Synergistic Effects of Combinations between Entomopathogenic Nematodes (Nemata: Rhabditida) and Chemical Insecticides on Mortality of Sphenophorus levis (Vaurie) (Coleoptera: Curculionidae)

Abstract

The effect of combinations of the entomopathogenic nematodes Heterorhabditis indica Poinar, Karunakar & David, 1992 (strain IBCB-n5) and Steinernema sp. (strain IBCB-n6) at doses of 2.4;12 and 60 IJ/cm2, with sub-doses of the chemical insecticides fipronil, thiamethoxam and imidacloprid were tested in laboratory conditions against adults and larvae of the sugarcane billbug, Sphenophorus levis Vaurie, 1978. Firstly, the effect of fipronil (200 g A.I./ha), thiamethoxam (250 g A.I./ha) and imidacloprid (700 g A.I./ha) were evaluated on the nematode viability. The insecticides were compatible with the nematodes, not affecting the viability of the infective juvenile after 24 hours. For the experiments with adults, the treatments containing nematodes and insecticides tested separately showed mortality below 25%, except for the two highest doses of Steinernema sp. (at 12 and 60 IJ/cm2), with mortality levels of 33.3% and 50% respectively. For the mixtures of the nematodes with sub-doses of thiamethoxam (62.5 g A.I./ha), the mortality levels increased significantly, reaching up to 66.7%, with the mixture of H. indica at 60 IJ/cm2, and 70% to 83.3% at all doses of Steinernema sp., showing synergistic effect at these combinations. The combination of H. indica and Steinernema sp. (12 IJ/cm2) with thiamethoxam (62.5 g A.I./ha) against larva, the nematodes tested alone or in mixture with the insecticides showed mortality levels between 75% and 87.5%, indicating no advantage of the mixture for improving the insect mortality.