MSc

The study entitled “Behavioural ecology of cowpea aphid, Aphis craccivora Koch (Aphididae: Hemiptera) mediated by tri-trophic interactions” was carried out in the farmer’s fields and Department of Agricultural Entomology, College of Horticulture, Vellanikkara. The objectives of the study include evaluation of temporal and spatial dynamics of population of A. craccivora and associated natural enemy fauna, monitoring the patterns of dispersal, polymorphic transition and phenological relationship of A. craccivora in cowpea and assessing the role of plant, pest and natural enemy mediated semiochemicals in the population build up of A. craccivora and natural enemy fauna in cowpea.


Field survey was carried out in the cowpea fields and glyricidia, a major alternate host of A. craccivora during May, 2007 – March, 2008 to study the population dynamics of A. craccivora (Koch) and its predators. The population of A. craccivora in cowpea was significantly high in November sown crop with the population reaching its peak in the first fortnight of December. In the field, the highest population was recorded from the East direction followed by North. Among the different plant parts, a significantly higher population of the aphid was recorded on leaves followed by pods. The aphid population showed a significant positive correlation with sunshine hours and wind speed whereas; the correlation with relative humidity though significant was negative.

The predatory complex of A. craccivora on cowpea comprised of the coccinellids Cheilomenes sexmaculata (Fabricius), Coccinella transversalis (Fabricius), Scymnus latemaculatus (Motschulsky), Scymnus pyrocheilus (Mulsant), Psedaspidimerus trinotatus (Thunberg), Micraspis discolor (Fabricius), 9Harmonia octomaculata Fabricius) and Brumoides suturalis (Fabricius) and the syrphids Dideopsis aegrota (Fabricius) and Ischiodon scutellaris (Fabricius). C. sexmaculata was the most abundant predator on A. craccivora in cowpea. The populations of C. sexmaculata, C. transversalis, and the total population of the syrphids had shown a significant positive correlation with the prey population suggesting their density dependant relationship with the prey.

In glyricidia, a higher population of A. craccivora was recorded during the second fortnight of December. The most abundant predator of A. craccivora in glyricidia was S. pyrocheilus. Correlations between the population of the aphid on glyricidia and weather parameters were not significant. However, temperature showed a significant correlation with the populations of the predators, C. sexmaculata, M. discolor and S. latemaculatus. C. sexmaculata and M. discolor also showed a significant negative correlation with relative humidity.

Pot culture experiment was conducted to study the population buildup of A. craccivora and development of alate morphs. Higher population build up of the alates was recorded on leaves followed by pods that coincided with 48th day after sowing. Studies on the dispersal pattern of the alate morphs using yellow sticky trap showed maximum dispersal of the alates during day compared to night. More dispersal was found to occur during the crop season, June – July than February-March. Maximum alates were caught in the sticky traps installed in the north direction of the field. Trap installed at 1m distance from the potted plants caught more dispersing alates.

The role of semichemicals originating from the host plant, the aphid and predators were studied using a four arm olfactometer. While A.craccivora responded more towards the water extracts of cowpea, the predatory coccinellids, C. sexamculata responded more to the hexane extract. Cowpea was found to be the most preferred host plant to A. craccivora in the olfactometer. Though the aphid showed more attraction towards the leaf of cowpea, it preferred the shoot tip to other plant parts in glyricidia.

While A. craccivora prefered low density of conspecific population, C. sexmaculata showed more attraction towards high aphid population density in the olfactometer. The coccinellid predators C. sexamculata and C. transversalis responded more to the olfaction cues of aphid infested leaves of the host plants than the uninfested leaves. The predators showed a negative response towards the eggs of these predators in aphid colonies.

In response to host plant odour, antennal and leg movements were observed in the aphid, the leg movement becoming faster as they reached closer. On locating the host plant, the antennal movement stopped in the aphid. The coccinellids also responded to prey odour by walking towards the prey. On reaching the prey, the predators tapped the aphid before they started feeding on it.