Western flower thrips are an invasive insect pest in agriculture. In many greenhouse crops, western flower thrips (Frankliniella occidentalis) are the most damaging thrips species. Western flower thrips are found on various plants, including many vegetables and ornamentals in greenhouses and on multiple herbs.
How to control western flower thrips in chilli crop
What do western flower thrips eat?
Western flower thrips feed on flowers and plants by inserting their modified left mandible into tissue and sucking fluid from the cells. Therefore, oviposition and feeding scars reduce ornamental plants’ aesthetic quality and marketability. If thrips feed on expanded leaves or petal surfaces, the damage appears as minor marks or silver spots.
Thrips leave greenish-black spots on plant surfaces as the thrips feed. If western flower thrips hatch inside developing buds, the damaged cells fail to produce as the flower or leaf expands, resulting in deformed flowers or leaves. Some plants grow a localized dead or discolored spot where the thrips eggs are inserted into the plant tissue.
|Western flower thrips
|Asia, North, South, Central America, the Caribbean, Europe, and Oceania. It has been recorded in Australia and New Zealand but not in Pacific Island countries.
Western flower thrips affected in Chilli crops in Nalgonda
Western flower thrips affected Chilli crops on thousands of acres in erstwhile Nalgonda. Chilli growers faced heavy losses due to western flower thrip’s attack on horticultural crops in thousands of acres of land in the Nalgonda and Suryapet districts. Farmers cultivated Chilli crops on 27,472 acres in the Suryapet district and 3,073 acres in Nalgonda.
Suryapet and Nalgonda districts produce more than 28 lakh tonnes of Chilli. This is because an acre will have an average of 12 tons of pepper crops. But, due to the western flower thrips attack, farmers could get only 10 percent of the expected yield, about six quintals per acre. This situation also affected the prices of Green Chilies in the vegetable markets. As a result, the cost of green chilies price has increased to Rs 80 per kg.
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Western flower thrips damage in Chilli crops
Western flower thrips vectors feed on plant viruses. Thrips cause injury by puncturing leaves and sucking plant sap. Punctured leaves have a silvery appearance that eventually turns to brown spots and can be confused with blown sand damage. Look for small, black spots in the damaged region to confirm thrips damage. Western flower thrips is the important vector of the thrips species and tomato spotted wilt virus to impatiens necrotic spot virus. Only the larval stage can get these tospoviruses.
They ingest the tospoviruses when they feed on infected Chilli plants and remain infectious for life. As the juveniles mature and develop wings, they fly to other crops and then spread the pathogen. Plant damage is similar to onion thrips. Still, this species can transmit different viruses, including tomato spotted wilt virus, tomato chlorotic spot virus, impatiens necrotic spot virus, and mangoant ring spot virus.
These viruses can cause significant plant damage and mortality to plants. A specialist should identify these diseases as symptoms vary greatly among plant species and environmental conditions. When bringing new plant material into your greenhouse or high tunnel, you should inspect it thoroughly to prevent accidental infection.
Western flower thrips description
|Adults are about 1 mm long, with females larger than males. The female varies from yellow to dark brown and has a more rounded abdomen. The male is always pale yellow and has a narrower abdomen.
|The yellow eggs cannot be seen because they are laid in plant tissue.
|Larvae develop through two instars and are distinctly yellow. Second instars become whitish before molting.
|Prepupae and Pupae
|Both prepupa and pupa are yellowish, quiescent non-feeding stages. The antennae and wing pads are typical of most thrip species.
Life cycle and appearance of western flower thrips
Western flower thrips develop through six stages: egg, two larval instars, prepupa, pupa, and finally, the adult moth. Western flower thrips eggs are laid in the tender parts of leaves, flower petals, and stems. They are inserted into the plant tissue with a saw-like ovipositor. Larvae are almost transparent white or yellowish to orange-yellow, with a large head and red eyes. Adult females are very variable in coloration. They range from nearly white to yellowish-orange to almost black.
Western flower thrips usually pupate in the soil, although pupae can also be found on leaves, flowers, or other shelters. Pre-pupal and pupal instars can be distinguished by their developing wing buds. Compared to the prepupa, the pupa has more extended, more developed wing buds and longer antennae curved above the head. Pre-pupal and pupal instars do not feed and only move when disturbed. Both pairs of wings are fully developed in adults.
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Western flower thrips control management
Plant management in and around plants, cultural practices, and biological control are essential factors in reducing the potential for damage from these thrips. When thrips are present on a crop, insecticides are the only viable control alternative.
Natural enemies are found feeding on thrips, including minute pirate bugs, predaceous mites, and lacewings. However, these natural enemies are susceptible to pesticide sprays and may not be present in fields where pesticides have been applied.
- Monitor fields regularly. Leaf stippling and distortion are good indicators of thrips feeding. Western flower thrips also leave black spots on the surface of leaves and flowers. Growers often feel damage before the pest is detected. However, the thrips population at this point is probably too high and has caused irreparable damage to the plants. These plants may also be infected with INSV or other viruses. Therefore, it is essential to monitor the thrips rather than wait for the damage to appear.
- Western flower thrips can be monitored with yellow sticky cards, although blue is generally more attractive to thrips and attractive to other insects. The yellow sticky cards should be staked above the crop canopy in the greenhouse. Move sticky cards up as canopy height increases.
- Visual inspection of plants is another easy method to detect thrips. Firstly, look on the underside of leaves for fast-moving larvae and fecal matter. Then, tap the flower heads on a sheet of paper and look for the thrips that have died.
- Indicator plants can also help monitor thrips and tospoviruses such as INSV. Indicator plants are species that are more attractive to thrips and Tospovirus than the production plant. Because it is more attractive and susceptible, the indicator plant will attract thrips and show signs of feeding and virus infection before harvest. This provides early warning to growers of developing thrips populations.
- Monitoring WFT populations is essential to detect early WFT problems in crops and to determine whether control measures have been effective. Early detection is necessary because feeding symptoms often go unnoticed until serious damage is done. Small populations are very easy to control compared to large populations.
- Counting thrips on affected plants is time-consuming and not cost-effective in commercial crops. So instead, counts of thrips on plants may be based on presence-absence assessments, in which the sampler notes the proportion of samples with thrips rather than counting the actual number of thrips in each sample.
- In the low desert region, thrips are a problem in January-March. However, in coastal areas, thrips numbers increase in April and continue throughout the year. Thrips usually feed on the undersides of leaves, found overall in the plant, especially in hard-to-examine and treat areas such as leaf folds.
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Several methods are available for monitoring thrips:
Carefully examine plant parts for signs of thrips and feeding, including folds in leaf tissue near the plant base. If 3 to 5 thrips are mainly found on a small plant, there are probably three times more hidden within the leaf layers or dispersed from the plant.
Place blue or yellow sticky traps along field edges when daytime temperatures exceed 17 to 18°C to indicate when adult thrips begin to migrate from adjacent plants into the field. Plants from different parts of the area on a sheet, tray, or sticky surface can be counted and identified. The best time to kill is in the morning, as adults are less active at this time.
- Western flower thrips feed on herbs, ornamentals, or other plants around fields that may be infected with the virus. Then they fly into the area and transmit the virus.
- Remove weeds and other potential hosts of Tospovirus from around fields.
- Quickly remove or shake plant debris from cut fields to reduce the spread of thrips in small fields.
- Avoid planting downstream crops that repel thrips, such as small grain crops.
- Sprinkler irrigation can help suppress thrips as it washes them off the plants.
- Cultural control options aim to prevent infection and reduce spread. Sanitation practices such as removing weeds, old plant material, and growing medium debris are the first lines of defense in reducing western flower thrips problems.
- Certain weeds, especially those in the Compositae and Solanaceae families and with yellow flowers, attract adults and serve as reservoirs for adult-transmitted (vectored) viruses. Therefore, weeds must be removed from within and around the perimeter of the greenhouse.
Officially acceptable organic practices to control western flower thrips
Use sprays of spinosad, azadirachtin, Isaria fumosorosa, Beauveria bassiana, or a combination of these products on organically certified plants.
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Chemical control of western flower thrips
Insecticide sprays may be required if thrips cause physical damage to the crop. However, there are problems with using insecticides to control thrips. Firstly, insects hide in the leaves of flowers and twigs. Second, the eggs are laid in the leaves, making it difficult for the spray to reach them. And third, thrips become increasingly resistant to insecticides, to the extent that there are large differences in the thrips sensitivity populations to commonly used products.
Use insecticides as follows, but remember that frequent use of broad-spectrum synthetic insecticides can also lead to insecticide resistance in thrips populations:
Use horticultural oil (made from petroleum), white oil (made from vegetable oil), or a soapy solution. The spray will not kill all the thrips but will suppress the population enough that predators and parasite numbers begin to build up and control them.
Several soaps or oil sprays will be required to bring thrips under control. It is important to thoroughly spray the undersides of leaves and terminal buds as these are areas where thrips congregate. It is best to spray between 4 and 6 pm to reduce the chance of sunburning leaves and flowers. Test the soap and oil on a few leaves or flowers.
- 3 tablespoons (1/3 cup) cooking oil in 4 liters of water.
- ½ teaspoon of dish soap.
- Shake well and use.
- Use soap (pure soap, not soap).
- 5 tablespoons of soap in 4 liters of water, or
- 2 tablespoons of dishwashing liquid in 4 liters of water.
Commercial horticultural oils can also be used. White oil, soap, and horticultural oil sprays block the breathing holes of insects, causing suffocation and death. Sprinkle under the leaves; the oil should contact the insects. Use neem to discourage adults from feeding and laying eggs on plants. Do not use broad-spectrum insecticides such as dimethoate, malathion, and permethrin. They have a more significant effect on the natural enemies of western flower thrips than on thrips.
Western flower thrips management in different plant growth stages
Check the seedlings to make sure they are free of symptoms. Growers should grow their seed or only source seed from nurseries screened with thrips-grade mesh and monitored for western flower thrips and TSWV. Remove weeds in and around crops. Thrips and TSWV have extensive host ranges, including many weeds. Grasses, however, are poor hosts and can be used around greenhouses and nurseries to reduce the need to manage other weeds that are hosts.
A 10 meters strip around greenhouses and nurseries or crops is sufficient. Bare ground is also adequate. Do not plant new crops with thrips-infected crops. Do not plant the same crop on the same land without a break: use rotation. And don’t plant new crops downstream from thrips-infected individuals. These steps are essential if an “old” crop is infected with TSWV.
- Monitor routinely for thrips. Use yellow or blue sticky traps placed about 10 cm above the crop, and check weekly.
- Prune any plants showing symptoms of the virus.
- Collect and dispose of crop debris by burying or burning it.
- Prevention tips for western flower thrips damage.
It is easier to prevent an infestation compared to managing an established one. Therefore, growers should try to reduce the number of thrips in greenhouses at the end of the growing season. Otherwise, thrips in the greenhouse will overwinter and result in a large population at the start of the next growing season. Growers should also avoid purchasing plants infected with thrips and introducing them into their greenhouse.
Tapping incoming plant flowers onto a white piece of paper is a quick way to screen incoming shipments. If present, thrips will die and appear on the white paper. WFT prefers to feed on flowers, so the longer a plant can be grown without flowers, the lower the thrips population will be. For example, some vegetatively propagated flower crop stock plants can be grown without flowers. In such cases, suppressing flowers or hand-picking and removing flowers as they develop can facilitate control of WFT populations.
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Weeds such as Galinsoga sp. and chickweed can serve as important reservoirs of both Tospovirus and thrips in greenhouse crops. Therefore, weeds inside and outside the greenhouse (within a 10-meter radius) should be destroyed as an essential part of thrips suppression. In addition, ornamental flowering plants should be removed to reduce the chances of large thrip populations developing adjacent to greenhouses.
Western flower thrips control with yellow traps
Alternatively, thrips can be monitored with a sticky trap or “tapping method.” Gently tapping the flowers or foliage of a few plants on a sheet of white paper will dislodge the thrips and make them visible. Tapping of such plants can be used to find if thrips are present and to estimate their numbers. For more precision, sticky nets can be used. Traps should be placed just above the crop canopy, about one per 200 square meters (about 2,000 square feet) in larger houses.
In houses, less than 2,000 square feet, use at least three traps, regardless of the size of the home. Place traps near doors, vents, and plants susceptible to thrips. Both the yellow and blue sticky traps will catch the WFT. Blue traps are a bit more practical. However, yellow traps are also attractive to whiteflies and other flying greenhouse pests, which can be essential for overall pest monitoring.
Western flower thrips are the primary thrips species encountered by greenhouse producers; they are highly polyphagous, feeding on a wide variety of horticultural crops grown in commercial greenhouses. Thrips feed using mouthparts to pierce plant cells and suck their contents. Damaged plant cells collapse, resulting in wilting of plants, wilting of flowers, or silvery spots and wrinkled leaves.
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