Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 15, 2002
Publication Date: November 20, 2002
Citation: Kanga, L.H., James, R.R., Boucias, D.G. 2002. Hirsutella thompsonii and Metarhizium anisopliae as potential microbial control agents of Varroa destructor, a honey bee parasite. Journal of Invertebrate Pathology. 81:175-184.
Interpretive Summary: The honey bee is of great economic importance not only for honey production, but also for crop pollination. Each year the bee industry in the U.S. contributes over $100 million of honey products and several billions of dollars worth in pollination of crops and vegetables. The parasitic mite, Varroa destructor, is currently the most serious threat to beekeeping. Infestations of Varroa mite in North America caused annual losses of untreated colonies as high as 50-80% in some areas of the United States. Apistan(R) (tau-fluvalinate) and Check-Mite(R) (coumophos) strips are the most widely used Varroa controls in the United States and Canada. Mites throughout Europe and North America are evolving resistance to miticides, threatening the bee industry. As a result, there is an urgent need for alternative control strategies that are sustainable and cost effective. We found that Hirsutella thompsonii and Metarhizium anisopliae were highly pathogenic to Varroa and mite mortality occurred 3-4 days after the spores were applied. Observation hives treated with H. thompsonii resulted in a significant mite mortality that was sustained over 42 days. The fungal pathogens were harmless to honey bee populations and did not affect the fecundity of the queen honey bee. These fungi show good potential for serving as effective biological control agents against the Varroa mite. We set up a cooperative research agreement with Sylvan Bioproducts Inc., which will assist us in developing methods for producing and formulating the fungi for Varroa control. Currently, we are developing more efficient application technology to reduce the time required per application, and make the treatments economically viable for beekeepers.
The potential of Hirsutella thompsonii Fisher and Metarhizium anisopliae (Metschinkoff) as biological control agents of the parasitic mite, Varroa destructor (Anderson and Trueman) was evaluated in the laboratory and in observation hives. Time required for 90% cumulative mortality of mites (LT90) was 4.16 (3.98-4.42) days for H. thompsonii and 5.85 (5.48-7.43) days for M. anisopliae at 1.8 x 10**8 conidia mm**-2. At temperatures similar to that of the broodnest in a honey bee colony, H. thompsonii (LD90=1.68x10**7 conidia mm**-2 at day 7) and M. anisopliae (LD90=1.29 x 10**9 conidia mm**-2 at day 7) showed a high level of pathogenicity to V. destructor. The applications of fungal pathogens to observation hives resulted in signficant mortality of mite populations, and reduction of the number of mites per bee for 21 and 42 days post-treatments; but the treatments did not significantly affect the levels of mite infestations in sealed brood. However, the pathogens must have persisted fairly well because mites were still infected [(82.97 (0.6)%] even 42 days post-treatments. In addition, the pathogens were found to sporulate on the host. A small percentage [(2.86(0.2)%] of dead mites found in the control hives also sporulated, suggesting that adult bees drifted between hives and disseminated the pathogen. H. thompsonii was harmless to the honey bees and did not have any deleterious effects on the fecundity of the queens. Microbial control with fungal pathogens provided promising new avenues for control of V. destructor and could be a useful component of an integrated pest management program in honey bee industry. Additional studies toward the development of more efficient application technologies, on the effects of the fungi on non-target organisms, and on an improvement of fungal formulations, should provide useful insights in the further development of these pathogens as biological control agents.