Spatial distribution and sequential sampling of aphid and their natural enemies on wheat

Abstract

Although many pests attack wheat, the damage due to wheat aphids has been on rise due to climate change phenomenon. Population of aphids increases during February to March and so the ladybird beetles to devour on aphids. With the aim of facilitating conservation of ladybird beetles, predator conditioned sampling plans were worked out. Wheat aphids, <em>Rhopalosiphum padi </em>L. and<em> Sitobion avenae</em> F. were observed infesting wheat. Major predatory coccinellid beetles,<em> Cheilomenes sexmaculata </em>(Fabricius) and<em> Coccinella septempunctata </em>(Linnaeus) were also found feeding on the wheat aphids. Spatial distribution of mixed population of the aphids and coccinellid beetles was analyzed using variance-mean ratio and regression models such as Taylor’s power law and Iwao’s mean crowding regression. The aphids and its predatory beetles were counted at different crop age. The mean population for wheat aphid was 7.88/plant and 0.72/ plant at 75 and 106 days after sowing (DAS), respectively. Variance-mean ratio indicated regular to aggregated distribution of aphids on the crop. Taylor’s power law aggregation parameter (b = 2.62) and density contagiousness co-efficient (β = 1.20) of Iwao’s mean crowding regression also revealed aggregated distribution of the wheat aphids. The mean population of coccinellid beetles varied from 0.72 to 1.43/plant during 68 to 100 DAS. Taylor’s power law aggregation parameter (b = 3.64) and density contagiousness co-efficient (β = 2.09) of Iwao’s mean crowding regression revealed aggregated distribution of the predators on the wheat crop. Sequential sampling plans were developed for wheat aphid management with and without predator’s effect through Taylor’s power law and Iwao’s mean crowding regression. Inclusion of predator effect in the regression models increased aphid population levels which necessitates need for management measures at higher population levels. Thus, the sequential sampling plans with predator effect are useful in avoiding unnecessary pesticide application on wheat crop for the aphid management.