21 14 ± 2 26 ± 4 11 ± 9 25 ± 2 13:11 2 4 53 ± 10 94 ± 22 9 ± 1 26 ± 4 10:14 2 6 52 ± 0 82 ± 17 12 ± 4 25 ± 6 16 Oct. 86a 16:8 17 12 15 ± 4 23± 5 11 ± 1 32 ± 4 Downloaded from https://academic.oup.com/ee/article/19/4/932/2480444 by guest on 27 September 2021 13:11 9 11 43 ± 15 74 ± 21 11 ± 9 30 ± 5 10:14 5 8 47 ± 10 66 ± 17 12 ± 1 27 ± 5 14 Nov. 86b 16:8 6 7 12 ± 2 25± 6 11 ± 1 32 ± 2 13:11 4 5 41 ± 14 66 ± 14 10 ± 3 26 ± 6 10:14 3 4 35± 9 49 ± 14 11 ± 4 39 ± 18 ANOV A statistics given in Table 3. a 50 A. comptana larvae placed at each photoperiod. b 20 A. camp/ana larvae placed at each photoperiod. in diapause termination, we began sampling A. 1988 at the Iowa State University Horticultural comptana larvae in the fall before the onset of low Research Farm, Gilbert. On 30 March 1989, three temperatures in the field. Lumite screen cages (1.8 by 1.8 by 1.8 m) (Chi- Effects of Temperature on Postdiapause De- copee Manufacturing Company, Gainsville, Ga.) velopmeni. Based upon the results of the study of were placed over these areas. The daily maximum diapause duration, we sampled overwintering lar- vae in February and March 1988 to determine the relationship between temperature and postdia- Table 2. Developmental responses to photoperiod by pause development. The larvae were placed in in- overwintering A. comptana", trom Ankeny, Iowa, 22OC; 1987-1988 dividual Petri dishes as previously described and maintained at five constant temperatures from 14 Sample date No. Mean no days ± SO to complete Photo- to 30"C and a photoperiod of 16:8 (L:D) (Table 4). adults development period These individuals were checked daily for pupation emerging and adult eclosion. L:O Larval Pupal Total Statistical Analysis. The mean number of days 24 Sept. 87a required for A. comptana pupation and adult eclo- 16:8 29 21 ± 3 9±2 30± 4 sion was calculated for each experimental condi- 10:14 8 57 ± 19 12 ± 3 69 ± 17 tion at each sample date. Data within a sample 15 Oct. 87 date were compared using one-way analysis of 16:8 27 14 ± 3 12 ± 3 26 ± 3 10:14 12 42 ± 9 12 ± 4 55 ± 9 variance (ANOVA) (P = 0.05). No significant dif- ference between long (16:8) and short (10:14) pho- 27 Nov. 87 toperiodic responses at a given temperature for a 16:8 38 13 ± 2 11 ± 1 24 ± 2 13:11 21 22 ± 11 12 ± 4 30 ± 7 sample date was used as the criterion for ending 10:14 18 22 ± 16 11 ± 4 32 ± 16 diapause in field populations of A. comptana. The reciprocal of the mean postdiapause devel- 22 Dec. 87 16:8 27 6± 2 14 ± 2 20 ± 3 (l/days) comptana opmental time for A. was re- 13:11 32 7± 6 11 ± 1 17± 5 gressed against temperature, and the resulting curve 10:14 31 6± 2 11 ± 1 16 ± 3 = was tested for linearity using ANOVA (P 0.05). 7 Jan. 88 The theoretical threshold for postdiapause devel- 16:8 43 6± 2 12 ± 1 18 ± 2 opment (tpd) was estimated by extrapolation, and 13:11 34 6± 1 11 ± 1 17± 1 the thermal constant (K) was calculated as 11m, 10:14 37 6± 3 11 ± 1 15 ± 1 where m is the slope of the line predicted from 26 Jan. 88 linear regression analysis (Campbell et al. 1974). 16:8 40 4± 9 ± 1 14 ± 2 Degree-day Summations for Postdiapause De- 13:11 31 5± 12 ± 1 16 ± 1 velopment. To determine the accuracy of the t 10:14 38 5± 10 ± 1 15 ± 1 and K values for predicting postdiapause devel- ANOVA statistics given in Table 3. opment under field temperatures, we allowed a 40 A. camp/ana larvae placed at each photoperiod on 24 known numbers of A. comptana to overwinter in September 1987;50 larvae placed in each photoperiod on all other three 3.2-m2 strawberry plots during the fall of dates. 934 ENVIRONMENTALENTOMOLOGY Vol. 19, no. 4 Table 3. ANOVA statistics for developmental responses by overwinteringA. comptana larvae to temperature aod photoperiod (see Tables I and 2) Larval Pupal Total Sample date 22· 14· 22" 14· 22· F df P F df P F df P F df P F df P 26 Sept.86 352 2,32 0.0001 ll8 2,43 0.001 3.6 2,29 0.04 NS 16 Oct.86 48 2,36 0.0001 48 2,44 0.0001 NS NS 14 Nov.86 16 2, 12 0.004 36 2,24 0.0001 NS NS 24 Sept.87 ll3 1,38 0.0001 13 1,35 0.001 138 1,35 0.0001 15 Oct.87 230 1,37 0.0001 NS 260 1,37 0.0001 27 Nov.87 II 2,95 0.0001 NS 6.5 2, 74 0.0025 22 Dec.87 NS 50 2,86 0.0001 4.6 2,86 0.01 Downloaded from https://academic.oup.com/ee/article/19/4/932/2480444 by guest on 27 September 2021 7 Jan.88 NS 8.4 2, III 0.0001 3.8 2,111 0.025 26 Jan.88 NS 33 2, 106 0.0001 30 2, 106 0.0001 and minimum temperatures were recorded from gens.