Effect of temperature on development and survival in post-diapausing leafcutting bee pupae (Megachile rotundata (F.)). II. Low temperature.

Abstract

At the time of the first male emergence leafcutting bee cells were exposed to temperatures of 4.4?, 15.6?, 18.3?, 21.1? and 23.9?C for one to 16 consecutive days; to a diurnal cycle of 15.6? and 29.5?C for four to 16 consecutive days; and to a series of alternating daily temperatures of 15.6? and 29.5?C to determine the effect of low temperatures on mortality. All temperature treatments resulted in a low or non-significant mortality. Development and emergence of pupae and teneral adults can be arrested for at least nine days when held at or below 18.3?C without increasing mor tality significantly. The alfalfa leafcutting bee, Megachile rotundata (F.), has been managed as an alfalfa pollinator since 1960 (Stephen and Torchio, 1961). The rate of increase in the bee depends on the control that can be exercised over the negative contributions of many biotic and abiotic factors. In leafcutting bee management, overwintered cells are kept under refrig eration at 2? to 4?C and subsequently incubated at 29? to 30?C on a schedule to synchronize their emergence with peak alfalfa bloom (Stephen and Os good, 1965). Cells are usually incubated in trays and once males begin to emerge, the trays are transferred to field domiciles which the subsequent emergent bees will populate. Ambient temperatures in the spring often fluc tuate widely and the extremes are suspected of causing substantial bee mor tality during the emergence period (Stephen and Undurraga, 1977). Brief exposure to 50?C during the emergence period have been shown to cause complete mortality to pupae and teneral adults (Undurraga and Stephen, 1980), but the effect of low temperatures during this period are unknown. This is a study on the effect of low and fluctuating temperatures on devel opment and survival of post-diapausing leafcutting bee pupae. Materials and Methods Bee cells from Winnett, Montana were incubated at 29.5?C and 25-30% RH following the procedures previously reported (Undurraga & Stephen, Received for publication 25 April 1980. This content downloaded from 157.55.39.163 on Wed, 23 Nov 2016 04:28:51 UTC All use subject to http://about.jstor.org/terms 678 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY Table 1. Effect of low temperatures on leafcutting bee emergence. Treatments? exposure intervals in?C No. No. bees cells emerged Emerg. Emerg. duration (days) Emerg. delay vs. control (days) 4.4??1 day 4.4??3 days 4.4??6 days 4.4??9 days 15.6??1 day 15.6??3 days 15.6??6 days 15.6??9 days 9. 29.5??control 52 52 55 49 56 53 53 49 54 50 50 51 43 53 49 51 43 51 96.2 96.2 92.7 87.8** 94.6 92.5 96.2 87.8**1 94.4 11 11 11 9 10 10 10 10 11 12 15 13 11 13 14 12 11 16 + 1 + 3 +7 + 8 + 1 + 2 +6 + 8 + 1 + 3 + 7 + 9 + 1 + 3 +6 + 9 Significance: **/> > 0.99. 1980). A total of 47 samples of 26 to 58 cells were accorded various tem perature treatments at the time the first adult males appeared (?21st day of incubation) to simulate field conditions. Cells were exposed to 4.4?, 15.6?, 18.3?, 21.1? and 23.9?C from 1 to 16 days (Tables 1 and 2); daily cycles of 15.6? and 29.5?C (Table 3); and alternating 24-hour periods of 15.6? and 29.5?C. Cells were transferred directly from one temperature regime to another, and equilibration, as determined by an intracell Cu-Co thermocouple, re quired from 5 to 10 minutes. Treated cells were returned to a constant 29.5?C until emergence was completed. Bee emergence was recorded daily. The normal deviate Z was used to compare the differences in survival among the treatments (Snedecor and Cochran, 1967). Results and Discussion There was no difference in survival among any of the constant or alter nating low temperature treatments, except for 4.4?C for 9 days (Table 1) and 15.6?C for 9 and 15 days (Tables 1 and 2). Even in the latter treatments, the mortality was minor compared to that recorded from some field-moni tored populations (30-35%). There was an expected decrease in survival associated with the increase in duration of the cold treatments (Tables 1 and 2). The differences in survival between the two treatments at 15.6? C for 9 days and the controls probably reflect sample variance. Mortality in bees exposed to 18.3?, 21.1? and 23.9?C for 4 to 16 days was not significant (Table 2). However, the time necessary to reach 50% and 90% emergence was altered in each sample (Emergence Delay, Tables 1 4). For example, all three samples treated at 4.4? and 15.6?C for 9 days (Tables 1 and 2) reached 50% and 90% emergence 9 days later than the This content downloaded from 157.55.39.163 on Wed, 23 Nov 2016 04:28:51 UTC All use subject to http://about.jstor.org/terms VOLUME 53, NUMBER 3 679 Table 2. Effect of low temperatures on leafcutting bee emergence (T?C). Treatments? exposure intervals in?C No. No. bees cells emerged Emerg. % Emerg. duration (days) Emerg. delay vs. control (days)