Impact of seasonal variation on insect population

Abstract

A periodical survey of incidence (percent) and intensity (percent) on popular insects was carried out for two consecutive years to study the impact of environmental changes on them in 17 villages of Tumkur district in Karnataka, The total sample size surveyed was 68 plots/month. The host crop selected was mulberry. Meteorological data wasgathered periodically and the survey was done by "Fixed plot" method. Percentage of Insect Incidence (PII) with corresponding insect population (PPI) was recorded and their association with weather parameters was correlated. The insects selected for the study were Spilosoma obliqua (Bihar hairy caterpillar), Diaphania pulverulentalis (Leaf roller) and Neorthacris acuticeps nilgriensis (Common wingless grasshopper). The PII and PPI of S. obliqua was maximum in September (15.23 percent and 1.56 Caterpillar/plant), moderate in winter (1.72 percent and 0.72 caterpillars/plant). In summer both were meager (PII=0.17 percent and PPI=0.01/plant). D. pulverulentalis exhibited maximum PII (22.21 percent) and PPI (5.46 larvae/plant) in November. It decreased in September (PII=9.91 percent and PPI=1.62 larvae/plant). In April-May, both parameters were negligible (PII=0.16 percent and 0.13 larvae/plant). Maximum PII (13.75 percent) of wingless grasshoppers was noticed in August, it decreased to 11.87 percent in November and negligible insect population was observed in summer (0.42 grasshoppers/microplot). Correlation and regression studies indicated a significant negative association of temperature with PII and PPI of these three insects (S. obliqua r=-0.82, D. pulverulenatlis r=-0.89, N. acuticeps r=-0.75). The study inferred that, the population density of these insects was temperature dependant. The threat to multiplication and spread of them under the changing climatic conditions with higher temperature (global warming) is significant and working out a suitable measure to protect them from declining in their population density appears to be very much necessary.