Field Tests with a Fluorescent Brightener to Enhance Infectivity of Fall Armyworm (Lepidoptera: Noctuidae) Nuclear Polyhedrosis Virus

Abstract

The nuclear polyhedrosis virus (NPV) of fall armyworm, Spodoptera frugiperda (J. E. Smith), was applied in combination with Fluorescent Brightener 28 (Calcofluor White M2R, Tinopal LPW) to whorl-stage corn. Concentrations of NPV ranged from 5 larval equivalents (1 LE = 6 x 109 polyhedral occlusion bodies) to 1235 LE per ha. Concentrations of fluorescent brightener ranged from 0.1 to 5% by weight in water and the water volume ranged from 234 to 926 liters per ha. Two days after treatment, fall armyworm larvae were collected from the treated plants and held on bean diet to observe mortality due to NPV, parasitoids, and ascovirus. The fluorescent brightener interacted significantly with virus concentration and with water volume to increase larval mortality. There was no increase in mortality due to NPV as the percent fluorescent brightener increased beyond 1%. In the higher volumes of water, 0.25% fluorescent brightener resulted in the highest percent mortality due to NPV. Cotesia marginiventris was the most abundant parasitoid recovered from fall armyworm in these tests, and as the percent mortality due to NPV increased, the percent mortality due to parasitoids and ascovirus decreased. Thus, the total mortality was not affected as greatly as the percent mortality due to NPV by changes in water volume or fluorescent brightener concentration. The reduction in mortality due to parasitoids did not appear to be a direct effect of the fluorescent brightener on the parasitoids. However, increased infectivity of the NPV and earlier mortality from NPV associated with the fluorescent brightener resulted in more host larvae dying of NPV before the parasitoids could complete development.