Tomato Spotted Wilt Virus: Thrips Control in Tomatoes

Tomato spotted wilt virus (TSWV) has no cure once a plant is infected. You can only slow its spread, and that means managing the thrips that carry it. This guide is for tomato growers who want to know which sprays, mulches and varieties work against TSWV, when to act, and what it costs. It draws on the University of California and University of Florida extension guides for tomato.

In short: plant varieties with the Sw-5 resistance gene, start with clean transplants, keep weeds down, use UV-reflective mulch, scout flowers each week, and rotate insecticide groups once thrips pass the threshold. Foliar sprays may limit in-field spread to some extent, but they do not protect a plant that is already infected. Our separate pages cover broader thrips control and other crop diseases; this page stays with tomatoes and the virus.

TSWV and thrips in tomatoes at a glance

What is tomato spotted wilt virus?

TSWV is a plant virus in the tospovirus group. It is spread by thrips, tiny insects that feed on leaves and flowers. According to the UC IPM tomato spotted wilt guidelines, the virus has an extremely wide host range, including many weeds and ornamentals as well as crops. It is one of the few plant viruses that infects both broadleaf plants and grass-like plants, such as tomatoes and onions.

The virus is found in many growing regions. UF/IFAS puts its known host range at over 926 plant species, and the Wikipedia summary of TSWV research gives over 1,000. Epidemics occur in many growing regions worldwide.

Crops affected by TSWV

Tomato is the best-known host, but the same virus hits many other crops. Growers who rotate or grow these crops nearby should treat them as part of the same problem:

• Tomato
• Pepper
• Potato
• Eggplant
• Lettuce and endive
• Celery
• Bean and cowpea
• Spinach
• Tobacco
• Peanut

In California, UC IPM lists tomato, pepper, lettuce, radicchio and fava bean as key crop hosts, and cheeseweed, sowthistle and prickly lettuce as key weed hosts.

Related tospoviruses

Two newer viruses look and behave much like TSWV. Tomato chlorotic spot virus (TCSV) and groundnut ringspot virus (GRSV) occur with TSWV in tomato and pepper in South Florida. The UF/IFAS guide to managing thrips and tospoviruses in tomato notes that TSWV is the only tospovirus found in tomato in North Florida and most of the continental US, apart from first reports of TCSV in Ohio and GRSV in South Carolina and New York. The same thrips spread all three, so the same steps help against all of them.

Symptoms of tomato spotted wilt virus

Symptoms change with the crop, plant age, temperature and the virus strain. Early detection matters because infected plants are a source for more spread.

Symptoms on tomato

• Bronzing of the upper side of young leaves, which later develop dark, dead spots.
• Cupped leaves that curl downward, sometimes with tip dieback.
• Ring spots on ripe fruit: pale yellow spots and blotches, often with rings inside each other.
• Raised areas on green fruit with faint rings.
• Stunting and wilting of shoot tips, mainly when plants are infected young.

UC IPM describes bronzing, necrotic spots, downward cupping and ring spots on fruit as the main signs in tomato.

General signs across crops

• Stunted growth and less vigour
• Yellow or bronze leaves
• Brown or black spots on leaves, stems and fruit
• Wilting of part or all of the plant
• Rings of discoloured tissue on leaves and fruit
• Misshapen leaves, stems or fruit

Signs in other key crops

• Pepper: yellow ring patterns on leaves, dead streaks on stems, deformed fruit.
• Potato: yellowing and curling of upper leaves, dead spots on tubers.
• Lettuce: yellow spots and dead patches on leaves, poor head formation.

Other problems can look similar. Nutrient shortages, herbicide drift and other viruses can cause bronzing or spots. A lab test from your state or regional plant clinic is the safest way to confirm TSWV before you change your spray plan.

How thrips spread TSWV

Thrips are very small, slender insects. Mature western flower thrips are about 1.5 mm long, and onion thrips about 1.3 mm. Both have two pairs of wings fringed with long hairs. You need a hand lens to see them well.

The way they carry the virus is unusual. A thrips must pick up TSWV as a young larva, while feeding on an infected plant. The virus then grows inside the insect. Once that larva becomes an adult, it can pass the virus to new plants for the rest of its life. Adults that did not feed on an infected plant as larvae cannot spread the virus. The virus does not pass from females to their eggs.

UC IPM’s tomato thrips guidelines say the adult thrips can transmit the virus for the remainder of their lives, which can last 30 to 45 days. Adults are the only stage that flies, but they are weak fliers. Wind, clothing and moved plants carry them between fields.

Thrips life cycle

• Egg: females lay eggs inside leaves, stems or flowers.
• Larva: two larval stages feed on plant cells. This is when the virus is picked up.
• Pupa: a non-feeding stage, usually in soil or plant debris.
• Adult: the winged stage that moves the virus from plant to plant.

At warm temperatures, each stage is short. The UF/IFAS guide gives about 6 days for eggs, 5 for larvae and 5 for pupae. Development slows in cool weather; thrips need at least about 10 °C (50 °F) to develop. A full generation can take 30 to 40 days in a North Florida winter.

Which thrips species matter

Not every thrips is a threat. The western flower thrips is the most efficient vector of TSWV. Onion thrips and some other species also carry it. UF/IFAS notes that eastern flower thrips and melon thrips are not able to spread TSWV (it lists chilli thrips as a non-vector too, though UC IPM names chili thrips among the vectors), and that eastern and Florida flower thrips do little harm to tomato. In fact, these harmless species compete with western flower thrips, so killing them with broad sprays can make the problem worse.

Factors that raise thrips numbers

• Warm, dry weather, which speeds up each generation.
• Nearby crops such as onions, garlic or cereals, where thrips build up and then move as those crops dry down.
• Greenhouses with cut flowers close to tomato fields.
• Weeds in and around the field.
• Too much nitrogen: UF/IFAS found that fertilising above recommended rates raised thrips numbers and TSWV incidence.

Chart

Thrips development time by stage

Days per stage at optimal temperatures, tomato-infesting flower thrips

▶ Western flower thrips: the damage they do to fruit and vegetable crops and how growers manage them. · 15:42

Thrips control in tomatoes before planting

Most of the work against TSWV happens before the first spray. Infection that starts early in the season causes the biggest losses, so the goal is to keep the virus and its vector out of young plants.

Choose resistant varieties

Plant tomato varieties with the Sw-5 resistance gene. UC IPM states that resistant varieties generally do not require insecticide applications for thrips to control tomato spotted wilt. UF/IFAS adds that Sw-5 also gives cross-resistance to TCSV and GRSV in South Florida trials.

There is a catch. Resistance-breaking strains of TSWV that overcome Sw-5 have been reported in Georgia, North Carolina and Alabama. Resistant varieties are still the base of any plan, but they need back-up from the other steps below.

Start with clean transplants

Buy transplants from greenhouses that manage thrips, and inspect them before planting. Manage thrips on transplants before they go to the field.

Plan the field layout

If you can, avoid planting tomatoes next to onions, garlic, cereals or cut-flower greenhouses. Avoid fields next to crops already infected with TSWV.

Lay UV-reflective mulch

Metalized, UV-reflective plastic mulch confuses thrips as they land, so fewer reach the young plants. UF/IFAS calls it very effective for reducing thrips numbers. Its effect fades as the canopy grows over the mulch. Some sprays also dull it: a single application of copper and mancozeb fungicide can reduce the reflectance by approximately 49%, according to UF/IFAS. Clemson HGIC also suggests silver or aluminium mulch for home gardens.

Chart

UV-reflective mulch after one fungicide spray

Relative reflectance of metalized mulch, before and after a single copper + mancozeb application (before = 100)

Organic treatment and control strategies

Organic growers have fewer tools against thrips, so cultural steps carry more weight. For a wider look at soil and crop practices, see our page on organic farming.

Cultural practices

• Weed control: remove weed hosts in and around fields, and control weeds and volunteers on fallow or idle land near next year’s field.
• Rogue early: UC IPM suggests removing infected plants at the seedling stage.
• Crop clean-up: promptly remove and destroy old tomato plants and other host crops after harvest.
• Rotation and spacing: keep new plantings away from old host crops.
• Balanced nitrogen: do not go above recommended rates.

Biological control

Natural enemies such as minute pirate bugs, predatory mites and lacewings feed on thrips. They do best when broad-spectrum sprays are avoided. Releases of predatory mites are more common in greenhouses than in open fields. Native flower thrips that compete with western flower thrips also count as a form of natural control.

Organic insecticides

Spinosad (Entrust) is listed by UC IPM as acceptable for use on organically grown produce. In a set of field trials published in Insects in 2020, Entrust at the standard rate reduced thrips’ densities by an overall 65.6% across onion and zinnia trials. That was well above the other organic products tested. Azera (pyrethrins plus azadirachtin), PyGanic (pyrethrins) and Neemix (azadirachtin) gave much weaker control in the same trials, about 19% to 29%. Read the result as a guide to how products ranked, not as a promise for your field.

Other natural insecticide options listed by UF/IFAS include terpenes (Requiem), kaolin clay (Surround) and Chromobacterium (Grandevo). Most give weak suppression and work best in a rotation.

Chart

Thrips control by organic insecticide

Mean % control versus untreated plots, weekly sprays, onion and flower thrips trials

Conventional control methods: tomato thrips chemical control

There is no chemical that cures TSWV. Sprays work on the thrips, and only slow the virus. UC IPM says foliar sprays early in the season and continuing through the season as needed may limit in-field spread to some extent. It also notes that soil-applied imidacloprid has not been shown to lower virus incidence.

Products listed by UC IPM and UF/IFAS

UF/IFAS also lists isocycloseram (Incipio) as a new active ingredient with high efficacy on thrips. Product registrations differ by country and state, and a product listed here may not be allowed where you farm.

Check the label and local rules. Always read and follow the product label, including rates, re-entry and pre-harvest intervals. Rules differ between the US, Canada, the UK, the EU, Australia and New Zealand. Ask your local extension service or a licensed crop adviser before you spray.

Resistance management

Western flower thrips develop resistance fast. The IRAC western flower thrips page lists field resistance to several major insecticide classes, including spinosyns in the US. UF/IFAS reports resistance in Florida to pyrethroids, carbamates and organophosphates, and some resistance to spinosyns.

• Rotate between IRAC groups.
• UC IPM advises not to use the same group more than twice per season.
• Spray only when scouting shows the threshold is reached.
• Favour products that spare natural enemies early in the season.

Monitoring and early detection

Good scouting tells you when to act and when to save the cost of a spray.

Scouting and thresholds

Thrips live in flowers, so sample flowers. UF/IFAS suggests placing a few flowers in a vial of 70% alcohol and counting the thrips. Its threshold for tomato is about one western flower thrips adult per flower, or about two larvae per fruit. Act once your field average reaches these numbers. Blue or bright yellow sticky traps help show when adults arrive (UC IPM).

Satellite crop monitoring

Satellite images will not show a single thrips. They can show patches of a field where plant vigour drops, which helps you decide where to walk first. The Farmonaut Satellite + AI Farm Monitoring System gives vegetation maps of each field through the season on web, Android and iOS. Pair those maps with flower counts and lab tests, not instead of them.

Lab confirmation

Quick test strips and lab tests can confirm TSWV in leaf samples. Many universities run plant clinics that test samples for a fee.

▶ How satellite NDVI maps show stressed parts of a large farm. · 4:09

Is a thrips spray program worth it?

Spraying costs money. The question is whether the plants you save from the virus pay for the sprays. The calculator below lets you test this with your own numbers. The default values are examples only, set close to break-even.

Try the numbers for your own farm below.

Case studies: managing TSWV in different crops

The same plan works across hosts, with small changes.

Tomato in Florida

In North Florida, western flower thrips is the main pest of spring tomatoes. UF/IFAS describes a program that combines resistant varieties, UV-reflective mulch and insecticides aimed at larvae; three to five weekly sprays are sometimes justified in North Florida. Weekly applications are usually enough to prevent secondary spread.

Tomato in California

UC IPM notes that insecticide treatments for thrips are usually not warranted in the Imperial Valley but may be needed in the Central Valley and coastal areas. Recent outbreaks in the San Joaquin Valley have been linked to nearby infested crops or weeds.

Pepper and eggplant

UF/IFAS runs a similar thrips program for pepper and eggplant, based on scouting flowers, conserving natural enemies and using reflective mulch.

Future perspectives in TSWV management

• New resistance genes to replace or back up Sw-5 where resistance-breaking strains spread.
• New insecticides, such as isocycloseram, that add another group to the rotation.
• Better forecasting of thrips flights from weather data.
• Remote sensing to target scouting at weak patches.
• Climate-resilient farming, since warmer seasons speed up thrips generations.

For a closer look at a related question, see our guide: How to Control Thrips with Best Systemic Insecticides.

Conclusion

TSWV in tomatoes is a thrips problem first. Start with Sw-5 varieties and clean transplants, keep weeds and old crops out, use reflective mulch, scout flowers weekly, and rotate insecticide groups when thrips pass the threshold. These steps lower the risk; none removes it. The challenges in agriculture from pests and viruses change each season, so review your plan with your local extension service every year.

Related pages on our site: app page, API services, developer documentation, apps for Android and iOS.

FAQs

What is the best chemical control for tomato spotted wilt virus?

No chemical cures TSWV. Sprays target the thrips. UC IPM lists spinetoram and spinosad at the top of its tomato thrips table. Rotate groups and follow the label.

Can a plant recover from TSWV?

No. Once infected, a plant stays infected. Remove infected seedlings early so they do not feed more thrips larvae.

Do resistant tomato varieties still need sprays?

Usually less. UC IPM says Sw-5 varieties generally do not need thrips sprays for TSWV. Watch for resistance-breaking strains, reported in some southeastern US states.

Does soil-applied imidacloprid stop TSWV?

UC IPM says soil-applied imidacloprid has not been shown to lower virus incidence in tomato.

What is the thrips threshold in tomato?

UF/IFAS uses about one western flower thrips adult per flower, or about two larvae per fruit.