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, 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.
Thrips can readily move long distances floating with the wind or transported on infested plants, and exotic species are periodically introduced. In Flowers there are mainly two types of Thrip species that attack Roses, Carnations and other flowers. The western flower thrips (Franklieniela occidentalis) and Thrips tabaci also known as Onion thrip. These Thrip species mainly feed on both leaves and flower petals with the majority of their damage to roses occurring throughout the growing period of the flowers. Their feeding may result in distorted buds that open only partially or abort prematurely.
Feeding on petals may result in petals streaked with silvery-white or brown as well as petals with browning edges. White and light-colored rose blossoms appear to be particularly attractive to thrips. Young leaves may be distorted and flecked with yellow as a result of thrips feeding.
Identification
Most adult thrips 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.
Feeding results in various tissue responses, including scar formation and distorted growth. Behavior, body appearance, and host plants help to distinguish among thrip species. For example, three dark spots on each forewing distinguish the adult predaceous six spotted 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. 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 certain insecticides. 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. For example, avocado thrips and greenhouse thrips superficially scar avocado fruit skin.
Citrus thrips and western flower thrips are also found in avocado but do not damage avocados. Citrus thrips occur on many species of plants but damage only blueberries and citrus.
Thrips hatch from an egg and 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 behavior and are called prepupae and pupae, even though thrips 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 lower leaf surfaces; 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. Thrips 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.
Damage
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 also vectors 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.
Management
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.
Monitoring
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.
Biological Control
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. 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.
Cultural Control
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.
Insect proof nets
Especially for plants grown under plastic cover (greenhouses/glass houses) some types of nets with a fine mesh can exclude thrips and other insects from plants.
Chemical Control
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 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.
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.