Determination of reapplication frequency required for the Cryptophlebia leucotreta granulovirus: a factor of rate of virus breakdown and larval behavior

Abstract: The Cryptophlebia leucotreta granulovirus (CrleGV) has now been used commercially for the control of the false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), on citrus in southern Africa for more than 10 years. Farmers need improved clarity on frequency of application required. Consequently, the rate of breakdown in the field, determined to be mainly due to ultraviolet radiation, was determined. This was done at regular intervals from zero to 28 days after application, by washing occlusion bodies off Navel orange fruit from both the northern (sunny) and southern aspects of trees and conducting dose-response bioassays against neonate T. leucotreta larvae in the laboratory. At 21 days after application, LD50 of CrleGV recovered from the northern side of trees was 15 times higher than from the southern side of trees. By 28 days after application, virulence of CrleGV on the northern side of trees was indeterminable, whereas on the southern side of trees, there was still a clear dose response. In a separate study, neonate T. leucotreta larvae were placed onto molasses-treated and water only-treated Navel oranges and behaviour filmed, recorded and analysed. Although distance traversed by the larvae on the fruit surface was significantly greater without molasses, larvae on molasses-treated fruit fed more actively, thus increasing the probability of viral ingestion before penetration. Combining the rate of breakdown of virus and larval feeding behaviour with and without molasses, a model is proposed for estimating necessary time intervals between CrleGV sprays.