Fungicide tests on adult alfalfa leafcutting bees (Hymenoptera: Megachilidae)

Chalkbrood is a serious disease of alfalfa leafcutting bee Megachile rotundata (F.) (Hymenoptera: Megachilidae) larvae, causing upward of 20% infection in the field. The causative agent is the fungus Ascosphaera aggregata. This bee is used extensively for alfalfa seed pollination in the United States. Using laboratory bioassays, we previously demonstrated that fungicides can reduce chalkbrood levels in the larvae. Here, we evaluate the toxicity of four fungicides, Benlate, Captan, Orbit, and Rovral, to adult bees by using three different bioassays. In the first test, fungicides were applied to bees' thoraces. In the second test, mimicking foliage residue, a piece of filter paper soaked in fungicide was placed on the bottom of a container of bees. The third test evaluated oral toxicity by incorporating fungicides into a sugar-water solution that was fed to the bees. The filter paper test did not discriminate among the fungicides well, and the oral test resulted in the greatest mortality. Toxicity to males was greater than to females. The use of fungicides for chalkbrood control is a logical choice, but caution should be used in how they are applied in the presence of bees.