Assessing habitat and dietary switching by predators in a cover crop system
Organic pear growers must balance the conflicting pressures between nitrogen and densities of pear psylla. We are assessing in 4 organic pear orchards whether an alfalfa cover crop leads to increased biological control of psylla; each alfalfa plot was paired with a control (grass) plot. Sampling has as yet failed to detect effects of the cover crop on biological control. Over 4,000 predator specimens were collected in a marking study to assess movement by predators from cover crop to tree (ongoing). A subset of those specimens will be analyzed for gut contents to confirm feeding on the target pest (ongoing). Nitrogen levels were higher in cover crop plots than grass plots.
1. Determine if an alfalfa cover crop leads to an increase in densities of generalist predators in the pear tree canopy;
2. Examine whether the increased densities of predators in trees having a cover crop understory is due to movement by predators from cover crop to tree;
3. Assess whether those predatdors then switch diet to attack pear psylla;
4. Determine if the combined effect of habitat and dietary switching then leads to biological control of pear psylla;
5. Forward results to growers and evaluate outcomes of the project.
Methods. Alfalfa cover crops were planted at 3 commercial orchards (3 replicate plots in each orchard) and (in 2006) at 1 experimental organic orchard (4 replicate plots); each alfalfa plot was paired with a grass control plot. Insect sampling was begun in 2008 at the experimental orchard and 2009 at the 3 commercial orchards. Densities of adults and immatures of the target pest (pear psylla) were assessed at approximately 3-week intervals between May and September in 2008, and May to the present in 2009. On each sampling date, predator densities were assessed using sweep nets (understory), beating trays (tree), and sticky cards (understory and tree). The understory was sprayed with a protein marker (egg white) on 3 dates in 2008 and 1 date so far in 2009. Predator specimens were then collected from both understory and tree to determine proportion of insects marked with the protein; presence of the marker in tree-collected specimens would be evidence that the specimen had originated from or had visited the understory. A subset of marked and unmarked specimens will be analyzed for presence of psylla proteins to assess whether those predators had been feeding on the target pest. Nitrogen levels in pear leaves were assessed in July 2008 and June 2009.
Results to date. Despite large numbers of generalist predators in the alfalfa cover crop, there is as yet no discernable effect on biological control of the target pest. Predator densities in trees are similar in control and alfalfa plots. Over 4,000 specimens have been collected in the marker trials. Those specimens are currently being analyzed using ELISA to detect presence of the marker protein. A subset of those specimens are to be macerated and assayed (with ELISA) to look for presence of psylla proteins within their guts; those specimens are being held in a freezer until they can be assayed. Nitrogen levels on the two sample dates were slightly higher in the alfalfa plots than the control plots. The 2008 results were presented to growers at an industry-sponsored research review held in Hood River, Oregon, in February 2009.
Work still to be accomplished. Sampling will continue until September 2009. A field day is planned for August 2009. A final marking trial is planned for early August 2009. Specimens collected in the marker trials and for gut contents assessments will be assayed for presence of the marker and psylla proteins, respectively, in winter 2009-2010. Nitrogen levels will be assessed on at least one more date. A report will be presented to industry at the Northwest Pear research review in February 2010. A publication will be written describing results in spring 2010.
Impacts and Contributions/Outcomes
The 2008 results were forwarded to growers at an industry-sponsored research review held in Hood River, Oregon, in February 2009. A field day is planned for August 2009. Benefits to growers from the project have yet to be determined.
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