Greenhouse - Recognizing Thrips and Thrips Damage

Thrips can be a major problem in greenhouses damaging flowers and transmitting viruses such as INSP and TSWV. The following is information on thrips.

Of the many species of thrips commonly found in greenhouses, two species, flower thrips (Frankliniella tritici) and western flower thrips (Frankliniella occidentalis) are most problematic. Adults of both species are about 1 mm long, slender and just visible to the naked eye. You may not notice adult thrips flying in the greenhouse but you can often detect both nymphs and adults in the open blossoms of your greenhouse crops. The most distinctive external feature of this group is found on the adults. They have 2 pairs of feather-like, long, narrow wings, which have few or no veins and bear fringes of long, fine hairs along their margins. The wings are held parallel along the back when at rest. Immature forms of thrips are wingless. Colors can vary from white to straw yellow to brown. When examining small insects suspected of being thrips, use a 10x magnifier to help you look for these morphological characteristics.

It is not possible to accurately identify which thrips species is infesting a crop even with a hand lens. Differences in microscopic structures on the adult female thrips are used to tell one species from another. Therefore, adult thrips must be inspected under a compound microscope to accurately determine the species. To distinguish between the flower thrips and western flower thrips, capture live adults and place them in a vial with alcohol. Take the sample to a Maryland Cooperative Extension office for identification. Species identification is not possible from sticky cards.

Thrips Damage

Thrips feed by piercing plant cells with their mouthparts and sucking out the cellular contents. The damage to plant cells caused by thrips feeding can result in deformation of flowers, leaves, and shoots. There is often silvery streaking and flecking on expanded leaves. Thrips often deposit tiny greenish-black fecal specks on leaves when they feed.

The distribution of western flower thrips was thought to be limited to west of the Mississippi, prior to the 1980s. This thrips has become the most persistent species attacking greenhouse plants throughout the United States, Canada, and many countries in Europe and Asia.
The western flower thrips has the ability to transmit tospoviruses impatiens necrotic spot virus and tomato spotted wilt virus (TSWV and INSV) to a wide variety of greenhouse plants. If thrips in your greenhouse are identified as western flower thrips and you also have plants infected with tospovirus, you must practice rigorous thrips control. There is no cure for tospovirus; infected plants must be destroyed. One western flower thrips adult can infect a plant after feeding for only 30 minutes. Because both the virus and the thrips attack such a wide variety of plants, including greenhouse crops and weeds, it may be difficult to eradicate the virus once it is found in a greenhouse. For more information on tospovirus visit the University of Maryland web site, at URL www.agnr.umd.edu/users/tospovirus.

Life Cycle

Most adult thrips seen in a greenhouse are females; in some species males are rare or unknown. Reproduction without fertilization is a frequent occurrence. Thrips are gregarious with large numbers often concentrated on the same leaf or flower.

The length of the life cycle is strongly influenced by temperature and humidity. Thrips in warm greenhouses have a shorter generation time than thrips outdoors. In the greenhouse, thrips development may continue uninterrupted throughout the year if suitable crops are available. Outdoors during warm periods in late afternoon, thrips sometimes swarm and are caught in wind currents to be dispersed over a wide area. Control of thrips is extremely difficult due to several biological characteristics. Thrips eggs are inserted into leaf or petal tissue, and are thus protected from insecticides. The eggs hatch into larvae, which usually remain protected in flower buds or foliage terminals. The insects pass through two larval stages, both of which feed in these protected areas. Toward the end of the second larval stage, the insects stop feeding and move down into the soil or leaf litter to pupate. The thrips pass through two "pupal" stages (prepupal and pupal), during which no feeding and little movement occurs. While in these pupal stages in the soil, they are protected from insecticides directed at the crop. There are currently no pesticides labeled as drenches to kill thrips pupae in soil. The adults can survive from 30 to 45 days. Female thrips lay 150 to 300 eggs depending on temperature and the host plant. Adults are found feeding in protected areas of the plant such as flowers and terminals. Hibernation takes place in the soil outside or in the soil under the greenhouse bench in unheated greenhouses.

The pest's rapid developmental time (egg to adult in 7 to 15 days at fluctuating temperatures), high reproductive rate, and preference for protected areas can make early detection difficult. Adults fly readily and can be carried on wind currents, or on clothing, to greenhouses near an infested field. They can fly from a sprayed to an unsprayed area, or can move into or out of a greenhouse through doors or greenhouse vents.

Extracted from "Thrips Management in Greenhouses" by Stanton Gill, Ethel Dutky and Michael Raupp, Maryland Cooperative Extension, University of Maryland