thrip  (38255 bytes)

Thrips

By Judy Sedbrook, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

Thrips are small insects, only about 1/20", but they can cause a lot of damage. At maturity, they are yellowish or blackish with fringed wings.  Nymphs have a similar shape but lack the wings. They are usually yellowish to white. Thrips are poor flyers. As a result, damage often occurs in one part of the plant then slowly spreads throughout it.

Size of thrip compared to a dime (21969 bytes)

Size of thrip compared to a dime

Thrips feed in buds, folded leaves, and other unexposed areas of plants. This makes them difficult to treat with an insecticide. They feed by sucking juices from the plant causing stippling, or small scars, on leaves, flowers and fruit. This results in stunting of the plant, leaf distortion and premature leaf drop. Flowers may be deformed and fail to open properly. Petals may show brown streaks and spots. Their excrement is black and shiny, which may be a clue to their presence.  In addition to this physical damage, thrips also transmit tomato spotted wilt virus and impatiens necrotic spot virus, for which there is no control.

Brown streaks and spots on rose as seen with thrips (53734 bytes)

Brown streaks and spots seen with thrips

Control of thrips is difficult. To look for their presence, shake the plant out over a sheet of white paper. If you find thrips there are a few steps you can take to dispatch them.

  • Dislodge them by applying a strong stream of water to the affected plant.
  • This is one time you will want to use overhead watering as it kills many of the thrips.
  • Placing aluminum foil mulches under the plants has been found in some instances to disorient the thrips.
  • Remove and discard affected blossoms and plant parts.
  • Thrips prefer tender new growth. Avoid excess pruning which may stimulate new growth.
  • Avoid planting near dry, weed or grassy areas. Thrips migrate from these areas into the garden.
  • The location of the insects makes it difficult to reach them with insecticides. Products that have been somewhat successful are: horticultural oils, insecticidal soaps, Neem, pyrethroids, acephate (non-food crops only). Two biological controls have shown promise in Colorado. They are Beauvaria bassiana (Naruralis O, Botanigard) and Spinosad.

Photos: Judy Sedbrook

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Date last revised: 01/05/2010