Grower 101: Western Flower Thrips By D. Casey Sclar, Ph. D.

Western flower thrips are very difficult to control, particularly in greenhouse situations. Thrips are found in many crop environments, both indoor and outdoor, and can survive the harshest winters in the Northeast. The most common months that thrips are problematic in greenhouse crop environments are from May through late October, although they may be present 365 days per year, infesting many greenhouse crops.

Thrips Biology

The pest's biology includes several stages that are difficult to contact. Eggs are laid in leaf and/or flower tissue, while nymphs I & II often feed in flower heads as well as leaves. Nymphs III & IV are non-feeding, immobile stages, which take place in the soil and probably in flower heads as well as any other protective location. Adults emerge from this stage and feed on leaves and flowers. Female thrips may reproduce without fertilization from males, and in peak summer months, an entire generation may occur in as little as 15 days. Development time is longer in winter months (as long as 45 days).

The pest's diet is variable. It can survive and reproduce on other insects and pollen in addition to or as a supplement to leaf and flower tissues. Particularly attracted to pollen sources, thrips will feed extensively on flower tissues and degrade flower quality. Reduced length of bloom and flower tissue damage is often the result. In addition, they transmit (vector) the tospoviruses Tomato Spotted Wilt Virus (TSWV) and Impatiens Necrotic Spot Virus (INSV) to more than 400 species of desirable ornamentals.

Controlling Thrips

Scouting and monitoring, particularly during peak season, are absolutely necessary to ensure proper detection of thrips before populations reach damaging levels. This may include the use of sticky cards (blue or yellow) or direct visual observation through tap sampling or blowing a gentle stream of warm air into flower heads. Reliance on any one of these methods alone is inadequate, as thrips are notoriously less conspicuous on cloudy and cool days. For example, thrips may be very active during early spring on susceptible host plants but will not move in great numbers to sticky cards of any color.

A variety of biological and cultural controls have been attempted, and there are always ongoing investigations in these areas for the management of this pest. However, the use of pesticides is often necessary to keep thrips populations below damaging threshold levels. Several different pesticides are currently used for thrips control and each has its own unique attributes. The table below discusses some of these in detail.



D. Casey Sclar, Ph. D.

D. Casey Sclar, Ph.D., is IPM Coordinator at Longwood Gardens, Kennett Square, Pa.



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