Nondormancy in Entomophaga maimaiga azygospores: effects of isolate and cold exposure

Azygospores (resting spores) of the fungal pathogen Entomophaga maimaiga are produced in later larval instars of the gypsy moth Lymantria dispar and normally enter constitutive dormancy. In the laboratory cadavers of recently dead larvae containing immature azygospores were placed on 1.0% water agar at 20 C for 2 wk after host death, allowing time for azygospore maturation. We found that some azygospores produced in this way did not enter dormancy. To investigate nondormancy, suspensions of azygospores from individual cadavers were transferred to moist, sterile soil at 15 C. Groups of gypsy moth larvae were exposed sequentially to azygospores for consecutive 4 d periods for 196 d. Infections first were seen among larvae exposed 24-28 d after bioassays began, and infection potential continued 196 d. Germination tests confirmed azygospore germination. Additional cadavers containing azygospores produced under the same conditions were maintained at 4 C for 1-8 mo, and each month new sequential bioassays were initiated. There was a general trend of earlier initiation of infection with longer durations of 4 C exposure; after 6-8 mo at 4 C first infections occurred 6-10 d after bioassays began. With 5-8 mo at 4 C infection levels declined after 96 d of sequential bioassays. Activity of azygospores differed by the individual larval cadaver in which they were produced; azygospores from 29.2% of cadavers yielded only 0-0.3% infection. Infection from nondormant azygospores did not differ among three Japanese isolates and two North American isolates, although azygospores from a third North American isolate caused no infections in 84 d, suggesting that dormancy had not been prevented.