The use of surfactants to enhance acaricide control of Phytonemus pallidus (Acari: Tarsonemidae) in strawberry

The tarsonemid mite, Phytonemus (Tarsonemus) pallidus ssp. fragariae (Zimmerman), is a serious pest of strawberry throughout much of Europe and North America. Many of the plant protection products previously effective against the mite, which causes stunting and distorting of leaves and buds, reducing fruit yield and quality, are no longer available. Finding alternative acaricides for control is a priority. In two field trials, we tested 9 foliar applied plant protection products to determine the efficacy for control of Phytonemus pallidus, and to assess their effects on natural enemies. The products tested as foliar applications (dose ha−1) were; abamectin 18 g/l EC (250, 500, 750, 1250 ml), tebufenpyrad 20% WP (500 g), fenpyroximate 51.3 g/l SC (1.0 l), bifenazate > 90% WP (400 ml), spiromesifen 24 g/l SC (500 ml), acequinocyl 15.8% SC (1.0 l), spirodiclofen 240 g/l SC (400 ml), etoxazole 110 g/l SC, and the surfactant, Silwett L-77 (Heptamethyltrisiloxane 0.05% (50 ml)). Effective products were used in admixture with the surfactant to investigate potential increased efficiency against P. pallidus. In each experiment, two applications were applied to plants previously inoculated with P. pallidus.None of the products controlled the mite effectively when applied alone, but the addition of Silwett L-77 greatly improved control of the pest, particularly in combination with abamectin, etoxazole and tebufenpyrad. These treatments reduced P. pallidus mite numbers by 98, 85 and 80%, respectively. Many of the acaricidal products that are normally active against other species of Acari probably fail to control P. pallidus because of the inaccessibility of the mite within the fold of the young leaflets. In addition, the leaves of strawberry plants are more waxy and hairy than most other crops and are difficult to wet. Hence, significant improvement in efficacy of some products can be achieved if mixed with the silicone surfactant.