Biology and predatory potential of rhynocoris marginatus (FAB.) (hemiptera: reduviidae) on insect pests of cowpea

Abstract

Insect pests form major threat to cowpea cultivation often causing a yield loss of 30 – 60 per cent. Even though insecticides are the most popular tools for pest management in cowpea their application throughout the crop period is not economically, socially and ecologically advisable. Biological control methods could be an option to reduce the insecticide load on the crop that is often constrained by lack of natural enemies that can effectively control several pests at a time. Members of the predatory family Reduviidae are gaining increased attention due to their broad host range as well as high density responsiveness. Rhynocoris marginatus belonging to the subfamily Harpactorinae of the family Reduviidae (Order: Hemiptera) has been found promising in managing a wide range of insect pests in various legume crop ecosystems. This predator reportedly has preference to lepidopteran caterpillars followed by nymphs and adults of hemipterans, two major insect groups regularly occurring in cowpea. In this context, the work on “Biology and predatory potential of Rhynocoris marginatus (Fab.) (Hemiptera: Reduviidae) on insect pests of cowpea” was carried out in the Department of Agricultural Entomology, College of Horticulture, Vellanikkara. The project aimed to study the biology and predatory potential of R. marginatus on leaf eating caterpillar of cowpea, Spodoptera litura (Fab.); to evaluate the predatory potential of R. marginatus on Aphis craccivora (Koch) and S. litura under caged condition and to study the field efficiency of R. marginatus against insect pests of cowpea. The egg stage recorded an average incubation period of 6.80 ± 0.91 days. It took 32.90 ± 1.81 days to complete nymphal development in five instars. Adult female lived longer than male. The preoviposition period was 14 ± 1.30 days and during the oviposition period of 66.13 ± 6.27 days, it laid 8.60 ± 0.97 batches of egg mass with mean fecundity of 377.20 ± 45.52 eggs. The predatory potential of second, third, fourth and fifth nymphal instars and adult were studied on third instar larvae of S. litura at prey densities, 2, 4, 6, 8 and 10 prey/ predator. Predation had strong correlation with the prey density and stage of the predator. When the number of prey killed by the predator was plotted against the prey density a Holling’s type II functional response curve was obtained. The predatory potential was also assessed against cowpea aphid, A. craccivora and leaf eating caterpillar, S. litura on caged cowpea plants. The third instar nymph of R. marginatus was released on cowpea plants in cages artificially infested with aphid and compared with control. Aphid population in predator released cages were significantly less than that in control. The predator was able to kill all the released third instar larvae of S. litura completely within four days. Field evaluation was carried out by releasing the predator at 30, 50 and 70 days after sowing of the crop. The release of the predator significantly reduced the aphid population on cowpea. The infestation by pod borer Maruca vitrata (Fab.) and pod bug, Riptortus pedestris (Fab.) were negligible. However, the number of pods damaged by pod bug was comparatively less in predator released field. At the same time, the predator had no impact on the population of coccinellid predators in the field.