Increasing the diversity of orchard understories to enhance the biological control of leafrollers (Lepidoptera: Tortricidae) : a dissertation submitted in partial fulfillment of the requirements for the degree of Bachelor of Horticultural Science with Honours at Lincoln University

Abstract

Leafrollers (Lepidoptera: Tortricidae) are key insect pests of orchards in New Zealand. The use of agrochemicals to control these and other pests in orchards and the subsequent loss of diversity has led to a negligible effect of natural enemies contributing to the biological control of leafrollers. The development of integrated and organic fruit production requires research to establish the potential for control of leafroller pests using natural enemies in reduced pesticide regimes. One method of enhancing natural enemy populations in orchards is through improving diversity in the system, which is achievable in a variety of ways. This work is based on providing floral resources for parasitic wasps of leafrollers, specifically pollen and nectar required by parasitoids for egg maturation. In this work, two trials were set up to test the effectiveness of buckwheat (Fagopyrum esculentum Moench) in attracting leafroller parasitoids into apple orchards managed under conventional and organic practices. Each trial was divided into eight blocks each containing a control ( overall grass in the organic orchard and grassed row and understorey kept weed free using herbicides in the conventional orchard). The treatment consisted of buckwheat drilled in the understorey of three trees in the centre of the plot. Two yellow water traps were placed in the centre of the 16 plots and were emptied weekly. Catches of parasitoids were compared between control and treatment plots for each sampling date. Significantly more (P<0.05) parasitoids were caught in buckwheat plots once flowering began. However, catches were dominated by Anacharis sp. (a parasitoid of the brown lacewing Micromus tasmaniae Walker.) and few leafroller parasitoids were caught. This is despite the known presence of one leafroller parasitoid in particular, Dolichogenidea tasmanica (Cameron). Parasitoid catches on commercially available yellow "Trappit" sticky traps placed in the trees of buckwheat and control plots showed on average 5 times the number of D. tasmanica in buckwheat plots than control plots. This demonstrates the importance of trap method and position used in studies of this nature. Results from release and re-collection of leafroller larvae in the 16 plots showed significantly higher (P=0.016) levels of leafroller parasitism by D. tasmanica in buckwheat plots compared with those in control plots. Mean parasitism levels in buckwheat and control plots were 34% and 21 % respectively. The implications of these results for enhanced biological control in orchards are discussed.