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

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Abstract
Trichogramma spp. egg parasitoids are generally absent in eggs of Helicoverpa armigera (Hübner) collected from chickpea, Cicer arietinum. In this study, the plant characters responsible for the absence of egg parasitoids and the feasibility of increasing parasitism levels on chickpea by mass-releasing Trichogramma chilonis Ishii were investigated. The residence time of female T. chilonison chickpea leaves was affected by trichomes and the acidic trichome exudates secreted on all green parts of the plant. The parasitoids 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 glabrous chickpea mutant than on washed leaves. When placed on unwashed chickpea 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 concentrations of malic and oxalic acids, the major components of the trichome exudate. 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 four-armed airflow olfactometer. No parasitized eggs were collected from a chickpea field in which T. chiloniswere released five times at a weekly interval at a rate of > 137,000 females ha–1. Sticky trap catches showed that no parasitoid population was sustained in the release field.