The current hot, dry weather can be conducive to outbreaks of thrips. Thrips are very small, slender, elongate, cigar-shaped insects, about 1 mm (1/25 inch) long. They differ from other insects by having thin strap-like wings that are fringed with hairs. The wings are usually folded lengthwise over the back when they are resting or feeding, as shown in the image below. They have asymmetrical mouthparts that have a well-developed left mandible and an underdeveloped right mandible. They feed by piercing plant cells by the mandible then sucking sap that oozes out of the punctured cells.

Smaller, tan thrips on left is the onion thrips (Thrips tabaci). Larger yellowish thrips on the right is the western flower thrips (Frankliniella occidentalis).
Photo by Alton N. Sparks, Jr., University of Georgia, Bugwood.org

Thrips generally have flowers as their preferred plant part but they also feed on leaves and fruits. They are found in flowers of many ornamental plants but also on various vegetables and fruit crops, including tomatoes, peppers, and cucumbers. Thrips are often overlooked due to their small size and their tendency to hide in protected places. When present at low density, thrips are usually not harmful to crops, but when they reach higher density, they can feed on the surface of fruits and cause injury.

The thrips species that infests many outdoor crops is Frankliniella tritici, which has the official common name of ‘flower thrips’, but which is widely known as the eastern flower thrips. The thrips species that is most common in greenhouses is the western flower thrips, Frankliniella occidentalis, which is generally much more difficult to control than other thrips because it is not highly susceptible to most insecticides. A third common species is the onion thrips (Thrips tabaci), which is a serious pest of dry bulb onions and green onions as well as cabbage.

Natural enemies of thrips are Orius flower bugs, which are very small predatory true bugs that prey on thrips as adults and nymphs. Orius insidiosus is a common species in Ohio that is frequently found on the moist fresh silks of sweet corn and on flowers of Queen Anne’s lace. There are also several species of predatory mites that prey on thrips.

Insidious flower bug (Orius insidiosus) feeding on an insect egg.
Photo by John Ruberson, Kansas State University, Bugwood.org

Orius bug feeding on a thrips.
Photo by Robert Webster / xpda.com / CC-BY-SA-4.0 via Wikimedia Commons.

Thrips can be monitored by shaking flowers over a paper or into a cup or zip-top sandwich bag, which should then be examined for the dislodged thrips running around on the surface. Action thresholds have not been developed for most crops, but on strawberry we use a threshold of 2 thrips per flower.

In the past, pyrethroid insecticides provided control of thrips, but there are widespread observations that pyrethroids are no longer very effective for thrips control at most locations. Newer insecticides used to control thrips on conventional crops include Radiant, Assail, and several others, as shown in Table 1 below. Products for thrips control on organic crops include Entrust and various others as shown in Table 2 below.

If a biological control approach is preferred, natural enemies are available for purchase from commercial insectaries for thrips control: Orius (predatory flower bugs), and two species of predatory mites: Amblyseius cucumeris and Ambylseius swirskii. Beneficial nematodes such as Steinernema feltiae are another option. Microbial options include sprays of the beneficial fungus Beauveria bassiana, which is found in several commercial products. Biocontrol is not feasible to begin once the thrips population is large but can be planned in advance at locations that have a consistent problem with thrips.

-Celeste Welty, Extension Entomologist

Although strawberries are not considered to be a vegetable crop, using VegNet is a good way to get information out to growers who have both vegetables and berry crops.

Strawberry fruit that have been injured by thrips are a dull or bronzed color, and are often small, hard, seedy, and fail to ripen. They can cause uneven maturity of fruit. When severe, their injury can make the strawberry crop completely unmarketable.

Thrips are an occasional serious pest of strawberries. This means that in most years, they are not a problem, but in some years, they can be a big problem. One such year was 2018 for some growers in Ohio. As far as we understand the problem, the reason for variability from year to year has to do with weather systems. In some years, conditions are right that large numbers of small insects such as thrips and leafhoppers are carried on strong weather fronts moving from the southern USA into Ohio during the time that strawberries are in bloom. In other years, this long-distance movement does not happen at all, or happens later, at a time when strawberries are no longer in bloom.

Thrips are small, slender, elongate, cigar-shaped insects, about 1 mm (1/25 inch) long. They differ from other insects by having narrow strap-like wings that are fringed with hairs (Figure 1). The wings are usually folded lengthwise over the back when they are resting or feeding (Figure 2). They have asymmetrical mouthparts (Figure 3) that have a well-developed left mandible and an underdeveloped right mandible. They feed by piercing plant cells by the mandible then sucking sap that oozes out of the punctured cells. Thrips generally have flowers as their preferred plant part. They are found in flowers of many species of plants. Thrips are often overlooked due to their small size and their tendency to hide in protected places. When present at low density, thrips are often not harmful to plants.

The thrips species that infests outdoor strawberries is Frankliniella tritici, which has the official common name of ‘flower thrips’, but which is widely known as the eastern flower thrips. It does not tolerate cold weather well so does not survive winter well in places like Ohio. The adults are yellowish brown, and the larvae are whitish-yellow. The larvae are similar to the adults in shape but smaller and without wings. On strawberries, the infestation starts by adult thrips during bloom but then can continue during fruit set by adults and their offspring larvae. Thrips hide under the cap of the berry or in grooves around the seeds on the berry.

A key to thrips management is frequent monitoring, at least once per week. Growers should examine early flower clusters on early cultivars. In each of five to 10 areas of the field, five to 10 blossoms should be tapped into a white cup, or into a zip-top sandwich bag, which should then be examined for the dislodged thrips running around on the surface. Count the number of thrips found, then calculate the average number of thrips per blossom. A rough action threshold for treatment with insecticide is the presence of 2 or more thrips per blossom. Once fruit are ¼ inch in diameter, an action threshold is 0.5 thrips per fruit. If thrips are above threshold, the trickiest part of management is to avoid spraying insecticide that will harm pollinators. Insecticide should be applied pre-bloom or before 10% of the plants have open blossoms. If thrips are found above threshold on early cultivars, then a preventive spray can be made on the later cultivars before their flowers open, to avoid harming pollinators.

Insecticides used to control thrips on conventional strawberries are Radiant, Assail, and Sivanto, all of which have thrips listed as a target pest on their labels. Thrips are well controlled by Lorsban, Brigade, and Danitol, which are allowed for use on strawberries, but thrips are not listed as a target pest of the label of these three products. Note that Lorsban has a 21-day pre-harvest interval. Products for thrips control on organic strawberries are Entrust and azadirachtin products such as Neemix and Aza-Direct.

If a biological control approach is preferred, several kinds of natural enemies are available for purchase from commercial insectaries for thrips control: Orius (predatory flower bugs), and two species of predatory mites: Amblyseius cucumeris and Ambylseius swirskii. Biocontrol is not feasible to begin once the thrips population is large but can be planned in advance at locations that have a consistent problem with thrips.

Figure 1. Typical appearance of a thrips.

Figure 2. A thrips with wings folded over its back.

Figure 3. Close-up of thrips head and mouthparts.

by Celeste Welty, Extension Entomologist

Now that the weather has turned hot and dry, it is likely that spider mite infestations will be found in various crops. In some fields, the mite infestation is worst on a field edge by a dusty road. Because mites are tiny, they are often overlooked or misdiagnosed as a disease. Infested leaves have fine webbing on the leaf undersides. Tomato leaves damaged by spider mites usually have yellow blotches, while bean leaves show white stipples or pin-prick markings from mite feeding. Pumpkins can tolerate moderate levels of mites, but watermelons are more sensitive to injury from mite feeding. A simple method of diagnosing spider mites is to shake leaves over a piece of paper and look for moving specks that are visible to the naked eye. A closer look with a magnifier can show the tiny mites that are white, marked with two large dark spots on the middle of the body.

Mites can be suppressed by overhead irrigation. Mites have many natural enemies that kill them, such as specialized predatory mites or generalist lacewings, ladybugs, and pirate bugs, but these helpful predators are often killed by pesticides. Chemical intervention can be needed to keep the crop alive if spider mites are abundant. When a mite infestation is limited to field edges, infested fields should be scouted, and a miticide applied as a spot treatment to isolated infestations. Mite control is better when higher volumes of water are used; 25 gallons of water per acre is better than 10 gal/A. Several pesticides are registered for spider mite control; some are restricted use and some are for general use, as shown for vegetable crops in Table 1 and for hops and fruit crops in Table 2. At some locations, organophosphates are still effective for mite control, with Dimethoate being the best bet and MSR (Metasystox-R) as another choice. Dimethoate is an option for melons but is not allowed on squash or cucumbers; it has been a preferred product for mite control on soybeans. Dimethoate is prohibited from use on ornamental crops in high tunnels and greenhouses but is not prohibited from vegetable crops in high tunnels and greenhouses. Where organophosphates are not effective, Agri-Mek (abamectin) is generally the most effective product for mite control but it is a restricted-use product, while Acramite (bifenazate) and Oberon (spiromesifen) are nearly as good but are not restricted-use products. Although Brigade (bifenthrin) and Danitol (fenpropathrin) are labeled for spider mite control when used at the high end of the rate range, they are generally not as effective as the true miticides. Dicofol is an old miticide that is still effective at some sites, but does not perform well at sites where resistant populations have developed. Vydate (oxamyl) is a restricted-use product that is registered for use on eggplant for mite control. On organic farms, insecticidal soap can be used for mite control but thorough coverage of the undersides of leaves is needed for good control.

-Celeste Welty, Extension Entomologist