Field efficacy of the nucleopolyhedrovirus of Anticarsia gemmatalis Hubner (Lepidoptera: Noctuidae): Effect of formulations, water pH, volume and time of application, and type of spray nozzle

The efficacy of the nucleopolyhedrovirus of Anticarsia gemmatalis Hubner (AgMNPV) was evaluated in relation to different application parameters of this biological insecticide, to elucidate some problems of its low efficacy in certain regions in Brazil, mainly in the state of Rio Grande do Sul (RS). Each experiment was conducted during four consecutive soybean seasons (1994/95 to 97/98), in Cruz Alta--RS, to evaluate the following parameters on AgMNPV field efficacy: available commercial formulations of the virus; pH of the viral aqueous suspension; spray volume; application time; spray nozzle type; and the mixture of the virus with mineral oil. Each plot consisted of 28 soybean rows of 10 m in length, spaced of 40 cm. The statistical design was the completely randomized block design, with four replicates. Samplings of AgMNPV infected or dead larvae (IL) were periodically made by the shake cloth method. The AgMNPV formulations produced by Nitral, Nova Era, Coodetec and Embrapa were efficient in controlling larval populations of A. gemmatalis when compared to the control. However, these formulations were significantly inferior to the crude extract of AgMNPV (maceration and filtration of dead larvae), indicating that current commercial formulations of this virus need to be improved. The AgMNPV was significantly affected by the water pH in the spray tank, as plots treated with a viral suspension at pH 6 resulted in a significantly higher number of IL than plots treated with suspensions at pH 2 and 10. The spray volume also influenced the control of A. gemmatalis larval populations by the AgMNPV. Volumes of 300, 200 e 100 L/ha were equally efficient in controling the insect. However, at 50 L/ha, the virus efficacy was significantly lower than that provided by the other spray volumes, apparently due to nozzle clogging at the lower application volume. The AgMNPV performance to control the insect was also significantly affected by the time of application, with higher efficiencies when spraying at 2:00 a.m. or 8:00 p.m., in comparison to 8:00 a.m. or 2:00 p.m. On the other hand, no differences were detected on the efficacy of the virus when sprayed with different nozzle types, and mineral oil addition to the viral suspension did not improve the field performance of the formulated AgMNPV.