Effects of mowing frequency on densities of natural enemies and select pests in the soil, in the ground cover, on the orchard floor, and in the tree canopy was documented at 3 reduced-pesticide pear orchards. Densities of natural enemies and prey in the ground cover increased dramatically with reduced frequency of mowing. The effects translated into higher densities of some natural enemy taxa in the soil and, more importantly, in the tree canopy. Parasitism rates of codling moth and pear psylla were unaffected by mowing regime, although low rates of parasitism in all treatments may have obscured effects.
1. Determine the effects of mowing frequency on insect densities in the ground cover, in the soil, on the orchard floor, and in the tree canopy in mating disruption pear orchards, emphasizing effects on biological control organisms and their prey;
2. Quantify natural enemy impact in different mowing regimes by estimating parasitism rates of pear psylla, codling moth, and leafminer;
3. Present project at annual Field Days.
Many pear growers in the Pacific northwest of the United States use mating disruption for controlling codling moth. The shift from a standard pesticide program to mating disruption has been accompanied by reduction in the use of broad-spectrum insecticides, in turn leading to higher densities of natural enemies in these orchards (Knight 1994, Gut and Brunner 1998), but also to higher densities of some secondary pests such as leafrollers (Gut and Brunner 1998, Walker and Welter 2001). Growers look increasingly to biological control to assist in managing secondary pests, and orchard practices that enhance biological control but that are also compatible with mating disruption would be valuable.
The impact of reductions in the use of broad-spectrum insecticides is unlikely to be limited to effects on tree-dwelling arthropods, but probably extends to inhabitants of the orchard floor or ground cover (Epstein et al. 2000, 2001; Miliczky et al. 2000). However, historical reliance on broad-spectrum insecticides in orchards has meant that we have a poor understanding of the numbers, types, and biology of arthropods that inhabit areas of the orchard other than the tree canopy. Furthermore, it is likely that beneficial arthropods associated with the ground cover or orchard floor could have a role in controlling orchard pests, especially pests that temporarily inhabit the ground cover or orchard floor (Riddick and Mills 1994) or pests that are restricted to the tree canopy but that are attacked by natural enemies that move between the tree canopy and the other habitats in the orchard (Bugg and Waddington 1994).
Pear growers differ in how they manage the ground cover in their orchards, including tolerance of weeds, herbicide use, and frequency of mowing. One especially noticeable difference among growers in ground cover management is with how often they mow. Frequency of mowing could affect arthropod numbers in the ground cover, through physical destruction of host plants, nectar sources, pollen sources, arthropod prey, or refugia. However, little is known of these interactions in orchard systems or other agricultural systems (Harris and Phillips 1986, Nentwig 1988, Stanyard et al. 1997).
Bugg, R.L., C. Waddington. 1994. Agric., Ecosyst. Environ. 50: 11-28.
Epstein, D.L., R.S. Zack, J.F. Brunner, L. Gut, J.J. Brown. 2000. Env. Entomol. 29: 340-48.
Epstein, D.L., R.S. Zack, J.F. Brunner, L. Gut, J.J. Brown. 2001. Biol. Control 21: 97-104.
Gut, L.J., J.F. Brunner. 1998. J. Agric. Entomol. 15: 387-405.
Harris, V.E., J.R. Phillips. 1986. J. Agric. Entomol. 3: 77-86.
Knight, A.L. 1994. J. Entomol. Soc. Brit. Col. 91: 27-36.
Miliczky, E.R., C.O. Calkins, D.R. Horton. 2000. Agric. Forest Entomol. 2: 203-215.
Nentwig, W. 1988. Oecologia 76: 597-606.
Riddick, E.W., N.J. Mills. 1994. Environ. Entomol. 23: 1338-1345.
Stanyard, M.J., R.E. Foster, T.J. Gibb. 1997. J. Econ. Entomol. 90: 595-603.
Walker, K.R., S.C. Welter. 2001. J. Econ. Entomol. 94: 373-380.
Sites. The studies were done at three pear orchards in 1999 and 2000 (sampling studies; parasitism; Field days) and at one pear orchard in 2001 (parasitism). The first site (1999-2001) was an experimental orchard maintained by USDA-ARS located at Moxee, WA (approx. 15 km east of Yakima, WA). The orchard comprised a 1.2 hectare block of 5-15 year old Bartlett pears. Pests were not controlled. The second site (1999, 2000) was located at Hood River, OR and comprised a 2.8 hectare orchard of Red Clapp, Red Anjou, Bosc, and Bartlett pear trees of mixed ages. Codling moth was controlled by pheromone. Other pests were controlled with oil, lime-sulfur, and amitraz. The third site (1999, 2000) was located in Peshastin, WA and comprised a 2.8 hectare block of 15-25 year-old Anjou pear trees. Codling moth was controlled using pheromone. Other pests were controlled with oil, sulfur, endosulfan, and pyriproxyfen.
Treatments. Three mowing treatments were used at each orchard, to provide a range of mowing frequencies: (1) mowed every 10-14 days (=control); (2) mowed once per month; (3) mowed once in mid-May and left unmowed thereafter. At each orchard, treatments were laid out in a randomized block design having 3 blocks per orchard. Treatment plots were 3-7 aisles wide by 30-150 meters long, depending upon size of the orchard. All plots were mowed first in mid-May, and treatments commenced following this initial mowing; treatments were maintained until early September.
Plant communities. We used a point-quadrat method to determine composition of the ground-cover in the different mowing treatments. A metal rod was dropped vertically in the aisles of each plot such that the end of the rod touched the soil surface. We then counted the number of contacts made along the length of the rod by plants, categorizing the plant contacts to species (broad-leaf plants) or grasses (all grass taxa combined). Drop locations within the plots were chosen randomly. Numbers of drops per plot were 20-50, depending upon plot size. Each plot was sampled on 6 dates during the growing season at 3-week intervals.
Objective 1: arthropod numbers. Sweep nets were used to sample the ground cover. Samples were taken every 3 weeks over the course growing season for a total of 6 samples per growing season. Sample size depended upon plot size but varied between 50-100 per plot. Beat trays were used to sample arthropods in the trees. Samples were taken every 3 weeks. Sample sizes were 10-25 trees per plot depending upon plot size. Pitfall traps (4 per plot) were placed in the herbicide strips in each plot to sample floor dwelling arthropods. Finally, soil samples were taken on 3 dates to determine effects of mowing on soil arthropods. Samples were 7.5 cm deep x 10 cm in diameter, taken using a circular soil cutting tool. Nine samples per plot were taken in the aisle areas, and 9 samples were taken in 1/3 of the plots in the herbicide strips.
Objective 2: parasitism rates. Pear psylla nymphs were collected on one date in July 2000 from each plot, taken to the laboratory and dissected to determine whether they had been parasitized by Trechnites insidiosus. Cardboard strips containing known numbers of diapausing codling moth were placed in the orchards in late September 2000. After 2 weeks in the field, strips were collected and taken to the laboratory. We recorded percentage of larvae parasitized and numbers of larvae that had disappeared from the strips (presumably due to predation). Lastly, on two dates in July 2000 (Moxee orchard only), leaves mined by the western tentiform leafminer were collected from each plot, taken to the laboratory, and dissected to determine status of the leaf miner larvae: healthy; killed by host-feeding wasp; or parasitized.
The studies were to be conducted again at the Moxee site in 2001, but a freak wind and hail storm destroyed our plots, snapping the trunks of a number of our trees. The trees were virtually completely stripped of leaves and fruit, and few insects (pest or otherwise) remained in the orchard in the months immediately following the storm.
Objective 3. Field days. Field days were held at the Peshastin orchard in August 1999 and August 2000, and at the Hood River orchard in March 2000.
Plant communities. Plots in all mowing treatments were dominated by grasses (9%-12% of contacts with metal rod). At all sites, broadleaf plants included primarily common dandelion and clovers, comprising 63-90% of the broadleaf plant contacts with the metal rod, depending upon orchard. Other broadleaf plants present included knapweed, common mallow, dock (Rumex spp.), lambsquarter, chickweed, vetch, black medic, bindweed, plantain, thistle, sowthistle, prickly lettuce, shpherd’s purse, salsify, and blackberry. Unsurprisingly, plant biomass was highest in the once-mowed plots, and lower in the monthly-mowed and control plots.
Objective 1. Arthropod numbers. Sweep nets. Natural enemy counts in sweep nets had similar seasonal patterns in the 3 orchards, with counts being low in May (1-3 per 25 sweeps) rising to a peak in August (10-45 per 25 sweeps). Numbers were highest at the orchard (Moxee) at which pests were not managed. The 3 most common taxa in the net samples were spiders, parasitic Hymenoptera, and predatory Heteroptera. Ladybeetles, hoverflies (Syrphidae), and lacewings were present, but at much lower numbers. Heteroptera included damselbugs (Nabidae) (20-95% of seasonal total predatory Heteroptera, depending upon orchard), big-eyed bugs (Lygaeidae) (0.5-53%), minute pirate bugs (Anthocoridae) (3-22%), and miscellaneous Miridae.
Counts in the net samples increased with decreased frequency of mowing. Magnitude of the increase varied by taxon. If counts are expressed as a proportion of the density observed in the control plots, then there was a 3 to 15-fold increase (depending upon taxon) in the once-mowed plots, and a 2 to 6-fold increase in the monthly mowed plots. The largest increases were noted for spiders, damselbugs, and ladybeetles, although parasitoids, big-eyed bugs, and minute pirate bugs also increased in numbers in the less frequently mowed plots.
Counts of various soft-bodied prey, including aphids, Lygus, and leafhoppers increased in the less-frequently mowed plots. Lygus is an occasional pest in pear orchards, thus its response to mowing treatment merits further attention.
Pitfall traps. We collected over 20,000 predatory arthropods in the pitfall traps at the three orchards. Trap counts were dominated by ground beetles (Carabidae) (71.5%, pooled among the 3 orchards), centipedes (7.6%), harvestmen (5.2%), spiders (4.7%), rove beetles (3.5%), and earwigs (2.0%). Of these taxa only the European earwig was affected by mowing treatment, decreasing in counts as mowing frequency decreased.
Beat trays. We recorded 1850 predatory and parasitic arthropods on beating trays. Counts were dominated by predatory Heteroptera (60.3% of total) due primarily to a single predatory mirid, Deraeocoris brevis (54.6% of total). Other Heteroptera included mullein bug (3.4%), minute pirate bug (0.8%), and damselbugs (0.8%). Other taxa included spiders (15.7%), earwigs (1.8%), lacewings (2.1%), ladybeetles (8.3%), and parasitic Hymenoptera. Counts of spiders and D. brevis increased significantly with decreased mowing (by 1.2 to 2-fold). Other taxa showed similar trends, but results were in general not statistically significant.
Soil samples. These data have yet to be analyzed statistically. Crude perusal of the numbers suggests that counts of certain predatory groups declined with increased mowing frequency. Counts of most taxa appeared to be substantially higher in the grassy aisles than in the adjacent vegetation-free herbicide strips.
Objective 2. Parasitism rates. Parasitism rates of pear psylla were not affected by mowing regime, varying between 12-15% in all plots (Moxee only; too few psylla were present at other 2 orchards). Predation rates (i.e., disappearance from cardboard bands) ranged between 2 and 20%, depending upon orchard. Parasitism was almost absent at 2 of the 3 orchards. At the third orchard, rates varied between 5 and 8%, but were independent of mowing regime. There was a weak (but non-significant) suggestion that parasitism rates of leafminer were highest in the least frequently mowed plots (63.5%) than in the control plots (54.8%). Total mortality (predation + parasitism) exceeded 80% in all plots by late July.
Objective 3. Field days. Field days were held on 3 dates: Peshastin, WA (Aug. 1999; attendance approximately 25); Peshastin, WA (Aug. 2000; attendance approx. 30); Hood River (March 2000; attendance approx. 20).
Results indicate that it is possible to increase natural enemy densities in orchards merely by reducing frequency that the ground cover is mowed. The increase in natural enemy densities could lead to increased biological control, although this needs to be tested directly. Also, potential negative effects (increased densities of pest species such as Lygus or spider mites, competition for water and nutrients between ground cover and trees) must be considered in any decision to change mowing frequency.
Educational & Outreach Activities
1. Horton, D.R., D.A. Broers, R.R. Lewis, D. Granatstein, R.S. Zack, T.R. Unruh, A.R. Moldenke, and J.J. Brown. 2003. Effects of mowing frequency on densities of natural enemies in three Pacific northwest pear orchards. Entomologia Experimentalis et Applicata (in press).
2. Horton, D.R. and A.R. Moldenke. Effects of mowing frequency on densities of soil arthropods in pear orchards. Environ. Entomol. (in prep.).
Horton, D.R. 1998. Effects of mowing frequency on ground cover insects in pear orchards. Proc. Wash. St. Hort. Assoc. 94: 144-147.
Horton, D.R. 2000. Know when to mow. Western Fruit Grower 120 (#6): 20D-20H.
Presentations at Scientific Meetings
Horton, D.R. 2002. Mowing frequency in pear orchards affects densities of natural enemies in ground cover and tree canopy. Northwest Symposium on Organic and Biologically Intensive Farming. Washington State University Center for Sustaining Agriculture and Natural Resources, Yakima, WA.
1. Horton, D.R. 2000. Orchard floor habitat management for beneficial insects. Workshop: Integrated fruit production – orchard floor management. Hood River, Oregon, Nov. 28, 2000. (Attendance approx. 40).
2. Horton, D.R. 2000. Habitat management to enhance biological control in orchards? Workshop: Annual Tilth Producers – Integrated Fertility Management. Chelan, Washington, Nov. 11, 2000. (Attendance approx. 40).
3. Horton, D.R. 2002. Extra-orchard and within-orchard habitats as possible sources of biological control in pear and apple orchards. Workshop: Environmentally Safe Practices for Control of Insect Pests in Orchards. Tilth Producers Annual Conference, Yakima, WA. (Attendance approx. 20).
Field days were held on 3 dates: in Peshastin, WA (Aug. 1999, approx 25 attending); Peshastin, WA (Aug. 2000, approx. 30 attending); and Hood River, Or (March 2000, approx. 20 attending).
Results were presented in February 2000 at the annual research review of the Washington State Tree Fruit Research Commission and Winter Pear Control Committee (the latter providing partial funding for the study). Attendance approximately 75.
Reports to Granting Agencies
A final report was provided to the Organic Farming Research Foundation, which provided partial support for the study.
Areas needing additional study
It is unclear as to the extent that ground cover insects move into the tree canopy resulting in biological control of pear pests in the tree. More study on the effects of ground cover management on quantified impact of pests is needed.