Thrips

Thrips are very small, elongate, cylindrical, gregarious insects ranging from 1/25 to 1/8 inch in length. The nymph are frequently pale yellow and highly active.

The antennae and legs are relatively short. Adults are usually black or yellow-brown, but may have red, black or white markings and often jump when disturbed. They may have wings or may be wingless. If wings are present, they are long, narrow and fringed with hairs. For this reason, thrips are commonly referred to as fringed-winged insects.

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.

Thrips are characterized by having a single mandible (jaw) used for rasping. This sword-like mandible is extruded when the mouth cone is compressed on plant tissue. The extruded mandible slashes open epidermal cells. The contents of the opened cells are then sucked in through the cone.

Western flower thrips feed on a wide variety of plants including chrysanthemums, gloxinia, impatiens, tomato, vegetables and grasses. Some plants species, varieties and cultivars are more attractive to the thrips than others. Thrips can be detected by beating flowers onto a piece of paper and looking for tiny walking dashes . They can be collected with a small paint brush and preserved in alcohol.

Life Cycle: 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.

Damage : Thrips feed on the foliage and flowers, as well as young tissues in shoot apexes where the leaves are expanding. They puncture the plant cells with their rasping-sucking mouthparts and withdraw cell sap. Feeding activities produce bleached, silvered or deformed leaves and necrotic spots or blotches on flower petals.

Eventually the damaged foliage becomes papery, wilts and drops prematurely. Thrips produce large quantities of a varnish-like excrement which collects on leaves, creating an unsightly appearance.

Thrips as Vectors : Thrips also pose a serious threat to crops by virus transmission. Thrips can transmit TOSPO viruses. The TOSPO viruses include the impatiens necrotic spot virus (INSV) and the tomato spotted wilt virus (TSWV). Both viruses can be fatal to floriculture crops and once a plant is infected it cannot be cured. Control options for TOSPO viruses are limited to suppression of vector populations (thrips) and eradication of infected plant material to reduce inoculum.

Symptoms of TOSPO viruses include necrotic lesions, wilting, yellowing, leaf distortion and ring spots. However, symptom expression depends on the type of plant infected and the environmental conditions of the growing area. In some situations infected plants may not exhibit any symptoms but can act as reservoirs.

To monitor for the presence of TOSPO viruses, it is a good idea to use indicator plants that are susceptible and express consistent, recognizable symptoms. Fava beans can be used as indicator plants for TOSPO viruses. Cultivar Little Toto is the most susceptible fava bean, but its availability is scarce. However, all fava beans are susceptible to the virus and will work as indicator plants. Certain varieties of petunias can also be used as indicator plants, but they are much more expensive and difficult to propagate.

Control : Controlling thrips is important because they are potential disease vectors. Total eradication is usually not possible, especially since thrips populations can develop pesticide resistance. Growers should use a combination of cultural, chemical and biological control strategies to manage thrips populations. Pesticides should be rotated to avoid resistance. Plants infected with TOSPO viruses should be discarded to reduce the chance of virus transmission.

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