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

Objectives1. Measure impact of greenhouse whitefly on

strawberry yields (damage calculation).

2. Incorporate environmental regulations regarding chemical use.

3. Account for commodity price cycle.

4. Develop a simple action threshold model to identify optimal chemical treatment dates.

5. Evaluate how control based on private incentives contributes to regional management of pest.

Policy Relevance1. Policymakers need to understand how

producers will act to mitigate their losses not just pest biology

2. Key Players:

EPA

CA Dept. of Pesticide Regulation (DPR)

CA Strawberry Commission & Industry

Calif. Dept. of Food and Agriculture

California Strawberries

• Coastal production

• California accounts for over 80% of U.S. production

• Florida accounts for around 12%

San Luis Obispo (5%)

Santa Barbara(10%)

Ventura (27%)

Orange (6%)

Monterey (33%)

Santa Cruz (18%)

Price Cycle: California Fresh Strawberries (1988-2002)

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25 30 35 40 45 50

Week

Fre

sh S

traw

berr

y P

rice

$/lb

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

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.

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)

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

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

Source: Dr. Tom Perring, UCR

Source: Dr. Nick Toscano, UCR - 1999

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

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

1500

2000

2500

3000

3500

4000

1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

WF Days (Jan 29 - Jun 5)

Yie

ld (g

/ pl

ant)

y = -396.9Ln(x) + 5593.7

R2 = 0.46

Damage Calculation

Marketable Value of Treated and Untreated Fall Planting: Watsonville

0.00.51.01.52.02.53.03.54.0

'000 $/acValue of BenchmarkYield per acreIncremental Value of2 spraysValue of 2 sprayyieldOptimal sprays: end

of Apr. & mid Aug.

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

Marketable Value of Treated and Untreated Fall Planting, Oxnard CA

-0.51.01.52.02.53.03.54.04.55.0

'000 $/ac Value of BenchmarkYield per acreIncremental value of 2spraysValue of 2 spray yield

Optimal sprays: early Jan & mid April

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

Other Issues1. Dynamic considerations may matter less

when total applications limited. Also, reinfestations from neighboring crops breaks the link between your actions now & whitefly population later.

2. Does market power make a shipper less concerned (risk preferences and also more inelastic demand).

3. Optimization errors by producers.

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.