1a. Objectives (from AD-416):
The alfalfa leafcutting bee is a critical pollinator for alfalfa seed production. Improved methods for managing alfalfa leafcutting bee health is needed so that growers can more effectively produce their own bees and not have to rely so extensively on importing bees from Canada. The objective of this project is to construct a model that combines information regarding rearing temperatures and post-overwinter development time, survival, and adult bee condition and provide recommendations on rearing temperatures that minimize development time and variability in development rates, while simultaneously maximizing bee survival and adult bee condition.
1b. Approach (from AD-416):
The approach is relatively straightforward, involving 1) collection of bees cells at different times of year from multiple sources, 2) dissection of large samples of bees cells to diagnose mortality factors, and 3) conducting life-table analyses. The latter is being done at Montana State University which has expertise on life table analyses, including accounting for irreplaceable mortality. The analyses will be done using a computer program developed specifically for these analyses. Overall, the study should provide new insights into benefits to be obtained from potential strategies for reducing developmental mortality of alfalfa leafcutting bees.
3. Progress Report:
Effective alfalfa leafcutting bee management requires an in-depth understanding of the role temperature plays on bee health and survival throughout their life cycle. We continued lab and field studies examining the effects of temperature on various bee attributes including fat content, egg development, second generation production, and survival. For two years running, bee cocoons (cells) were collected in late October and placed on a thermal gradient to simulate variable late-fall storage temperatures and determine whether bee survival depends on temperature and the date bees are moved into winter storage (4 C). After bees emerged the following year, we also measured fat content. We found that an early transfer of cocoons to winter storage (such as by Nov.) and low prior storage temperatures provided the best winter survival and fatbody levels in emerging adults. We also found that rearing temperature affects fat stores of emerging adult females, and hypothesized that, if fat content is important to nesting bees, we should see an effect on their egg production and a rapid decline in fat content among nesting females after their emergence as adults. To test the latter, we collected females at nest boxes throughout the summers of 2010 (in MT) and 2011 (MT and UT). Fat declined 29-39 percent within 1 wk after bee release, probably because it was being quickly metabolized for egg production. Fat stores then remained low for the rest of the summer, reflecting the fact that females have few options for renewing fat after emergence. The potential management implication of this is that, if bees are reared at suboptimal temperatures, it may not only affect development rate and survival, but reduce post-emergence fecundity and the production of new bees. In the lab, we also simulated variable temperatures experienced by larvae during the nesting season and found that the date bees were removed from the field affected the proportion of second generation bees emerging. Among cells removed on July 31, the ratio of successfully overwintering bees to those that emerged as second generation was about 5 to 1, while for those removed on Aug. 16 it was less than 2 to 1. Temperatures experienced by larvae also affected production of second generation bees. In some samples, larvae reared above 30 C (86 F) produced mostly second generation bees. If this result holds up in subsequent experiments, it suggests that problems with second generation bees are somewhat at the mercy of field conditions (high temperatures) that cause developing larvae to emerge early.