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
Objectives
1. 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 Relevance
1. 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%
Santa Cruz (18%)
Monterey (33%)
San Luis Obispo (5%)
Ventura (27%)
Santa Barbara
(10%)
Orange (6%)
Fresh Strawberry Price $/lb
2.5
Price Cycle: California Fresh
Strawberries (1988-2002)
2
1.5
1
0.5
0
0
5
10
15
20
25
Week
30
35
40
45
50
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
Damage Calculation
4000
y = -396.9Ln(x) + 5593.7
R2 = 0.46
Yield (g / plant)
3500
3000
2500
2000
1500
1000
1500
2000
2500
3000
3500
4000
WF Days (Jan 29 - Jun 5)
4500
5000
5500
6000
Marketable Value of Treated and
Untreated Fall Planting: Watsonville
00
0
/2
00
12
/2
3
12
/2
/
20
00
0
/2
00
0
11
/1
1
/2
/2
00
20
9/
9/
00
0
/2
8/
19
00
0
/2
00
7/
29
20
7/
8/
00
0
/2
6/
17
00
0
/2
00
5/
27
20
5/
6/
00
0
/2
00
0
4/
15
/2
00
3/
25
20
3/
4/
00
0
/2
2/
12
00
0
/2
1/
22
1/
1/
20
00
Optimal sprays: end
of Apr. & mid Aug.
9/
30
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10
/2
1
'000 $/ac
00
0
Value of Benchmark
Yield per acre
Incremental Value of
2 sprays
Value of 2 spray
yield
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
Value of Benchmark
Yield per acre
Incremental value of 2
sprays
Value of 2 spray yield
00
0
/23
/2
00
0
12
/2/
2
00
0
12
11
/11
/2
00
0
/2
/21
10
9/3
0/
2
00
0
00
0
9/9
/2
00
0
9/
2
8/1
9/
2
00
0
00
0
7/2
7/8
/2
00
0
7/
2
6/1
7/
2
00
0
00
0
5/2
5/6
/2
00
0
5/
2
4/1
5/
2
00
0
00
0
3/2
3/4
/2
2/1
2/
2
00
0
Optimal sprays: early
Jan & mid April
00
0
2/
2
1/2
1/1
/2
00
0
'000 $/ac
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
-
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 Issues
1. 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.