Thrips

How to Identify Thrips

Here the tiny size of thrips is apparent against this human finger.

Thrips are tiny insects, 1/16 of an inch or smaller, that feed on plant leaves and flowers. They are very small, but are a bit easier to see than some pests since they crawl fairly quickly and are found in large numbers. They have large pointed abdomens in proportion to their thorax and head, and small stick-like wings. A hand lens is helpful in identifying these pests. Thrips range in color from yellow, to dark brown, to black. You are most likely to find them on the newest growth of plants. Gently tap a flower head or young leaf on a piece of white paper. If thrips are present you will see the tiny insects being dislodged onto the paper, much like pepper from a pepper shaker.

On the top, F. occidentalis and on the bottom, H. haemorrhoidalis feed on leaves.

(top) Photo courtesy of UC Statewide IPM Project and (bottom) Photo courtesy of UC Davis Dept. of Entomology

Two species of of thrips are particularly destructive in greenhouses, the western flower thrip, Frankliniella occidentalis, and the greenhouse thrip, Heliothrips haemorrhoidalis. The two species vary in color over their lifecycles. Generally though, the western flower thrip is yellow with a darker back and the greenhouse thrip is black. It is advisable to seek an expert's opinion by e-mailing a high resolution photo to the county agricultural commisioner's office or university extensions. If you need to take a sample in to a local office, do so by carrying the pest in a sealed plastic container. Fortunately, the management of both species is similar.

How to Identify Damage Caused by Thrips

The necrotic spots and streaks on these petals are the result of thrips.

Photo courtesy of Whitney Cranshaw, Colorado State University

Thrips tend to cause damage on tender new growth of plants, and will commonly cause cosmetic damage to flower petals. Discolored spots or streaks on petals, and disfigured or wilted looking flowers are common symptoms of thrips damage. Thrips will also act as pollinators - just when you don't want them to. Premature flowering, wilting, and senescence can be caused by the action of thrips.

Necrotic spots on these leavs are due to the feeding of thrips.

Photos courtesy of the Horticultural Development Company

Thrips may also feed on foliage, particularly new foliage. Small yellow or brown spots will appear across the leaf surface as they use their mouthparts to pierce plant cells and suck out the contents. If the damage is severe enough leaf yellowing, leaf malformations, and leaf drop may occur. Thrips will often leave behind a shiny tar-like waste product on the leaves.

This begonia exhibits symptoms of the impatiens necrotic spot virus.

Photo courtesy of UC Statewide IPM Project

Another problem with thrips is that they are capable of spreading the tomato spotted wilt virus and impatiens necrotic spot virus. Their feeding action from plant to plant is similar to a hypodermic needle. These viruses cause concentric rings of dead tissue on leaves and fruit. Other symptoms include wilting and yellowing of affected tissues.

Greenhouse Conditions

The greenhouse is not just a place where growers produce crops. It is an environment that attracts, supports, and produces plant-eating pests. Keeping this fact in mind, we can realize and deal with the fact that our management of the greenhouse can control pests.

Since thrips can be a very persistent problem, the best approach is to exclude them if they are not already present in the greenhouse. Quarantine new arrivals and inspect these plants to be sure they are not harboring thrips. Another important aspect of excluding thrips is the use of mesh screens over greenhouse openings (vents, windows, rooftops, etc.) with insect meshes of 145 microns or smaller. Such meshes will often be marketed as "virus" mesh rather than insect mesh, since its ultimate purpose is to prevent spread of tomato spotted wilt and impatiens necrotic spot virus.

In many cases thrips will feed on pollen, nectar, and even other insects. They are pragmatic little insects that will live on a variety of plant hosts. Keeping the greenhoue and surrounding area free of weeds and leftover plant material (dead leaves, old stock, etc.) eliminates shelter for these pests. In this same vein of thinking, it is a good idea to periodically sterilize soil under greenhouse benches. Thrips spend a portion of their lives as pupae in soil. The soil may be sterilized of them using insecticides or hydrated lime. If it is economically feasible, one can clear the greenhouse after crop production for a week, leave all vents and windows closed, wait for thrips pupae to emerge from the soil as adults, then keep temperature in excess of 102 degrees F for two days to kill them off. At this point it would also be possible to kill the adults using insecticides. However, it is pointless to spray before the adults have emerged, since the eggs and pupal stages of thrips are tolerant of insecticides.

While chemical pesticides are often used against thrips, these creatures are known to rapidly develop resistance to insectides. It is vital to resist falling into routine when using chemicals. Thrips reproduce rapidly, giving rise to a new generation approximately every fourteen days. For insectides to remain effective, each generation should be sprayed with a different class of insecticides. If the contiguous generations are exposed to the same class of insecticide the population will quickly become immune to that class. For chemicals to work properly, a carefully thought out schedule of pesticides must be made up.

What Are the Natural Enemies of Thrips?

Thrips have many natural enemies that can be employed in the greenhouse. Several mite species and some insects are used. Two mites that have proven effective are Amblyseius swirskii and Hypoaspis miles.

This A. swirskii (leftmost) has caught what appears to be a thrip nymph.

Photo courtesy of the Horticultrual Development Company

A. swirskii is a beneficial mite that preys on the larval stages of various thrips. They are also known to survive on whitefly eggs, whitefly larva, spider mites, and pollen. Their ability to survive on alternate food makes them an ideal candidtate for long-term pest population control and preventative control of thrips. They live on the plant leaves where they hunt their prey; they have a hearty appetite for thrips and feed continuously. A. swirskii has been evaluated by the USDA as an effective predator of thrips on pepper. These mites do well in warm humid conditions (often the case in greenhouses) and are hampered by cool dry conditions. A. swirskii is available in slow-release packages and is approved on most ornamental crops.

H. miles attacks this thrip nymph.

Photo courtesy of the Horticultural Development Company

H. miles is also a predatory mite used for the control of thrips. This mite is particularly useful because it is a soil-dwelling creature. It seeks out the thrips pupae in the soil where they lay out of reach of other control methods. They are also known to feed on other greenhouse pests such as fungus gnats and springtails. H. miles will survive on detritus if necessary and will establish themselves for the long-term in the soil. Employing these mites can be a useful tool in cutting down on pesticides and time-consuming procedures for cleansing the greenhouse environment.

Here O. insiduiosus is seen searching for insect prey on a leaf. In this case it has found whitefly eggs and pupae.

Another natural enemy of thrips is the minute pirate bug, Orius insidiosus. These little bugs will seek out adult and larval thrips, aphids, spider mites, butterfly eggs, and pollen. They work well with the predatory mites mentioned above in the control of thrips in greenhouses. O. insidiosus is a winged predator so it has the advantage of being able to fly quickly from plant to plant. They have a strong drive to feed and work well in lowering thrips populations. They are also good candidates for long-term control because they will feed on other pests and pollen in the absence of thrips. The minute pirate bug thrives in warm conditions, but will become inactive when temperatures drop below 50 degrees F.

How Can I Get These Natural Enemies?

The H. miles and O. insidiosus are commonly used in greenhouses. A. swirskii is relatively new in the trade but is gaining popularity rather quickly. All three of theses species are readily available online from reputable companies, or in some cases from local insectaries. Keeping track of pest populations using sticky traps will help you determine when it is appropriate to order natural enemies and schedule releases. More information on using sticky traps to monitor whitefly populations can be found on the UC Statewide IPM Project website (http://www.ipm.ucdavis.edu/PMG/r280390411.html)

These natural enemies are like any other pest control method in that they will come in easy-to-use packaging and have instructions on how to use them.