How to Manage Pests
Pests in Gardens and Landscapes
Thrips, order Thysanoptera, are tiny, slender insects with fringed wings. They feed by puncturing their host and sucking out the cell contents. Certain thrips species are beneficial predators that feed only on mites and other insects. Beneficial species include black hunter thrips and the sixspotted thrips. Pest species (often in the family Thripidae) are plant feeders that scar leaf, flower, or fruit surfaces or distort plant parts. Other species of thrips feed on fungal spores and pollen and are innocuous.
Most adult thrips are slender, minute (less than 1/20 inch long), and have long fringes on the margins of both pairs of their long, narrow wings. Immatures (called larvae or nymphs) are similarly shaped with a long, narrow abdomen but lack wings. Most thrips range in color from translucent white or yellowish to dark brown or blackish, depending on the species and life stage. A few species are more brightly colored, such as the distinctive reddish orange abdomen of larvae of the predatory thrips, Franklinothrips orizabensis and Franklinothrips vespiformis.
In many species, thrips feed within buds and furled leaves or in other enclosed parts of the plant. Their damage is often observed before the thrips are seen. Discolored or distorted plant tissue or black specks of feces around stippled leaf surfaces are clues that thrips are or were present. However, some abiotic disorders, pathogens, and certain other invertebrates can cause damage resembling that of thrips. For example, lace bugs, plant bugs, and mites also stipple foliage, and lace bugs and certain plant bugs produce dark, watery fecal specks. Look carefully for the insects themselves to be certain that pest thrips are present and the cause of damage before taking control action.
Behavior, body appearance, and host plants help to distinguish among thrips species (Tables 1, 2, and 3). For example, three dark spots on each forewing distinguish the adult predaceous sixspotted thrips from pest thrips. Adults of western flower thrips and onion thrips, are noticeably larger than avocado and citrus thrips adults, so mature body size helps to distinguish them when they occur together on the same host plant. Nonprofessionals may be able to identify thrips using resources such as online keys listed in Suggested Reading. However, thrips can be positively identified to species only by an expert. Fortunately, most thrips are susceptible to some of the same controls, such as exclusion and pesticides.
It is more important to distinguish among thrips species in situations where integrated pest management methods are used. For example, predatory thrips or other natural enemies are highly specific to certain pests (Tables 3 and 4) and are likely to help control only certain species of plant-feeding thrips. Certain thrips occur on many different plants but damage only a few of the plant species on which they are found, so identifying the thrips species may reveal that it is harmless in that situation and no control action is needed. For example, avocado fruit skin is scarred by avocado thrips and greenhouse thrips, but citrus thrips and western flower thrips are harmless in avocado. Citrus thrips occurs on many species of plants but damages only blueberries and citrus.
Thrips are poor fliers but can readily spread long distances by floating with the wind or being transported on infested plants. New thrips introductions can pose serious threats and complicate identification. A recent introduction of Klambothrips myopori has caused serious leaf and shoot galling damage to Myoporum laetum (ngaio tree) and Myoporum ‘Pacificum’ (a groundcover) along the coast of California. This thrips was both a new introduction and an undescribed species, so that initially not even the experts knew what to call it or how it might be managed. This species is now well established and from its original detection site in San Diego has spread north along the coast to at least as far as Santa Barbara. It is expected to continue to spread to wherever Myoporum species have been planted.
The thrips life cycle includes the egg, two actively feeding larval (nymphal) stages, nonfeeding prepupal (propupal) and pupal stages, and the adult. Thrips have a metamorphosis that is intermediate between complete and gradual. Last-instar larvae change greatly in appearance, and they are often called pupae even though thrips do not have a true pupal stage.
Thrips eggs are elongate, cylindrical to kidney-shaped, and relatively large in relation to the female. Females of most plant-feeding species insert their tiny eggs into plants, commonly into leaves or buds where larvae feed. The pale prepupae and pupae of most species drop to the soil or leaf litter or lodge within plant crevices. Greenhouse thrips pupate openly on lower leaf surfaces while pupae (and eggs) of some gall-making species, such as Cuban laurel thrips, occur on leaf surfaces but are enclosed within distorted plant tissue. Thrips have several generations (up to eight or more) a year. The life cycle from egg to adult may be completed in as short a time as 2 weeks when the weather is warm.
Thrips prefer to feed in rapidly growing tissue. Feeding by thrips typically causes tiny scars on leaves and fruit, called stippling, and can stunt growth. Damaged leaves may become papery and distorted. Infested terminals may discolor, become rolled, and drop leaves prematurely. Petals may exhibit “color break,” which is pale or dark discoloring of petal tissue that was killed by thrips feeding before buds opened. Thrips cause silvery to brownish, scabby scarring on the avocado and citrus fruit surface, but this cosmetic damage does not harm the internal fruit quality. Feces may remain on leaves or fruit long after thrips have left. Where thrips lay eggs on grapes, dark scars surrounded by lighter “halos” may be found on the fruit. Thrips feeding on raspberries, apples, and nectarines can deform or scar developing fruit; sugar pea pods may be scarred or deformed. Citrus thrips feeding severely distorts blueberry shoot tips and foliage, reducing fruit yield.
Western flower thrips are primarily pests of herbaceous plants, but high populations occasionally damage continuously- or late-blossoming flowers on woody plants such as roses. When thrips populations are high on roses, flower buds may become deformed and fail to open. Petals may be covered with brown streaks and spots. Western flower thrips can also vector certain tospoviruses including impatiens necrotic spot virus and several strains of tomato spotted wilt virus. Some plant-feeding thrips are also predaceous on other pests, such as spider mites. In young cotton seedlings in California, western flower thrips is considered beneficial because it feeds on spider mites.
In comparison with woody shrubs and trees in landscapes, herbaceous ornamentals and certain fruit and vegetable crops are generally more susceptible to serious injury from thrips’ feeding and thrips-vectored viruses, especially when plants are young. Thrips feeding on woody plants can damage fruit and very noticeably affect plants’ cosmetic appearance. But thrips rarely kill or threaten the survival of woody plants unless the thrips populations are very high and cause serious feeding damage resulting in premature leaf drop or stem dieback.
Healthy woody plants usually tolerate thrips damage; however, high infestations on certain herbaceous ornamentals and developing fruits or vegetables may justify control. If control is necessary, use an integrated program of control strategies that combines the use of good cultural practices and conservation of natural enemies with the use of least-toxic insecticides, such as narrow-range oils. Greenhouse thrips biology and management differs some from that of most other pest thrips, as discussed in a separate section.
Monitor thrips adults and larvae by branch beating or shaking foliage or flowers onto a sheet of paper or a beating tray or sheet. Adult thrips can also be monitored using bright yellow sticky traps. Blue sticky traps are most effective for capturing western flower thrips, but thrips are harder to discern on this darker background. Remember that the presence of thrips does not mean that damage will result from their feeding. Even large numbers of thrips in traps or adults at flowers feeding on pollen do not necessarily indicate that control action is needed. Plants suspected of being infected by thrips-vectored viruses such as impatiens necrotic spot virus or tomato spotted wilt virus can be reliably diagnosed only by a laboratory test of plants with symptoms or, in certain instances, by using specialized test kits discussed in the publication, Integrated Pest Management for Floriculture and Nurseries.
Predatory thrips (Table 3) and other beneficial insects and mites, including minute pirate bugs and predaceous mites (Table 4), help to control certain plant-feeding thrips species. Although certain predators and parasites of thrips are produced commercially and can be purchased through the mail, little or no research has been conducted on the effectiveness of releasing thrips predators or parasites in landscapes and gardens. Releasing purchased natural enemies in most situations is unlikely to provide satisfactory pest control. Conserving naturally occurring populations of beneficials by controlling dust and avoiding persistent pesticides is the most important way to encourage biological control of pest thrips.
Thrips species that feed on many different plant species often move into gardens and landscapes when plants in weedy areas or grasslands begin to dry in spring or summer, so it is wise to avoid planting susceptible plants next to these areas or to control nearby weeds that are alternate hosts of certain thrips. In small gardens, thrips can be knocked off plants with a spray of water. Vigorous plants normally outgrow thrips damage; keep plants well irrigated, but avoid excessive applications of nitrogen fertilizer, which may promote higher populations of thrips. Remove and dispose of old, spent flowers. Investigate the availability of resistant cultivars. For example, western flower thrips damage to roses is less of a problem in cultivars with sepals that remain tightly wrapped around the bud until just before blooms open.
Prune and destroy injured and infested terminals when managing a few small specimen plants in the landscape. Regular pruning of infested parts can be especially effective with the gall-making Cuban laurel thrips. Avoid shearing plants. Shearing, which is clipping dense foliage to maintain an even surface on formal hedges, stimulates susceptible new growth. Prune by cutting plants just above branch crotches and nodes instead of shearing off terminals. Pruning the interior of citrus trees can increase predaceous mite populations in the exterior canopy, thereby reducing fruit-scarring by citrus thrips.
Avocado thrips damage may be reduced by modifying fertilization (amount, application method, formulation, and timing) and pruning (the extent and time of branch removal) to induce plants to continue to produce tender foliage during about May and June, when avocado produce most young fruit. For example, instead of pruning avocado during February through April, January pruning may induce additional growth flush during May fruit set, which may reduce thrips tendency to move from leaves (where thrips are usually harmless) to young fruit (which are scarred by thrips feeding).
Row covers, hot caps, and other types of cages can exclude thrips and other pests from vegetables and other young herbaceous plants. Any type of covering that excludes insects but allows light and air penetration can be used. Wood, wire, or plastic frames covered with muslin, nylon, or other mesh can be used for several years. Floating row covers can be placed on top of beds with no frames or hoops. The crop itself lifts the fabric as it grows. Vented polyethylene, spunbonded polyester, point-bonded polypropylene, and woven plastics are available for this use. Floating row covers are useful on sturdy crops that do not grow too tall. Use hoops, plastic tunnels, or wire strung between posts to hold up covers on plants that grow upright or have sensitive tips that might be damaged when pushing against covers.
Apply row covers during planting or before crops emerge. Plants are normally covered or caged only while they are young and most susceptible to damage. Once plants get larger or temperatures get warmer, remove covers to provide enough growing space and to prevent overheating. A drip or furrow irrigation system is necessary when using row covers.
Reflective mulch or mesh confuses and repels certain flying insects searching for plants, apparently because reflected ultraviolet light interferes with the insects’ ability to locate plants. Most uses of reflective mulch have been against winged aphids, but infestation of young plants by other pests including leafhoppers, thrips, and whiteflies has also been prevented or delayed. In flower and vegetable crops that are especially sensitive to viruses, the added cost of reflective mulch may be justified because the mulch can be significantly more effective than insecticides in preventing the spread of viruses and other diseases vectored by insects. It is most effective during early growth when plants are small; as plants grow larger, it is less effective, and other methods may be needed. Reflective mulches cease to repel insects when the plant canopy covers more than about 60% of the soil surface.
Transplant through holes in the mulch or apply the mulch before plants emerge from the soil by leaving a thin mulch-free strip of soil along the planting row. Liquid reflective mulches are also available that can be sprayed on the soil and plants emerge through them. Reflective mesh is also available for application over the top of a crop that can lift this lightweight material as it grows. Various materials, such as polyethylene plastic film, can be used. Silver or gray is the most effective color for reflective mulch or mesh, but white also works. Commercially available products include aluminum-metalized polyethylene and silver-embossed polyethylene from suppliers listed at the end of this publication. Aluminum foil is also effective but is expensive, delicate to handle, and probably not feasible on a large scale but may be fine for a home garden.
Reflective mulch may improve crop growth beyond that provided by pest control, possibly due to warmer night soil temperatures, more even soil moisture, and increased light levels. Certain mulches have other beneficial or negative effects, such as weed control, water conservation, or increasing crop susceptibility to root diseases, so investigate which material is likely to work best in your situation.
Disposing of plastics can often be a problem because most recyclers will not accept plastics with soil on them; consequently, most plastic mulches in California are disposed of in landfills. If they are handled carefully, however, plastic mulches may be usable for more than one season.
Although thrips damage to leaves is unsightly, thrips activity does not usually warrant the use of insecticide sprays. For instance, while thrips damage on citrus or avocado fruit may look unpleasant, it does not harm trees or affect the internal fruit quality. When damage is noticed on ripening fruit or distorted terminals, the thrips that caused the injury are often gone. It’s not until later when tissue grows and expands that injury caused earlier becomes apparent. While viruses vectored by thrips may cause plant loss, insecticide sprays are not recommended to prevent viruses because thrips are not killed fast enough to prevent the transfer of the virus to new plants. Prevention of thrips infestations is the only way to prevent infection by thrips-vectored viruses.
No pesticide provides complete control of thrips. In comparison with other insects, most thrips are difficult to control effectively with insecticides. Reasons include thrips’ tiny size, great mobility, hidden feeding behavior, and protected egg and pupal stages. Improper timing of application, failure to treat the proper plant parts, and inadequate spray coverage are also common mistakes and can be more important in influencing the effectiveness of treatment than choosing which pesticide to apply. Before using a pesticide, learn more about your specific plant situation and the biology of your pest species. Often you will learn chemical control cannot be effective until the next season when new plant growth develops. If insecticides are used, they will only be partially effective and must be combined with appropriate cultural practices and conservation of natural enemies. Greenhouse thrips is an exception; because it is sluggish and feeds in groups on exposed plant parts, thoroughly applying most any insecticide will kill this species.
Narrow-range oil (Sunspray, Volck), azadirachtin (Safer BioNeem), neem oil (Green Light Garden Safe), pyrethrins combined with piperonyl butoxide (Garden Safe Brand Multi-purpose Garden Insect Killer, Spectracide Garden Insect Killer), and (at least for greenhouse thrips) insecticidal soaps (Safer), can be somewhat effective for temporary reduction of thrips populations if applied when thrips are present and damage first appears. These materials have the benefit of allowing at least a portion of the natural enemy populations to survive because they do not leave toxic residues. Sprays must be applied to thoroughly cover susceptible plant tissue, such as new leaf growth and buds. On plants with a history of severe, unacceptable damage, begin treatment early when thrips or their damage is first observed. Repeat applications (usually 5 to 10 days apart, depending on temperature) are usually required because these insecticides only kill newly hatched thrips and recently emerged adults.
Other insecticides for thrips include spinosad (Conserve, Green Light Lawn & Garden Spray Spinosad, Monterey Garden Insect Spray) and (available only to licensed pesticide applicators) abamectin (Avid). These materials are derived from beneficial microbes and have low to moderate adverse impact on natural enemies. Abamectin and spinosad should be applied no more than once or twice a year, and can be more effective against thrips than the previously listed insecticides. The beneficial fungus Beauveria bassiana (BotaniGard) can be applied to commercial landscapes but is not available for use in home gardens or residential landscapes.
With most thrips species, eggs are protected within plant tissue and prepupae and pupae are in the soil and will not be killed. No pesticide treatment will restore the appearance of injured tissue; plants will remain damaged until leaves drop, injury is pruned off, or new unblemished fruit is produced.
For ornamental nonfood plants, a licensed pesticide applicator can use the systemic organophosphate insecticide acephate (Orthene), but acephate can be highly toxic to natural enemies and it commonly causes spider mites to become abundant and damage plants within a few weeks after its application. Another systemic insecticide, imidacloprid (Bayer Advanced Garden Tree & Shrub Insect Control, Merit) provides some suppression of foliage-feeding thrips only, but it is also toxic to some natural enemies of thrips. Avoid the use of organophosphate insecticides (e.g., malathion), carbamates (carbaryl), or pyrethroids (e.g., cyfluthrin, fluvalinate, and permethrin) because all these materials are highly toxic to natural enemies, can cause dramatic increases in spider mite populations, and are not particularly effective against most thrips.
Greenhouse thrips infests many perennial plants, usually those with harder leaves. It occurs primarily on the underside of leaves, on touching fruit clusters, and on plant parts that touch each other. Greenhouse thrips is a sluggish species with adults that tend not to fly. Individuals feed in groups and populations usually begin in a limited part of the plant and spread slowly, so pruning off colonies can be effective if the undersides of leaves on susceptible plants are regularly inspected to allow early detection and removal of new infestations. In addition to the materials listed above for the control of thrips on ornamental nonfood plants, greenhouse thrips is readily controlled with thorough application of contact sprays such as oil or pyrethrins (plus piperonyl butoxide) to the underside of infested leaves where it feeds. However, because populations can rapidly resurge, repeat applications may be necessary. Even though it is easier to control temporarily with pesticides than other thrips, greenhouse thrips also often has effective natural enemies, so it is important to assess whether spraying is warranted and to select materials that are least toxic to natural enemies.
Megaphragma mymaripenne is an important parasite in coastal avocado, often killing about 25 to 50 percent of greenhouse thrips eggs. Parasitized eggs develop a relatively large round hole, usually in the middle of the egg blister, where the Megaphragma mymaripenne adult emerged. When a greenhouse thrips emerges, part of the egg shell is often visible at the side of the egg blister.
Another parasitic wasp, Thripobius semiluteus, that attacks only greenhouse thrips has been effective in controlling this pest in greenhouses and southern California avocado orchards. There is no information on the effectiveness of Thripobius in landscapes. Observe whether any greenhouse thrips larvae are parasitized and, if Thripobius is present, conserve parasites whenever possible. The tiny, black and yellowish female Thripobius lays its eggs in young thrips nymphs. Parasitized thrips become swollen around the head, and about 2 weeks before the wasp’s emergence, the parasitized larvae turn black, in contrast to the pale color of unparasitized greenhouse thrips larvae. Unlike healthy black mature thrips, the black parasitized larvae are smaller and do not move. Thripobius develops from egg to adult in about 3 weeks when temperatures average 70°F.
Suppliers of Reflective Mulch
Blake Enterprises 844 E. Dinuba Ave Reedley, CA 93654 (559) 638-3631
Sonoco 1 North Second Street Hartsville, SC 29550-0160 (843) 383-7000 www.sonoco.com
Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2001. Integrated Pest Management for Floriculture and Nurseries. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3402.
Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359.
Flint, M. L. 1998. Pests of the Garden and Small Farm: A Grower’s Guide to Using Less Pesticide. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3332.
O´Donnell, C. A., L. A. Mound, and M. P. Parrella. Multilevel Identification System for Thrips Associated with Flower Crops in North America. Available online.
O´Donnell, C., G. Moritz, L. Mound, S. Nakahara, and M. Parrella. Pest Thrips of North America. Available online.
Authors: : S. H. Dreistadt, UC Statewide IPM Program; P. A. Phillips, UC Statewide IPM Program, Ventura Co.; and
C. A. O’Donnell, Entomology, UC Davis
PDF: To display a PDF document, you may need to use a PDF reader.