Verticillium lecanii (formerly known as Cephalosporum lecanii) was first described in 1861 and is a cosmopolitan fungus found on insects. It is a common pathogen of scale insects in tropical and subtropical climates. V. lecanii is known as a "white-halo" fungus because of the white mycelial growth on the edges of infected scale insects. The conidia (spores) of V. lecanii are slimy and attach to the cuticle of insects. The fungus infects insects by producing hyphae from germinating spores that penetrate the insect's integument; the fungus then destroys the internal contents and the insect dies. The fungus eventually grows out through the cuticle and sporulates on the outside of the body. Infected insects appear as white to yellowish cottony particles. Diseased insects usually appear in 7 days. However, due to environmental conditions, there may be some considerable lag time from infection to death of insects. V. lecanii works best at temperatures of 15 to 25°C and a relative humidity of 85 to 90%. The fungus needs high humidity for at least 10 to 12 hours. This can be a problem as many plant pathogenic fungi (e.g. Botrytis) favor these same environmental conditions. V. lecanii spores are damaged by ultra-violet radiation. In greenhouses, heating pipes may reduce the effectiveness of the fungus, because this creates a microclimate where the air is drier and humidity is lower. In addition, V. lecanii is generally not useful in interiorscapes due the low humidity conditions in these environments.
The fungal mycelium of V. lecanii produces a cyclodepsipeptide toxin called bassianolide, which has been shown to kill silkworm. The fungus produces other insecticidal toxins such as dipicolinic acid. The activity of V. lecanii depends on the strain of the fungus. V. lecanii strains with small spores infect aphids, whereas fungal strains with large spores infect whiteflies. Certain strains have also been reported to be pathogenic on rust fungi.
Higher doses of the fungus result in faster kill. Virulence depends on the density of spores and rate of sporulation, which is dependent on environmental conditions. Fungal virulence varies with the method of conidial production. Less virulent conidia are obtained from fermented media as compared to shaken liquid or solid media. Formulated products from conidial production can last up to 1 year. These products are easy to wet and dilute for spraying. Also, the fungus can stick to the surface of leaves and host insects. Studies have shown that combining entomopathogenic fungi with an insecticide may enhance its performance as the fungi create wounds that makes it easier for the insecticide to enter the insect.
V. lecanii has been commercially available in Europe for control of aphids (Vertalec®) and whiteflies (Mycotol®). Vertalec® has been used to control green peach aphid, Myzus persicae, on chrysanthemums in greenhouses in England. One application provided control for 3 months. In addition, the melon aphid, Aphis gossypii, has been controlled with applications of V. lecanii. Infected aphids serve as an additional source of inoculum and spores can easily disperse within greenhouses. The material works best on chrysanthemums that are black clothed. This raises the relative humidity, which enhances the performance of the fungus. In addition, this protects the fungal spores from ultra-violet radiation. However, if there are too many aphids, then large numbers of dead white corpses can detract from the plants aesthetic appearance. Mycotol® has been used to control whiteflies on gerbera daisies, cucumbers and tomatoes. In greenhouse grown cucumbers and tomatoes, V. lecanii sprays have been demonstrated to cause 90% mortality of whiteflies. In addition, it has been reported to infect western flower thrips.
V. lecanii is compatible with most parasitic and predatory arthropods. Although the fungus can kill immature Encarsia formosa parasitizing greenhouse whitefly, the fungus has no effect on adults. As a result, V. lecanii may be readily incorporated into pest management programs that utilize biological control agents. However, it is not currently registered for use in North America.
Ray Cloyd, University of Illinois
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