Laboratory evaluation of antifeedant activity of Trichoderma spp. isolates in aphid biocontrol

Fungi offer several potential strategies against pest insects to reduce crop losses. Indeed, fungi and their metabolites can influence, by different modalities, several traits of the insect biology, such as survival, development, growth, fecundity and feeding behaviour. Nonetheless, in general, very few data are available on the repellency of fungi towards insect pests. We started a laboratory study aimed at evaluating the potential of secondary metabolites synthesized by fungal isolates belonging to different species of the genus Trichoderma (Deuteromycetes) as antifeedant compounds towards phytophagous insects such as aphids. The final goal aimed to understand the mechanism behind the capacity of aphids to detect certain metabolites and walk off. Repellency offers an alternative management strategy interfering with aphid processes of host plant selection and feeding behaviour and may lead to a reduction of both direct feeding damage and virus transmission. Tests were carried out utilising three species of aphids (Homoptera: Aphididae): Schizaphis graminum (Rondani), one of the most important pest of cereal crops; Megoura viciae Buckton, an insect pest of leguminous crops; and Myzus persicae (Sulzer), a cosmopolitan species that attacks plants in the field, readily infests vegetables and ornamental plants grown in greenhouses, and is highly noxious due to its ability to transmit plant viruses. The fungi isolates were administered to aphids in the form of a fungal culture that was cultured on rice kernels and than dried and finely ground. Trials, carried out utilising adult aphids pointed out an antifeedant activity of some isolates belonging to different species of the genus Trichoderma against aphids of the three species. A repellent effect was detected over the entire duration of the trials lasting 9 hours. However, a different sensitivity was observed across the winged and wingless morphs of S. graminum towards the fungal cultures applied on leaves of the host plant. Indeed, some isolates were repellent for winged but not for wingless morphs. In terms of the sensory organs involved in the perception of metabolites, electroantennographic studies, carried out with two species, S. graminum and M. viciae, showed that volatile compounds of fungal cultures were not able to stimulate the olfactory sensilla of antennae of both aphid species; as a consequence these substances cannot be responsible for the repellent effect. Taste sensilla lodged on the mesothoracic distal tarsomere of both aphid species were more strongly stimulated by aqueous solutions of fungal cultures than aqueous solutions containing the rice kernels finely ground alone. The observed aphid behaviour might be due to their sensitivity to water soluble compounds, with a repellent activity, produced by Trichoderma isolates.