Entomopathogenic nematodes heteroharbditis indicaand steinernema karli as control agents of legume pod borer maruca vttrata syn testulalis in cowpea

Abstract

Cowpea (Vigna unguiculata (L.)Walp) is one of the major legumes largely cultivated in tropical and sub tropical regions of the world. In Kenya and other parts of the world, the social and economic importance of cowpea include livestock fodder, legume vegetable food crop, green manure, nitrogen fixation, cover crop, fibre extraction, medicinal uses, shade tolerant and drought tolerant among others. In Africa, cowpea production fluctuates mainly due pests and diseases with insect pests playing the greatest role. The legume pod borer Maruca vitrata is particularly deadly attacking both the vegetative and reproductive parts of cowpea causing yield' losses of between 20-80%. Several control options for M vitrata exist including crop rotation, pesticide treatment and biological agents for biological agents available, entomopathogenic nematodes Heterohabditis indica andSteinernema karii have been known to prey on insect larvae but their effectiveness as agents for biological controlhas not been elucidated on Maruca. This work aimed to determine pathogenicity of entomopathogenic nematodes (EPNs) H. indica and S. karii on M vi/rata under laboratory and screen house conditions and determine the net effect of treatment on cowpea infested with the legume pod borer. The experiments were laid in a complete randomized design, with five treatments; zero (control), 100, 200, 300 and 400 Dauver juveniles (DJs) per insect with three replicates. The DJs concentration of 400 with the highest mortality in the laboratory was subjected to screen house trial. In the screen house trials, five Maruca larvae were released on each potted cowpea plant. Data on number of dead larvae and damaged flowers was recorded on 24-hour interval from the start of both laboratory and screen house experiments: 24, 48, 72, 96 and 120 hours. Cadavers were dissected to confirm nematode infection and progeny yield. There was a negative correlation between increase in nematode concentration and the total number of DJs recovered. At 300 dose of H. indica and S. karii, M vitrata cadavers yielded 24000 and 17000 .DJs respectively while at 400 dose the yield was14400 and 15450 DJs respectively. In all the nematode concentrations tested in the laboratory no larvae survived beyond 120 hours with both H. indica and S. karii. However, H. indica higher dose of 400 nematodes per larvae achieved 100% mortality within 72 hours while at the same dose S. karii caused mortality of 66%. The difference between the control and treatments using the EPNs was significant p<0.05 in the laboratory. In the screen house trials the treatment was significant p<0.05, though the mean larval mortality was low in the screen house (1.733) than in the laboratory (3.600). The mean flower damage in H. indica, S. karii treated plants and control was 1.667, 2.067 and 3.200 respectively. Overall, it was found that EPNs H. indica and S. karii were pathogenic to M vitrata larvae in both laboratory and screen house conditions. It is concluded that, H. indica and S. karii are potential biological control for the legume pod borer in cowpea and this provides a platform for development of eco-friendly biopesticide using EPNs as viable component of integrated pest management (IPM). More studies are recommended to elucidate the effect of >400dose of the EPNs, repeated application of EPNs H. indica and S. karii on Mvitrata in the semi field and determination of their pathogenicity on M vitrata under field conditions are necessary.