Abstract
Thrips attack a number of vegetable, fruit, and flower crops, causing
considerable economic damage. Their population levels may be monitored
using sticky cards and by direct examination of plant parts using
a hand lens. Several cultural control options are available, including
avoiding planting thrips-susceptible crops following small grains,
managing vegetation in the fields and field edges, using colored
mulches, and avoiding high nitrogen levels. Some cabbage and onion
varieties are somewhat resistant to thrips attack. Several beneficial
insects suppress thrips levels. Organically acceptable pesticides
are available for thrips control.
Table of Contents
Introduction
Thrips are known to be serious pests on a wide range of fruit,
vegetable, flower, and agronomic crops. Thrips are members of the
order Thysanoptera, which contains a number of genera and species.
For example, there are at least two species of thrips that attack
onions: onion thrips (Thrips tabaci) and western flower
thrips (Frankliniella occidentalis). Both species have
a wide host range, including cereals and broad-leafed crops. (1)
The information contained in this publication is largely generic
and applies broadly to most kinds of thrips. Onions are often emphasized,
however, since much research has been directed to thrips control
in this crop.
Thrips feed by rasping the leaves and other tissues of plants to
release the sap, which they then consume. This feeding reduces the
plant's ability to produce food and interferes with transportation
of foliar nutrients to the bulb. The resulting damage is usually
measured as an overall reduction in bulb size and weight of onions
and flower corms produced. There may also be effects on the number,
size, and appearance of flowers. The injury caused by thrips' rasping
of the leaves enables various plant pathogens to gain entry, thus
increasing disease problems. In addition, thrips carry plant pathogens
on their mouth parts from one plant to another. In onions, entire
fields can be destroyed, especially in dry seasons. In fruit crops,
thrips damage may also result in the scarring of fruit and significant
loss to culling.
The stage of growth when an infestation occurs seems to determine the extent
of yield loss. In onions, it appears that early and late-season
infestations diminish yields less than those occurring in mid-season
during the bulbing stage. (2)
For more basic information see the THRIPS
computerized knowledge database. THRIPS covers nine species
of thrips occurring in the U.S. and a major thrips predator, the
minute pirate bug. The site provides detailed information on identification,
biology, scouting, life cycles, and control using pirate bugs.
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Monitoring Thrips
Monitoring to identify when thrips arrive and to determine population
levels is helpful in designing an appropriate control strategy.
In onions, thrips generally migrate into a field when plants in
surrounding areas begin to dry, so monitoring efforts should be
timed accordingly.
Thrips numbers should also be determined. The University of California
recommends sampling at least five (onion) plants from four separate
areas of each field. Leaves should be pulled apart and examined
carefully with the assistance of a hand lens, and all thrips counted.
(1) The numbers present can be used to determine
action thresholds for applying pesticides. In corm-propagated flowers,
blue sticky cards are sometimes used for trapping and monitoring
thrips populations. (3) However, research at
the University of California has indicated that hot-pink sticky
cards outperform blue-colored traps for this purpose. (4)
Coviello et al. (1) note that reliable treatment
threshold levels for onions are speculative. They are, in the least,
variable, depending on whom you ask. In California, a threshold
of 30 thrips per plant at mid-season has been used successfully
for dry bulb, fresh market, and drying onions, using conventional,
synthetic pesticides. This number would be adjusted downward for
very young plants and upward for larger, mature plants. In New York
State a conservative action threshold of three thrips per leaf has
been suggested, and one thrips per leaf for Spanish and green bunching
onions. (5) Georgia entomologist David Riley
suggests an initial treatment threshold of one thrips per plant
and then waiting until they have reached five thrips per plant for
a second treatment. (6)
When considering appropriate action threshold levels, it is important
to remember that no single number will always be a reliable guide.
Climate is a factor. Hot and dry conditions favor thrips damage,
and cool rainy weather hinders it. In drier years fewer thrips per
leaf can be tolerated before yield losses result. (2)
In onions, thrips must be controlled before the crop reaches the
early bulbing stage, so that populations do not exceed manageable
levels. (1)
Plant architecture can also influence thrips population levels.
In onions, cultivars with flat-sided leaves and a compact growth
point (where the leaves are closely compressed) protect thrips from
natural enemies, weather, and insecticides. Conversely, round, openly
spaced leaves reduce thrips' hiding places. (2)
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Cultural Control Methods
The choice of cover crops can affect the number of overwintering
thrips. Thrips-susceptible crops should not be planted following
wheat or rye, which provide excellent over-wintering sites for thrips.
Oats is a better choice, but requires later fall planting than rye
or wheat—a factor that constrains its use as a cover crop
in colder regions. (7)
Weed destruction in the field and surrounding margins can help
to reduce thrips populations, since these areas serve as overwintering
and re-infestation sites. Growers should take care, however, that
vegetation management does not conflict with strategies designed
to reduce soil loss (through maintaining soil cover), to increase
biodiversity, and to make the farm system more sustainable.
Drought stress increases the susceptibility of onions to thrips
damage. Adequate irrigation throughout the growing season is a critical
factor in minimizing damage. (2)
The fact that thrips are color-sensitive suggests that colored
mulches may be effective in their control. Louisiana researchers
conducted a study to see whether aluminum-coated mulch would repel
the pest. (8) Black plastic was spray-painted
in the field with aluminum paint. The reflective mulch repelled
33 to 68% of the thrips. Ultaviolet-absorbing plastics—used
to build walk-in field tunnels—have proved effective in protecting
crops from western flower thrips, as well as sweet potato whitefly,
cotton aphid, and the diseases they vector. (9)
Soil fertility management may also affect thrips infestation and
damage. According to one source (10), a lack
of adequate soil calcium may invite higher populations of thrips.
Another writer suggests that nutritional balance can reduce thrips
attack. High nitrate levels will invite thrips, and the effects
of excessive nitrate are compounded by shortages of potassium, sulfur,
boron, and manganese. Foliar applications of soluble calcium and
kelp will balance the excess nitrogen. These nutrient levels can
be monitored on a weekly basis, using plant tissue analysis, to
make accurate adjustments. (11)
In iris, gladiolus, daylilies, and other vegetatively propagated
flowers, severely infested plant parts should be cut away and destroyed.
Dig corms early in the fall and cut off the tops before thrips move
down into them. Destroy the remaining debris. (3)
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Genetic Resistance
Among onions, varietal resistance in some sweet Spanish types is
noted in the literature. It is suggested that these may be older
cultivars with more open canopy growth. (5)
There is also some genetic resistance among cabbage cultivars: Masada,
Brutus, Galaxy, and Amtrack are reported to be highly resistant;
Fresco, SuperElite, Satelite, Protector, Bartolo, and Supergreen
demonstrate moderate resistance. (12)
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Biological Control
Many beneficial organisms work to suppress thrips. These include
lady beetles, minute pirate bugs, ground beetles, big-eyed bugs,
lacewings, hover flies, predatory mites, and spiders. (5)
Unfortunately, these predators and parasites may be hampered by
the fact that thrips feed under close-fitting leaves and down in
the leaf sheaths where they are difficult for predators to find.
Insecticides, even those cleared for use in organic production,
also tend to work against beneficial predators and parasites. They
should be used minimally and applied with caution.
On occasion, thrips populations may explode, as large numbers migrate
from nearby vegetation that is mowed, harvested, or drying up. In
such circumstances, in-field populations of beneficial predators
and parasites may not be adequate. The grower can increase the numbers
of predatory and parasitic insects and arachnids on the farm by
providing protective habitats for them. These habitats—often
called refugia—can be integrated into crop rotation planning,
and can supplant "weedy" field borders and waste areas.
For more details on refugia, see the ATTRA publication
Farmscaping
to Enhance Biological Control.
Naturally occurring fungal diseases can also devastate thrips populations.
(5) Limiting the use of foliar fungicides by
using forecasting systems and cultural controls helps to protect
beneficial fungi and maximize this natural control mechanism.
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Alternative Pesticides
Several alternative pesticides are available for controlling thrips.
Sulfur, insecticidal soap, and diatomaceous earth have all demonstrated
efficacy in suppressing thrips in several crops. (3,
13, 14) Being contact
pesticides, however, their effectiveness in onions would probably
be limited, because the thrips can hide between the leaves. In contrast,
three applications of superfine sulfur are recommended at monthly
intervals in fruit crops for spring thrips control. Lime sulfur
has also been suggested as an alternative. (14)
Spray formulations of the biological agent Beauvaria bassiana
(e.g., Naturalis-O®, BotaniGard®, Mycotrol®) are also
useful for thrips control. The agent is a fungus that grows and
reproduces in the host; therefore, effective control may not be
observed until 7 to 10 days after application. B. bassiana
is most effective when used early, before large thrips populations
have built up. (14)
The botanical pesticides garlic, rotenone, ryania, pyrethrum, neem,
and nicotine have been suggested for thrips control. (10,
16, 17, 18)
Another botanical presticide, sabadilla, also has a record of controlling
thrips and is suggested when other botanicals fail. (3)
Organic growers should be aware that nicotine and other tobacco-based
pesticides are prohibited in organic production. The formulations
of many other botanical products are also prohibited, and organic
producers are encouraged to consult their certifier before purchasing
them.
Noted organic proponent J. Howard Garrett recommends use of a homemade
botanical spray formulated from garlic and pepper. (17)
Garrett's garlic/pepper tea is made by liquefying two bulbs of garlic
and two cayenne or habañero peppers in a blender 1/3 full
of water. Solids are strained out, and enough water is added to
make one gallon of concentrate. Garrett mixes ¼ cup concentrate
with two tablespoons of vegetable oil and enough water to make 1
gallon of ready-to-use spray mix. (18) A commercial
product called Hot Pepper Wax®, containing capsaicin (the active
ingredient of cayenne pepper), is also recommended for thrips control.
(19) A highly refined horticultural oil can
be used during the growing season for control of thrips and several
other insects. (20)
Spinosad (e.g., Conserve®) may also be useful in controlling
thrips in the field. Spinosad is a recently discovered insecticide,
derived from the fermentation of Actinomyces bacteria commonly found
in the soil. (21) The National Organic Standards
Board has recommended that spinosad be allowed in organic production.
Organic growers should consult their certifier before using.
Surround™ Crop Protectant is labeled for suppression of thrips
and several other insects for apples, stone fruits, citrus, small
fruits, and onions. Surround is a kaolin clay film sprayed onto
the crop as a barrier to insect pests. Supplemental control measures
may be needed in addition to Surround. ATTRA has more information
on the use of Surround in fruit crops, available on request.
Always follow label instructions when using any pesticide, and
keep in mind that the decision to use a pesticide should be made
only when other approaches to pest management fail to provide adequate
crop protection. Again, be aware that some "natural" pesticides
may be unacceptable or limited in certified organic production.
Check with a certifying agent before purchasing or using any such
products.
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References
- Coviello, R., W.E. Chaney, and
S. Orloff. 1993. Onion and Garlic Pest Management Guidelines.
University of California Statewide IPM Program. Davis, CA.
UC
ANR Publication 3453.
- Fournier, Francois, Guy Boivin,
and Robin Stewart. 1995. Effect of Thrips tabaci (Thysanopters:
Thripidae) on yellow onion yields and economic thresholds for
its management. Entomological Society of America. Vol. 88, No.
5. p. 1401-1407.
- Ellis, Barbara W., and Fern Marshall
Bradley. 1992. The Organic Gardener's Handbook of Natural Insect
and Disease Control. Rodale Press, Emmaus, PA. 534 p.
- Anon. 1998. Pink thrips. B.U.G.S.
Flyer. Winter. p. 3.
- Hoffmann, Michael P., Curtis H.
Petzoldt, and Anne C. Frodsham. 1996.
Integrated Pest Management
For Onions. Cornell University. Cornell, NY. 78 p.
- Hatfield, Brooke. 2003. Thrips
a growing disease problem for Vidalia onions farmers. The Vegetable
Growers News. March. p. 18-19.
- Anon. 1992. Thrips on onions and
cole crops. IPM Practitioner. May-June. p. 13.
- Quarles, William. 1990. Reflective
mulch and thrips-vectored virus. IPM Practitioner. November-December.
p. 7.
- Antignus, Y., N. Mor, R. Ben-Joseph,
M. Lapidot, and S. Cohen. 1996. Untraviolet-absorbing plastic
sheets protect crops from insect pests and from virus diseases
vectored by insects. Environmental Entomology. Vol. 25, No. 5.
p. 919-924.
- Rateaver, Bargyla, and Gylver
Rateaver. 1993. Organic Method Primer Update.
The Rateavers, San
Diego, CA. 596 p.
- Cantisano, Amigo. 1999. Onion
thrips cause trouble in many regions. Growing for Market. December.
p. 8-9.
- Williams, Greg, and Pat Williams.
1998. Resistance of cabbage cultivars to onion thrips damage.
HortIdeas. May. p. 53.
- Flint, Mary Louise. 1990. Pests
of the Garden and Small Farm. University of California. Oakland,
CA. 276 p.
- Metcalf, C.L., W.P. Flint, and
R.L. Metcalf. 1962. Destructive And Useful Insects: Their Habits
And Control, 4th ed. McGraw-Hill Book Co., New York, NY. 1087
p.
- Porter, Patrick. No date. New
Pesticide for Less Toxic Insect Control. West Virginia University,
Morgantown, WV.
- Yepson, Roger B. 1976. Organic
Plant Protection. Rodale Press, Emmaus, PA.
p. 588-589.
- Garrett, J. Howard. 1993. J.
Howard Garrett's Organic Manual. The Summit Group, Fort Worth,
TX. 203 p.
- Anon. 2003. Peaceful Valley
Farm Supply 2003 Main Catalog. Peaceful Valley Farm Supply, Grass
Valley, CA p. 106-108.
- Garrett, J. Howard. 1989. J.
Howard Garrett's Organic Manual.
Lantana Publishing Co., Dallas,
TX. p. 89.
- Anon. 1998. Peaceful Valley
Farm Supply 1998-1999 Main Catalog.
Peaceful Valley Farm Supply,
Grass Valley, CA. p. 88.
- Lazano, D., and G. Kilchher.
1998. How to choose oil to safely kill bugs. The Press Democrat.
June 21. p. 1.
- Grossman, Joel. 1998. Entomological
Society of America's 1997 Conference—Part 3. The IPM Practitioner.
April. p. 10.
Thrips Management Alternatives in the Field
By George Kuepper
NCAT Agriculture Specialist
Cole Loeffler, HTML Production
IP 132
Slot 118
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