Title: Health status of alfalfa leafcutting bee larvae in commercial U.S. alfalfa seed fields Authors
Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 8, 2013
Publication Date: N/A
Interpretive Summary: The alfalfa leafcutting bee is used extensively for pollinating alfalfa seed crops. Its commercial value as a pollinator is second only to honey bees. Unfortunately, like honey bees, it suffers from health issues that have made it more difficult to propagate. We conducted a geographically broad survey at the end of the nesting season to evaluate the causes for bee production losses in alfalfa seed fields. Healthy larvae were found in only 47% of the nest cells. The largest production losses were due to a condition called pollen ball (a nest cell with food but no larva)(16.7%), larvae killed by a disease called chalkbrood (8.0%), and larvae that died for no apparent reason (15.5%). Frequency of pollen balls was related to the number of adult bees used per hectare and the age of the alfalfa stands. The unknown mortality corresponded with the USDA plant hardiness zone. Since warmer climates have higher plant hardiness zone designations, this mortality appears to be associated with warmer climates, but the results were not conclusive. The frequency of chalkbrood was correlated with using fewer bees per hectare and more shelters, but the correlations were not strong and so may be due to other factors that were not accounted for. Interestingly, occurrence of this disease was not affected by nesting board disinfection practices. Vapona is an insecticide used to control parasitoids in the parent bees. This fumigant treatment was associated with an increase in both chalkbrood and the onset of winter dormancy in the larvae. How this adult treatment could affect the offspring in this way can only be speculated. This survey quantifies the variation in the quality of alfalfa leafcutting bees in commercial populations produced across most of the U.S. alfalfa seed production area.
Technical Abstract: We conducted a geographically large survey to quantify production losses in the alfalfa leafcutting bee (Megachile rotundata, Hymenoptera: Megachilidae), a solitary pollinator used extensively in alfalfa seed production. Healthy prepupae were found in only 47.1% of the nest cells collected at the end of the season. Most other cells contained pollen balls (typified by a provision but no larva)(16.7%), unknown causes of larval mortality (15.5%), or larvae killed by chalkbrood (8.0%). Prevalence of pollen balls was correlated positively with bee release rates and negatively with alfalfa stand age. The unknown mortality was correlated with the USDA plant hardiness zone, and thus, some of the mortality may be caused by high temperature extremes, although the nesting season degree days was not correlated. Chalkbrood prevalence was correlated with possible nesting-resource or crowding related factors, such as the number of bees released per hectare and the number of shelters used, but not with nesting board disinfection practices. Vapona® is used to control parasitoids when the parent bees are incubated prior to release. This fumigant was associated with an increase in both chalkbrood and diapausing offspring, although any reason for these correlations are unknown. This survey quantifies the variation in the quality of alfalfa leafcutting bee commercial populations produced across much of the U.S. alfalfa seed production area.