Why Trichogramma (Hymenoptera : Trichogrammatidae) egg parasitoids of Helicoverpa armigera (Lepidoptera : Noctuidae) fail on chickpea

Trichogramma spp. egg parasitoids are generally absent in eggs of Helicover pa armigera (Hubner) collected from chickpea, Cicer arietinum. In this stud y, the plant characters responsible for the absence of egg parasitoids and the feasibility of increasing parasitism levels on chickpea by mass-releasi ng Trichogramma chilonis Ishii were investigated. The residence time of fem ale T. chilonis on chickpea leaves was affected by trichomes and the acidic trichome exudates secreted on all green parts of the plant. The parasitoid s spent a longer time on chickpea leaves where the acidic trichome exudates had been washed off than on unwashed leaves, and longer on leaves of a gla brous chickpea mutant than on washed leaves. When placed on unwashed chickp ea leaves, 6.8% of the parasitoids were trapped and killed by the exudates. In a filter paper bioassay, female T. chilonis were deterred by high conce ntrations of malic and oxalic acids, the major components of the trichome e xudate. Acetone and hexane extracts from the surface of chickpea leaves did not elicit a response from the parasitoids in the bioassay. Similarly, the parasitoids did not respond to volatiles emitted by chickpea plants in a f our-armed airflow olfactometer. No parasitized eggs were collected from a c hickpea field in which T, chilonis were released five times at a weekly int erval at a rate of > 137,000 females ha(-1). Sticky trap catches showed tha t no parasitoid population was sustained in the release field.