First steps towards a robust non-chemical thrips control strategy in field grown strawberries

Abstract: Strawberry production in summer in the Netherlands predominantly occurs in the field. In the open field production system the crop is flowering during c. 4 weeks overlapped and followed by growing and ripening of the fruit. A key insect pest is thrips, predominantly Thrips tabaci, T. fuscipennis and T. major. Adults and larvae hide in and feed on the flowers, resulting in deformed and discolored fruit. Because of the overlap between flowering and fruit development, chemical thrips control is a particular concern with respect to food safety requirements. Biological control can help to control thrips, but in the current production system not to a satisfying extent, making growers feel to be in a Russian roulette without chemical insecticides.Several non-chemical control measures are known to have efficacy, to varying extents (stability of efficacy). Predatory mites and bugs and the use of flower strips or banker plants, providing alternative food as well as shelter to predators, may improve thrips control. Also entomopathogenic fungi like Lecanicillium muscarium have some efficacy to control thrips. The combination of these measures into a strategy could provide a cumulative effect, comparable to the use of insecticides.A strip trial was carried out on a commercial farm to test the principle of cumulative measures. Three subsequent strawberry plantings, on 11 and 25 May and 9 June, were used to facilitate predators – once released – to migrate along with the flowering and thrips from one planting to another, thus reducing the total numbers of predators to be released. Orius majusculus was used as predatory bug, with Alyssum sp. as a banker plant species. L. muscarium would be applied if on average more than two thrips per flower were counted. Both bug and thrips counts were made twice a week during flowering, to assess both migratory aspects and pest and predator development over time. Fruit quality was assessed in the white fruit stage and at harvest.Thrips counts exceeded the threshold twice in the first planting and once in the second, followed by L. muscarium sprays The results in the first two plantings were promising, with similar product quality as the surrounding practice field as claimed by the grower. In the third planting average thrips numbers did virtually not cross the damage threshold, resulting in refraining from additional sprays. Product quality however was poor, with various possible explanations. The trial is both informative and questioning in many ways, demonstrating the necessity to integrate many components to build a reliable alternative to chemical control, and in the meantime get sufficient grip on all of these components. After all, growers will only embrace it as an alternative if both stability and reliability of the system are comparable to chemical control.