The effect of humidity on germination and infection of termites by the hyphomycete, Metarhizium anisopliae.

The effect of relative humidities (r.h.) from 90 to 100% on germination of a termite-active isolate of Metarhizium anisopliae (isolate FI25 and FI610) was studied using a liquid germinating medium to which the appropriate amount of glycerol had been added. Germination was increasingly delayed at water activities equivalent to 99, 98, and 96% r.h. and completely inhibited at 94, 92, and 90%. Twenty-one isolates were then screened for germination at 96 and 100% r.h. All isolates showed delayed germination at 96% r.h. but most isolates eventually gave a high final percentage germination at this humidity. Two isolates, FI527 and FI638, were markedly slower to germinate at both humidities. The susceptibility of two species of termites, Nasutitermes exitiosus and Coptotermes acinaciformis, to FI610 was tested at r.h. down to 86%-the lowest humidity at which the insects would survive. No consistent effect of humidity on pathogenicity was detected. Mortality occurred over the range range of humidities tested; sporulation from the disease-killed termites, however, occurred only at r.h. above 93%. It is concluded that the microclimate around living termites is usually sufficiently humid to ensure infection under most field conditions and that humidity is unlikely to limit the efficacy of the fungus in controlling termites.