Thrips

Thrips are very small, slender insects that can be easily seen with a hand lens. Mature onion thrips are about 0.05 inch (1.3 mm) while western flower thrips are slightly larger at 0.06 inch (1.5 mm). The most distinctive characteristic of thrips is two pairs of wings that are fringed with long hairs. Adults are pale yellow to light brown. The immature stages have the same body shape as the adult stage, but unlike adults, they are wingless and lighter in color.

When viewed under a microscope, western flower thrips can be distinguished from onion thrips by the hairs on the front of the thorax (the body segment after the head), as well as their 8-segmented antennae. In contrast, onion thrips lack these hairs, and their antennae are 7-segmented. These structures are best seen through a compound microscope at 400x.

Thrips are the most common insect pest of onion, causing widespread damage. They are found wherever onions are grown in California. Thrips are present every year, but infestations vary widely depending on weather conditions. Both onion thrips and western flower thrips have a wide host range that includes cereal crops, broadleaved crops, and many weed species. Both onion thrips and western flower thrips attack onion, but onion thrips are believed to be more prevalent and injurious than western flower thrips.  Many areas have both onion thrips and western flower thrips, and their numbers can vary over the season.

Thrips thrive in hot and dry conditions and are usually more damaging in areas where these climatic conditions prevail for most of the production season. They may also transmit iris yellow spot virus, which is only vectored by onion thrips.

High numbers of thrips can reduce both yield and storage quality of onions. Onion thrips also can be an occasional problem on garlic but are generally not as serious a pest as they are on onion.

Thrips are most damaging when they feed during the early bulbing stage of onion development. They feed under the leaf folds and in the protected inner leaves near the bulb but may also be found feeding on exposed leaf surfaces when their numbers are high. Their feeding may lead to leaf scarring, which is most serious on green onions. Severe scarring can reduce the effective photosynthetic area and creates an entry point for foliar diseases.

Both adults and nymphs cause damage. When foliage is severely damaged, the entire field takes on a silvery appearance.

Natural enemies, including predaceous mites, minute pirate bugs, and lacewings, are often found feeding on thrips. However, natural enemies are susceptible to broad-spectrum insecticide sprays such as pyrethroids, and may not be reliable or effective in fields where broad-spectrum insecticides have been used.

Avoid planting onions near grain or alfalfa fields if possible, because thrips often increase in those fields in the spring and migrate to onion fields when the grain senesces, or when the alfalfa is cut. Overhead irrigation and rainfall suppress thrips numbers, but pesticide applications are often still necessary.

Use biological and cultural controls in an organically certified crop. Sprays of the Entrust formulation of spinosad, azadirachtin, and entomopathogenic fungi such as Beauveria bassiana are also acceptable for use on organically certified crops.

Thrips are most damaging to yields during the early bulbing stage. However, management should start before onions reach the bulbing stage, so that they do not reach levels that are difficult or impossible to control. Onions can tolerate higher numbers of thrips closer to harvest. However, in the case of hand-topped onions, applying pesticide closer to harvest may also be desirable because thrips can be a nuisance to harvest crews.

Evaluate thrips numbers in a field by randomly sampling entire onion plants (see Identifying Pests of Onion and Garlic and Their Damage for thrips photos and identification tips):

• Sample at least five plants from four separate areas of the field.
• Pull the leaves apart.
• Use a hand lens to count all the thrips on the inner leaves near the bulb as well as those under the leaf folds.
• Use a hand-held click counter for more efficient counting. Thrips are highly mobile and may disperse or hide before they can be counted.
• Use the thrips monitoring form (PDF) to record your results.

If you wish to make a cursory evaluation of thrips infestation, randomly sample leaves to evaluate thrips numbers and damage under leaf folds. Keep in mind that sampling only the leaves often fails to detect thrips hidden between leaves near the bulb.

An economic threshold has not yet been developed for thrips, but an action threshold of 30 thrips per plant mid-season (lower for very young plants and higher for larger, mature plants) has been used for making successful pesticide application decisions for dry bulb, fresh market, and drying onions. Other thresholds that use thrips per leaf (rather than thrips per plant) are used in New York and other states, in which all thrips are counted on a plant and then divided by the number of leaves. This method accounts for the size of the onion plant; however, the reliability of the method has not yet been evaluated in California.

Because the marketability of green onions (those marketed fresh with the leaves attached) is severely reduced by thrips scarring, apply insecticide at the first sign of thrips feeding. On onions grown for seed, thrips can reduce yield and quality of seed production during seed set, but no treatment thresholds have been established.

While resistance to organophosphate and pyrethroid insecticides has not been evaluated in California, it has been documented in other states and is suspected in California. For this reason, alternate insecticides with different modes of action when multiple pesticide applications are needed per season. A common practice for minimizing the risk of resistance development in thrips populations is to apply insecticides with the same mode of action for a maximum of two consecutive applications, and use an insecticide with a different mode of action to control the next generation of thrips if a third application is necessary. A total of three applications is often enough. Thorough coverage is essential for control, as most thrips feed in protected areas of the plant.