Submitted to: Apidologie
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/29/2004
Publication Date: 1/1/2005
Citation: James, R.R. 2005. Temperature and chalkbrood development in the alfalfa leafcutting bee. Apidologie. p. 15-23. Interpretive Summary: The alfalfa leafcutting bee is used to pollinate alfalfa seed, but managed bees suffer from a fungal disease of the larvae called chalkbrood. This disease is more common in the western U.S., than in the interior northern states and Canada. We tested the effect of temperature on growth and spore germination for the fungus that causes the disease, and on the levels of disease in larvae. The lowest levels of chalkbrood disease in larvae occurred at 35°C, yet this temperature was the optimum for fungal germination and growth on agar, and is stressful to the insect. The fungus was most likely to produce spores in infected cadavers at 25°C, a moderate temperature. Daily exposures to high temperatures (40°C for 6 h) did not affect chalkbrood occurrence, but it reduced spore production. It is not clear why the greatest likelihood for disease does not occur at optimum temperatures for fungal growth, or at the temperatures most detrimental to the insect. A few hypotheses have been proposed, but further studies are needed.
Technical Abstract: Ascosphaera aggregata, the causative agent for chalkbrood in the alfalfa leafcutting bee (Megachile rotundata), is a major mortality factor when the bee is used as a pollinator in commercial fields. We tested the effect of temperature on A. aggregata hyphal growth, spore germination, and disease incidence. The lowest incidence of chalkbrood occurred at 35°C, yet this temperature was the optimum for fungal germination and growth on agar, and is stressful to the insect. Sporulation of M. rotundata cadavers that were infected with the fungus was highest at 25°C. Daily exposures to 40°C for 6 h did not affect disease incidence, but it negatively impacted spore production. A similar disease response to temperature has been seen by others for this bee, and other Ascosphaera spp in other bees, but it is not clear why the greatest likelihood for mycosis does not occur at optimum temperatures for the fungus, or at least at the temperatures most detrimental to the insect. A few hypotheses have been proposed, but empirical data are lacking.