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Thrips causing damage to crops is not new but Western Flower Thrips were only introduced to Australia in 1993. Western flower thrips are similar in appearance and biology to the onion and tomato thrips which are common in Australia. However, due to its efficiency at spreading tomato spotted wilt virus and the feeding damage caused by western flower thrips, millions of dollars of crop damage has been reported overseas and in Australia. Australian researchers have developed this CD to help you identify and The CD contains very well developed videos in English and Vietnamese to give you a good understanding of how to manage the pest including:
Identifying the pest : Western Flower Thrips, referred to as WFT, is a serious pest of horticultural crops in Australia. It not only causes feeding damage but also transmits a virus to plants. This virus is Tomato Spotted Wilt Virus or TSWV. WFT, like many other thrips species, is about 1mm long and ranges in colour from pale yellow through to light brown. It looks very similar to other thrips species like tomato thrips and onion thrips. A high magnification microscope is needed to positively identify WFT. So if you suspect your crops could be home to this pest, you should take a sample to be analysed by a specialist. Thrips and TSWV can cause serious damage to crops and a small infestation of thrips can quickly cause a major problem if they’re not controlled. It’s important to regularly check for thrips in your crops. Early warning of infestations is crucial in managing the pest. There are two ways to check for thrips. The first is by using yellow sticky traps. These should be hung about 10cm above the canopy of the crop and inspected weekly. While sticky traps are a useful monitoring tool for thrips, crop inspection is a much more reliable method and it also allows you to check for TSWV at the same time. Thrips like to hide in flowers and buds or on young plant growth. To check your crop, either tap flowers and leaves over a sticky trap or container and check under a microscope, or collect a sample for examination by an expert. Thrips infestations in crops can be quite patchy so it is important to check plants in quite a few areas of the crop and surrounding vegetation. How the damage is caused : Western flower thrips causes damage in two ways. By feeding on plants and by transmitting Tomato Spotted Wilt Virus. There are three other thrips species that transmit TSWV, tomato thrips, onion thrips and melon thrips. The virus is transmitted like this….. An egg is laid onto an already infected plant. When the larval stage of the thrips hatches out, it doesn’t have any wings so it usually feeds on the infected plant it was laid on. Thrips can only contract the virus by feeding at the very early larval stages. The thrips then develop into a pupal stage, which with WFT is usually below the plant in the litter layer. When the adult thrips emerge it will be carrying the virus. The adult then feeds on other plants and the virus is spread. WFT needs to feed for as little as 5 minutes to transmit the virus to a plant. The virus then multiplies throughout the plant and the symptoms develop. Symptoms of Tomato Spotted Wilt Virus include blotchy and deformed fruit. Common symptoms of the virus on leaves include spots of dead tissue, distortion and browning. Stunting and plant death are more advanced symptoms of TSWV. Cucumber plants can also contract the virus but show no symptoms. Non-chemical control : There are a number of strategies growers can adopt to manage thrips and viruses. One of the most important things growers can do is control weeds in and around their crops. Weeds can hide both thrips and virus and these can easily move across to your crops. WFT don’t like to cross areas of bare ground and distances of as little as 10 metres can be a barrier to them. So to help protect your crops, remove weeds from within the crop and maintain bare ground or closely mown grass for at least 10 metres around your crops. You should also destroy old crops. Thrips and virus can live in them and then move into new or adjacent crops. If possible avoid sequential planting or sowing susceptible crops close to each other. Use non-host species or fallow between plantings. Remove any plants showing virus symptoms. These are a reservoir for thrips to transmit the virus to uninfected plants. Purchase seedlings from virus-tested nurseries or propagate on site. TSWV is not transmitted through seed. Plant TSWV resistant varieties where possible. WFT has a number of predators. Some of these occur naturally in crops and some can be purchased from biological control suppliers. If using natural enemies you need to be careful that any pesticides you apply do not affect the predators. Chemical control : Western Flower Thrips develops resistance to chemicals very quickly. It is already resistant to a number of commonly used chemicals. For this reason it is important you choose chemicals for WFT control very carefully. When using insecticides you must ensure that you get good coverage of your crops. Spray equipment should be checked and calibrated regularly to make sure it is working properly. Control with insecticides is complicated by limited access to WFT at different lifestages. Eggs are laid into leaves are unable to be contacted by chemicals. However when the larvae hatch they live and feed on the plant surface which makes them much easier to kill with chemicals. At the pupal stage they’re hidden again in the leaf litter layer and the pupae has a thickened covering. Chemical sprays are generally ineffective at this stage of their development. Once they grow into adults, thrips again feed on the plant surface making them easier to target. Because they prefer secluded spots, good spray coverage is needed to reach them. For best results you should target WFT at both the larval and adult stages of the lifecycle. Sprays should be applied at intervals. The length of time between sprays depends on temperature. A series of 3 consecutive sprays should be applied at the appropriate intervals. After the 3 sprays you should again check for thrips levels. Remember that with some sprays it can take a few days for their full effect to be seen. If you decide a further series of sprays is needed, use a chemical from a different chemical group to avoid resistance developing. After the second series of sprays you shouldn’t need additional control for at least 2 to 3 weeks. If monitoring shows the need to spray earlier then it could mean the thrips have become resistant to the insecticide you’re using, your spray application is not appropriate, or your non-chemical controls are not adequate. You will need to change to a new chemical, or look for other control methods. Managing thrips and viruses takes time and planning, but is achievable. Getting some professional advice at this stage could save you a lot of time and a lot of money. |
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