Bioassay of selected fungicides for control of chalkbrood in alfalfa leafcutter bees, Megachile rotundata

Abstract - Bioassays were carried out to determine the effects of 5 selected fungicides on various developmental parameters on the incidence of chalkbrood in Megachile rotundata. The fungicides were bioassayed by incorporation into the natural provisions of leafcutter bees at the egg stage. Half of the fungicide-treated individuals were challenged with 1 x 10⁶ ascospores of Ascosphaera aggregata 24 h after treatment with fungicide. Addition of spore inoculum increased mortality, but chalkbrood levels remained under 50%. All fungicides tested had no effects on bee mortality of non-challenged larvae at doses ≤ 100 ppm. At 1 000 ppm, increased mortality occurred in the Ascocidin, Rovral and enilconazole treatments. Treatment with Rovral resulted in earlier mortality and a slightly prolonged developmental time. Ascocidin, Benlate and Rovral reduced levels of chalkbrood but were toxic to larvae. Consequently these compounds had no effect on total mortality. Treatment with DFMO resulted in increased levels of chalkbrood. Mean developmental times for dying larvae measured by the time to defecation were increased with Rovral. Treatment with Ascocidin, Benlate, DFMO and Rovral resulted in heavier sporulating cadavers compared with controls. This increase in weight of cadavers may translate to increased spore loads.