DEVELOPMENT OF NUCLEAR POLYHEDROSIS-VIRUS FOR CONTROL OF GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) IN ONTARIO .2. REDUCTION IN DOSAGE AND EMITTED VOLUME (1989 AND 1990)

A double application of 1.25 x 10(12) (total 2.5 X 10(12)) polyhedral inclusion bodies (PIB) of Disparvirus (nuclear polyhedrosis virus) in an emitted volume of 10.0 L per ha gave acceptable control of gypsy moth, Lymantria dispar (L.), larvae in 1988. More aerial spray trials were conducted in 1989 and 1990 to test a reduced dosage and volume of Disparvirus. Dosage on all plots, applied when larvae were mainly in their first instar, was a double treatment of 5 X 10(11) PIB per ha, 3-5 days apart, giving a total of 10(12) PIB per ha. The aqueous tank mix contained 25% (v/v) molasses, 10% (w/v) Orzan LS, and 2% (v/v) Rhoplex B60A sticker. Emitted volumes of 10.0 L per ha and 5.0 L per ha were each tested on three replicated plots in 1989, and 5.0 L per ha and 2.5 L per ha were each tested on three replicated plots in 1990. In both 1989 and 1990, three untreated check plots were paired with treated plots on the basis of pre-spray egg-mass densities. The treatments were assessed by counts of pupae in burlap traps, estimates of defoliation, and change in numbers of egg masses in the treated and check plots. Better control was obtained with the reduced dosage at 10.0 L per ha and 5.0 L per ha than at 2.5 L per ha. Using the aqueous formulation described above, a double application of 5 x 10(11) PIB per ha at 5.0 L per ha applied at the peak of the first larval instar is the current recommendation for control of gypsy moth in Ontario using nuclear polyhedrosis virus.