A Chemical Resistant Invader: Greenhouse Whitefly in Strawberries

1. A Chemical Resistant Invader: Greenhouse Whitefly in Strawberries Colin A. Carter, James A. Chalfant, Rachael E. Goodhue, & Greg McKee University of California-Davis PREISM Workshop, Aug. 2004

2. Objectives • Measure impact of greenhouse whitefly on strawberry yields (damage calculation). • Incorporate environmental regulations regarding chemical use. • Account for commodity price cycle. • Develop a simple action threshold model to identify optimal chemical treatment dates. • Evaluate how control based on private incentives contributes to regional management of pest.

3. Policy Relevance • Policymakers need to understand how producers will act to mitigate their losses  not just pest biology • Key Players: EPA CA Dept. of Pesticide Regulation (DPR) CA Strawberry Commission & Industry Calif. Dept. of Food and Agriculture

4. California Strawberries • Coastal production • California accounts for over 80% of U.S. production • Florida accounts for around 12% Santa Cruz (18%) Monterey (33%) San Luis Obispo (5%) Ventura (27%) Santa Barbara (10%) Orange (6%)

5. Price Cycle: California Fresh Strawberries (1988-2002)

6. Traditional Season • Fall planting (90% of acreage) • Planted late Sept. (Oxnard) – Oct. (Watsonville) • Harvested • December – June in Oxnard area • March – October in Watsonville area • Summer planting (10% of acreage) • Planted in July (Oxnard) & August (Watsonville) • Harvested • September – December in Oxnard area • October – May in the Watsonville area

7. Calif. Industry Has Closed the Southern Hemisphere Window • CA Strawberries are now available essentially year-round: no more from Australia/NZ. • Did growers inadvertently also provide a host for whiteflies year-round? • Southern Calif: filled gap in season for whitefly. • Northern Calif: provided convenient, better host late in year.

8. Greenhouse Whitefly: Resident Invader • Common pest along CA coast • Emerged in strawberries in 1999-2000 • Strawberries not previously a host • Invaded primarily Oxnard and Watsonville areas • Heavy infestation in 2002 • Possible explanations for invasion: • Increased summer acreage • Expansion of total acreage • Urban hosts closer to strawberry fields • Nursery stock (Oxnard)

10. Economic Impact of Whitefly Feeds on the sap of strawberry plant • Reduce total yield up to 25% • Reduce marketable yield • Decrease nutritional content (less sugar, citric acid) • Helps spread plant viruses

11. Greenhouse Whitefly Management • Control is complicated • Difficult to kill (resistant to traditional chemicals) • Feeds on underside of leaf • Few chemical products registered for control • Admire (used at planting): not registered • Esteem: not registered • Other chems. provide limited control of adults • Eliminating plant hosts is another option • Crop clean-up • Reducing overlap of strawberry plantings • Break continuous whitefly cycle by eliminating plant hosts

12. Source: Dr. Tom Perring, UCR Source: Dr. Nick Toscano, UCR - 1999

13. Economic issues • Price cycle creates economic incentive to plant “host” crops • Continuous “host” plants allows year-round population development • Summer plantings • Second year plantings • Late harvest for processing • Alternative hosts (Oxnard) • Lack of grower coordination in whitefly management • Environmental regulations of chemical control

14. Esteem (pyriproxyfen) • Esteem provides effective post-plant whitefly control • Application costs approx. $40/acre • Effective for up to nine weeks • Sometimes used in conjunction with Admire (Imidacloprid) • Emergency registration for 2004 • Restricted to two applications per acre per year

15. Damage Calculation

16. Marketable Value of Treated and Untreated Fall Planting: Watsonville Optimal sprays: end of Apr. & mid Aug.

17. Comments on Watsonville Case • WF population peaks in March – April • Late April Esteem spray may not fully control Whitefly population • Likely to promote larger overall population • Watsonville monoculture • August spray reduces carryover into fall plants, transplanted in Oct– Nov

18. Marketable Value of Treated and Untreated Fall Planting, Oxnard CA Optimal sprays: early Jan & mid April

19. Comments on Oxnard Model • WF population peaks in late March – April • Spray in March or April will control the historical spike in WF population • If only fall plantings, entire harvest season could be protected • Increasing summer acreage makes this unlikely • Multiple hosts makes reinfestation likely

20. Other Issues • Dynamic considerations may matter less when total applications limited. Also, reinfestations from neighboring crops breaks the link between your actions now & whitefly population later. • Does market power make a shipper less concerned (risk preferences and also more inelastic demand). • Optimization errors by producers.

21. Conclusion • If growers focus on the value of strawberries instead of the number of whiteflies, this delays the first spray of Esteem until April & may not result in total whitefly control in the Watsonville area. • Significance: individual grower spray decisions may not completely control the greenhouse whitefly, and the severity of the invasion could worsen.