FIELD-TESTS WITH A FLUORESCENT BRIGHTENER TO ENHANCE INFECTIVITY OF FALL ARMYWORM (LEPIDOPTERA, NOCTUIDAE) NUCLEAR POLYHEDROSIS-VIRUS

The nuclear polyhedrosis virus (NPV) of fall armyworm, Spodoptera frug iperda (J. E. Smith), was applied in combination with Fluorescent Brig htener 28 (Calcofluor White M2R, Tinopal LPW) to whorl-stage corn. Con centrations of NPV ranged from 5 larval equivalents (1 LE = 6 x 10(9) polyhedral occlusion bodies) to 1235 LE per ha. Concentrations of fluo rescent brightener ranged from 0.1 to 5% by weight in water and the wa ter volume ranged from 234 to 926 liters per ha. Two days after treatm ent, fall armyworm larvae were collected from the treated plants and h eld on bean diet to observe mortality due to NPV, parasitoids, and asc ovirus. The fluorescent brightener interacted significantly with virus concentration and with water volume to increase larval mortality. The re 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 t o NPV. Cotesia marginiventris was the most abundant parasitoid recover ed from fall armyworm in these tests, and as the percent mortality due to NPV increased, the percent mortality due to parasitoids and ascovi rus decreased. Thus, the total mortality was not affected as greatly a s the percent mortality due to NPV by changes in water volume or fluor escent brightener concentration. The reduction in mortality due to par asitoids did not appear to be a direct effect of the fluorescent brigh tener on the parasitoids. However, increased infectivity of the NPV an d earlier mortality from NPV associated with the fluorescent brightene r resulted in more host larvae dying of NPV before the parasitoids cou ld complete development.