ARS | Publication request: Laboratory Bioassays to Evaluate Fungicides for Chalkbrood Control in Larvae of the Alfalfa Leafcutting Bee, Megachile rotundata (Hymenoptera: Megachilidae)

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 7, 2008
Publication Date: October 1, 2008
Citation: Huntzinger, C., James, R.R., Bosche, J., Kemp, W.P. 2008. Laboratory Bioassays to Evaluate Fungicides for Chalkbrood Control in Larvae of the Alfalfa Leafcutting Bee, Megachile rotundata (Hymenoptera: Megachilidae). Journal of Economic Entomology.

Interpretive Summary: A serious problem with alfalfa seed production arises from a fungal disease called chalkbrood that causes mortality in the alfalfa leafcutting bee, the principal pollinator of alfalfa. Laboratory bioassays were conducted with four fungicides, Benlate®, Captan®, Orbit¿, and Rovral®, to begin screening fungicides to identify those with the potential for chalkbrood control. Fungicides were tested for efficacy against growth of fungal cultures, and effects on incidence of chalkbrood disease and toxicity to the alfalfa leafcutting bee larvae. Both Benlate® and Rovral® reduced fungal growth and incidence of chalkbrood with minimal toxicological effects on larval bees. Orbit¿ was effective in reducing fungal growth, but did not reduce incidence of chalkbrood and was toxic to bee larvae. Captan® was not effective in reducing hyphal growth or chalkbrood incidence, and was toxic to bee larvae. Fungicides that reduce incidence of chalkbrood and larval mortality in this laboratory study are candidates for further study for chalkbrood control.

Technical Abstract: Chalkbrood, a fungal disease caused by Ascosphaera aggregata, is a major mortality factor in populations of the alfalfa leafcutting bee (Megachile rotundata) used in commercial alfalfa seed production. Four formulated fungicides, Benlate®, Captan®, Orbit¿, and Rovral®, were tested in the laboratory for efficacy against hyphal growth of A. aggregata cultures, effects on incidence of chalkbrood disease, and toxicity to larvae of M. rotundata. Both Benlate® and Rovral® reduced fungal growth and incidence of chalkbrood with minimal negative toxicological effects on larval bees. Orbit¿ was effective in reducing hyphal growth, but did not reduce incidence of chalkbrood and was toxic to bee larvae. Captan® was not effective in reducing hyphal growth or chalkbrood incidence, and was toxic to bee larvae. Fungicides that reduce incidence of chalkbrood and larval mortality in this laboratory study are candidates for further study for chalkbrood control.