How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PESTS

Thrips are tiny insects that have four featherlike wings, each consisting of a thick supporting strut with fine hairs on the front and hind edges. Thrips go through six life stages: egg, first instar, second instar, prepupa, pupa, and adult. Thrips insert eggs into plant tissue. The first two instars and the adults feed by piercing and removing the contents of individual plant cells.

Western flower thrips. This thrips has three color forms that vary in abundance depending on the time of year. There is a pale form that is white and yellow, except for slight brown spots or blemishes on the top of the abdomen; an intermediate color form with a dark orange thorax and brown abdomen; and a dark form that is dark brown. The intermediate form is present throughout the year, but in spring the dark form predominates while the pale form is most abundant at other times throughout the year. The dark form is an overwintering form that is usually found in foothill or mountain areas. Its presence in greenhouses in spring indicates thrips are migrating into the houses. Western flower thrips usually feed in enclosed tissues such as flowers, buds, or growing tips. Adults also feed on pollen and on spider mites. The prepupa and pupal stages take place in the soil beneath infested plants. Females will lay male eggs if unmated and female eggs are produced once mating has occurred. Development times to complete one generation of western flower thrips varies from 11 days (77° to 87°F), to 44 days (50° to 60°F).

Greenhouse thrips. Adult greenhouse thrips are tiny, black, insects with whitish to translucent wings folded back over their thorax and abdomen. Legs are also a whitish color. Nymphs are whitish to slightly yellowish in color and produce a globule of fecal fluid at the tip of their abdomen. These globules of fluid increase in size until they fall off and another one begins to form, resulting in a characteristic spotting of the infestation area with black specks of fecal material.

DAMAGE

Western flower thrips primarily feeds on flowers but also sometimes on new vegetative growth, whereas greenhouse thrips feeds primarily on foliage. Direct feeding damage includes streaking, spotting, and tissue distortion. On leaves, feeding often occurs along veins and appears as an outlining of the veins. Western flower thrips can vector tomato spotted wilt virus as well as many other viruses. Western flower thrips may cause premature senescence of flowers, such as African violets, because they prematurely pollinate the flowers. On orchids, western flower thrips feeding and egg laying will leave translucent 'pimpling' spots on petals and leaves. Greenhouse thrips stipple the foliage of numerous field and greenhouse grown plants. The stippling damage caused by thrips feeding on individual cells is often confused with mite stippling. Thrips feeding is often accompanied, however, by black, varnishlike flecks of dried excrement whereas mite stippling is often accompanied by webbing or shed skins.

BIOLOGICAL CONTROL

Three commercially available predators to help control western flower thrips are the minute pirate bug, Orius tristicolor, and two predatory mites, Neoseiulus cucumeris and Hypoaspis miles. Minute pirate bugs are polyphagous and will also feed on aphids, mites, and small caterpillars. Orius are released at a rate of 2000 to 4000 per acre, while Neoseiulus cucumeris are released at a rate of 10 to 50 mites per plant for each of 2 to 3 weeks. These mites will also feed on spider mite eggs, pollen, and fungi. Hypoaspis miles are soil-inhabiting predators that feed on thrips prepupae and pupae in the soil. These mites are generally released in the soil at planting and are most successful at controlling thrips when there is plant-to-plant contact that facilitates movement of the predators between plants. A commercially available parasite of greenhouse thrips is Thripobius semileteus.

CULTURAL CONTROL

Because western flower thrips and greenhouse thrips feed on a large variety of plant species, keep production areas free of weeds, which can serve as hosts for thrips populations. Most commercially available screens have pore sizes slightly larger than the width of the western flower thrips thorax (145 microns), meaning that some winged adults can penetrate these openings. However, covering openings to the greenhouse with fine screens does exclude most thrips. Be sure that the ventilation system on an existing greenhouse can accommodate the reduced flow caused by a fine screen or else the system will need to be modified.

Carefully inspect plants being brought in to start a new crop to ensure that they are free of thrips and other pests. A holding area where plants are kept for about 11 to 12 days is useful so that plants can be inspected for any infestations that may develop. Treat any infested plants if necessary.

MONITORING and WHEN TO TREAT

Blue sticky cards are most attractive to western flower thrips. However, yellow cards are good predictors of western flower thrips populations, are easier to count and are more commonly used for general-purpose insect monitoring. Place yellow sticky cards vertically in the crop canopy, with the lower one-third of the trap in the leaves and the upper two-thirds above the leaves. As the crop grows, the traps will need to be raised.

There is little research on the most effective trap density to use or on treatment thresholds. Research in California greenhouse roses suggests that three traps per cultivar is adequate. In greenhouses with many different cultivars, place traps in the most sensitive varieties, usually yellow or white flowers. In large greenhouses of the same or similar cultivars, there should be at least eight traps per 100,000 square feet. The treatment threshold for roses is 25 to 50 thrips per card per week (25 for more sensitive yellow- and white-flowered varieties, 50 for reds). In other crops place one card per 10,000 square feet. Consider treating if an average of 5 to 10 thrips per card per week is present.

It is important to note that correct identification of pest thrips is essential in a monitoring program. There may be several species of thrips present on a sticky card but only the western flower thrips and greenhouse thrips should be counted when making treatment decisions.

Most insecticides must be applied at least two times, 5 to 7 days apart, for efficacy against western flower thrips.

TREATMENT

Selected Materials Registered for Use on Greenhouse or Nursery Ornamentals
Read and follow the instructions on the label before using any pesticide. Before using a pesticide for the first time or on a new crop or cultivar, treat a few plants and check for phytotoxicity.

PUBLICATION

UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries
UC ANR Publication 3392
Insects and Mites
K. L. Robb, UC Cooperative Extension, San Diego Co.
H. S. Costa, Entomology, UC Riverside
J. A. Bethke, Entomology, UC Riverside
M. P. Parrella, Entomology, UC Davis
Acknowledgment for contributions to the insects and mites section:
R. S. Cowles, Connecticut Agricultural Experiment Station, Windsor, CT

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