Joint action of nuclear polyhedrosis virus and chemical insecticides against the black cutworm, Agrotis ipsilon (Hufn.).

A polyhedrosis virus disease was isolated from the larvae of Agrotis ipsilon. Isolation of causative agent from the infected larvae again showed to be positive. Microscopic examination of the infected larvae (by means of a light microscope) gave an evidence of the development of the polyhedral inclusion bodies (PIB) in all typical tissues where the virus is known to develop. Examination by an electron microscope showed various irregular shaped polyhedra; their diameter being about 1.5 to 3.5 mu. Most of these polyhedra were hexagonal and they measured about 2.2 mu. Experimental studies were conducted to investigate its pathogenicity alone and when combined with chemical insecticides. The joint action of the nuclear polyhedrosis virus of A. ipsilon (AINPV) combined with four chemical insecticides namely Fenvalerate, Decamethrin, Cypermethrin and Methomyl was assessed against the second instar larvae of A. ipsilon. Results showed that Decamethrin was the most toxic compound followed by Cypermethrin and Fenvalerate while Methomyl was the least toxic, the median lethal concentrations (LC50) were 0.25, 0.77, 14 and 450 ppm, respectively. The LC50 value of AINPV was 12 x 10(6) PIB/ml. Mixtures containing sublethal concentrations (LC25 and less) of both AINPV and any of the tested insecticides showed synergistic effects. On the other hand, an additive effect was observed with combinations containing doses of chemical insecticides over the range between LC25 and LC50 values.