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United States Department of Agriculture

Agricultural Research Service

Title: Impact of Disinfecting Nesting Boards on Chalkbrood Control in the Alfalfa Leafcutting Bee

Author
item James, Rosalind

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 16, 2005
Publication Date: August 1, 2005
Citation: James, R.R. 2005. Impact of disinfecting nesting boards on chalkbrood control in the alfalfa leafcutting bee. Journal of Economic Entomology. 98(4):1094-1100.

Interpretive Summary: The alfalfa leafcutting bee has been used in large numbers to pollinate alfalfa seed crops since the 1960s. These bees are kept in nesting boards that are essentially a solid board with holes drilled or molded into them. However, bee reproduction has been greatly affected by chalkbrood, a larval disease caused by a fungus. In the U.S., attempts to control this disease have been fairly unsuccessful, but include removing the overwintering larvae from the nesting boards, and then disinfecting the boards with heat treatments or a fumigant before they are used again the next year. The problem is that many boards are made of polystyrene (so heat cannot be used), and very few fumigants are registered for this use. In this study, the effectiveness of heat and methyl bromide fumigation were compared to a new disinfection method, ozone exposure. Ozone did not kill many of the fungal spores in fumigation chambers, and did not reduce chalkbrood levels in the field. Methyl bromide and heat treatments did kill large numbers of spores, but did not reduce chalkbrood levels in the field. Surprisingly, new nesting boards (boards free of spores) also had the same amount of chalkbrood in the field as used (contaminated) boards. Disinfecting nesting material may be necessary for controlling chalkbrood, but it is not enough, in and of itself. Enough spores were present in the field from someplace other than the boards to keep infections levels high. I hypothesize that this source was the emerging female bees, but the source needs to be determined so that it can be targeted for control.

Technical Abstract: The alfalfa leafcutting bee (Megachile rotundata (Fab.)) is a solitary, cavity nesting bee that has been managed in large numbers to pollinate alfalfa seed crops since the 1960s. Propagation of these bees from one year to the next has been seriously hampered by chalkbrood, a larval disease caused by the fungus Ascosphaera aggregata Skou. In the U.S., attempts to control the disease have been fairly unsuccessful, but include removing nests from the nesting boards, and then disinfecting the boards with heat treatments or a fumigant. The problem is that many boards are made of polystyrene (so heat cannot be used), and very few fumigants are registered for this use. In this study, ozone was tested as a fumigant and compared with heat treatments and methyl bromide fumigation. Ozone was found to be inadequate for killing A. aggregata spores, and for reducing chalkbrood levels in the field. Methyl bromide and heat treatments did greatly reduce spore viability in the boards, but did not reduce chalkbrood levels in the field. Surprisingly, larvae in new nesting boards (boards free of contamination) and chalkbrood infection levels were similar to those nesting in contaminated, used boards. Disinfecting nesting material may be necessary for controlling chalkbrood, but the results reported here are an indication that it is not sufficient in and of itself. Some other source of spores was present in the field that was greater than the effect of contamination from the boards. I hypothesize that this source was the emerging female bees, but the source needs to be determined so that it can be targeted for control.

Last Modified: 11/23/2014
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