Thrips, order Thysanoptera, are tiny, slender insects with fringed wings. They feed by puncturing the epidermal (outer) layer of host tissue and sucking out the cell contents. This results in stippling, discoloured 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 discolour and scar leaf, flower, and fruit surfaces. 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. Also, it may reduce flower longevity. Certain thrips are beneficial predators that feed on other insects and mites.
Thrips can readily move long distances floating with the wind or transported on infested plants. For example, myoporum thrips causes severe galling of Myoporum laetumand M. pacificum. Originally from New Zealand, this thrips was introduced into Southern California. Spreading to most areas of the state where its hosts are grown.
Thrips Life Cycle
Thrips hatch from an egg. Develop through two actively feeding larval stages and two nonfeeding stages, the prepupa and pupa, before becoming an adult. Late-instar larvae change greatly in appearance and behaviour. They are called prepupae and pupae, even though they do not have a true pupal stage.
Females of most plant-feeding species lay their elongate, cylindrical to kidney-shaped eggs. On or into leaves, buds, or other locations where larvae feed. The pale prepupae and pupae of most species drop to the soil or leaf litter or lodge within plant crevices or galls.
Greenhouse thrips pupate openly on the lower leaf surface. While pupae (and eggs) of some gall-making species. Such as Cuban laurel thrips and myoporum thrips, occur on leaf surfaces but are enclosed within distorted plant tissue. They have several generations (up to about eight) a year. When the weather is warm. The life cycle from egg to adult may be completed in as short a time as 2 weeks.
Extremely active, thrips feed in large groups. They leap or fly away when disturbed. Host plants include onions, beans, carrots, squash and many other garden vegetables, and many flowers, especially gladioli and roses. Both adults and the wingless larvae are attracted to white, yellow and other light coloured blossoms and are responsible for spreading tomato spotted wilt virus and impatiens necrotic spot virus.
Their 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 discolour and become rolled. Petals may exhibit “colour break,” which is pale or dark discolouring of petal tissue that was killed by thrips feeding before buds opened.
On some plants, they 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.
Thrips are difficult to control. If management is necessary, use an integrated program that combines the use of good cultural practices, natural enemies, and the most selective or least-toxic insecticides that are effective in that situation.
If thrips are a suspected cause of plant damage, the adults and larvae can be monitored by branch beating or gently shaking foliage or flowers onto a light-coloured 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 harbouring 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.
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 beneficial, avoid creating dust and consider periodically rinsing the 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 eggshell is often visible at the side of the egg blister.
Thripobius semiautos parasitizes greenhouse thrips larvae. Thrips parasitized by this wasp’s larvae become swollen around the head and turn black, in contrast to the pale colour of unparasitized greenhouse thrips larvae. Unlike healthy black mature thrips, the black parasitized larvae are smaller and do not move.
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 harbour 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-coloured, 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.
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. . 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, 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.
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.
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 the 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.
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 the 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 behaviour, 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 by reading the label and consulting the Active Ingredients Database in the online version of this Pest Note at www.ipm.ucanr.edu. Often you will learn chemical control cannot be effective until the next season when new plant growth develops. Certain products are available only by hiring a professional applicator. If insecticides are used, combining their use with appropriate cultural practices and other methods usually improves the pest control.
Insecticides Most Compatible with IPM
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 (AzaMax, Safer Brand BioNeem), insecticidal soaps (Safer), narrow-range oil (Bonide Horticultural Oil, Monterey Horticultural Oil), neem oil (Green Light Neem, Schultz Garden Safe Brand Neem Oil), and pyrethrins, which many products combine with piperonyl butoxide (Ace Flower & Vegetable Insect Spray, Garden Tech Worry-Free Brand Concentrate).
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.
Spinosad (Captain Jack’s Deadbug Brew, Green Light Lawn & Garden Spray with Spinosad 2, Monterey Garden Insect Spray) is generally more effective against thrips than the products above. Spinosad lasts 1 week or more and moves short distances into sprayed tissue (has translaminar activity) to reach thrips feeding in protected plant parts. Adding horticultural oil to the spray mix can increase its persistence within plant tissue. This insecticide is a fermentation product of a naturally occurring bacterium, and certain formulations are organically acceptable. Spinosad can be toxic to certain natural enemies (e.g., predatory mites, syrphid fly larvae) and bees when sprayed and for about 1 day afterwards do not apply spinosad to plants that are flowering.
Insecticides More Toxic to Thrips and Beneficial Insects
Systemic insecticides are absorbed by one plant part (e.g., roots) and moved (translocated) to other plant parts. Trunk spray or injection of an effective, systemic, neonicotinoid insecticide can provide relatively rapid control. With soil drench or injection, there is a longer time delay between neonicotinoid application and insecticide action. Neonicotinoids vary in effectiveness for thrips control. For example, dinotefuran (Safari), available to professional applicators, can provide good control. Imidacloprid (Bayer Advanced Tree & Shrub Insect Control, Merit) commonly fails to provide satisfactory thrips control, and imidacloprid generally is not recommended.
Neonicotinoids have a low, moderate, or severe adverse impact on natural enemies and pollinators varying with the product, situation, and the species and life stage of invertebrate. Neonicotinoid insecticides can translocate to flowers and may harm natural enemies and pollinators that feed on nectar and pollen. Delay systemic insecticide application until after plants have completed their seasonal flowering, unless the product label directs otherwise. Where the roots of nearby plants grow near treated plants. Those other plants may also take up some of the soil-applied insecticides. Delay soil application, when possible, until after the nearby plants are also done flowering.
Insecticides to Avoid
The systemic organophosphate acephate (Lilly Miller Ready-to-Use Systemic, Orthene) is available for ornamental, nonfood plants. Avoid using it. Acephate can be highly toxic to natural enemies. Pollinators and can cause spider mites to become abundant and damage plants after its application.
Avoid foliar sprays of other organophosphate insecticides (e.g., malathion), carbamates (carbaryl), or pyrethroids (e.g., bifenthrin, cyfluthrin, fluvalinate, and permethrin). These materials are highly toxic to natural enemies and pollinators. They can cause spider mite outbreaks, and are not particularly effective against most thrips. Because their use in landscapes and gardens can run or wash off into storm drains. Can contaminate municipal wastewater. These insecticides are being found in surface water and are adversely affecting nontarget, aquatic organisms.
Greenhouse Thrips Management
Greenhouse thrips can infest many plant species but primarily is a pest of evergreen, broadleaved perennials. It occurs mainly on the underside of leaves and on fruit clusters or other plant parts that touch each other. They are sluggish and the adults tend not to fly. Individuals feed in groups and populations usually begin in a limited part of the plant and spread slowly. If the underside of leaves on susceptible plants are regularly inspected. To allow early detection and removal of new infestations, pruning off colonies can be effective.
Greenhouse thrips is readily controlled with a thorough application of contact sprays. Such as horticultural oil, natural pyrethrins (plus piperonyl butoxide), or insecticidal soaps to the underside of infested leaves. Repeat applications may be necessary. Keep in mind that greenhouse thrips have natural enemies in the landscape (discussed above). Assess whether spraying is warranted and select materials that are least toxic to natural enemies.