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:
Growers that rear alfalfa leafcutting bees must maintain them within temperature ranges that promote bee survival, fitness, and reproduction, while allowing high synchrony of emergence with crop bloom and low 2nd generation bee production. In 2011, we continued our studies of the effect of temperature on leafcutting bees, adding an assessment of the lipid content of adult females collected from the field. We also published a paper on a three-year study of the effect of pre-release temperatures on the emergence success and health of M. rotundata (Environmental Entomology 40: 917-930) and another on seasonal changes in pollen use by bees (Apidologie 42: 223-233). Below, we summarize data from three on-going studies. 1. Fall storage study: Bee cells were removed from field shelters in early September and placed on a lab temperature gradient to simulate the effect of variable early-fall storage temperatures. Cells kept at 25C (77F), or lower, had higher emergence success the following summer; 79 percent of cells stored at or below 25C produced adults later, but only 21 percent of those above 25°C did so, a nearly 4-fold decline in emergence. Among bees that emerged, both males and females had a higher proportion of body lipids (fats), a likely indicator of better bee health. Even temperatures as low as 7-15C (45-59F) allow high emergence rates and lipid content. These results, as well as preliminary findings from our late-fall rearing studies (done in cooperation with ARS, Logan, UT), confirm that relatively low fall temperatures are beneficial to pre-wintering bees and that there is a sharp drop-off in bee survival and health above the 25C threshold. 2. Lipid content of bees in the field: If lipid content is important to adults bees, especially females which use stored fat to make eggs, then it should decline rapidly after females begin summer nesting activity. To study this, we collected bees in 2011 from the start of field activity until late August. After analyzing lipid content, it was evident that it declined rather sharply (by about 50 percent) within two weeks of the beginning of the flight season and remained fairly constant after that. The slight rise in average lipid content by the end of August may reflect the appearance of 2nd generation females that have not yet used up much of the stored fat. 3. Summer temperatures and the production of a 2nd generation: We have been conducting two studies of the effect of summer field temperatures on bee health and the appearance of 2nd generation bees. During two summers, we kept isolated whole nests at temperatures of 18-32C (64-90F). The number of 2nd generation adults per nest increased with temperature both years. We are in the process of analyzing the data further to determine sex ratio of the 2nd generation, and the effect of temperature on emergence success of those bees that enter diapause.