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Least-Toxic Control of Thrips Choose a different pests

Identification

Pest type: Insects

In-depth information:

According to UC IPM:
Most adult thrips, order Thysanoptera, are elongate, 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 oblong or slender and elongate and lack wings. Most thrips range in color from translucent white or yellowish to dark brown or black. A few species are brightly colored, such as the distinctive reddish-orange larvae of the predatory thrips, Franklinothrips orizabensis and F. vespiformis.

Thrips feed by puncturing the epidermal (outer) layer of host tissue and sucking out the cell contents, which results in stippling, discolored flecking, or silvering of the leaf surface. Thrips feeding is usually accompanied by black varnishlike flecks of frass (excrement). Pest species are plant feeders that discolor and scar leaf, flower, and fruit surfaces, and distort plant parts or vector plant pathogens. Many species of thrips feed on fungal spores and pollen and are often innocuous. However, pollen feeding on plants such as orchids and African violets can leave unsightly pollen deposits and may reduce flower longevity. Certain thrips are beneficial predators that feed on other insects and mites.

It is more important to distinguish among thrips species in situations where integrated pest management methods are used. For example, each species of natural enemy preys on and helps to control only certain species of thrips or other pests. Certain thrips occur on many different plants but damage only a few of the plant species on which they are found. Identifying the species of thrips may reveal that it is harmless in certain situations and no control action is needed.

Is it a problem?

According to UC IPM:
Thrips feeding on plants can damage fruit, leaves, and shoots and very noticeably affect plants’ cosmetic appearance. However, thrips rarely kill or threaten the survival of trees and shrubs. Herbaceous ornamentals, and certain vegetable crops, are more susceptible to serious injury from thrips feeding and thrips-vectored viruses, especially when plants are young.

Thrips feeding can stunt plant growth and cause damaged leaves to become papery and distorted, develop tiny pale spots (stippling), and drop prematurely. Infested terminals may discolor and become rolled. Petals may exhibit “color break,” which is pale or dark discoloring of petal tissue that was killed by thrips feeding before buds opened. On some plants, thrips can cause severe stunting to the early season flush of terminal growth. Cuban laurel thrips create tightly rolled, podlike leaf terminals on Ficus and form galled foliage from midsummer through fall. Myoporum thrips can create tightly bunched and twisted terminal growth that resembles a gall, within which large numbers of thrips can survive and reproduce.

Western flower thrips is primarily a pest of herbaceous plants; but high populations can damage flowers on woody plants, such as roses. Rose petals may develop dark streaks and spots from feeding injury that occurred before the buds opened, or the flower buds may deform and fail to open. Western flower thrips are also vectors for Impatiens necrotic spot virus and Tomato spotted wilt virus, which can severely damage or kill certain vegetable crops and herbaceous ornamentals.

Thrips cause brown to silvery, scabby scarring on the avocado and citrus fruit surface but do not harm the internal quality or flavor of the fruit. Where thrips lay eggs on grapes, fruit may develop dark scars surrounded by lighter “halos.” Thrips feeding on apples, nectarines, and raspberries can deform or scar developing fruit. Citrus thrips feeding severely distorts blueberry shoot tips and foliage, reducing fruit yield.

In many thrips species, by the time their damage is observed, such as after buds open, the thrips may no longer be present. Some abiotic disorders, pathogens, and certain other invertebrates can cause damage resembling that of thrips. For example, various true bugs and mites also stipple foliage; and some true bugs produce dark fecal specks. Before taking control action, look carefully for the insects themselves to be certain that pest thrips are present and the cause of the damage.

Pest prevention practices

Remove potential habitat
Foster natural resilience

In-depth information:

According to UC IPM:
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. Avoid planting susceptible plants next to these areas, and control nearby weeds that are alternate hosts of pest thrips. Grow plants that are well-adapted to conditions at that site. For example, plants adapted to grow in full sun can be stressed when planted in shady conditions and may be more susceptible to thrips damage. Provide appropriate cultural care to keep plants vigorous and increase their tolerance to thrips damage. Keep plants well irrigated, and avoid excessive applications of nitrogen fertilizer, which may promote higher populations of thrips. Old, spent flowers can harbor thrips, so their removal and disposal is sometimes recommended. However, the general benefit of this practice in landscapes is unknown; and old blossoms also commonly shelter beneficial predators of thrips.

Investigate the availability of resistant cultivars. For example, western flower thrips more often damages fragrant, light-colored, or white roses. Rose cultivars, with sepals that remain tightly wrapped around the bud until just before blooms open, have fewer thrips problems. Where Cuban laurel thrips is a problem on Indian laurel fig, you can plant Ficus microcarpa, “Green Gem,” which is mostly resistant to this pest.

Monitoring and record-keeping

According to UC IPM:
If thrips are a suspected cause of plant damage, thrips adults and larvae can be monitored by branch beating or gently shaking foliage or flowers onto a light-colored sheet of paper, beating tray, or small cloth. For thrips that feed in buds or unexpanded shoot tips, clip off several plant parts suspected of harboring thrips, place them in a jar with 70% alcohol (ethanol), and shake vigorously to dislodge the thrips. Strain the solution through filter paper so thrips can more readily be seen. Adult thrips can also be monitored by hanging bright yellow sticky traps in or near host plants.

Be aware that the presence of thrips does not mean that damage will result from their feeding. Large numbers of thrips in traps, or adults in flowers feeding on pollen, do not necessarily indicate that control action is needed. Plants suspected of being infected by thrips-vectored viruses can be reliably diagnosed only by sending properly collected samples from symptomatic plants to a laboratory that tests for plant pathogens.

Non-chemical and mechanical controls

In-depth information:

According to UC IPM:

Pruning

Prune and destroy injured and infested terminals when managing a few small specimen plants in the landscape. Avoid shearing plants, which is the clipping of dense foliage to maintain an even surface on formal hedges or creating specific shapes (topiary). Shearing stimulates thrips-susceptible new growth. Prune by cutting plants just above branch crotches and nodes instead of shearing off terminals.

Prune during specific times of the year to help control certain thrips. Prune off galled, rolled terminals of Indian laurel fig during winter to greatly reduce thrips damage the next summer. Relatively few Cuban laurel thrips can survive the winter outside of the protection provided by the leaves they gall. Instead of pruning avocado during February through April, January pruning may reduce thrips scarring of fruit. January pruning can induce additional avocado growth flush during May fruit set and reduce thrips scarring of fruit. When succulent foliage is abundant in spring thrips tend to remain and feed on leaves and not move to fruit. Pruning the interior of citrus trees can increase predaceous mite populations in the exterior canopy, thereby reducing fruit scarring by citrus thrips.

Row Covers

Row covers, hot caps, and other types of cages with a fine mesh can exclude thrips and other insects from vegetables and other young herbaceous plants. Apply row covers before crops emerge or to pest-free plants during planting. Plants are normally covered or caged only while they are young and most susceptible to damage. Once plants become larger or temperatures get warmer, remove covers to provide enough growing space and to prevent overheating. Drip or furrow irrigation is generally necessary when using row covers.

Any type of covering that excludes insects but allows light and air penetration can be used. With sturdy crops that do not grow too tall, floating row covers (vented polyethylene, spunbonded polyester, point-bonded polypropylene) can be placed on top of beds with no frames or hoops. The crop itself lifts the fabric as it grows. For plants that grow upright or have sensitive tips that might be damaged when pushing against covers, use hoops, plastic tunnels, or wire strung between posts to hold up covers. Wood, wire, or plastic frames covered with muslin, nylon, or other fine mesh can be used for several years.

Reflective Mulch

Mulch or mesh that reflects light interferes with certain flying insects’ ability to locate plants. If the plants are initially pest-free, and relatively small in comparison with the surface area that is covered with reflective material, reflective mulch can delay or reduce the extent to which young plants become infested by winged aphids and adult leafhoppers, thrips, and whiteflies. In flower and vegetable crops that are especially sensitive to insect-vectored viruses, the cost and effort of using reflective mulch may be justified; because the mulch can be significantly more effective than insecticides in preventing or delaying infection of small plants. As plants grow larger, reflective mulch becomes increasingly less effective and other management methods may be needed. Reflective mulch ceases to repel insects by the time the plant canopy covers more than about half of the soil surface.

Silver or gray is the most effective color for synthetic reflective mulch or mesh, but white also works. Some organic mulches (e.g., straw) and living mulches (e.g., interplanting the crop with buckwheat, Fagopyrum esculentum) may also repel certain pests, but this is not as well documented.

Synthetic mulch application methods include:

Transplant seedlings through holes in the mulch.

Apply the mulch before plants emerge from the soil and leave a narrow, mulch-free strip along the planting row.

Lay lightweight material that allows light and air penetration over the top of a crop that is sturdy enough to lift the material as it grows.

In addition to temporarily repelling certain flying insects, mulch may improve growth of certain crops by increasing light levels, keeping soil warmer overnight, reducing weed growth, and conserving soil moisture. Mulch can also have negative effects. It can increase crop susceptibility to root diseases, prevent the use of overhead watering, and make it more difficult to know whether you are maintaining proper soil moisture in the rooting zone of a plant. Most recyclers will not accept plastics with soil on them; therefore, plastic mulches typically are disposed of in landfills. Investigate which material and methods are likely to work best in your situation.

Biological controls

According to UC IPM:

Predatory thrips, green lacewings, minute pirate bugs, mites, and certain parasitic wasps help to control plant-feeding thrips. To conserve and encourage naturally occurring populations of these beneficials, avoid creating dust and consider periodically rinsing dust off of small plants, avoid persistent pesticides, and grow a diversity of plant species.

Where thrips are a problem, learn whether that pest has specific natural enemies important in its control. For example, a minute pirate bug, Macrotracheliella nigra, and green lacewing larvae are important predators of Cuban laurel thrips. Euseius species mites are important predators of citrus thrips. With greenhouse thrips in Southern California, up to 50% of its eggs are killed by a tiny wasp, Megaphragma mymaripenne. After feeding inside during its larval stage then pupating, the emerging adult parasite leaves a relatively large round hole in the tiny thrips egg. Conversely, when a greenhouse thrips emerges from an unparasitized egg, part of the egg shell is often visible at the side of the egg blister. Thripobius semiluteus parasitizes greenhouse thrips larvae. Thrips parasitized by this wasp’s larvae become swollen around the head and 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.

There is little research-based information on the effectiveness of releasing thrips natural enemies in gardens and landscapes. Releasing purchased natural enemies, in most situations, is unlikely to provide satisfactory thrips control.

Least-toxic chemical options as a last resort

According to UC IPM:

Although thrips damage is unsightly, it does not usually warrant the use of insecticides in gardens and landscapes. Feeding injury typically does not become apparent until after tissue grows and expands. Thus, by the time damage is noticed on ripening fruit or distorted terminals, the thrips that caused the damage are often gone. No pesticide application will restore the appearance of injured tissue; plants will remain damaged until leaves drop, injury is pruned off, or new unblemished fruit is produced. Where plant viruses are a problem, insecticides typically do not kill thrips fast enough to prevent the transfer of virus from thrips to plants. Using row covers or other methods to prevent thrips infestation is the most effective way to prevent infection by thrips-vectored viruses.

Thrips can be difficult to control effectively with insecticides, partly because of their mobility, feeding behavior, and protected egg and pupal stages. Improper timing of application, failure to treat the proper plant parts, and inadequate spray coverage when using contact materials are common mistakes that can prevent potentially effective insecticides from actually providing control. Before using a pesticide, learn more about the biology of your pest species and the characteristics of available products. If least-toxic pesticides are used, combining their use with appropriate cultural practices and other methods usually improves the pest control.

Contact insecticides that do not leave persistent residues can be effective for greenhouse thrips and other species that feed openly on plants. These products have low toxicity to people, pets, and pollinators and relatively little adverse impact on biological pest control; because they do not leave toxic residues that would kill natural enemies migrating in after their application. Contact insecticides include azadirachtin (TreeAzin, AzaMax, Safer Brand BioNeem), insecticidal soaps (Safer), narrow-range oil (Bonide Horticultural Oil, Monterey Horticultural Oil), and neem oil (Green Light Neem, Schultz Garden Safe Brand Neem Oil). To be effective, contact sprays must be applied to thoroughly cover buds, shoot tips, and other susceptible plant parts where thrips are present. On plants with a history of unacceptable damage, begin treatment early when thrips or their damage is first observed. Unless directed otherwise by the product label, periodically repeat the application as long as pest thrips and susceptible plant parts are both present.

Chemicals to Avoid

Look at your product labels and try to avoid products containing those chemicals listed below:

(A = acute health effects, C = chronic health effects, SW = surface water contaminant, GW = ground water contaminant, W = wildlife poison, B = bee poison, LT = long-range transport)

Acephate (C, SW, W, B)

Bifenthrin (A, C, SW, W, B)

Carbaryl (A, C, SW, GW, W, B)

Cyfluthrin (A, C, W, B)

Dinotefuran (C, B)

Etofenprox (C, W, B)

Fluvalinate (W, B)

Imidacloprid (A, C, SW, W, B)

Malathion (A, C, SW-URBAN, GW, W, B)

Permethrin (A, C, GW, W, B)