Dahlias in bloom in mid-July (Photo: Margaret Pickoff)

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In the past several weeks, scouts and growers have observed damage from a major flower crop pest insect. They are tiny in size, but have the potential to render stems unmarketable. Who are they?

Thrips! Let's meet these six-legged critters.

If you've grown flowers in high tunnels, you are likely already familiar with thrips, though they can be a big issue for field-grown cuts as well. Thrips are small enough to be difficult to see with the naked eye (1/20 inch long), with slender bodies and fringed wings.

Slender, yellow-brown insect on a green leaf — Western flower thrips (Photo: Frank Peairs, Colorado State University, Bugwood.org)

They have rasping-sucking mouthparts, and feed on shoots, leaves, flowers, and fruits of a wide range of plant species. Their feeding, which involves puncturing a plant tissue's epidermis and sucking out the contents of its cells, causes a stippled appearance on leaves that might be confused with mite or leafhopper feeding (more on these flower pests another time). They also tend to leave behind dark frass as they feed.

Green leaves with light stippling damage — Thrips feeding damage on verbena (Photo: Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org)

Thrips feeding damage can cause overall stunting of plant growth, as well as premature leaf drop, and discoloration or distortion of petals and flowerheads.

Pink flower with white streaks — Discoloration on gladiolus petals from thrips feeding (Photo: Whitney Cranshaw, CO State University, Bugwood.org)

Pink-petaled flower with distorted flowerhead — Distorted dahlia flowerhead (Photo: Kelly Piccioni)

Thrips can also vector plant viruses that impact floral crops, like Impatiens Necrotic Spot Virus and Tomato Spotted Wilt Virus.

Green leaves with yellow markings — Dahlia with Tomato Spotted Wilt Virus (Photo: Margaret Pickoff)

To confirm the presence of thrips, plant stems can be tapped or shaken over a piece of white paper. The thrips, while tiny, will often be visible on the paper. If thrips are suspected to be feeding inside flower buds, a bud can be clipped from the plant and placed into a jar of 70% ethanol alcohol, agitated, strained onto filter paper, and observed with a hand lens.

Managing thrips can be very difficult, in part because they often feed in closed buds and other tight quarters that are difficult to see and impossible to reach with contact insecticides. Some growers have success using fine mesh insect netting to exclude thrips from high tunnels. Biological controls, like lacewings, predatory thrips, predatory mites, parasitic wasps, and Orius (minute pirate bug), can be effective when released in high tunnels and greenhouses.

White insect netting on hoophouse with flowers growing inside — Flowers in hoophouse. Photo: Margaret Pickoff

Mowing or controlling weeds around growing areas can reduce thrips populations in crops by eliminating alternate hosts. Dandelion, lamb's quarters, sow thistle, jimson weed, and many other common weedy species are hosts of Western flower thrips. Reflective plastic mulches can also be used at the base of plants to deter and disorient thrips.

Pyrethrin, insecticidal soaps, Spinosad, and plant-based oils can help to keep thrips populations in check. Because of their rapid reproductive rate, thrips are prone to developing resistance to products used repeatedly, so rotation of insecticide active ingredients is important.

In addition to thrips, we're observing Japanese beetles, leafhoppers, and June bugs. We'll be covering these over the coming weeks. Just a note that we'll be taking a summer hiatus next week, and there will be no update on July 25^th. We’ll be back with a new update on August 1^st!

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