Species composition and population dynamics of some arthropod pests in cotton fields of irrigated and semi-arid regions of Punjab, Pakistan

The present study aimed to record seasonal dynamics and diversity of different insect pests in cotton agroecosystems. Two well-known cotton growing districts i.e. district Layyah and Vehari were selected for the study from the cotton belt of Punjab, Pakistan. Sampling was done bi-weekly for two consecutive years from July to October during 2018 and 2019. Sweep netting, visual counting, hand picking, wet finger method, beat sheets, aspirator and pitfall trapping methods were used for collection. Shannon-Wiener and Simpson indices were used to compute diversity while Menhinick and Margalef indices were used for the estimation of species richness. A total of 94,343 individuals representing 43 species, 40 genera, 28 families and 6 orders were recorded. Family Aleyrodidae dominated over other pest families. Bemisia tabaci (Gennadius, 1889) of family Aleyrodidae was the dominant species with 39.68% share among all pest species. Estimated species richness of all arthropod pest species belonging to both districts were about 94%. The densities of pests fluctuated with time. The peaks of sucking pest densities were observed in July and August while densities of chewing pests peaked in late September or early October each year. Population densities of jassids Amrasca biguttula (Ishida), armyworm Spodoptera litura (Fabricius), and pink bollworm Pectinophora gossypiella (Saunders), showed strong negative correlation with temperature, humidity and rainfall while thrips population density showed positive correlation with these parameters. Minor loss due to pests are acceptable everywhere, but it is only possible when their populations remain below their economic threshold levels. Present study will aid to design pest management strategies in cotton growing areas round the globe.

Aphid honeydew quality as a food source for parasitoids is maintained in Bt cotton

Bt-transgenic cotton has proven to be highly efficient in controlling key lepidopteran pests. One concern with the deployment of Bt cotton varieties is the potential proliferation of non-target pests. We previously showed that Bt cotton contained lower concentrations of insecticidal terpenoids as a result of reduced caterpillar damage, which benefited the aphid Aphis gossypii. It is thus important that non-target herbivores are under biological control in Bt cotton fields. The induction or lack of induction of terpenoids could also influence the quality of aphid honeydew, an important food source for beneficial insects. We therefore screened A. gossypii honeydew for cotton terpenoids, that are induced by caterpillars but not the aphids. We then tested the influence of induced insect-resistance of cotton on honeydew nutritional quality for the aphid parasitoid Lysiphlebus testaceipes and the whitefly parasitoid Eretmocerus eremicus. We detected the cotton terpenoids gossypol and hemigossypolone in A. gossypii honeydew. Although a feeding assay demonstrated that gossypol reduced the longevity of both parasitoid species in a non-linear, dose-dependent manner, the honeydew was capable of sustaining parasitoid longevity and reproduction. The level of caterpillar damage to Bt and non-Bt cotton had no impact on the quality of honeydew for the parasitoids.These results indicate that the nutritional quality of honeydew is maintained in Bt cotton and is not influenced by induced insect resistance.