Abstract
The substantial impact of cotton insects and the lack of effective control strategies are the main limiting factors for cotton production. In greenhouse experiments, the vertical and horizontal distribution of Helicoverpa armigera (Hubner) eggs was manipulated. Trichogramma minutum (Riley) and T pretiosum (Riley) differed significantly in searching behaviour as measured by parasitization rates on three cotton cultivars. Parasitization rates were higher on the upper and lower leaves than on the middle leaves. Furthermore, parasitization rates were negatively correlated with distance between the releasing site and egg batches on the cotton plants. Morphological traits, i.e. presence of black glands or trichome densities of the cotton cultivars played a significant role. The parasitization rates on cultivars with glands and lower trichome density were higher than with no glands and high trichome density. Moreover, GC-MS analysis revealed that volatiles and the phytosterol composition of leaves were significantly different for cotton cultivars. These chemical traits of host plants are considered in relation to Trichogramma behaviour.