Western flower thrips (WFT) are serious pests of many garden and crop plants, especially in glasshouses. They cause bleached patches on the underside of leaves, and if infestation is severe the whole plant may be defoliated.
Direct feeding buy WFT is often less of a concern than transfer of diseases such as tomato spotted wilt virus.
In Europe WFT has become resistant to many pesticides since it became an economic pest of horticulture crops in the early 1980's.
WFT is most threatening in environments with low diversity and high chemical and pesticide inputs. Generally good organic management will encourage self-reliant plants and a range of predators able to cope with all but the worst infestations of WFT.
WFT can complete its life cycle in 2-7 weeks, laying 2-5 eggs day for about 6 weeks. The actual time depends on temperature, and can vary from 15 – 25 days in temperatures of 20 – 30o C to 25 – 36 days in temperatures of 10 – 20o C). Prolonged higher temperatures will help to kill adults and dehydrate pupae and larvae. They usually pupate in the soil, where they can over-winter in the soil as a pupa.
General hygiene is very important. Keep rampant weeds away from the main crop. They may host WFT and WFT can transfer disease from weeds to the crop. A bare area or closely mown grass for 10 metres around the crop will form a barrier and reduce pest transfer. Pull spent blooms from flowering crops, compost or lightly turn in all organic waste, and most importantly do not transfer WFT between farms. If visiting areas where WFT is established, change clothes and brush hair before returning home. Only Propagate from healthy plants and closely examine seedlings before planting or bringing them home from the nursery.
Selecting planting dates when WFT numbers are low may help to reduce populations. Generally numbers are highest in summer and early autumn, when temperatures have been high for long enough to permit multiple generations.
Maintain vigilance in the crop. Note thrips if they are active, and be alert for signs of virus transfer. Th locate thrips, blow gently into flowers or shake the flowers over a piece of white paper. You may need to use a 10x hand lens.
Yellow or blue sticky traps (non-drying glue) are recommended. Traps for monitoring of WFT in filed and greenhouse crops are part of the National Strategy for the Control of WFT, at one per 100m2 (10 x 15cm traps, located just above the top of the crop). For control in an organic situation they may need to be used at a greater density, with some traps located in the crop, and combined with other techniques.
Installing insect screens on some glasshouses will greatly help, but may also reduce air transfer to the outside, causing management and production problems.
Cultivating lightly (shallowly) in winter may help to destroy pupae by exposing them to sun or to predators. It may help to seal glasshouses for 7 days prior to planting, The high temperatures will cause pupae to mature, and if the house is empty, the newly emerging adults will be unable to feed or shelter or lay eggs. Many will die from temperature, others can be killed using an acceptable organic pesticide.
The common red and black ladybird and lacewings are both very effective predators, but large numbers may be required for control in commercial situations.
Pesticides are only effective on larvae and adults, due to physical isolation and protection of the other growth stages (eggs are laid inside the plant, and pupae are mainly buried in the soil). Therefore it is necessary to devise a spray program which can control new adults emerging from eggs. Time to hatching is governed by temperature, but six applications 3 - 5 days apart will generally control all emerging larvae and pupae (3 in hot weather, 5 in cool conditions). Organic insecticides with some action against thrip include neem and spraying oils, sulphur or lime sulphur (not to be used in hot weather) and garlic sprays. Derris may also be effective, but pyrethrum is generally not very effective on thrips.
Two products from Organic Crop Protectants Pty Ltd, Neemazal®, containing 1% Azadirachtin (an extract from neem) and Eco Oil®, containing canola oil and tea tree oil extracts are the very promising for control of WFT.
Monitor for WFT with sticky traps, or by growing susceptible potted plants such as petunia, gloxinia and broan beans (they also reveal if tomato spotted wilt virus is around). Move pots into the crop to monitor WFT activity – they will show signs of TSWV (including wilting and bronzing of leaves and concentric yellowish rings and splotches on fruit) only a few days after infection.