Field evaluation of a granulosis virus for control ofPieris rapae [Lep.: Pieridae] in the United Kingdom

Abstract

A purified granulosis virus isolated fromPieris brassicae (L.) was tested in the field against an introduced population ofPieris rapae (L.) larvae on cabbage (cv January King) in small experimental plots at Littlehampton, Sussex. Experiments were designed to compare the relative efficacy of single and multiple applications of virus (2.1×1012 and 3.7×1012 or 2.1×1014 and 3.7×1014 virus capsules/ha) in reducing numbers ofP. rapae larvae. An experiment was carried out in June 1978 and repeated in August to coincide approximately with the 2 natural generations ofP. rapae in southern England. Larval populations were monitored by regularin situ assessment of plants and by destructive sampling. Within 10 days of spraying virus there was a significant reduction in the mean larval population on all virus-treated plots compared with untreated controls. Sprays of 2.1×1014 and 3.7×1014 capsules/ha reduced larval numbers more quickly than 2.1×1012 and 3.7×1012/ha treatments. In the 1st experiment, three sprays of virus at either 2.1×1012 or 2.1×1014 capsules/ha gave no increase over the final level of control achieved by a single spray. However, in the 2nd experiment, a single spray of 3.7×1012 capsules/ha did not significantly reduce the numbers of larvae. It is likely that this failure could be accounted for by a combination of the larger “natural” population ofP. rapae recorded midway through the 2nd experiment and the rapid inactivation of virus deposits which left little infectious virus to infect these larvae. Virus inactivation was so rapid that only 7–33 % of the initial virus deposits remained 1 day after application. These results suggest that further understanding of virus formulation, persistence and dosage rates are needed before such a virus can be used in a rational manner.