Effects of cowpea intersowing and insecticide application on Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) and its natural enemies in pigeonpea intercropped with sorghum

Abstract

In a split-plot experiment, Helicoverpa armigera eggs, larvae, and H. armigera egg predators were sampled weekly or biweekly to test whether intersowing of cowpea could increase the natural control of Helicoverpa armigera in pigeonpea that had been relay intercropped with sorghum, and to test the effect of an insecticide treatment on the H. armigera population and on its natural enemies. Cowpea intersowing resulted in higher numbers of H. armigera eggs and larvae per pigeonpea plant in the first 3 weeks ofsampling, whereas the predator species studied were not significantly affected, possibly due to the small overlap between the composition of natural enemies in sorghum and pigeonpea: the two beneficial species Orius tantillus (Motschulsky) (Heteroptera: Anthochoridae) and Menochilus sexmaculatus F. (Coleoptera: Coccinellidae), very common predators in sorghum, occurred in low numbers in pigeonpea, where the most common predators were Cheiracanthium inornatum O.P.-Cambridge (Aranea: Clubionidae) and anthicids (Anthicus sp., Formicomus sp.) (Coleoptera: Anthicidae). Insecticide treatment resulted in significantly higher number of H. armigera eggs per plant compared with no insecticide treatment, the effect being most pronounced immediately after the treatment. Insecticide had no significant effect on the number of H. armigera larvae per plant, but it had a strong adverse effect on C. inornatum and anthicids.