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ity to produce food and interferes with trans- Monitoring Thripsportation of foliar nutrients to the bulb. Theresulting damage is usually measured as an Monitoring to identify when thrips arrive andoverall reduction in bulb size and weight of to determine population levels is helpful in de-onions and flower corms produced. There may signing an appropriate control strategy. In on-also be effects on the number, size, and appear- ions, thrips generally migrate into a field whenance of flowers. The injury caused by thrips’ plants in surrounding areas begin to dry, sorasping of the leaves enables various plant monitoring efforts should be timed accordingly.pathogens to gain entry, thus increasing diseaseproblems. In addition, thrips carry plant patho- Thrips numbers should also be determined. Thegens on their mouth parts from one plant to University of California recommends samplinganother. In onions, entire fields can be de- at least five (onion) plants from four separatestroyed, especially in dry seasons. In fruit crops, areas of each field. Leaves should be pulled apartthrips damage may also result in the scarring of and examined carefully with the assistance of afruit and significant loss to culling. hand lens, and all thrips counted.(1) The num- bers present can be used to determine actionThe stage of growth when an infestation occurs thresholds for applying pesticides. In corm-seems to determine the extent of yield loss. In propagated flowers, blue sticky cards are some-onions, it appears that early and late-season times used for trapping and monitoring thripsinfestations diminish yields less than those oc- populations.(3) However, research at the Uni-curring in mid-season during the bulbing stage. versity of California has indicated that hot-pink(2) sticky cards outperform blue-colored traps for this purpose.(4)For more basic information see the THRIPS com-puterized knowledge database at <http:// Coviello et al.(1) note that reliable treatmentwww.gladescropcare.com/tech-thrips.html>. threshold levels for onions are speculative. TheyTHRIPS covers nine species of thrips occurring are, in the least, variable, depending on whomin the U.S. and a major thrips predator, the you ask. In California, a threshold of 30 thripsminute pirate bug. The site provides detailed per plant at mid-season has been used success-information on identification, biology, scouting, fully for dry bulb, fresh market, and drying on-life cycles, and control using pirate bugs. ions, using conventional, synthetic pesticides. This number would be adjusted downward for very young plants and upward for larger, ma- ture 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 ento- mologist David Riley suggests an initial treat- ment 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 toler-Figure 1. Thrips life cycle. ated before yield losses result.(2) In onions,Used with permission of California Statewide IPM Project. thrips must be controlled before the crop reaches the early bulbing stage, so that populations do not exceed manageable levels.(1)PAGE 2 //THRIPS MANAGEMENT ALTERNATIVES IN THE FIELD
Plant architecture can also influence thrips duce thrips attack. High nitrate levels will in-population levels. In onions, cultivars with flat- vite thrips, and the effects of excessive nitratesided leaves and a compact growth point (where are compounded by shortages of potassium,the leaves are closely compressed) protect thrips sulfur, boron, and manganese. Foliar applica-from natural enemies, weather, and insecticides. tions of soluble calcium and kelp will balanceConversely, round, openly spaced leaves reduce the excess nitrogen. These nutrient levels canthrips’ hiding places.(2) be monitored on a weekly basis, using plant tis- sue analysis, to make accurate adjustments.(11)Cultural Control Methods In iris, gladiolus, daylilies, and other vegetatively propagated flowers, severely infested plant partsThe choice of cover crops can affect the num- should be cut away and destroyed. Dig cormsber of overwintering thrips. Thrips-susceptible early in the fall and cut off the tops before thripscrops should not be planted following wheat or move down into them. Destroy the remainingrye, which provide excellent over-wintering sites debris.(3)for thrips. Oats is a better choice, but requireslater fall planting than rye or wheat—a factorthat constrains its use as a cover crop in colder Genetic Resistanceregions.(7) Among onions, varietal resistance in some sweetWeed destruction in the field and surrounding Spanish types is noted in the literature. It is sug-margins can help to reduce thrips populations, gested that these may be older cultivars withsince these areas serve as overwintering and re- more open canopy growth.(5) There is alsoinfestation sites. Growers should take care, how- some genetic resistance among cabbage culti-ever, that vegetation management does not con- vars: Masada, Brutus, Galaxy, and Amtrackflict with strategies designed to reduce soil loss are reported to be highly resistant; Fresco,(through maintaining soil cover), to increase SuperElite, Satelite, Protector, Bartolo, andbiodiversity, and to make the farm system more Supergreen demonstrate moderate resistance.sustainable. (12)Drought stress increases the susceptibility of Biological Controlonions to thrips damage. Adequate irrigationthroughout the growing season is a critical fac-tor in minimizing damage.(2) Many beneficial organisms work to suppress thrips. These include lady beetles, minute pi-The fact that thrips are color-sensitive suggests rate bugs, ground beetles, big-eyed bugs, lace-that colored mulches may be effective in their wings, hover flies, predatory mites, and spiders.control. Louisiana researchers conducted a (5) Unfortunately, these predators and parasitesstudy to see whether aluminum-coated mulch may be hampered by the fact that thrips feedwould repel the pest.(8) Black plastic was spray- under close-fitting leaves and down in the leafpainted in the field with aluminum paint. The sheaths where they are difficult for predatorsreflective mulch repelled 33 to 68% of the thrips. to find. Insecticides, even those cleared for useUltaviolet-absorbing plastics—used to build in organic production, also tend to work againstwalk-in field tunnels—have proved effective in beneficial predators and parasites. They shouldprotecting crops from western flower thrips, as be used minimally and applied with caution.well as sweet potato whitefly, cotton aphid, andthe diseases they vector.(9) On occasion, thrips populations may explode, as large numbers migrate from nearby vegeta-Soil fertility management may also affect thrips tion that is mowed, harvested, or drying up. Ininfestation and damage. According to one such circumstances, in-field populations ofsource(10), a lack of adequate soil calcium may beneficial predators and parasites may not beinvite higher populations of thrips. Another adequate. The grower can increase the num-writer suggests that nutritional balance can re- bers of predatory and parasitic insects and //THRIPS MANAGEMENT ALTERNATIVES IN THE FIELD PAGE 3
arachnids on the farm by providing protective Noted organic proponent J. Howard Garrett rec-habitats for them. These habitats—often called ommends use of a homemade botanical sprayrefugia—can be integrated into crop rotation formulated from garlic and pepper.(17)planning, and can supplant “weedy” field bor- Garrett’s garlic/pepper tea is made by liquefy-ders and waste areas. For more details on refu- ing two bulbs of garlic and two cayenne orgia, ask for the ATTRA publication Farmscaping habañero peppers in a blender 1/3 full of wa-to Enhance Biological Control. ter. Solids are strained out, and enough water is added to make one gallon of concentrate.Naturally occurring fungal diseases can also Garrett mixes ¼ cup concentrate with two table-devastate thrips populations.(5) Limiting the spoons of vegetable oil and enough water touse of foliar fungicides by using forecasting sys- make 1 gallon of ready-to-use spray mix.(18) Atems and cultural controls helps to protect ben- commercial product called Hot Pepper Wax®,eficial fungi and maximize this natural control containing capsaicin (the active ingredient ofmechanism. cayenne pepper), is also recommended for thrips control.(19) A highly refined horticul-Alternative Pesticides tural oil can be used during the growing season for control of thrips and several other insects. (20)Several alternative pesticides are available forcontrolling thrips. Sulfur, insecticidal soap, and Spinosad (e.g., Conserve®)may also be useful indiatomaceous earth have all demonstrated effi- controlling thrips in the field. Spinosad is a re-cacy in suppressing thrips in several crops.(3, cently discovered insecticide, derived from the13, 14) Being contact pesticides, however, their fermentation of Actinomyces bacteria com-effectiveness in onions would probably be lim- monly found in the soil.(21) The National Or-ited, because the thrips can hide between the ganic Standards Board has recommended thatleaves. In contrast, three applications of super- spinosad be allowed in organic production.fine sulfur are recommended at monthly inter- Organic growers should consult their certifiervals in fruit crops for spring thrips control. Lime before using.sulfur has also been suggested as an alternative.(14) Surround™ Crop Protectant is labeled for sup- pression of thrips and several other insects forSpray formulations of the biological agent apples, stone fruits, citrus, small fruits, and on-Beauvaria bassiana (e.g., Naturalis-O ® , ions. Surround is a kaolin clay film sprayed ontoBotaniGard ®, Mycotrol ®) are also useful for the crop as a barrier to insect pests. Supple-thrips control. The agent is a fungus that grows mental control measures may be needed in ad-and reproduces in the host; therefore, effective dition to Surround. ATTRA has more informa-control may not be observed until 7 to 10 days tion on the use of Surround in fruit crops, avail-after application. B. bassiana is most effective able on request.when used early, before large thrips populationshave built up.(14) Always follow label instructions when using any pesticide, and keep in mind that the deci-The botanical pesticides garlic, rotenone, ryania, sion to use a pesticide should be made only whenpyrethrum, neem, and nicotine have been sug- other approaches to pest management fail togested for thrips control.(10, 16, 17, 18) An- provide adequate crop protection. Again, beother botanical presticide, sabadilla, also has a aware that some “natural” pesticides may berecord of controlling thrips and is suggested unacceptable or limited in certified organic pro-when other botanicals fail.(3) Organic growers duction. Check with a certifying agent beforeshould be aware that nicotine and other tobacco- purchasing or using any such products.based pesticides are prohibited in organic pro-duction. The formulations of many other bo-tanical products are also prohibited, and or-ganic producers are encouraged to consult theircertifier before purchasing them.PAGE 4 //THRIPS MANAGEMENT ALTERNATIVES IN THE FIELD
References 11) Cantisano, Amigo. 1999. Onion thrips cause trouble in many regions. Growing for Market. December. p. 8–9.1) Coviello, R., W.E. Chaney, and S. Orloff. 1993. Onion and Garlic Pest Management 12) Williams, Greg, and Pat Williams. 1998. Re- Guidelines. University of California State- sistance of cabbage cultivars to onion thrips wide IPM Program. Davis, CA. UC ANR damage. HortIdeas. May. p. 53. Publication 3453. 13) Flint, Mary Louise. 1990. Pests of the Gar-2) Fournier, Francois, Guy Boivin, and Robin den and Small Farm. University of Califor- Stewart. 1995. Effect of Thrips tabaci nia. Oakland, CA. 276 p. (Thysanopters: Thripidae) on yellow onion yields and economic thresholds for its man- 14) Metcalf, C.L., W.P. Flint, and R.L. Metcalf. agement. Entomological Society of America. 1962. Destructive And Useful Insects: Their Vol. 88, No. 5. p. 1401-1407. Habits And Control, 4th ed. McGraw-Hill Book Co., New York, NY. 1087 p.3) Ellis, Barbara W., and Fern Marshall Brad- ley. 1992. The Organic Gardener’s Hand- 15) Porter, Patrick. No date. New Pesticide for book of Natural Insect and Disease Control. Less Toxic Insect Control. West Virginia Rodale Press, Emmaus, PA. 534 p. University, Morgantown, WV.4) Anon. 1998. Pink thrips. B.U.G.S. Flyer. 16) Yepson, Roger B. 1976. Organic Plant Pro- Winter. p. 3. tection. Rodale Press, Emmaus, PA. p. 588- 589.5) Hoffmann, Michael P., Curtis H. Petzoldt, and Anne C. Frodsham. 1996. Integrated 17) Garrett, J. Howard. 1993. J. Howard Pest Management For Onions. Cornell Uni- Garrett’s Organic Manual. The Summit versity. Cornell, NY. 78 p. Group, Fort Worth, TX. 203 p.6) Hatfield, Brooke. 2003. Thrips a growing 18) Anon. 2003. Peaceful Valley Farm Supply disease problem for Vidalia onions farmers. 2003 Main Catalog. Peaceful Valley Farm The Vegetable Growers News. March. Supply, Grass Valley, CA p. 106-108. p. 18-19. 19) Garrett, J. Howard. 1989. J. Howard7) Anon. 1992. Thrips on onions and cole Garrett’s Organic Manual. Lantana Pub- crops. IPM Practitioner. May-June. p. 13. lishing Co., Dallas, TX. p. 89.8) Quarles, William. 1990. Reflective mulch 20) Anon. 1998. Peaceful Valley Farm Supply and thrips-vectored virus. IPM Practitioner. 1998-1999 Main Catalog. Peaceful Valley November-December. p. 7. Farm Supply, Grass Valley, CA. p. 88.9) Antignus, Y., N. Mor, R. Ben-Joseph, M. 21) Lazano, D., and G. Kilchher. 1998. How to Lapidot, and S. Cohen. 1996. choose oil to safely kill bugs. The Press Untraviolet-absorbing plastic sheets protect Democrat. June 21. p. 1. crops from insect pests and from virus dis- eases vectored by insects. Environmental 22) Grossman, Joel. 1998. Entomological Soci- Entomology. Vol. 25, No. 5. p. 919-924. ety of America’s 1997 Conference–Part 3. The IPM Practitioner. April. p. 10.10) Rateaver, Bargyla, and Gylver Rateaver. 1993. Organic Method Primer Update. The Rateavers, San Diego, CA. 596 p. //THRIPS MANAGEMENT ALTERNATIVES IN THE FIELD PAGE 5