Introduction

Onion thrips, Thrips tabaci (Lindeman), is a regular and potentially serious pest of onions and cabbage in Ontario. Thrips are widely distributed in Canada and the United States and feed on a variety of vegetables, field crops and weed species. Thrips first appear in most vegetable fields in mid-to-late June and have several overlapping generations throughout the summer months.

Description

Thrips are small, slender and fast-moving. The adults are approximately 2 mm in length, pale yellow to brown in colour and pointed at both ends (Figure 1). The male of the species is wingless and is rarely found. The females have four very slender wings fringed with close-set long hairs. When at rest, the wings are laid over the abdomen and extend slightly past it. The nymphs are similar in shape but smaller in size and pale yellow in colour.

 

Figure 1. Adult thrips feeding on cabbage.

Biology

In Ontario, both adults and nymphs overwinter on winter grains, clover or alfalfa. Migration to vegetable crops usually occurs in early summer when winter grains begin to dry down. Once in an onion or cabbage field, the thrips seek cover between or under leaves, where they begin to feed. The females insert white, bean-shaped eggs into the leaf tissue. The eggs hatch in 5 to 10 days. The nymphs grow in size and pass through four distinct stages, the last two of which are spent in the soil. Development form egg to adult requires from 10 to 30 days, depending on temperature. Once mature, females soon begin to lay eggs. The females are able to reproduce without mating and, therefore, are not dependent on the presence of males of the species. Consequently, increases in the thrips population can occur very rapidly, especially during periods of hot, dry weather. Eggs, nymphs and adults may be found together at any time during the summer and produce many overlapping generations.

Damage

Onions

Damage is caused by both nymph and adult feeding. Thrips use their rasping and sucking mouthparts to scrape the leaf surface and suck up the exuding plant juices. This feeding produces silvery-white, mottled lesions on the leaf surface (Figure 2). Initially, injury is difficult to detect, due to the concealed location of the thrips. A close look between the leaf sheaths is necessary to locate active thrips or leaf injury. Large numbers of thrips can cause serious damage to onions. The lesions may become so numerous that the entire plant takes on a white or straw-coloured appearance. Continued feeding results in leaf distortion, followed by wilting, browning and premature lodging.

Figure 2. Thrips damage on onions. Note the silvery-white lesions.

During hot, dry seasons, damage from thrips becomes most serious and may result in reduced onion yields. Feeding also puts added stress on the plant, making the onion more susceptible to bacterial rot and fungal attack.

Cabbage

Figure 3. Thrips damage on cabbage appears as small brownish-gray wartlike growths on the leaf surface.

In Ontario, injury to cabbage by thrips is usually most serious in storage cabbage or heads harvested in late summer and early fall. Feeding by thrips results in a proliferation of cells protruding from the injured epidermal layer of the head. The result is small, brownish-gray water-like growths on the leaf surface (Figure 3). The wrapper leaves may also show silver coloured mottled lesions along with black fecal material. Feeding by thrips represents more of a cosmetic problem than a loss in yield. If feeding injury is severe or several thrips are present on the cabbage, heads may be unacceptable for the fresh and processing market. Some varieties with tighter heads are less susceptible to injury and should be considered if thrips become a perennial problem.

Monitoring and Control

Onions

Yellow sticky board traps used for monitoring the onion maggot can also be used to detect the migration of thrips from their overwintering hosts.

Once the migration has begun, individual onion plants in several sections of the field should be inspected on a regular basis. To find thrips, the leaves must be parted to reveal the youngest emerging leaf in the centre of the plant (Figure 4). Thrips prefer this area because it is the most succulent part of the plant and provides excellent protection from weather and insecticides. Adult thrips may also be found on older leaves, particularly where leaves have folded over, providing a sheltered area. Areas where thrips are most likely to be found are the field borders, near woods or in the vicinity of grain or field crops. Then examining the plants, look for injury as well as for the presence of the insects.

Figure 4. Separating the leaves of onion reveals both damage and active thrips.

At least 50 plants should be examined when scouting for thrips; in addition it is necessary to estimate the average leaf stage of the crop prior to plant sampling. Carefully record the approximate number of thrips per plant that you find. Then divide this total by the average leaf stage of the crop to give you a "thrips per leaf" estimate. Threshold guidelines are as follows: 3 thrips per leaf for cooking onions, 1 thrip per leaf for Spanish onions and green bunching onions". Spanish onions have a lower tolerance because they are very susceptible to bacterial diseases which thrip's damage can enhance. Green onions also have a lower tolerance because the tops are sold and must be free of blemishes.

Cabbage

Sticky board traps are useful to determine the time that populations of thrips are migrating into the crop. Traps should be located along the outer edges of the field and examined twice weekly. Once an influx of thrips has been detected, scouting should become more intense. Scouting for thrips can be integrated with scouting for lepidopterous larvae (caterpillars). Twenty to thirty plants should be examined two times a week with emphasis on plants around the borders of the field. Thrips are normally found on the underside of the leaf or within the leaves of the developing head. They are often difficult to find, making it important to also look for signs of feeding. After head formation, damage can be detected by peeling off the outermost leaves. Pesticide applications, if necessary, are more effective prior to head formation. Thorough insecticide coverage is essential for good control of thrips. Due to the behavior of the insect, sprays should be directed at both the top and underside of the leaves. Once thrips are between the leaves in a head, they are difficult to contact with an insecticide.

Thrips have many natural enemies which include ladybird beetles, minute pirate bugs, lacewings, spiders and predacious and parasitic wasps. The fungus Entomopthora thripidum also infects thrips providing some natural control. In a season with low insect pressure, these natural control measures can prevent thrips from causing economic losses. Heavy rains will also reduce thrip numbers by washing them from the plants. Reduction in thrip numbers has also been observed with the use of overhead irrigation.

For a list of recommended materials and timing of spray applications, refer to OMAFRA Publication 363, "Vegetable Production Recommendations", "Integrated Pest Management for Crucifers in Ontario" (Order No. 701), and "Integrated Pest Management for onions, carrots, celery and lettuce in Ontario" (Order No. 700), or consult your local Horticultural Crops Advisor.