How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Thrips are very small, slender insects that are best seen with a hand lens: mature onion thrips are about 0.05 inch (1.3 mm) long and flower thrips are slightly larger at 0.06 inch (1.5 mm) long. The most distinctive characteristic of thrips are two pairs of wings that are fringed with long hairs. Adults are pale yellow to light brown in color. The immature stages have the same body shape as adults but are lighter in color and are wingless. When viewed under a microscope, western flower can be distinguished from onion thrips by its red eyes and 8-segmented antennae, while onion thrips' eyes are gray and its antennae are 7-segmented.

Both onion thrips and western flower thrips have a very extensive range of hosts, including cereals and broadleaved crops. Both species attack onions, but onion thrips are believed to be more prevalent and injurious. They also can be a problem on garlic, but generally are not as serious a pest as they are on onion. Onion thrips thrive in hot, dry conditions and are usually more damaging in areas where these climatic conditions prevail for most of the production season.

DAMAGE

Thrips are the most common and serious insect pest of onions, and are found wherever onions are grown in California. High populations of thrips can reduce both yield and keeping quality of onions. Thrips are most damaging when they feed during the early bulbing stage of plant development. Scarring of leaves is a serious problem on green onions.

Thrips have rasping-sucking mouthparts and feed by rasping the surface of the leaves and sucking up the liberated plant fluid. They feed under the leaf folds and in the protected inner leaves near the bulb. When population levels are high, thrips can also be found feeding on exposed leaf surfaces. Both adults and nymphs cause damage. When foliage is severely damaged, the entire field takes on a silvery appearance. Severe scarring also creates an entry point for foliar leaf diseases.

MANAGEMENT

Biological Control
Natural enemies, including predaceous mites, minute pirate bugs, and lacewings, are often found feeding on thrips. These beneficials are very susceptible to insecticide sprays, however, and may not be important in fields where insecticides have been used.

Cultural Control
Avoid planting onions near grain fields, if possible, because thrips numbers often build up in cereals in spring. Overhead irrigation and rainfall provide some suppression of thrips populations, but treatments are often still necessary.

Organically Acceptable Methods
Biological and cultural controls as well as sprays of the Entrust formulation of spinosad are acceptable for use on organically certified crops.

Monitoring and Management Decisions
Although thrips feeding during the early bulbing stage is the most damaging to yields, thrips must be controlled before onions reach this stage so that populations do not exceed levels that can be adequately controlled. Onions can tolerate higher thrips populations closer to harvest; however, in the case of hand-topped onions, thrips can be extremely annoying to harvest crews and treatment closer to harvest may be desirable.

To make a cursory evaluation of thrips infestation levels, randomly sample leaves and evaluate thrips numbers and damage under leaf folds. A far more reliable means of evaluating thrips populations, however, is to randomly sample entire onion plants. This way leaves can be pulled apart and, using a hand lens, all the thrips on the inner leaves near the bulb can be counted as well as those under the leaf folds. Sample at least five plants from four separate areas of the field. A reliable treatment threshold has not been developed; however, a threshold of 30 thrips per plant mid-season (lower for very young plants and higher for larger mature plants) has been used successfully for dry bulb fresh market and drying onions.

For processing onions, monitor thrips by examining the entire top growth of the onion plant and counting the number of thrips. Sample 10 plants from four areas of the field. Sample weekly, or more frequently when counts exceed 20 thrips per plant. Calculate the average number of thrips per plant on two successive sample dates. Divide the average by the number of days between samples to get the number of thrips per plant per day or thrips-days. Add up the thrips-days on the sample day to get the cumulative thrips-days (CTD) during crop growth. Research indicates that significant yield loss occurs when 500 to 600 CTD or more accumulate. This is the equivalent of 50 to 60 thrips per plant per day for 10 days, 25 to 30 thrips per plant per day for 20 days, and so on.

The marketability of green onions (those marketed fresh with the leaves attached) is severely reduced by thrips scarring; apply treatments 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 insecticides has not been evaluated in California, it has been documented in other states and is suspected in California. For this reason, alternate insecticides from different chemical families when multiple treatments are needed during a season. Thorough coverage is essential for control, as most thrips feed in protected areas of the plant.

PUBLICATION

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453
Insects and Mites
R. L. Coviello, UC Cooperative Extension, Fresno Co.
W. E. Chaney, UC Cooperative Extension, Monterey Co.
S. Orloff, UC Cooperative Extension, Siskiyou Co.
G. J. Poole, UC Cooperative Extension, Los Angeles Co.

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