Thrips and Tospoviruses
Friday, 30 March 2007 14:18
Thrips populations are increasing with the warmer temperatures. I've observed thrips feeding upon numerous greenhouse crops including ivy geraniums, double impatiens, tomato bedding plants, plus greenhouse weeds including chickweed, galinsoga, etc.
Tospoviruses have also been detected on greenhouse crops including impatiens, double impatiens etc.
How do thrips damage plants?
They pierce plant cells and cells collapse resulting in deformed flowers, leaves and shoots. Silvery flecking and small black droppings can be seen on expanded leaves and flowers. When thrips feed in the buds, the buds are deformed. Thrips also spread tospoviruses, including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV). Because thrips are so small and prefer to hide in flowers and buds, sticky cards need to be used to detect the thrips before their feeding damage is evident.
How do thrips acquire and transmit tospoviruses?
Thrips cannot transmit the virus unless they acquire tospoviruses as first instar larvae. First instar larvae must feed on virus-infected plants. Infected thrips are produced in the greenhouse only if infected plants (including weeds), which can support thrips development, are present. Winged adult thrips are primarily responsible for viral spread. An infected thrips is able to transmit tospoviruses to at least one plant per day until its death.
Tospoviruses have a wide host range and infect over 600 different plants. Double impatiens, New Guinea Impatiens, begonias, as well as many different herbaceous perennials including penstemon, monarda, campanula, phlox, platycodon, etc. may become infected
To confirm that plants are infected with tospoviruses, plants can be submitted to a diagnostic laboratory or growers can purchase easy to use immunostrips that are available from Agdia. (www.agdia.com)
If plants are infected with tospovirues, there is no cure. Infected plants should be rogued from the greenhouse by placing them in a plastic bag and removing them. (Thrips are more attracted to virus infected plants, so by bagging the plants, you are less likely to move thrips throughout the greenhouse.)
What is the threshold for thrips?
Little research has been done to establish threshold for ornamental crops. A grower's tolerance for thrips varies depending upon the crop and the time of year. If the virus is present, the tolerance is zero. Many growers use an action threshold of 10 thrips per card per week during for their spring crops, if the virus is not present. Tapping the foliage over a sheet of white paper is another way to monitor for adult thrips as well as larvae, especially in cooler houses with herbaceous perennials and or vegetative petunias, etc.
When is the best time to spray against thrips?
Thrips also have two mass flights per day, so sprays in the early evening may contact more thrips. Small droplet sprays, repeated applications (two to three sprays about 5 days apart) and treating before you see a peak in adult numbers on yellow sticky cards are critical. Adult thrips numbers on cards tend to peak every two to three weeks. Apply insecticides before this peak, so adults will be killed before they lay eggs.
Thrips are notorious for their ability to development resistance, as are most other greenhouse pests. What is effective for one grower may not work for another grower. Rotation is critical to conserve the effective life of materials labeled for thrips. Researchers suggest using a material for one generation (two to three weeks depending upon temperatures) before switching to an insecticide in another class. Consider using contact materials with a non-specific mode of action including horticultural oils(MOA Group M), insecticidal soaps (MOA Group M) (read labels carefully for plant safety information and precautions) and Beauvaria bassiania (Botanigard, Mycotrol O) (MOA Group M) in your rotation to help suppress thrips. For a complete list of insecticides labeled for use against thrips, see the New England Greenhouse Floriculture Guide.
Photos: Thrips injury
INSV symptoms
More INSV photos, 2, 3, 4
Leanne Pundt
