Sustainable Management of the Western Flower Thrips in Strawberry Crops

Thrips are an ancient order of insects (order Thysanoptera) that have been around since the time of the dinosaurs. They are so adaptable that they are found from the Arctic to the tropics, feeding on a wide range of plant, animal and fungal hosts. Thrips are small in size, typically around 2mm long, and their delicate fringed wings make them weak fliers. This makes them unlikely candidates for causing devastating crop loss, yet thrips cause billions of pounds loss to the global economy every year. The most damaging species spread plant viruses, as well as causing direct feeding damage with their piercing-sucking mouthparts. The western flower thrips, Frankliniella occidentalis, is considered the most economically damaging of these on a global scale. This article explored why WFT has become such a devastating pest in one crop (strawberry) and how it can be managed in a sustainable way. Originally from western USA, WFT has spread rapidly around the world since the 1970s. Several factors have contributed to this spread: this highly adaptable species has been recorded on over 240 host plants including over 80 crop species, so it can establish in new habitats relatively easily; its ability to break down toxins has meant that it has developed resistance to all the pesticides commonly used against it; WFT evade capture by predators and humans by hiding in small cracks and crevices, which has allowed them to spread around the world through the increasingly global plant trade; global warming may also contribute to their gradual spread Northwards. WFT was first recorded in the UK in 1986, when it became a major pest of glasshouse crops such as cucumber, pepper and chrysanthemum. The occurrence in strawberry coincided with the increased use of polytunnels, which provide a warmer environment in which WFT thrive. At 25°C, adult WFT females have a 2-week generation time and lay about 7 eggs per day on a flowering strawberry crop, so the potential for a rapid population increase is massive, and growers can lose a crop in a short time when thrips are uncontrolled. Faced with this threat, growers turned initially to chemical insecticides, but with few chemicals available and increasing levels of pesticide resistance, control broke down and WFT became the most economically damaging pest of UK strawberry. Both adult and larval WFT feed on strawberry fruit, causing it to turn a bronze colour which makes it unmarketable. The annual loss of UK strawberry production due to WFT was estimated at 10–15%, with individual farmers losing entire crops. In a survey of growers, the breakdown in WFT control in UK strawberry crops could be linked to four main causes: 1. WFT has become resistant to all the pesticide products registered for use on strawberry. 2. There was a large carry-over of pests from the previous season in crops with poor control, either from over-wintered crops, or from reused, untreated growbags, resulting in damage following first-flowering. 3. Predators were released too late, or in insufficient quantities. 4. Crop protection products that are harmful to predators were used during the time when predatory mites were being used, or there was repeated use of slightly or moderately harmful pesticides, which prevented predator establishment and interrupted thrips control. Some growers consistently manage thrips with minimal fruit damage. These growers release predatory mites routinely from flowering, and combined biological control with mass trapping in one-year crops, and are careful to avoid broadspectrum, long-residual pesticides during the time when the predators were working. If insecticide treatment is required, these growers are careful to select products that will have limited impact on predators, then re-introduce predators after treatment. Therefore, the most effective approach to thrips control is to integrate all methods.