Presence of pathogen-killed larvae influence nesting behavior of the alfalfa leafcutting bee, (Hymenoptera:Megachilidae)

Authors: Klinger, Ellen; WELKER, DENNIS - Utah State University; James, Rosalind

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2021
Publication Date: 3/17/2021
Citation: Klinger, E.G., Welker, D., James, R.R. 2021. Presence of pathogen-killed larvae influence nesting behavior of the alfalfa leafcutting bee, (Hymenoptera:Megachilidae). Journal of Economic Entomology. 114(3):1047-1053. https://doi.org/10.1093/jee/toab030.
DOI: https://doi.org/10.1093/jee/toab030

Interpretive Summary: Bees (alfalfa leafcutting bees) used for pollinating alfalfa plants can get a fungal disease called chalkbrood. While there are many cases of diseased insects changing their behaviors to alter disease spread, there is no known behavior of this bee when infected with fungal spores. We would like to fully understand if there are behaviors, as some may reduce the amount of offspring females can produce, leading to costly losses for alfalfa pollinators. We exposed alfalfa leafcutting bees to three treatments (no diseased bees, diseased bees, and diseased bees that had been heated to kill the disease) over three years and observed how many offspring they made. We found that the bees exposed to the heat-killed diseased bees had the most success in offspring. We conclude that potential grooming behaviors and ability to stay in the nesting site were behaviors most likely caused by the presence of a diseased bee. If we can find a way to mimic behaviors caused by the non-infective diseased bees, we can potentially improve bee nesting.

Technical Abstract: The alfalfa leafcutting bee, a commercial pollinator used for alfalfa seed production, is susceptible to chalkbrood disease via ingested fungal spores. Diseases of insects can elicit behavioral changes in their hosts, but there are no recorded behaviors of alfalfa leafcutting bees in response to fungal exposure. We conducted field studies to determine if bees in pathogen-dense environments altered their nesting patterns, specifically if bees exposed to fungal spores produced higher numbers of nest cells and whether the proportions of nest cells that failed as eggs or small larvae (a state known as “pollen ball”) were greater. We found that our control bees, non-treated bees which were not exposed to chalkbrood spores other than those in the natural environment, had the highest proportion of pollen ball cells. Bees experimentally exposed to infective spores created the lowest number of nests and the fewest cells. Bees experimentally exposed to heat killed non-infective spores produced the greatest number of nests and cells overall and the greatest number of healthy progeny. We conclude that that there are underlying behaviors that are elicited in response to presence of chalkbrood spores that reduce the proportion of failed nest cells (grooming) and increase retention of bees at nesting sites (delay of bee emergence). Through further study of these behaviors bee managers can potentially increase the productivity of their bee populations.