Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2002
Publication Date: 3/20/2003
Citation: Horton, D.R., Broers, D., Lewis, R., Granatstein, D., Zack, R., Unruh, T.R., Moldenke, A., Brown, J. 2003. Frequency of mowing and effects on densities of natural enemies in the ground cover, in the tree canopy, and on the orchard floor at three Pacific Northwest pear orchards. Entomologia Experimentalis et Applicata. 106:135-145. Interpretive Summary: Pear growers differ substantially in how often they mow the ground cover in orchards, and it is unclear whether different mowing practices affect numbers of beneficial insects in orchards. We systematically varied mowing frequency at 3 pear orchards in the Pacific northwest, and monitored effects on natural enemies in the ground cover, on the orchard floor, and in the tree. Reduced frequency of mowing led to dramatic increases in number of predators and parasites in the ground cover, with lesser effects in the trees, and essentially no effects on insects inhabiting the orchard floor. Effects in the trees were most noticeable for spiders and a predatory true bug. These results suggest that pear growers may enhance biological control in their orchards merely by reducing frequency of mowing.
Technical Abstract: Effects of mowing frequency on ground cover composition and on counts of predatory, parasitic, and phytophagous arthropods in the ground cover of pear orchards were determined. Samples taken in the tree canopy (with beating trays) and on the orchard floor (with pitfall traps) tested whether counts of natural enemies in these habitats were also affected by mowing regime. A reduction in frequency of mowing from 2¿3 times per month to once per month or once per growing season led to increased cover of grasses, broadleaf plants, and broadleaf plants in flower. Sweep net counts of natural enemies in the ground cover were dominated numerically by spiders, parasitic wasps, and predatory Heteroptera, with lesser numbers of other taxa. Decreased mowing frequency led to significantly higher counts. The immature stages of several common predatory taxa were present in the most frequently mowed plots, suggesting that disruption of communities caused by mowing did not prevent reproduction in the these plots or prevent migration into the plots by the immature life stage. Counts of aphids and Lygus increased significantly with decreased mowing frequency. For the pitfall samples, only the European earwig exhibited a change in counts associated with mowing treatment; numbers of earwigs declined as mowing frequency decreased. Mean tray counts for almost all natural enemy taxa were higher in the less frequently mowed plots, but significantly so only for two taxa: spiders and a predatory mirid. It is unclear whether biological control of pests in the tree canopy was enhanced by manipulating mowing frequency. That is, questions remain as to the extent of movement by predators and parasitoids between the ground cover and tree canopy, and as to the importance of plot size in demonstrating consistently significant treatment effects in the tree canopy.