Millennium Grove Avocado Demonstration

PROBLEM: Walnut husk fly is the key pest in much of California’s walnuts. As growers find alternatives to broad–spectrum pesticides, such as against codling moth, naturally occurring beneficials drive all pests below the level of economic damage with exception of walnut husk fly..

SIGNIFICANCE: When growers spray to control this pest, they disrupt the natural enemy complex. The disruption forces them to rely on toxic materials to control other walnut pests in a "spray for one; spray for all" cascade of effects. By maintaining bare ground in walnut orchards, a complete soil food fails to develop and soil–dwelling husk fly predators are too few to suppress the pest. The situation is complicated by the harvesting requirements of walnuts. Many cover crops could potentially enhance the soil food web. Unfortunately, these same plants may interfere with collecting. Ground cover may encourage mold on fallen nuts because of greater ground moisture.

STRATEGY: Build a complete soil food web so that the ground stage of the husk fly is attacked by natural enemies. Select and determine how to manage ground covers that feed the soil organisms without impeding harvest. Compare husk fly damage with neighboring orchards.

ACCOMPLISHMENTS: A nine–acre grove in the Santa Ynez Valley was planted with a variety of perennial and self–seeding annual grasses and legumes to determine which species established and could be managed. At harvest time, a combination of low mowing and natural drying provided a substrate compatible with harvesting requirements. Mounds of horse–stall bedding straw were placed near each tree to encourage spiders, predator beetles, predator mites, fungi, etc. Husk fly damage has been low during several years of the study, but unusual weather and a variety of other circumstances complicate the interpretation.