Champion Daylilies
Thrips
Thrips, order Thysanoptera, are tiny, slender insects with fringed wings. They feed by puncturing their host plant or animal prey and sucking up exuding contents. Certain thrips species are beneficial predators that feed only on other insects and mites. Beneficial species include black hunter thrips and the sixspotted thrips. Pest species are plant feeders that scar leaf, flower, or fruit surfaces or distort plant parts. Other species of thrips simply feed on fungal spores and pollen.
Identification
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. 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 nymphs of the predatory thrips, Franklinothrips vespiformis.
Many thrips species feed within buds and furled leaves or in other enclosed parts of the plant. Their damage is often observed before the thrips can be 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 lacebugs, and certain plant bugs produce dark, watery fecal specks. Look carefully for the insects themselves to be certain that pest thrips are present before taking control action.
Life Cycle
The thrips life cycle includes the egg, two actively feeding nymphal stages, nonfeeding prepupal (propupal) and pupal stages, and the adult. Thrips have a metamorphosis that is intermediate between complete and gradual. Thrips nymphs are often called larvae; last-instar nymphs 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 species insert their tiny eggs into plants, commonly into leaves or buds where nymphs 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.
Damage
Feeding by thrips 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. Avocado, citrus, and greenhouse 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. Thrips are primarily pests of herbaceous plants, but high populations occasionally damage continuously or late-blossoming flowers on woody plants such as roses and ornamentals. When thrips populations are high on roses and ornamentals, flower buds may become deformed and fail to open. Petals may be covered with brown streaks and spots. Thrips 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 some situations thrips are considered beneficial because they feed on spider mites.
Thrips prefer to feed in rapidly growing tissue. They are poor fliers but can spread long distances by floating with the wind or being transported on infested plants. Herbaceous ornamentals and certain fruit and vegetable crops are generally more susceptible to thrips; infestations may reduce the aesthetic quality of landscapes but usually do not seriously harm or kill woody plants.
Management
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 differs some from that of most other pest thrips; special management information, which is in addition to that given for all thrips, is discussed in a separate section.
Monitoring
Monitor thrips adults and nymphs by branch beating or shaking foliage or flowers onto a sheet of paper, a beating tray, sheet, or clipboard. 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.
Biological Control
Beneficial insects and mites including minute pirate bugs and predaceous mites 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. Conserving naturally occurring populations of beneficials by controlling dust and avoiding persistent pesticides is the most important way to encourage biological control of thrips.
Cultural Control
Thrips often move into gardens and landscape plantings 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.
Chemical Control
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. Also, by the time damage is noticed on ripening fruit, the thrips that caused the injury are usually gone. 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. Furthermore, most thrips are difficult to control effectively with insecticides because they are protected within plant parts that surround them as they feed. If insecticides are used, they will only be partially effective and must be combined with appropriate cultural practices and conservation of natural enemies.
For ornamental nonfood plants, several applications of a systemic insecticide such as the organophosphate acephate (Orthene) can provide temporary control of thrips, but this product can be highly toxic to natural enemies. Another systemic insecticide, imidacloprid (Bayer Advanced Garden, Marathon, and Merit), is also available. This material will provide some suppression of foliage-feeding thrips only. Other materials available to licensed pesticide applicators include the microbial-derived materials abamectin (Agri-Mek and Avid) and spinosad (Conserve and Success), which have low to moderate impact on natural enemies. None of these materials, however, provides complete control 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, will cause dramatic increases in spider mite populations, and are not particularly effective against thrips.
Greenhouse 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. 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 rapidly resurge, repeat applications may be necessary.
Thrips