MSc

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.