Comparison of sprays from a fixed overhead spray boom with overall air-assisted knapsack spraying for control of Tetranychus urticae and the effects on phytoseiids in raspberry

Abstract: Spotted wing drosophila (SWD), Drosophila suzukii, has established in the UK and this fruit pest is currently controlled with insecticide spray programmes, coupled with good farm hygiene. Given that much of the control for other pests in raspberry crops, such as the two-spotted spider mite, Tetranychus urticae, relies on biological control it is important to develop compatibility strategies for biocontrol by predatory mites with insecticide sprays for control of SWD. To determine whether the method of spraying could be important, experiments were done in small purpose-built poly-tunnels to compare the same insecticide spray programme applied by two different spraying methods, overall canopy spraying using an air-assisted knapsack sprayer and a system of overhead spraying to give spray deposits mainly on the upper leaf surface only. Populations of T. urticae and the predatory mite complex were recorded throughout the raspberry growing season and compared to an untreated control, over two seasons. The numbers of SWD emerging from picked raspberries were counted. The numbers of T. urticae were higher in the sprayed plots. In 2016, the natural phytoseiids were affected by the spray treatments, but the effect was mitigated by spraying from above, where there was markedly less spray on the underside of the leaves in the overhead spray treatment. Introduced Phytoseiulus persimilis was less affected by the spray programme than anticipated. In both years the numbers of SWD were lower in the treated plots compared to the untreated control, the overhead spraying performing as well as the overall spray.