Added floral resources in Hawkes Bay agroecosystems may not increase parasitism of larval leafroller, Epiphyas postvittana (Lepidoptera: Tortricidae)

Abstract

In agroecosystems, the efficacy of biological control exerted by many parasitoids is predicted to be enhanced where the availability of floral resources is increased. Such resources may enhance parasitoid longevity and fecundity. To test this prediction, parasitism of larval leafroller, Epiphyas postvittana (Lepidoptera: Tortricidae), was measured following the addition of floral resources to the understorey of apple orchards. In 2001, five angiosperm treatments (grass/broadleaf blends) in 80m2 plots were established in mature organic and Integrated Fruit Production (IFP) apple orchards in Hawkes Bay. This replicated design within ~0.5ha also included a weedy control. During two successive summer trials, parasitism was measured by adding potted apple plants (Malus domestica) inoculated with leafroller larvae, which were later recovered and reared in the laboratory. Overall, parasitism rates did not differ significantly between treatments. In December, the parasitism rate in the IFP orchard (<10%) was lower than that recorded in the organic orchard (40-60%). In both orchards during March, parasitism ranged from ~80-100%. Additionally, time searches over 5-months measured abundance and parasitism of natural leafroller infestations in these treatments. Parasitism did not differ significantly from controls, and ranged from 51-94%. In orchard trials and searches, ~95% of the parasitoids were Dolichogenidea spp. (Hymenoptera: Braconidae). In 2003, a commercial vineyard was used to measure the effect of flowering plants on leafroller parasitism. The vineyard, with no naturally occurring floral resources and few weeds, contrasted the orchards used in 2001-02. Potted flowering alyssum (Lobularia maritima) (Brassicaceae) was added to plots containing potted apple plants inoculated with leafroller larvae. This replicated design included four treatments (full flower, one- and two-thirds flower and a non-flowering control). No significant effect of alyssum on parasitism was observed. Parasitism was significantly, negatively correlated with distance from a nearby orchard. During two trials, ~90% of the parasitoids were Dolichogenidea spp. This experiment provided evidence that local floral resources within a few meters of the leafrollers, were not as important for biological control as a large nearby refuge population that was twenty to several hundred meters distant, depending on replicate. During a three-week interval between vineyard trials, a further study monitored parasitoid dispersal from an orchard into an adjacent grazed pasture, and sought to determine if parasitism in the pasture matrix was negatively correlated with distance from the orchard. Using a replicated design potted apple plants inoculated with leafroller larvae were positioned at intervals in the pasture. The orchard contained controls. Dolichogenidea spp., the only parasitoid found, dispersed into the pasture to 100m, the maximum distance tested; it actively foraged for hosts within this matrix. Logistic modelling found the probability of orchard parasitism was 49%. The probability of parasitism in the pasture matrix was negatively correlated with distance from the orchard, though relatively high rates of parasitism were still observed at 100m. Improving the efficacy of lepidopteran biological control through the addition of floral resources rests on our ability to gather empirical data. On the basis of the experimental design used in the orchards and the vineyard, I was unable to achieve my research objectives. From this study then, the implications of Dolichogenidea spp. dispersal, which appear to encompass tens of meters, suggests in future the spatial design of similar experiments may be a critical factor.