The following article, by Professor A.M. Shelton of Cornell University, appeared in the March 2007 issue of Onion World and is reprinted by permission.

Onion Thrips

Scientific Name:
Thrips tabaci

Geographic Distribution:
Worldwide. Found in most areas where onions are grown.

Identification

Immature: White to pale yellow in color, onion thrips in the immature stage are very small, usually 0.5 to 1.2 mm in length. These thrips have elongated, elliptical and slender bodies with dark colored eyes that are easy to see. Unlike adults, immatures do not have wings and, hence, cannot fly.
Pupae: This is the intermediate stage between the immature and adult stages. Thrips pupae are pale yellow to brown in color and have short antennae and wing buds that are visible but nonfunctional.
Eggs: White or yellow in color, eggs are microscopic and are difficult to see without the aid of a microscope. Females typically insert the eggs, one by one, into the plant tissue. Adults prefer to lay their eggs in leaf, cotyledon or flower tissues.
Adults: Adults are about 2 mm in length and have fully developed wings that are fringed with long hairs. When at rest, the wings are folded along the back of the insect. Adults are pale yellow to dark brown in color.

Host

Preferred hosts are onions, garlic and other related plants. Onion Thrips also attack various crops including cabbage, cotton, celery, tomatoes, beans, cucumber and pineapple. Thrips can be found on almost any cultivated and weedy plants.

Biology and Ecology

Typical life cycle is 14 to 30 days, however, when temperatures are over 90˚ F, the life cycle can be as short as 10 or 11 days. Adults may live up to 20 days. Thrips do not need to mate for reproduction. Females that do not mate will produce only female offspring. Each female can produce up to 80 eggs. It is important to remember that a single thrips can produce a substantial population within a very short time.

Damage

Thrips can cause considerable economic damage. They begin feeding by piercing and rasping the leaf surface with their mouth parts to release the liquids from the plant cells. Later, they suck up plant contents. Thrips prefer to feed on the young plant tissue on the newest emerged leaves, so scouting for thrips should begin there. When the leaf grows, the previous damage produced by the thrips enlarges, leaving silvery patches or streaks on the leaves. Small black fecal matter is also commonly seen on the leaves. When damage is severe, these small patches can occupy much of the surface of the leaf and the plant cannot adequately photosynthesize. The plant loses more water than normal through the damaged tissue and plant pathogens can easily penetrate the injured plant. In severe attacks, the whole plant can turn white or silver and leaves can wither. In injured plants, bulb size is reduced. Thrips may also serve as vectors of some viruses such as Iris Yellow Spot Virus and other plant diseases, including the fungus, purple blotch (Alternaria porri).

Control

Onion thrips populations and damage are most severe during hot, dry weather. The reasons are that rainfall is normally a major mortality factor and more generations are produced in hot weather. The main method of control is the use of insecticides, but populations of thrips in some areas have developed resistance to some insecticides, especially pyrethroids. Best control results are obtained with an integrated strategy that includes making sure transplants are free from thrips, plots are planted away from infested areas, culls and volunteers are removed from the field (since they can serve as a source of infestation), and the plants are not stressed by other factors such as poor irrigation. Because of the occurrence of insecticide resistance in some regions, check with your local extension service to determine which insecticides might be useful in your area. It is best to rotate classes (i.e. different modes of action) of insecticides every two weeks to slow the development of resistance to any single class. Before using insecticides, use the economic threshold for onions in your particular area as a guide for when to treat. Modify the general treatment guidelines based on the weather, cultivar and market value of the crop. Special attention needs to be observed when the weather is hot and dry since thrips populations can explode. Due to growing insecticide resistance problems around the world, it is very important to use insecticides as little as possible in an IPM program.