The potential of the fungus, Verticillium Lecanii as a control agent of glasshouse aphid pests

Abstract

The ecology of the fungus, Vertlciilium lecanii, and its potential as a microbiological insect cide against the aphids, MyzuS persicae, Macrosiphoniella sanborni and Brachycaudus helichrysz, on glasshouse chrysanthemums have been examined.

To achieve reproducibility, a single-spore isolate of V. lecanii, C-3, was used and designated as standard. It produced conidiospores on solid media and, more abundantly and readily, blastospores in liquid media. In nutritional studies, submerged production of conidiospores was not achieved. In aqueous suspension, conidiospores survived longer than blastospores. Conidiospores exposed below 80% relative humidity soon died, the speed of death depending on the spore microenvironment.

A bioassay technique was developed and is believed to be the first statistically reliable method of quantifying fungal spore pathogenicity towards insects. With M. sanborni.the semi-weighted mean lethal concentration 50% of 28 assays was 1.83 x 10"' spore /ml suspension (fiducial limits, 1.45 and 2.31). All but one of 15 V„ lecanii strains, some pathogens of other insects and some probable hynerparasxtes of rust fungi, were similar in pathogenicity to C-3, Five other strains, all from rust or mildew fungi, were apathogenxc. B ..astospores of C—3 were approximately twice as pathogenic as conidiosporeso With single- and multi-spore isolates, repeated passage through apaids or subcu, ture on agars scarcely altered pathogenicity, but subculture greatly chanced colonial morphology.

In glasshouses, blastospores and conidiospores produced similar levels of aphid control. Spore sprays established excellent control of My. persicae within 2-3 weeks, maintaining control for up to 8 weeks. Also, B. helichrysi was controlled in vegetative plant tips but not on tightly closed flower buds. Control of M. sanborni was usually unsatisfactory. In laboratory bioassays, all 3 aphid species were equally susceptible to V. lecariii suggesting that behavioural and ecological factors explain differences in glasshouses. V. lecanii could be used advantageously in integrated control systems on chrysanthemums.

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