Plants affected
Many plants can be attacked by various thrips species, including glasshouse crops, houseplants, and a range of ornamental garden plants, such as gladiolus, rose, privet and honeysuckle, and vegetables such as peas (see below), onion and leeks.
Symptoms
Leaves that have been damaged by thrips become dull green and develop a silvery-white discoloration of the upper surface. There may be tiny black spots on leaves due to the insect’s excrement. Eggs are deposited on young leaves and in flower buds. Shoot tips and flower buds can become distorted if they have been damaged by the insects’ feeding. Flower petals may be flecked white where pigment has been lost, and flower buds may fail to open.
Onion thrips (Thrips tabaci) and
western flower thrips (Frankliniella occidentalis)
can transmit tomato spotted wilt virus, which affects many
ornamental plants as well as tomatoes.
Cause
There are many different pest species of these insects, including Thrips, Kakothrips, Dendrothrips, Parthenothrips, Heliothrips and Frankliniella species. Adult thrips vary in colour from yellow-brown to black and some have a banded appearance.
Thrips, which are also sometimes known as thunder flies, feed by sucking sap from leaves and flowers. The adults have narrow elongate bodies up to 2mm in length and two pairs of narrow wings that are folded back over the abdomen. The immature stages are similar in shape, but wingless and usually paler in colour than the adults.
Control
Insecticide sprays containing bifenthrin (Scotts Bug Clear Gun, Doff All In One Garden Pest Killer, Bayer Sprayday Greenfly Killer Plus) or imidacloprid + methiocarb (Bayer Ultimate Bug Killer aerosol) or imidacloprid + sunflower oil (Bayer Ultimate Bug Killer Concentrate), acetamiprid (Scots Bug Clear Ultra), thiacloprid (Provado Ultimate Bug Killer Ready To Use), or thiamethoxam (Westland Bug Attack) can be used to control thrips on ornamental plants; thiamethoxam is for use on container-grown ornamental plants only. Check the manufacturer's instructions if it is necessary to spray an edible plant, as it is important to use a product that is approved for use on that particular type of plant.
Plant oils or extracts (Growing Success Fruit & Veg Bug Killer, Vitax Organic 2 in 1 Pest and Disease Control, Scotts Bug Clear for Fruit & Veg), rotenone* (Bio Liquid Derris Plus*) or pyrethrum (Py Spray Garden Insect Killer, Doff All in One Insecticide Spray, Scotts Bug Clear Gun for Fruit & Veg) are alternative organic pesticides. These can be used on most food plants and ornamentals but check the manufacturer's instructions first.
Predatory mites (Amblyseius species) can be obtained from some mail order suppliers of biological controls for release in greenhouses or on houseplants to control western flower thrips (Frankliniella occidentalis).
* Rotenone/derris-based insecticides are being withdrawn from sale. Gardeners who have this pesticide may continue using it until 10 October 2009. Any unused products after that date should be disposed of safely by handing them to the staff at a local authority manned waste disposal facility.
Disposing of old chemicals