Formation and germination of Entomophaga maimaiga azygospores

Abstract

Azygospores (resting spores) of the gypsy moth fungal pathogen Entomophaga maimaiga are produced in abundance during late spring and early summer in late-instar gypsy moth larvae (Lymantria dispar). Azygospores subsequently form, each from an individual hyphal body. Development of azygospores occurs asynchronously over several days; by 5 days after host death, greater than 60% of fungal cells had matured from hyphal bodies to the final double-walled resting state. Azygospores undergo constitutive dormancy and, under field conditions, will not germinate for approximately 9 months after production. Azygospores do not require nutrients to germinate. Germination of field-collected resting spores under laboratory conditions began more than 2 days after transfer from the field to the laboratory. Higher levels of germination occurred with a 14 h L: 10 h D cycle compared with 13 h L: 11 h D or 12 h L: 12 h D. Azygospores germinate relatively slowly and germination rates were greatest between 4 and 8 days, with a total of 71.8 or 72.5% germination by 16 days at 14 h L: 10 h D and 15 or 20 °C, respectively. During 1994 and 1995, resting spores began causing infections in experimental larvae in early May, about 1 – 2 weeks prior to gypsy moth egg hatch, and ceased causing infections in mid to late June, when late instars were present. This latter timing is a correction of previously reported information. Bioassays investigating resting spore activity determined that during 1994, once resting spores began germinating in the field, levels of infection were positively associated with soil moisture. Key words: azygospores, resting spores, entomopathogenic fungi, Entomophaga maimaiga, Lymantria dispar, biological control.