The nature of the interaction between ground cover and the pear tree in low-pesticide and organic pear orchards is not well-understood, particularly with respect to whether ground cover is a valuable source of natural enemies in the orchard. The present study was done to determine whether mowing frequency affects natural enemy densities in the ground cover and pear trees of mating disruption and organic pear orchards. I showed that reduced frequency of mowing prompted large increases in densities of natural enemies in the ground cover, presumably in response to increased availability of pollen and nectar from flowering plants and to increased availability of small, soft-bodied prey such as aphids. Taxa showing the largest increases in densities included lacewing larvae, spiders, damselbugs (Nabidae), parasitoids, ladybug beetles, syrphid flies, and minute pirate bugs. Ground-dwelling predators such as ground beetles, harvestmen ("daddy-long legs"), and certain spiders were much less affected by mowing frequency, and if anything showed some slight reduction in numbers in the less frequently mowed plots. In the pear tree, counts of spiders and parasitoids were higher in less frequently mowed plots. For pest species, densities of spider mites in the tree and in the ground cover were higher in the less frequently mowed plots than in the frequently mowed plots. Virtually no spider mites were observed in the ground cover of the weekly mowed plots. Lygus spp. and stinkbugs associated with the ground cover increased in abundance as mowing frequency decreased. However, neither pest was recovered in damaging numbers on beat tray samples taken in the pear tree. Finally, for certain taxa (ground beetles, harvestmen, earwigs), there were extremely large density differences between blocks within orchards that easily swamped the much smaller differences within blocks caused by the different mowing treatments. Observations suggest that tree age (canopy openness?) and block location (i.e., nearness to nonagricultural habitats) dramatically affected densities of these specific predators.
Although it is clear from this study that mowing frequency had striking effects on densities of natural enemies in the orchard, it remains unclear just what impact this has on orchard pests. I monitored parasitism rates of pear psylla nymphs in all three mowing regimes, but showed no affects of mowing treatment. However, parasitism rates were atypically very low (<<10%) all season, and the study should be repeated when rates are at more typical levels. I also estimated predation rates of codling moth larvae by monitoring disappearance of larvae from cardboard strips stapled to the trunk of the pear tree. I failed to show any effects of mowing frequency on predation rates. However, strips were left in the orchards for only 48 hours (due to concerns about larvae migrating from strips to trees), and predation rates were thus fairly low. There was a positive correlation between disappearance rates and densities of earwigs in the plot, suggesting that earwigs are predaceous upon codling moth larvae. The impact portion of the study will be repeated next year at the same locations and using the same mowing treatments, but efforts will be made to improve methods and to increase sample sizes.
