Increasing the diversity of orchard understories to enhance the biological control of leafrollers (Lepidoptera: Tortricidae)

Abstract

Leafroller caterpillars (Lepidoptera: Tortricidae) are significant pests of New Zealand export fruit crops. A dependence on high agrochemical inputs for leafroller control has led to low diversity within orchard systems and natural enemies of leafrollers have been reduced to a negligible role in biological control. Information is needed to establish the potential of natural enemies of leafrollers in a low pesticide regime, especially if plant diversity in the orchard can be enhanced. One method for doing the latter is to provide floral resources for the Hymenoptera which parasitise leafrollers - the female parasitoids need pollen and nectar for egg maturation. In this work two successive trials were carried out in stonefruit and apple orchards in the summers of 1994-95 and 1995-96, respectively. Orchards were divided into eight blocks, each containing one replicate each of a control and a treatment. The treatment had buckwheat (Fagopyrum esculentum Moench.) drilled (as 3 m x 2 m plots) in the centre of each replicate, while the control areas were kept weed free with herbicides. Four yellow pan traps for catching insects were placed in the centre of each of the 16 plots and were emptied weekly. The catches of parasitoids were compared between the control and treatment areas. In the first trial, on average ten times more parasitoids were captured in the buckwheat plots than in the controls. However, few of the parasitoids were natural enemies of leafrollers; the catches were dominated by Anacharis sp., parasitoids of the brown lacewing (Micromus tasmaniae Walker.), itself a beneficial insect. Leafrollers were very rare in this orchard, which explains the absence of their parasitoids in the traps. In the second season, with the experiment conducted in an orchard with a history of leafroller populations, higher numbers of Anacharis sp. were again caught in buckwheat than in control plots. However, significantly higher (34% compared with 20%) levels (p<0.05) of parasitism (by Dolichogenidea tasmanica (Cameron)) of released leafroller larvae were observed in buckwheat plots than in controls, despite the small plot size. Also, higher numbers of D. tasmanica were captured in yellow sticky traps 2 m above the buckwheat than in the equivalent position in the control plots.