Western Flower Thrips

t.b. violets

Western Flower Thrips


Thrips — I wish I had a picture to share but I’ve killed all mine for now so you’ll have to go look it up if you want to see some. Sorry. I have had them on occasion, I just didn’t take any pictures. They move too quick!

Thrips are the african violets most dangerous pest according to some growers. Mites can be nearly impossible to get rid of and kill your plant if left untreated and a lot of growers will just toss a plant with mites rather than try to save it. But thrips are everywhere! It seems like they target your beautiful violets at every turn. And they can carry INSV — Impatient Necrotic Spot Virus and spread it to wipe out your whole collection. So can any pest infestation left untreated. Penn State University Extension has a very good page on INSV and says » The virus causes a wide variety of symptoms including wilting, stem death, stunting, yellowing, poor flowering, ‘chicken pox-like’ sunken spots on leaves, etches or ring spots on leaves, and many others.» Their page also has information on Western Flower Thrips.

Western Flower Thrips are a big problem in commercial greenhouses, orchards and nurseries where they monitor their activity with sticky traps to get an idea of the population numbers so they can know when to spray to keep down the damage done. They never expect to totally eradicate them because they come in on the wind and it’s a big windy world outside! They can’t kill every thrip in the world at the same time so the ones that live through an insecticide spray will develop the resistance to it as a survival mechanism. That’s the problem.

In a greenhouse environment, they can never kill them all. So they might be a big deal to the commercial grower but not so much to the average home grower. with a good strategy. The home growing environment is a lot different than the open commercial growing environment where you should have a lot more control in a smaller confined area. But we tend to use some of the same products on our violets that commercial growers do and the industry does provide lower concentrations for home users.

In the greenhouse, they rotate their insecticides to kill the resistant strains that may have survived another insecticide and become resistant to it. Does that make sense? So far so good. You can do that at home too if you think your last insecticide dip or spraying was not very thorough. But thrips can come in on the wind through your screens so how do you know if you have new ones or old insecticide resistant ones that survived? You don’t so the best prevention is to rotate your insecticides at home too.

My favorite product is spinosad and I use it in low dosage on every plant that comes into my house. I don’t intend to leave any survivors to become resistant. But, I also rotate my insecticides when I do have to use them on all my plants as a preventative.

I don’t want to repeat any information that is already out there so I can just tell you about my experience and point you to a few good research sources so you can gather correct information to put with your own growing experiences.

According to the University of Florida thrips in general can cause all kinds of damage. «Common symptoms include bronzed, curled, or stunted leaves. Infested plants decline over time, and severe infestations can cause total leaf loss.» University of Florida on Thrip Damage

In my experience, the best preventative is a pro-active spraying of a 3 in 1 product weekly or bi-weekly. Most bugs find neem oil and essential plant oils in general distasteful so it is a deterrent and will not harm your plants but you need to shake it up as you apply it to get even coverage.

As a home grower, you do not want to just cut down on the number of thrips in your growing area because you can completely control them unlike an outdoor operation where they can’t. So, you want to kill them all and you can kill them all at once in your little enclosed environment (at least until the next time).

When you kill them all, there are none left to develop a resistance to your product. The new ones you get after that will all be new brought in from the outside and not necessarily resistant to your application. That being said, there is no way to know for sure if they are already resistant from other insecticide applications to whatever product you use.

According to the University of California western flower thrips can cause leaves to drop. «Thrips feeding can stunt plant growth and cause damaged leaves to become papery and distorted, develop tiny pale spots (stippling), and drop prematurely.» UC IPM on Western Flower Thrips

So, Western Flower Thrips are a real threat to african violet growers but not the only thrip that can attack them. The best way to fight them is to learn all about them and have a preventative plan in place.



There are more than 6,000 thrips species sucking the life from plants all over the world. Get rid of them naturally without resorting to toxic sprays by using these SAFE, organic methods.

A common pest found in greenhouses and indoor/ outdoor gardens, thrips damage plants by sucking their juices and scraping at fruits, flowers and leaves. Plant leaves may turn pale, splotchy, and silvery, then die. Injured plants are twisted, discolored and scarred.

Adults are very small (less than 1/25 inch) straw-colored or black slender insects with two pairs of feathery wings. Without the use of a hand lens, they resemble tiny dark threads.


Extremely active, thrips feed in large groups. They leap or fly away when disturbed. Host plants include onions, beans, carrots, squash and many other garden vegetables, and many flowers, especially gladioli and roses. Both adults and the wingless larvae are attracted to white, yellow and other light colored blossoms and are responsible for spreading tomato spotted wilt virus and impatiens necrotic spot virus.

Life Cycle

Adults and pupae overwinter in garden soil. In spring, newly emerged females insert eggs into the tissues of flowers, leaves or stems. (They do not need to mate for reproduction.) Each female can produce up to 80 eggs, which hatch within days in warm weather or weeks to months in colder weather. They become wingless larvae (nymphs), which feed on plant sap. After two or more nymphal stages, many thrips drop to the soil to pupate. Emerging adults fly to the plant and repeat the cycle. There may be 12-15 generations per year with the entire cycle from egg to adult requiring less than 16 days in warm weather.


Thrip management is a matter of garden maintenance — reducing the places where thrips may breed — and requires removing plant debris while it’s still on the ground and green. Thrips lay their eggs in slits they cut in live plant stems. Vigilance — spotting problems early and responding to them — is also required. Check your plants for damage and clusters of the pests at the place where leaves are attached to stems. Don’t wait to take action. Take the measures listed below. And be sure to use the safest, most proven products.

  1. To get rid of thrips remove weeds and grass from around garden areas to eliminate alternate hosts. Clean up crop debris in the garden, especially onion leaves after harvest. (Dry mulch will not attract thrips. Green mulch will.)
  2. Inspect all plants you import into the garden for signs of thrips or their damage. Discard any infested plants by securely bagging and putting in the trash.
  3. Blue sticky traps are helpful for monitoring adult populations.
  4. If found, use the Bug Blaster to hose off plants with a strong, encompassing spray of water to reduce pest numbers.
  5. Release commercially available beneficial insects, such as minute pirate bugs, the effective thrips predator (feeds on eggs and larvae before they can become adults), ladybugs, and lacewing, (especially effective in green houses) to attack and destroy all stages of this pest. For best results, make releases after first knocking down severe infestations with water spray or other method.
  6. BotaniGard ES is a highly effective biological insecticide containing Beauveria bassiana, an entomopathogenic fungus that attacks a long-list of troublesome crop pests – even resistant strains! Weekly applications can prevent insect population explosions and provide protection equal to or better than conventional chemical pesticides.
  7. Severe populations may require a least-toxic, short-lived botanical insecticide (pyrethrin) to reduce pest numbers. Follow-up with predatory insects to maintain control.
  8. Safe, smothering insecticidal soaps made from naturally occurring plant oils and fats, are also effective for knocking down heavy infestations (and won’t harm most naturally occurring beneficial insects). Spinosad and neem oil can be used to spot treat heavily infested areas.

Tip: Thorough coverage is necessary when using natural contact insecticides, especially on the undersides of leaves and where leaves attach to stems, a favorite place for thrips to congregate.

Recommended Products

Blue Sticky

Uses a special color attractant to protect plants from thrips and leafminers.

Thrips Predator (Thripex)

This very small, tan colored predator moves quickly to capture various thrips species.

Bon-Neem II

Specially formulated to kill mites, aphids, whitefly and more on contact.

Swirski-Mite Plus

Hang the sachets in the affected area and let these natural forces do their work!

3-in-1 Spray

Contains sulfur and insect killing soap derived from natural fats and plant oils.

BotaniGard ES

Contains active Beauveria bassiana spores that attack a long-list of troublesome pests.

70% Neem

For broad spectrum use on vegetables, flowers, trees and more — indoors or out!

Take Down

Combines the fast knockdown of pyrethrin and the residual activity of canola oil.


Thrips On Roses: How To Kill Thrips In Your Rose Garden

By Stan V. Griep
American Rose Society Consulting Master Rosarian – Rocky Mountain District

In this article, we will take a look at thrips (flower thrips and even some known as the chili thrips) as one of the pests we may have to deal with in our rose beds. Thrips are tough customers when it comes to controlling them once they have set in upon our roses.

Identifying Thrips on Roses

Thrips are extremely active slender brownish yellow winged insects. They seem to favor the lighter colored blooms and will typically leave red spots and brown streaks on the petals. The flower buds are often deformed and typically will not open.

The chili thrips will attack the foliage and basically the entire host plant. The amount of damage they can do in a very short time is astounding! The chili thrips will kill the entire host rose bush or plant quickly if not treated immediately upon noticing the earliest stages of an attack upon the rose bushes or plants in the gardens.

Controlling Thrips on Rose Bushes

One of the reasons thrips can be so hard to control is that they live inside the buds and blooms of the roses and other flowering plants in the garden. Both the young and mature thrips feed on the sap within the petals by rasping the tissue of the petals to suck the sap out. The thrips usually start out breeding on various grasses and weeds. Once those sources are cut down, they move onto attacking the ornamentals in the garden.

The number of thrips attacking our gardens can grow very quickly once they have found the blooms of our gardens. The complete life cycle for the thrips can occur in two weeks time, so their numbers do indeed rise very quickly if a method of control is not started as soon as possible.

To gain control of a problem with thrips, using a systemic insecticide may prove to be the most effective. The systemic insecticides move throughout the tissues of the rose bushes treated, thus getting into even the most seemingly hidden tissues where the thrips love to attempt to hide, feed and breed. As always, the use of an insecticide is no light or easy choice. Using an insecticide that has the best chance of controlling the problem quickly will mean using less of it over time with hopefully less impact.

Take the time to read the labels well on the insecticides available in your area, and be sure that thrips are, in fact, listed as one of the insects controlled. Most insecticides will help in gaining control of the extremely nasty and tough chili thrip; however, the key is to spray frequently. Even though I do not like to use the insecticides, especially the systemic types, the amount of damage these pests can do in such a very short time warrants serious consideration. Staying on top of, or better yet ahead of, a major attack is extremely important.

Many people today use drip irrigation in their gardens or some form of automatic irrigation. The big problem with that is that the rose bushes or plants in our gardens, typically, do not get the close up inspection as when we water by hand. Thus, when an insect or fungal attack happens, it can gain control quickly and easily. By the time the problem is noticed, the choices are very limited as to what will gain control and do so quickly.

Remember, the garden grows best when the shadow of the gardener is there frequently. Take a garden walk to truly look over the foliage of your rose bushes and other plants at least weekly, even then a problem can get ahead of us.


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The Western flower thrips is one variety of thrips that can cause extensive damage on over 500 species of plants.

Lyle Buss/Univeristy of Florida


Age-Old Wisdom meets Modern Tools

Have you seen thrips damage on your plants? Here are tips for identifying and getting rid of thrips in your garden.

What Are Thrips?

Thrips are tiny insects about as fat as a sewing needle that dine on many plants worldwide. Also known as thysanoptera or thunderflies, thrips are sucking insects that can cause some damage to plants. However, their damage can be much worse when they transmit viruses to plants.

Thrips Life Cycle: The life cycle depends on the species of thrips as well as the location, host plant, and other factors. Adult thrips overwinter in plant debris, bark, or other materials. They become active in early spring and lay eggs in plant tissue. These eggs hatch after 3–5 days, and the nymphs then feed for 1–3 weeks before resting to molt in 1–2 weeks. Thrips can have up to 15 generations per year outdoors. Adult thrips live short lives of about one month.


Identifying Thrips

Adult thrips are slender and tiny, at ⅕0- to ½5-inches long. Their colors can be anywhere from yellow to brown or black, and if you try to get close to them, they will probably leap or fly away. They have narrow, fringed wings. The nymphs look like even smaller adults, though they tend to be light green or yellow rather than darker colors. Their wings are also not fully developed, and they sometimes have red eyes.

Thrips appear to be tiny dark slivers on your plants. It is hard to see their bodies well without a magnifying glass, but up close, they look a bit like lobsters. Shake them onto a white background in order to see them well.

Thrips Damage

Thrips damage includes streaks, silvery speckling, and small white patches. This happens because the thrips suck plant cells from many garden plants, flowers, fruits, and shade trees. If you have a major infestation of thrips, your plants might be stunted with damaged flowers and fruit. The damage that you notice might instead come from the virus that the thrips spread (usually tomato spotted wilt virus).

Photo Credit: University of Florida. These streaks of damage on roses come from chilli thrips.

Control and Prevention

How to Get Rid of Thrips

  • To keep thrips populations under control, try using yellow or blue sticky traps.
  • Shaking branches to remove the thrips and catching them on a cloth underneath is one easy way to quickly remove the thrips from your plants.
  • For onion thrips: Take a dark piece of paper into the garden and knock the onion tops against it; if thrips are present, you will spot their tan-colored bodies on the paper. A couple of treatments with insecticidal soap kills them. Follow the package directions. Spray the plants twice, three days apart, and the thrips should disappear.
  • For fruit trees: Spray dormant oil on the trees.
  • As a last resort, dust the undersides of leaves with diatomaceous earth.

Photo credit: Steven Arthurs/University of Florida. Black thrips and thrip damage on a fig tree.

How to Prevent Thrips

  • For flower thrips on gladiolus: Mix 1 tablespoon Lysol household cleaner with 1 gallon water. Soak gladiolus corms in the liquid and plant while still wet to prevent thrips.
  • You can plant various flowers to attract beneficial insects that are natural predators to thrips. Some good predators include pirate bugs, lacewings, and lady bugs. Learn more about attracting those predators.
  • For onion and western flower thrips, try releasing minute pirate bugs or the predatory mite Amblyseius cucumeris.
  • Reflective mulches can help to hide your plants from thrips.
  • If there is a very big thrips problem in your area, some plants have resistant varieties.
  • Try not to overfertilize plants, as this can lead to more thrips damage.

Do you have more tips for controlling thrips? Let us know below!


Ministry of Agriculture, Food and Rural Affairs

Thrips in Greenhouse Crops — Biology, Damage and Management

Agdex#: 290/621
Publication Date: 01/14
Order#: 14-001
Last Reviewed: 01/14
History: Replaces OMAFRA Factsheet 03-075, Management of Thrips in Greenhouse Crops, and OMAFRA Factsheet 03-077, Biology of Thrips in Greenhouse Crops
Written by: Graeme Murphy — Greenhouse Floriculture IPM Specialist/OMAFRA; Gillian Ferguson — Greenhouse Vegetable IPM Specialist/OMAFRA; and Les Shipp — Greenhouse Entomologist/Agriculture and Agri-Food Canada

Table of Contents


Thrips are a major pest of greenhouse crops in Ontario. A number of thrips species are commonly found including western flower thrips (Frankliniella occidentalis), eastern flower thrips (Frankliniella tritici), onion thrips (Thrips tabaci), and Echinothrips. However, western flower thrips is the predominant species and the most difficult to control.

Figure 1. Comparison between adult western flower thrips (right) and adult Echinothrips (left).

Adult western flower thrips are approximately 1-2 mm in length and generally yellowish-brown in colour. Identification to the species level is difficult (especially among western flower thrips, eastern flower thrips and onion thrips) because they are so small and their colour varies. Adults are the only stage that can be identified to species. Identification should be done by specialists.

Life history

The life cycle consists of five stages: egg, larval, prepupal, pupal and adult. Female adult western flower thrips live up to 30 days and lay 2-10 eggs per day. At 20°C, development from egg to adult takes approximately 19 days. At 25°C, it takes 13 days. The eggs are inserted into soft plant tissues, including flowers, leaves, stems and fruit. In sweet pepper, egg hatch gives the leaves a speckled appearance, with the degree of speckling corresponding to the number of hatched eggs. The larval stage (see Figure 2) consists of 2 instars that feed and develop on the leaves, flowers and fruit. The prepupal and pupal stages often complete their development on the ground or growing medium, but pupation can also take place on the plant. The pupa (see Figure 3) is a non-feeding stage during which the wings and other adult structures form.

Figure 2. First and second larval instars plus adult of western flower thrips.

Figure 3. Pupal stage of western flower thrips.

The adults are weak fliers, usually taking short flights from leaf to leaf or plant to plant. Nevertheless, they disperse rapidly throughout the greenhouse. Adult thrips can be transported on wind currents and will enter the greenhouse through vents and doorways. At all stages they may be dispersed on workers’ clothing and on infested plants, growing media or farm implements.


The adult and larval stages feed by piercing the plant surface with their mouthparts and sucking the contents of plant cells. This causes white or brown spots on the leaves where the plant cells have been destroyed. These spots are also speckled with dark fecal droppings from the thrips.

Vegetable Crops

In cucumber (see Figure 4) and tomato, thrips damage is noticed first on the lower leaves. In sweet pepper (see Figure 5), it is evident in the upper youngest leaves. Heavy infestations reduce the ability of the plants to photosynthesize, reducing the yield. On vegetable flowers, thrips feeding creates silvery white streaks on the petals. Fruit damage varies according to the crop. For instance, in cucumber fruit, feeding creates severe distortion and curling as well as white streaks (see Figure 6). Feeding on sweet pepper (see Figure 7) causes silvery or bronze streaks or spots on the fruit. Thrips also feed on the calyx, causing it to turn up and expose the fruit to bacterial infections. On tomato, thrips may lay eggs in the fruit, creating ghost-spotting (see Figure 8). Ghost-spotting can also occur with sweet pepper and cucumber.

Figure 4. Thrips feeding damage on cucumber leaves.

Figure 5. Thrips feeding damage on pepper leaves.

Figure 6. Thrips feeding damage on cucumber fruit.

Figure 7. Egg-laying scars and feeding damage on sweet pepper.

Figure 8. Thrips egg-laying scars on tomato

Ornamental Crops

Western flower thrips has a host range of hundreds of plant species, including many major commercial floriculture crops. Damage includes feeding scars and leaf distortion (see Figures 9 and 10). Thrips are particularly attracted to flowers, where they cause damage such as streaking and scarring of petals, distortion of flowers and flower buds and incomplete petal expansion (see Figures 11 and 12).

Figure 9. Thrips feeding damage on roses. (Photo credit: Colleen Teerling, Agriculture and Agri-Food Canada)

Figure 10. Thrips feeding damage on chrysanthemum leaves.

Figure 11. Thrips feeding damage on chrysanthemum.

Figure 12. Thrips feeding damage on gerbera.

Virus Transmission

Western flower thrips is the most important vector of a group of viruses called tospoviruses. Tomato spotted wilt virus (TSWV) and impatiens necrotic spot virus (INSV) are the most common tospoviruses in greenhouse crops. In Ontario, TSWV is generally found in vegetable crops and some ornamental crops such as chrysanthemum, while INSV is more common in ornamental crops. In vegetables, symptoms of this disease vary according to the host, cultivar and stage of plant development, but it can severely reduce or even stop plant growth. Other general symptoms include stunting, bronzing and curling of the leaves, and distortion of affected plant areas. In addition, infected fruit are misshapen and ripen unevenly, often with a necrotic ring pattern (see Figures 13 and 14).

Figure 13. TSWV symptoms on pepper fruit.

Figure 14. TSWV symptoms on pepper leaves.

In ornamental crops, many different species serve as hosts for INSV. Symptoms and susceptibility vary widely (see Figures 15-20) but include:

  • ring spots and line patterns on leaves
  • necrotic lesions
  • black streaking on veins and stems
  • stunting
  • death of growing points and crown
  • plant death in some crops (e.g., gloxinia)

Figure 15. INSV symptoms on kalanchoe: concentric ring patterns.

Figure 16. INSV symptoms on Aphelandra: necrotic leaf lesions.

Figure 17. INSV symptoms on cineraria: stem lesions.

Figure 18. INSV symptoms on gloxinia: ring spots and leaf lesions.

Figure 19. INSV symptoms on gloxinia: extreme necrosis leading to death.

Figure 20. INSV symptoms on Exacum: complete plant collapse.



Monitoring the population levels of western flower thrips is critical for successful pest management. In vegetable crops, monitoring should begin during propagation and continue after transplanting. In floriculture crops, thrips can be present at damaging levels year-round, although populations are usually smaller during winter. Commercially available blue or yellow sticky traps can be used to monitor the population densities of adult thrips (see Figure 21). Blue traps are more attractive to western flower thrips, although yellow traps are more attractive to other pests such as whiteflies and aphids. Your choice depends on how many pests you need to monitor, the susceptibility of the crop to thrips and/or tospoviruses and your need to detect thrips populations at low levels.

Figure 21. Sticky cards: blue (left) and yellow (right)

When setting up a monitoring program, use 1 trap per 100-200 m 2 . The exact number will depend on the layout of the greenhouse. A large open range will require a lower total density of cards than a greenhouse made up of a several smaller areas. Place the sticky cards in a grid pattern throughout the greenhouse. Check the traps weekly and record the average number of thrips per trap. Be aware that this is not an absolute measure of the population; rather, it measures increases and decreases in thrips numbers throughout the year. As you become more aware of how the numbers on sticky cards relate to the population in the crop, you can use the monitoring data to help you make pest management decisions. In greenhouse ornamentals, visually inspecting simple flowers, such as impatiens, can provide good estimates of thrips numbers in the crop. However, in more complex flowers, visual counts can be less reliable. In sweet pepper and cucumber crops, precision-level sampling programs have been developed for monitoring adult western flower thrips in the flowers. These sampling programs vary the number of samples taken according to the population level of the pest and accurately predict the pest density to set precision levels. Contact the OMAF Greenhouse Pest Management Specialist or your IPM Consultant for more detailed information before implementing your monitoring program.

Cultural control

Sanitation is the first and most important step in implementing an effective pest management program. Effective sanitation will reduce or even eliminate thrips as a pest problem. For example, in cut roses, removing all flower buds (including non-marketable flowers) can significantly reduce thrips populations in that crop. Cultural control measures also include maintaining a healthy crop and an optimal greenhouse environment (such as 80% relative humidity), creating less favourable conditions for a rapid increase in the density of thrips populations.

Physical control

An influx of outside pests, including thrips, can overwhelm your greenhouse IPM program, making it difficult to plan ahead. To prevent this, use screens to restrict the movement of insects into the greenhouse. For more information on screening, see the OMAF Factsheet Screening of Greenhouses for Insect Exclusion.

Biological control

Because thrips have developed resistance to most registered pesticides, biological control is now the primary strategy for controlling thrips in greenhouse crop production. Biological control agents include predatory mites such as:

  • Neoseiulus (= Amblyseius) cucumeris
  • Amblyseius swirskii
  • Iphesius (= Amblyseius) degenerans
  • Stratiolaelaps scimitus (= Hypoaspis miles)
  • Gaeolaelaps gillespiei
  • Gaeolaelaps aculeifer (= Hypoaspis aculeifer)
  • minute pirate bugs (Orius insidiosus)
  • nematodes (Steinernema feltiae)
  • the fungal insect pathogen Beauveria bassiana

N. cucumeris (see Figure 22) and A. swirskii are the most extensively used predatory mites and look very similar. These mites control western flower thrips on the foliage by feeding on the first instar larvae. A. swirskii can also feed to a lesser extent on second instar thrips. As such, it takes a number of weeks for their impact to be seen in the greenhouse, and it is unlikely that they will completely eliminate thrips populations. The life cycle for N. cucumeris is completed in approximately 10 days at 20°C and 6 days at 25°C. A. swirskii has a higher optimal temperature for development than A. cucumeris and performs better in summer conditions. Its development time is similar to that of A. cucumeris but depends on the number and type of prey available.

Figure 22. Adult and egg of Neoseiulus cucumeris.

Predatory mites should be introduced at the beginning of the crop or as soon as thrips are detected. Sanitation at the beginning and end of a cropping season is extremely important and will delay any thrips infestation until the biological control agents can be effective. Regular introductions of either N. cucumeris or A. swirskii are necessary, either by dispersing bran mixed with mites on plants or growing medium or by hanging a slow-release rearing sachet on plants (see Figure 23). The sachet system provides a continuous release of mites to the plant and should be replaced monthly. In ornamental production, many growers are now using new slow-release mini-sachets, which reduce the cost substantially and can be used on individual containers (e.g., hanging baskets or even 15-cm pots). Applying a supplemental food source such as apple pollen to chrysanthemum may help A. swirskii to get established when thrips levels are low. The number of introductions depends on the crop and level of thrips infestation (contact the OMAF Greenhouse Pest Management Specialist or your IPM Consultant). Control of the thrips should be achieved in 5-9 weeks. When using N. cucumeris or A. swirskii, it is important to maintain at least 70% relative humidity in the greenhouse and avoid using any persistent pesticides for several months before introducing the mites.

Figure 23. Methods for introducing predatory mites: directly on the plants (top), using a bag rearing system (middle) and piling bran on rockwool cubes or other growing medium (bottom).

Orius is effective in controlling thrips (see Figure 24). Unlike N. cucumeris and A. swirskii, Orius will feed on all stages of thrips. It is often found in the flowers, where it feeds on pollen as an alternative food source. Because pollen is not often present in ornamental crops, Orius is not as effective in flower crops as it is in vegetables. However, recent research has shown ornamental peppers can be used as a banker plant for Orius in other ornamental crops, allowing a population to establish, develop and disperse within the greenhouse. Some ornamental and vegetable growers are using this strategy to take advantage of the control potential offered by Orius. Development time for Orius from egg to adult is 31 days at 20°C and 19 days at 25°C. Orius enters reproductive diapause when there are less than 12 hr of light per day. Thus, Orius is only effective as a biological control agent from March to September.

Figure 24. Adult Orius preying on western flower thrips.

Orius is best released when the pest level is low. One or two releases are usually enough to provide thrips control in approximately 3-5 weeks, depending on the level of thrips and the type of host crop. For greenhouse vegetable crops, Orius is most successfully used on peppers and cucumber. Introduce adults in several locations where they can naturally disperse by flying throughout the greenhouse. Flower sampling is the best method to monitor the presence of Orius.

Iphesius degenerans (see Figure 25) differs from N. cucumeris and A. swirskii in its appearance and its ability to tolerate less humid conditions. It is dark and very agile. Because it reproduces very well on pollen, it performs best in crops with a pollen source (e.g., greenhouse peppers) but is unlikely to be the best option for floricultural crops.

Figure 25. The predatory mite Iphesius degenerans.

Stratiolaelaps scimitus and Gaeolaelaps gillespiei (see Figure 26) are soil-dwelling predatory mites that feed on a variety of soil organisms, including thrips pupae. Apply either of these to the growing medium (e.g., rockwool, peat mixes) once only, at the beginning of the crop. Although it is difficult to determine the exact impact of these predators on a thrips population, research has estimated they can kill up to 30% of pupae. Because they are unlikely to provide enough control on their own, they are better used in combination with other predators.

Figure 26. The predatory mite Stratiolaelaps scimitus.

Nematodes are frequently used by ornamental growers in Ontario. Research in Ontario and Europe has shown that they effectively control thrips pupae when applied to the growing medium on a weekly basis. To reduce costs, this is best done by overhead application in propagation, when the plants are pot tight.

Beauveria bassiana is a fungal pathogen of thrips. It is usually mixed in water and applied as a spray. Like many fungi, it is more effective under high humidity. Therefore, to treat ornamentals, it is most often applied in propagation. In vegetables, it can be either sprayed onto the crop or distributed via bumble bees that are supplied with hives specially equipped with dispensing trays. These trays contain Beauveria bassiana spores that are diluted with a powdered carrier. The bees must walk through the trays to leave the hives. In the process, some of the spore mixture sticks to their bodies. The spores become distributed in the crop when the bees fly in search of nectar and pollen and when they pollinate the crops. When thrips come into contact with spores on the crop surface, they become infected and die.

Chemical control

Chemical control of western flower thrips can be difficult. They are resistant to most pesticides and feed deep within the flower head or on developing leaves. This makes them a difficult target for insecticides, so thorough coverage is essential. If you use pesticides to control thrips, follow these general guidelines:

  • Begin applications early, before the thrips population grows too large. Thrips are more easily managed when population levels are low.
  • Although it is important to rotate chemical classes, use only one chemical class for the duration of the thrips’ life cycle. This generally means using a different class every 2-3 weeks, depending on the time of year.
  • Apply pesticides in early morning or late afternoon, when flight activity of thrips is at a peak. This increases exposure of the thrips to the pesticides.

For more information:

This Factsheet was authored by Graeme Murphy, Greenhouse Floriculture IPM Specialist, Economic Development Division, OMAF, Vineland; Gillian Ferguson, Greenhouse Vegetable IPM Specialist, Economic Development Division, OMAF, Harrow; and Les Shipp, Greenhouse Entomologist, Agriculture and Agri-Food Canada, Harrow.


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