2002-2004 Specialty Crops Research Program
University of California

Principal Investigators: Mark Hoddle, Michael Brownbridge and Paul De Ley

Project title: Assessing Organic Mulches for Thrips Control in Avocado Orchards

Avocado production in California (CA) has been severely impacted by avocado thrips (AT), Scirtothrips perseae, an invasive species from Latin America. Expenses associated with AT control, cost the avocado industry an estimated $8-11 million/year. New cultural and biological control strategies are urgently needed for AT. Approximately 78% of thrips larvae drop from avocado trees to pupate beneath the host plant. Our studies indicate that composted organic mulches used for biological control of avocado root rot also significantly reduce adult thrips emergence. We have shown that mulch harbors a diverse community of natural enemies (arthropod predators, entomopathogenic fungi and nematodes) and suspect that it provides an environment that enhances the impact of these beneficial organisms. Research proposed herein seeks to develop a greater understanding of the mechanisms involved in thrips suppression and identify ways of enhancing the effectiveness of this cultural-biological control technique. The project has four objectives: (1) Determine effects of abiotic factors on avocado thrips survival in mulch. (2) Evaluate natural enemies originally recovered from mulch for AT control. (3) Determine if natural enemy combinations suppress thrips in mulch more effectively than individual species. (4) Document and demonstrate the suppressive effect of mulches in a commercial 600-acre avocado orchard.

1. Objectives. The arrival of in California (CA) in 1996 AT (Nakahara, 1997), has forced avocado growers from a production strategy historically reliant on biological control to one that is almost exclusively reliant on chemical pesticides. Novel cultural and biological controls are urgently needed to promote a return to more ecologically acceptable crop protection practices. We have shown that composted municipal yard waste (mulch), laid around avocado trees, significantly reduces survival of avocado thrips pupating beneath them. We propose to investigate mechanisms involved in thrips suppression, determine whether selected natural enemies can be used to bioenhance mulches, and demonstrate the impact and utility of this management approach as follows: (1) Determine effects of abiotic factors (humidity and temperature) on avocado thrips survival in mulch. (2) Evaluate natural enemies (micro-arthropods, fungi, nematodes) originally recovered from avocado orchards for control of avocado thrips. (3) Determine if combinations of natural enemies suppress thrips in mulch more effectively than individual species. (4) Document and demonstrate the suppressive effect of mulches on pupating thrips in a commercial avocado orchard.

2. Justification. In 1999-2000, the CA avocado harvest was worth $339.5 million (CAC Annual Report, 2001). In the past, natural enemies have kept pests in avocado orchards below economically injurious levels and pesticide use has been minimal (Fleschner, 1954; McMurtry, 1992). However, the introduction of AT from Latin America has forced growers to utilize insecticides for pest management. Crop losses caused by thrips feeding, combined with the additional expenses incurred for its control, costs the industry an estimated $8-11 million/year (Hoddle & Morse, 1998a,b). Pesticide use poses considerable environmental and human-health risks, and has led to a resurgence of many pests (Hoddle & Morse, 1998a,b). Development of an effective cultural control strategy that enhances natural enemy activity against avocado thrips would significantly reduce insecticide use and prolong the active lifetime of registered insecticides.
Many species of phytophagous thrips drop from host plants to pupate in soil or leaf litter; pupation rates are affected by abiotic (e.g., moisture) and biotic (e.g., pathogens) factors (Lewis, 1973). Most avocado orchards in CA have very little vegetative diversity and the ground is either bare or covered with a layer of dead avocado leaves, an environment that is not conducive to the development of robust natural enemy populations. Provision of a habitat that allows these beneficial organisms to flourish is likely to lead to a greater reduction of AT.