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.

Whiteflies

How to Identify Whiteflies

Whiteflies of various growth stages live on the undersides of leaves.

Whiteflies are present in many growing systems, including greenhouses. They are small but easily seen on leaves. As the name suggests, they are white to whiteish yellow. Simply shake a leaf and they will fly rapidly in small circles before settling once more on the leaves.

On the left Bemisia argentifolii and on the right Trialeurodes vaporiorum

Two species of whitefly are troublesome pests in the greenhouse, the silverleaf whitefly (Bemisia argentifolii) and the greenhouse whitefly (Trialeurodes vaporariorum). The adults are very similiar in appearance except for the the way they hold their wings at rest. The silverleaf whitefly holds its wings folded like the peaked roof of a house, while the greenhouse whitefly lays its wings flatly over its back.

The pupal stages of B. argentifolii (left) and T. vaporarium (right).

Photo courtey of UC Statewide IPM Program

A more accurate method of telling the difference between the two species is to examine the immature stages of the insect. The pupal stage of the greenhouse whitefly is round with flat raised sides like a cake, and has waxy filaments protruding from the top edges. The same growth stage in the silverleaf whitefly is oblong, has no filaments, and resembles a flattened turtle shell. 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.

How to Identify Damage Caused by Whiteflies

Sooty mold is growing in honeydew on these avocado leaves.

Photo courtesy of UC Statewide IPM Program.

Whichever species of whitefly, the damage caused is the same. While the whitefly does feed directly on the contents of plant cells, the economic losses are usually due to the pests producing huge amounts of honeydew. Honeydew is a waste product that is high in sugar. The pest flicks it onto leaf surfaces and this sticky substance fosters rapid development of sooty mold on plant tissue. The sooty mold makes greenhouse plants unsalable. Plants covered in sticky honeydew, grey fungus, and flying insects are not readily accepted into many retail outlets, let alone by consumers.

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.

Some greenhouse conditions are known to encourage whitefly populations. The easiest and most effective thing we can do to control whiteflies is to exclude them. If you do not already have whiteflies, quarantine and inspect all incoming stock carefully for adults, immature stages, and eggs on the undersides of leaves. Discard any infected material. Exclusion of the winged adults can be accomplished by covering greenhouse openings (vents, windows, rooftops, etc.) with insect meshes of 405 microns or smaller. Since whiteflies feed on a variety of host plants, keeping the greenhouse clear of weeds can reduce the potential for pest problems to begin there and spread to the crop. Like many insects, whitefly reproduction cycles are sped up by warmer temperatures. Keeping greenhouse temperatures at the minimum requirement for the intended crop will help to slow insect infestations and save money on heating.

The immature whiteflies on the underside of this leaf do not have working legs and are stationary.

Insecticide applications are not generally recommended to combat whitefly infestations. These insects live much of their lives as immobile immature stages on the undersides of leaves. Oil sprays that are effective against them must make contact with them on the underside of the leaf to kill them. This makes applying sprays very time consuming and difficult. In addition to this, the eggs and pupal stages of whiteflies are tolerant of many insectides. This leaves a substantial population alive even after a succesful spray.

Whitefly populations are well known to build up resistance to pesticides very quickly. When an insect population is subjected to pesticide, only insects with resistance to the poison live. These survivors reproduce in an environment that is free of competition and predators. Their offspring are also resistant to the pesticide. Whiteflies go through many cycles of reproduction quickly, and this is why many chemicals are simply ineffective in controlling them. In fact, their numbers are known to increase after in pesticide application, mainly because their natural enemies have been eliminated.

What Are the Natural Enemies of Whiteflies?

Left E. eremicus, natural enemy of silverleaf whitefly and right E. formosa, natural enemy of greenhouse whitefly.

Photos courtesy of Koppert Biological Systems and UC Statewide IPM Project

Both the silverleaf whitefly and the greenhouse whitefly have very effective natural enemies. These enemies are parasitoids that lay their eggs into the whitefly pupa. Each one is very specific to its whitefly host. Eretmocerus eremicus is the parasitoid of the silverleaf whitefly and Encarsia formosa is the parasitoid of the greenhouse whitefly. Each will only go after their specific host, so it is vital that you have the correct identification of your whitefly pest. Your local agricultural commisioner's office or university extension should be able to help. A high resolution photo sent by e-mail is often effective, or if you need to you can usually bring the pest in to a local office in a sealed plastic container.

The E. formosa adult lays her egg into the greenhouse whitefly pupa. The young wasp develps in the safety of the pupa case, then emerges as an adult ready to lay her own eggs.

Image courtesy of UC Statewide IPM Project

These natural enemies are tiny wasps. Their life cycles depend on the presence of whitefly, so they are highly motivated to seek out whitefly pupa in order to lay eggs in them. The wasps are winged insects that move easily from plant to plant. The real beauty of this is that each whitefly that is destroyed in this process then hatches another wasp that repeats the cycle. Application of pesticides can do great harm to populations of these beneficial insects. However, if an application is absolutely necessary, these wasps are relatively cheap and easy to replace. These little insects can greatly reduce whitefly populations and continue to work well when the pest populations are very low. Keeping a working amount beneficials in the greenhouse keeps pests in check for the long-term.

How Can I Get These Natural Enemies?

This easily purchased card will release parastic wasps into the greenhouse.

Photo courtesy of UC Statewide IPM Project

Both E. eremicus (enemy of silverleaf whitefly) and E. formosa (enemy of greenhouse whitefly) are well established as control methods for whiteflies. They 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.

Spider Mites

How to Identify Spider Mites

A two-spotted spider mite feeding on a leaf.

The spider mite is a very common pest in greenhouses. It is tiny, 1/16 of an inch or smaller. Most spider mites are oblong in shape, have eight legs, and are dull or dusty yellow to red colors. The two-spotted spider mite usually has two distinct spots visible on the creature. A magnifying lens is useful in identifying them, but they are usually much more noticeable due to their habits.

Adult spider mites can be seen in the webbing surrounding the growing shoot of this gardenia.
They can also be seen stationary on the shiny leaf surface.

Spider mites colonize on leaf surfaces near the midvein, usually near the upper new growth on plants. They often stay close together. Wherever spider mites are, their tell-tale webbing will also be on the plant. They are very slow creatures and will crawl very slowly when poked or if air is blown on them.


How to Identify Damage Caused by Spider Mites

Spider mite damage on these roses has caused a yellow stippling effect on the leaves.
Photo courtesy of UC Statewide IPM Project

These pests have rasping-sucking mouthparts that they use to eat the contents out of plant cells. A yellow stippling appears on the leaves as the plant cells die. The cosmetic damage can cause many greenhouse plants to be unsalable. Severe damage reduces the plant's ability to photosynthesize, reducing size and yield. Plant death can occur.

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.

Some greenhouse conditions are known to encourage spider mite populations. Hot and dry conditions have this effect. When plants are water-stressed the mites feed on more cells to sustain themselves and end up spreading. The warmth of the greenhouse encourages rapid reproduction cycles. The closeness of greenhouse plants allows mites to crawl from plant to plant if the leaves are touching.

Keeping plants adequately watered will reduce damage from mites. As much as it is possible, temperatures should be kept below 85 degrees Fahrenheit. Simply spacing plants so that canopies do not overlap can stop the potential of spider mite populations to spread rapidly across the greenhouse.

While an application of insecticides is sometimes necessary, it will usually end up hurting beneficial arthropods in the area. Consider that an initial reduction in pests after insecticide application is often followed by a greater resurgance soon after. This is because all the predators of the pest have been killed and the few pests that survived the insecticide are now free to infest the greenhouse. Reducing chemical sprays as much as possible allows a healthy system of beneficials to remain in the greenhouse and control pests continually. Cutting back on insecticide applications will save time and money.


What Are the Natural Enemies of Spider Mites?

Interestingly enough, other mites are commonly used to prey on spider mites. In particular, the species Phytoseiulus persimilis and Amblyseius californicus are effective.

P. persimilis has caught this spider mite and is feeding on it.
Photo courtesy of UC Statewide IPM Project.

P. persimilis hunts down and kills spider mites. It prefers to feed on younger stages of the spider mite, but will attack older. P. persimilis can only feed on spider mites so you can be assured that they will hunt down and eat every spider mite they can as long as they are alive in the greenhouse.

A. californicus has sought out these spider mite eggs.
Photo courtesy of UC Statewide IPM Project.

A. californicus operates in much the same way as P. persimilis but has the added benefit of being able to survive without spider mite prey for weeks at a time. This makes it an option for preventative control. It is also tolerates a range of chemical pesticides, making it a valuable addition to conventional pest control.

The F. acarisuga larva is going after and feeding on two-spotted spider mites and their eggs.
Image courtesy of BioBest Biological Systems

Another predator of spider mites, Feltiella acarisuga , can be used to control them in the greenhouse. This insect is a gall-midge that lays its eggs into spider mite colonies. The larvae feed voraciously on eggs, nymphs, and adults of the pest. Not only does the gall-midge thrive in the humidity of a greenhous, but it is able to fly from plant to plant laying its eggs. It works well in conjunction with P. persimilus and A. californicus and has the ability to survive in the absence of the spider mites for some time.

How Can I Get These Natural Enemies?

All of these natural enemies are easily purchased online from several reputable companies. They 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.

What is Bio-Control and Why Should I Care?

Biological control (Bio-Control), is any activity of one species that reduces the adverse effects of other species.

Simply put, the insects that plague your greenhouse operation have enemies. Many people are aware that ladybeetles eat aphids; a classic example of insect destroying insect while growers reap the benefits. Thrips, whiteflies, and spider mites also have natural enemies which can be easily purchased and put to use.

Bio-control does not mean "organic". While it can be a very useful part of organic operations, it can also be a very effective tool for conventional growers. A carefully integrated pest management system combining chemicals and bio-control can have a great rate of success in keeping losses down and production up.

Some benefits of using bio-control are:

Reduction of pests numbers to below economic threshold

Low cost of natural enemies

Reduction in costs spent on pesticides

Learning to monitor and apply chemicals only when necessary (saving money)

Less pest resistance to chemicals

Less toxic chemicals for workers to deal with

Lower chances of sudden pest outbreaks due to presence of natural enemies

Long-term control

Here a big-eyed bug is feeding on whiteflies of various growth stages on a leaf.

Welcome

Hi, and welcome to Bio-Control in the Greenhouse! If you are a growing plants indoors, then you've probably already met up with Thrips, Whiteflies, and Spider Mites. This site should outline some easy ways to identify these pests and help you choose some beneficial insects to control them.

While this site is intended to educate people in using certain natural enemy species to control thrips, whitefly, and spider mites, there are far more comprehensive guides to biological control. Both the UC Statewide Integrated Pest management website http://www.ipm.ucdavis.edu/ and The Natural Enemies Handbook written by Flint and Dreisdadt were invaluable resources in creating this page.