Crop Protection Research Institute
The Benefits of Insecticide Use: Strawberries
Spider Mite Damage
Spider Mite Predator
Lygus Bug Damage
Slug on Strawberry
March 2009
Leonard Gianessi
CropLife Foundation
1156 15th Street, NW #400 Washington, DC 20005
Phone 202-296-1585 www.croplifefoundation.org
Fax 202-463-0474
Key Points
• Uncontrolled spider mite infestations reduce strawberry yield by 25%.
• Most growers (90%) use predatory mites in combination with insecticides for
spider mite control.
• The feeding of lygus bugs causes strawberries to become deformed.
• Slugs would damage about 20% of the strawberries on treated acres.
Technical Summary
California produces more than 80% of the fresh market and processed strawberries grown
in the U.S. California produces about 20% of the world’s supply of strawberries.
Strawberries are grown on 36,000 acres in California with annual production of 2.1
billion pounds valued at $1.2 billion. Strawberry yield in California averages 47,000
pounds/acre with a production cost of $33,300/A [28]. Currently, it is estimated that
100% of California strawberry acres are treated with insecticides and that a 60% yield
loss would occur if insecticides were not used and alternatives were used instead
(predator/parasite releases, vacuum machines) [24]. California strawberry growers spend
about $580/A on insecticides which represents about 2% of the cost of growing
strawberries [28]. Florida ranks second in the US in the production of strawberries
providing 15% of the total US crop and 100% of the domestically-produced winter
cdrop.In Florida, North Carolina, and Michigan, the costs of insecticides for strawberries
are $559/A, $660/A, and $72/A respectively [30]-[32].In states other than California,
strawberry yield losses without insecticides are estimated at 11-80%[24].
Two-spotted Spider Mites
The two-spotted spider mite is an economic pest of strawberry in all U.S. production
areas. The two-spotted spider mite is the major arthropod pest of California’s strawberry
industry [8]. Two-spotted spider mites are very small (about the size of a pencil point)
insect-like creatures that feed on strawberry foliage. They appear like moving dots.
Mites form colonies and may be most noticeable by the spider-like webbing that they
produce that gives them their name. The two-spotted spider mite survives on many hosts,
including fruits, vegetables, and weeds. Spider mites do not fly but are wind-borne in the
silk webbing and thus newly planted strawberry fields can soon become infested. The
mites overwinter as adults and begin reproducing as temperatures increase in the spring.
Their rapid rate of development (mites complete their life cycle in five days) and high
reproductive capacity (50-100 eggs per female) enable them to reach damaging
population levels very rapidly. High population levels are accompanied by large amounts
of webbing under which eggs are deposited. Eggs are spherical and translucent. Mites
feed and deposit eggs on the undersides of leaves. Reproduction is usually continuous
from early spring until late fall; or, in warmer areas they can reproduce year-round.
Mating and egglaying typically occur year round in all coastal strawberry-growing
regions in California [1]. Two-spotted spider mites attack new strawberry plantings in
the fall and their populations build up rapidly. Controls are usually needed every year in
all California growing regions to prevent yield losses [4]. Yield loss can be significant at
all mite infestation levels exceeding one mite per leaflet. More than 80% of California
strawberry acreage is fall-planted and mite control during the first 4-6 months is critical
[5].
The mites suck the sap from the strawberry plants and cause a general loss in vigor, yield
reduction, and permanent stunting or death. Photosynthesis and transpiration rates are as
much as 50% lower in plants with high late season mite populations than in plants where
mites are suppressed. Photosynthesis and transpiration reductions in stressed plants are
accompanied by decreases in fruit weight, number and size [2]. A seven-year study
concluded that yield loss resulting from spider mite feeding in untreated plots versus
plots treated with insecticides averaged 25% [3]. The mite feeding caused a significant
reduction in the number of berries produced per plant.
In the 1950s tests with insecticides showed that substantial reductions in spider mite
populations could be achieved with applications of demeton (94%) and dicofol (99%)
[6][7]. With the increased adoption of the annual planting of strawberries, the two-spotted
spider mite became the most serious pest of strawberries in California.
Since 1964, entomologists at the University of California have been conducting research
to develop methods for managing the mite on strawberries [2]. Initial studies indicated
that natural predation, primarily by six-spotted thrips, did not prevent spider mite
populations from reaching damaging levels [9].
Initially, a classical biological control approach was tried. A predatory mite
(Phytoseiulus persimilis) that feeds exclusively on spider mites and consumes two or
three adults or several dozen eggs in a day was imported into California from both Chile
and Italy [8]. P. persimilis is an aggressive feeder that multiplies and spreads rapidly and
can reduce spider mite populations quickly. P persimilis was released into strawberry
fields and became established, but did not provide an economic level of control [8].
Researchers then attempted inundative release programs. P.persimilis effectively
controlled the two-spotted spider mite where they were released early at 320,000 per acre
before the latter species reached one mite per leaf [10]. Strawberry yields in the P.
persimilis plots were ten tons per acre higher than in the untreated check plot [11].
However, the cost of releasing 320,000 predacious mites per acre was not economically
feasible due to the high costs of mass production [12]. The cost to the grower was
approximately $2,000 per acre [14].
Research in the 1980s demonstrated that early releases of P. persimilis at 30,000 per acre
followed by one to two applications with selective miticides that have minimal
detrimental effect on the mite predators allowed the predators to provide ongoing
baseline control of the pest [8][13]. The cost of the release of 30,000 predator mites was
estimated at $300/A while two supplemental insecticide applications cost $141/A [15].
The net benefit of the combined treatment was $2,810/A while the two insecticide
applications delivered a $2,000 net benefit per acre and the predator mite release
delivered a $900 net benefit per acre [15]. The total yields for the different treatments
were (pounds/acre): no treatment (60,000), insecticide (72,000), predator mites (65,000),
and combined predator mites and insecticide (75,000) [15]. The new selective
insecticides have to be applied at one hundred times the label rate in order to kill half of
the predators [29]. Approximately 90% of California strawberry acres receive predator
mite releases.
Organic strawberry growers in California rely on the release of the mite predators for
control of two-spotted spider mites. Organic growers make four releases for a total
release of 40,000 predators per acre [16]. The cost of the releases totals $260/A which
includes $220/A for the predators and $40/A for the labor involved in releasing them. As
a result of relying on a low rate of predator release, organic strawberry growers do incur a
yield loss.
In Florida, spider mites are the main pest. Before the introduction of new effective
miticides, Florida strawberry growers had an estimated 40% adoption of predatory mite
programs. With the introduction of the new miticides that rate has dropped to 25% even
though the two systems are roughly the same cost. Growers report the desire to have
“clean plants” rather than those with mites on them [34].
Lygus Bug
The lygus bug ( the Western tarnished Plant Bug [WTPB]) is the primary late-season pest
of summer-season strawberries. This insect is a native species and feeds on a broad range
of winter broadleaf weeds.The established economic threshold for lygus is extremely low
at one bug per ten plants. When left uncontrolled, lygus feeding can cause damage to 40
to 60% of the summer-season fruit [17]. In central coastal California, lygus bugs
overwinter as adults feeding on weeds. In spring when the rains cease and the weeds dry
out, the adults rapidly move into strawberry fields. Lygus bugs are strong fliers.
Whereas strawberries are not a preferred host of lygus in California, the absence of other
more attractive plants in late spring is perhaps the basis for extensive colonization of
strawberries [18]. Lygus is an important insect pest of strawberry on California’s central
coast, including the Watsonville and Santa Maria growing regions. It is rarely a pest in
Southern California, where picking occurs primarily in the early spring months [33]. At
least two generations develop between April and August in strawberries in this region.
Because of the importance of lygus as a key pest of strawberries in the central coast of
California, control efforts are targeted at this insect pest annually [20]. Lygus bugs prefer
to feed on the reproductive parts of plants. The stylet of the lygus bug is fine and pointed
with sharp serrations angled back toward the head, and well suited for penetrating and
lacerating plant tissues [21].
The cultivated strawberry produces an aggregate fruit composed of seeds arranged in a
spiral fashion on an enlarged receptacle [19]. The fleshy receptacle forms the edible part
of the berry. Growth of the receptacle is principally a function of cell enlargement
controlled by plant growth regulators released from the seeds. Lygus bugs feed by
sucking out plant juices with straw-like mouth parts. Lygus bugs damage fruit by
puncturing individual seeds and sucking out juices that contain growth hormones; this, in
turn, stops development of the berry in the area surrounding the feeding site. Fine stylets
are inserted into the plant tissue. Repeated plunging of the stylets during feeding renders
considerable damage to the tissue. The cells surrounding the feeding collapse. In
addition, enzymes may be injected into the fruit via saliva, causing further cellular
breakdown [19]. The berry stops developing in the immediate area of the destroyed
seeds. Growth of the fruit tissue beneath and surrounding the damaged seeds stops. The
most serious damage is fruit deformation or “catfacing.” Such fruit cannot be sold fresh,
but instead are salvaged for processing [20]. Deformed berries increase a grower’s
harvest cost because they must be sorted from fresh market quality berries.
WTPB prefer to lay eggs on the strawberry receptacle because of the complexity of its
surface and the small distance between achenes on an enlarging berry. This reduces the
accessibility of WTPB eggs to Anaphes iole .Economic damage is estimated to occur at
densities of 1 or 2 WTPB per 20 strawberry plants sampled.
Malathion was the most important treatment for lygus control and was applied at a rate of
2 LBS AI/A to about 55% of strawberry acres [5]. Malathion is effective against the
young lygus immediately after hatching. A study of the use of malathion showed a net
return of $1170/A for malathion applications costing $34/A [17].
Bigeyed bugs are the most important natural predator of lygus bugs. They feed on eggs
and young lygus. Damsel bugs also feed on eggs and young lygus and minute pirate bugs
feed on lygus eggs. None of these natural enemies, however, is successful in keeping
lygus from reaching damaging levels when there is a heavy migration of adults into
strawberry fields when wild vegetation dries up in the spring [5]. A parasitic wasp which
attacks lygus eggs is available commercially. In experimental plots, augmentative
releases of wasps at 37,500/week provided 43% lygus suppression and a 22% reduction
in berry damage [35] [18]. However, because thresholds for this pest are very low and
adults moving into the field from external sources are not controlled, economically
acceptable control is not achieved with the wasp releases. While these predators feed on
WTPB, they often do not keep summer populations below the economic injury level [36].
Efforts to modify release strategies and enhance performance of the parasitoid by
integration of Anaphes releases into trap crops has not been recently feasible due to
commercial unavailability and high cost of parasitoids[36].
One approach to reducing lygus bug populations is the use of tractor-mounted suction
devices, referred to as “bug vacuums.” Research demonstrated that the bug vacuums
provide limited lygus control in strawberries [23]. Vacuuming once a week for twenty
weeks reduced the number of damaged fruit by 37 % while the use of insecticides
reduced the damage by 82 % [23]. Weekly use of the vacuum is necessary since the
vacuums remove adults but have limited impact on the young lygus. Lygus adults are
mobile and rapidly migrate into strawberries after passage of the vacuum [18][36].
Vacuums may increase problems with powdery mildew and gray mold by spreading the
pathogens that cause these diseases [22][36]. Vacuum devices are not selective and
substantially reduce the populations of some important natural enemies such as bigeyed
bugs and spiders [4][36]. Secondary outbreaks of whiteflies, cutworms, and cabbage
loopers sometimes have followed the use of vacuum devices in strawberries, probably
because vacuuming removed adult parasites of these pest species [4]. Approximately
60% of organic strawberry growers use the vacuum devices.
Insecticidal soap sprays are acceptable for use on organically certified strawberries. Three
applications of insecticidal soap totaling six gallons at a total cost of $152/A would have
to be made [16]. A single application will reduce young lygus populations by no more
than 50% and will have little effect on adults [22]. The soap applications also kill about
50% of the predatory mite eggs [5].
Organic strawberry growers do not use the chemical pesticides registered for
conventional lygus bug control nor can they routinely use any organically-compliant
materials economically, due to the expense and low efficacy of the products [36]. Instead,
organic strawberry growers on the Central Coast rely on field isolation from hosts,
adjustments in fresh-market harvest schedules, and whole field deployment of tractormounted insect vacuums. Recently, organic strawberry growers have become interested
in experiments with plantings of attractive non-crop vegetation as trap crops which might
simultaneously attract WTPB away from strawberry fields ( where they can be removed
by the use of a bug vac) and increase levels of WTPB-associated predators and
parasitoids. In the 2001 experiment, the average reduction in adult WTPB in the alfalfa
trap crop was 70% in a single pass.
In 2001, the WTPB egg parasitoid Anaphes iole was made available by a commercial
insectary in May. This parasitoid was released into 4 alfalfa trap crop replicates and
average parasitism of WTPB eggs was 35% in the trap crop within ten days of release of
2,000 parasitoids. 40% parasitism was detected in the first adjacent strawberry row but
average parasitism decreased rapidly outward to row 4 (4%) and no parasitism was
detected in row 8. Of the plants that had WTPB eggs present, the average percent
parasitism per ‘lygus egg/leaf’ was 16% overall and ranged from 4 to 24%[37].This egg
mortality was not sufficient to have a suppressive population impact[36].This project was
discontinued due to rearing difficulties at the commercial insectary and subsequent
unavailability of parasitoids due to expense and a limited market demand[36].In 2002,
treatment significance could not be detected in August, and trap crop attractiveness may
have been compromised by lack of irrigation water. It is extremely important to maintain
the horticultural quality of the trap crop throughout the production season and especially
to maintain irrigation water supply [36].
Vacuum treatment of an alfalfa trap crop reduces beneficial insects (generalist predators
and parasitoids) by 40% after a single pass. Successive passes 2 and 3 lowered beneficial
insect numbers approximately 11% more [36].
Trap crops can accumulate WTPB at 5-10 times greater density than strawberries and
while they also accumulate beneficial insects, they only do so at a slightly higher rate
than in the strawberry crop [37].Therefore, trap crops alone may not lead to a substantial
reduction in either WTPB densities or a reduction in damage caused by WTPB in
adjacent strawberries [37].
Tractor-mounted vacuuming of a trap crop can only begin in May, when all drainage
ditches are closed and the threat of heavy spring rain and muddy row conditions has
diminished [36].
Slugs
Slugs are mollusks that feed on a wide range of plants. The two types of slugs commonly
encountered on California’s central coast are the garden slug (1 inch) and the little gray
slug (0.5 inch). Both slugs have no shell and are slimy. Slugs are sensitive to dryness,
and seek out moisture, making the humid environment under the mulch of strawberries
attractive to them.
The head of the slug has two sets of tentacles. The eyes are on the tips of the upper
tentacles. The lower tentacles, which are shorter, are used for tasting and smelling. The
mouth is located between and below the lower tentacles and is equipped with a radula, a
tooth-covered rasp that the slug uses to grate plant tissue. The slug glides along on a
muscular foot. This muscle constantly secretes mucus, which later dries to form the
silvery slime trail that signals the presence of these pests.
The slug mates throughout the warm months. It is hermaphroditic, meaning that an
individual slug has both male and female reproductive organs. An individual may start
out as a male, then become both male and female, and finally become solely female.
Certain species may even self-fertilize, which means they can produce a viable offspring
without mating. After mating, clusters of eight to sixty clear jellylike eggs are laid in
sheltered areas on the ground. Newly hatched slugs resemble the adults but are much
smaller. The average life span of the slug is from nine to thirteen months, and an adult
can lay from three hundred to five hundred eggs during its lifetime.
On strawberries, slugs can be highly destructive because they feed on the fruit. They
make small, moderately deep rough holes in the berries that can be recognized by the
slime that is deposited on the surrounding fruit surface [25]. Upon drying, these slime
trails take on a silvery appearance. Slug-damaged strawberries are unmarketable [26].
The IPM recommendation for slug control in strawberries is the application of
metaldehyde baits which attract and kill the slugs [26]. In the U.S. commercial baits
containing metaldehyde have been on the market since 1938. In one experiment, the
application of metaldehyde reduced the number of slug-damaged strawberries in a 50 foot
row from 108 to 0 [27].
It has been estimated that metaldehyde is applied to 12% of California’s strawberry acres
for slug control [24]. Without the use of a pesticide to control slugs on the treated acres,
strawberry yields would likely decline by 20% [24].
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