Factors affecting the production of resting spores by Zoophthora radicans in the spotted alfalfa aphid, Therioaphis trifolii f. maculata

Abstract

Resting spore production by Zoophthora radicans (Brefeld) Batko in infected cadavers of the spotted alfalfa aphid, Therioaphis trifolii (Monell) f. maculata, varied according to a number of factors. For four isolates of the fungus tested, the proportion of cadavers containing resting spores was consistently greater at 10 °C than at higher temperatures. At 20 °C, a saturated atmosphere (100% RH) favoured resting spore production, and the proportion of infected cadavers that contained resting spores was nearly twice the amount of that at 95% RH. Individual isolates of the fungus were found to differ in their ability to form resting spores in infected cadavers; some formed resting spores in few or no cadavers, whereas others formed resting spores in many, under similar conditions. The ability to produce resting spores sometimes varied following serial culturing on solid media. Interestingly, resting spore production was found to correlate with inoculum density. Also, inoculation of aphids with two fungal isolates resulted in a marked increase in resting spore production compared with that of either isolate alone. It is presumed that some form of genetic combination is involved. Indirect evidence suggests that this is more likely to be cytoplasmic than nuclear in origin.