Assessing habitat and dietary switching by predators in a cover crop system

View the project final report

• 2009 annual report

Commodities

• Fruits: pears, general tree fruits

Practices

• Crop Production: cover crops, organic fertilizers
• Education and Training: demonstration, extension, on-farm/ranch research
• Natural Resources/Environment: biodiversity
• Pest Management: biological control, integrated pest management
• Production Systems: agroecosystems

Cover crops in orchards lead to noticeable increases in densities of generalist predators on the orchard floor. However, there are uncertainties about whether the higher densities actually translate into biological control of pests in the tree. These uncertainties have slowed implementation of this tactic in commercial orchards as a means to improve levels of biological control. A lack of consensus about the value of cover cropping to biological control is due to methodological shortcomings in many studies combined with an incomplete understanding of predator biology. Methodological limitations may include any of the following: lack of replication, insufficiently sized experimental plots, lack of pest suppression data, and a paucity of work done in commercial orchards. Incomplete understanding of predator biology has also slowed implementation. Two questions are especially important: (1) At what rate do predators move from cover crop into tree?; and (2) how readily do these tree colonists then switch diet to include the target pest? We will use a unique combination of biochemical methods to assess movement and feeding by predators under field conditions, and will examine these processes in a system comprising an alfalfa cover crop and pear, emphasizing biological control of a pest of pears (pear psylla, Cacopsylla pyricola). The studies will be done in large (0.5 acre) replicated plots at four certified organic orchards, under commercial growing conditions, and with grower cooperation. Assessment of predator movement will be done using methods that allow us to mark insects in large areas and to monitor their movement from cover crop to tree. Concurrently, we will use biochemical methods to detect psylla proteins in the guts of predators, to assess whether predators that switch habitats also switch diets to attack the target pest. Movement and feeding by predators will be compared between cover crop plots and plots having a typical grass understory. Predator densities will be monitored on the orchard floor and in the tree canopy in both control and cover crop plots. Lastly, we will monitor densities of the pest and damage to fruit, to assess whether a cover crop leads to increased biological control of psylla. Grower cooperators will be responsible for plot maintenance and orchard management, including irrigation, mowing of the orchard floor, weed control, and pest control. Results of the project will be provided to growers and the scientific community through publications, presentations, and field days. Educational products include a color insert to be published in a local trade journal, and summaries of methodology and results placed on the WSU Tree Fruit Research Center web site. The short-range outcome will include a better understanding of how a cover crop affects biological control of pear psylla. Medium-range impacts would be shown by grower adoption of cover crop practices beyond the two-year study, or by adoption of practices by other growers. We will assess adoption of practices through discussion with the grower community at field days and at an annual research review.

1. 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 colonists then switch diet to attack pear psylla; 4. Determine if the combined effect of habitat and dietary switching then leads to biological control of psylla and a lowering of damage to fruit; 5. Forward results to growers and evaluate outcomes of the project.