Analyzing the results of a biodiversity experiment: enhancing parasitism of Platynota idaeusalis (Lepidoptera: Tortricidae).
A common goal of conservation biological control is to enhance biodiversity and increase abundance and effectiveness of predators and parasitoids thus increasing sustainability of pest management. Although many studies report an increase in abundance of natural enemies, it has been difficult to document increases in rates of biological control. To enhance parasitism of the leafroller, Platynota idaeusalis (Tortricidae), alternate food was provided by interplanting peaches with extrafloral nectaries into apple orchards. Laboratory studies showed that the presence of peach extrafloral nectar increased longevity and parasitism rates by Goniozus floridanus (Bethylidae), the dominant parasitoid in West Virginia, USA. In orchard studies we found the total number of Hymenopteran parasitoids was higher on peach trees than on adjacent apple trees. Abundance of Hymenoptera was also significantly higher on the side of traps facing away from rather than toward peach trees, indicating attraction to peach trees producing extrafloral nectar. However, total parasitism rates of P. idaeusalis by all species of parasitoids were not affected by the presence of peach extrafloral nectar in any field studies. Insect injury to fruit at harvest showed that fruit from orchards with interplanted peach trees had less injury from San Jose scale (Quadraspidiotus perniciosus) and stink bugs (Pentatomidae) than fruit from an apple monoculture control orchard. Although interplanting with peach trees did not result in detectable increased biological control, the experiment did have beneficial results for pest management. By collecting data on the response of an ecosystem service (e.g., fruit quality) we were able to document a reduction in damage of two pests by the interplanting of peach trees with extrafloral nectaries into apple orchards. This demonstrates the need for a more holistic approach to evaluating habitat manipulation experiments. Without information on the response of yield quality we would have concluded that the experimental addition of peaches had no effect on insect pest damage. But, the yield data showed that pentatomids and scales responded with decreased damage, through an as yet undetermined mechanism, from the experimental manipulation. A cost:benefit analysis of the habitat manipulation is needed before recommendations can be made. It appears promising, however, that the sustainability of apple ecosystems can be enhanced by increasing its biodiversity with the addition of peach trees with extrafloral nectaries.