Quantifying biological control of pear psylla in a cover crop system

Effects of an alfalfa cover crop on biological control of pear psylla and tree nutrition was assessed. The bullet points below summarize findings.

Experimental Orchard (Moxee)
• substantial increase in predator densities on orchard floor associated with alfalfa cover crop
• no correlative effect on predator numbers in trees
• no effects of cover crop on psylla densities
• ca. 2% increase in pear leaf nitrogen in alfalfa plots
• evidence for movement between orchard floor and tree by some predator taxa (especially lacewings and ladybeetles), but no striking differences between cover crop and grass plots (data still being analyzed)
• gut contents of predators that moved from orchard floor to tree to be assessed using ELISA (specimens still being assayed)

Commercial Orchards
• Study expanded to 3 commercial organic orchard (SARE and CSANR funding)
• Minimal build-up of natural enemies in alfalfa, apparently due to frequent mowing
• Very low pest and predator densities in trees

Plans for 2010 (CSANR)
• Determine whether mowing of alfalfa prompts movement by natural enemies into tree

Grant Information

  • Project ID: 075
  • Project Status: Complete


  • Principal Investigator(s): Horton, D.
  • Investigator(s): Jones, V., Unruh, T.


Manuscript to be submitted winter 2011

Additional Funds Leveraged

Western Region SARE (2008-2009): $121,092; Washington Tree Fruit Research Commission (2007-2009): $45,000


The primary natural enemies of the targeted pest (pear psylla) are mostly confined to pear tree, and are little affected by cover crop; i.e., our movement trials showed little back-and-forth movement between cover crop and tree for the important predators of pear psylla. Thus, the alfalfa cover crop was found not to contribute extensively to biological control of the targeted pest in this system. Nitrogen content in pear trees increased in cover crop plots compared to control (grass understory) plots.