Hatch of Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae) eggs exposed to two different overwintering and hatch regimes.

Diabrotica virgifera virgifera LeConte eggs from South Dakota collected adults were exposed to two overwintering regimes (field conditions and 8.5? C) and two hatch regimes (simulated field conditions and 25? C). Thermal units (TU) for each hatch regime were calculated at base tem perature of 11? C. No significant differences were found among treatments for percent hatch. No significant differences were found in TU except for those that were overwintered in the field and had a hatch regime of 25? C. Comparisons between this study and others provided evidence that throughout the midwestern corn-belt, first hatch could be predicted to occur at a TU accumulation of 232 ? 21 {X ? SD) and 50% occurrence of hatch could be predicted to occur near a TU accumulation of 292 ? 69. The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, has been one of the most important pests of corn in the USA for over three decades. There have been numerous publi cations and reviews concerning this insect, however, only a few have addressed the use of thermal units (TU) for predicting egg hatch (Sechriest, 1969; Weiss and Mayo, 1983; Wilde, 1971; Wilde et al., 1972). In the aforementioned studies temperature accumulations were not given or were not comparable among studies and/or the number of eggs upon which the results were based was not given. The objectives of this study were 1) to determine the TU for egg hatch under field temperature conditions; and 2) determine if the TU differed for eggs that were kept under normal laboratory temperature conditions (eggs given 5 months chill at 8.5? 0.5? C for diapause termination then placed at 25? 1? C for hatch) from eggs that were kept under field temperature regimes. methods and materials: All eggs used in this study were obtained by the methods of Jackson (1986) from adult WCR that had been collected in the late summer of 1986 in a field near Madison, SD. The procedures used for overwintering of eggs were as follows: eggs were allowed to remain in the original oviposition dishes for 3 weeks at 25? 1? C. They were then placed in screened (top and bottom with 180 ^m opening screen) 100 mm x 100 mm x 15 mm dishes (ca. 250 eggs/dish) that contained a moist layer (3-5 mm) of 80 mesh (180 fim) sieved soil. The eggs were then covered with another layer (3-5 mm) of 80 mesh soil and covered with the screened lid. The egg dishes were placed for the winter either at 17 cm deep in a field plot of bare surfaced soil (eight dishes) or buried near the center of a soil filled 5 liter bucket (four dishes, one dish per bucket) and placed in a constant temperature chamber at 8.5? 0.5? C. Field temperatures were monitored with a recording thermograph that had a sensor placed at 17 cm deep in the field. In the spring when the field temperatures warmed to near 8? C, the dishes with eggs were removed from the field soil plot and from the constant temperature chamber (Julian Day [JD] 105). Eggs were washed from the screened dishes through a 60 mesh (250 urn opening) sieve; removing the soil but retaining the eggs. Eggs from each group were then counted and placed in groups of 25 on the surface of moist 80 mesh sieved soil in a 5 cm x 5 cm x 1.5 cm plastic capped dish. Each group of eggs (overwintered in the field or overwintered at a constant temperature) was subdivided for hatch treatments. Group 1, 500 eggs (10 replications of 50 eggs each) had been exposed to overwinter field temperatures at 17 cm deep and then placed at simulated field temperatures for 10 cm deep on JD 105. These field temperatures were simulated in a refrigerated incubator that was controlled by a Watlow? controller. The controller directly responded to a hard-wired remote ther mocouple placed in a bare soil plot at 10 cm deep. Accuracy of the incubator temperatures to the field soil temperatures was substantiated by comparing the daily charts from a recording thermograph that had a sensor placed at 10 cm deep in the field, adjacent to the remote thermocouple and a recording This content downloaded from 157.55. 39.49 on Mon, 26 Sep 2016 05:34:28 UTC All use subject to http://about.jstor. org/terms 608 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Table 1. Diabrotica virgifera virgifera, percent hatch, first hatch occurrence, and 50% hatch oc currence when exposed to two overwintering and two hatch regimes. Overwintering Total c Julian day (X ? SD) regime Hatch regime exposed % Hatch First hatch 50% hatch Field Fieldd 500 58 ? 12 170.8 ? 1.7 174.3 ? 0.9 8.5? C Fieldd 200 67 ? 14 170.0 ? 1.1 173.6 ? 1.1 Field 25? C 500 62 ? 11 124.9 ? 1.2 129.0 ? 1.8 8.5? C 25? C 200 68 ? 8 120.3 ? 0.7 124.3 ? 1.4 a Means (X ? SD) followed by the same letters do not differ significantly from each other; Student Newman-Keuls test for multiple comparisons; P ? <0.05. b ANO VA: F = 15.62; d.f. = 3, 52; P = 0.0001. c ANOVA: F = 16.72; d.f. = 3, 52; P = 0.0001. d Field simulated temperatures. thermograph with the sensor placed in the incubator. Group 2, 200 eggs (four replications of 50 eggs each) had been exposed to 8.5? 0.5? C over the winter and then placed in the field temperature simulation incubator on Julian Day 105. Group 3, 500 eggs (10 replications of 50 eggs each) had been exposed to field temperatures over the winter and then placed in a constant temperature incubator at 25? 0.5? C. Group 4, 200 eggs (four replications of 50 eggs each) had been exposed to a temperature of 8.5? 0.5? C over the winter, and then placed in a constant temperature incubator at 25? 0.5? C on Julian Day 105. Hatch and mortality from all groups were observed daily or as needed from Julian Day 105 until Julian Day 258. Data for the purposes of this study were compiled for first hatch and 50% hatch. TU were recorded daily with an Omnidata? biophenometer placed in each regime on Julian Day 105. The biophenometer records TU so that on any given sample date the accumulated TUs were as follows: