Silicon adjuvant combats tarsonemid mite

Control of the tarsonemid mite - one of strawberries' most widespread and damaging pests - can be greatly improved by adding a silicon-based adjuvant to the pesticide being used.

This has been shown in an HDC-funded trial carried out in a poly-tunnel by EMR entomologist Dr Jerry Cross.

The everbearer strawberry plants involved in the trial were artificially heavily infested with tarsonemid mite and sprayed twice in August with a range of contact-acting miticides, with and without the addition of the silicon wetter Silwet L-77.

The miticides used in the study included Masai (tebufenpyrad), Dynamec (abamectin) and UKA378b, a ketoenole compound being developed by Bayer CropScience that is not expected to be available commercially for at least two years.

Cross said: "None of the treatments was very good on its own but with the addition of Silwet Dynamec and UKA378b gave almost 100 per cent per cent control of the pest."

He explained that the pest is usually very difficult to combat by spraying because it lives on the upper leaf vein between the folds of young unfolded leaves, so it tends to be inaccessible to sprays.

This problem is exacerbated by the waxy and hairy nature of strawberry leaves - something not generally known by growers - and by the fact that very few miticides are systemic (entering the plant through its roots and shoots) and therefore cannot reach the unexposed target.

Dynamec has a SOLA for use on strawberries at a recommended rate of 500ml/ha.

Neil Helyer of Fargro - which markets Dynamec - reckons that the reason for the adjuvant warning is because "somewhere along the line (the combination) might have damaged a crop" and so the warning is purely a precautionary one.

To be on the safe side, Helyer and Cross advised growers to try Dynamec plus Silwet initially on a small number of plants and inspect them after about seven days to see if any phytoxicity has developed. Even if it has not, treatment of the crop would be at growers' risk.

Cross also pointed out that growers' first line of attack against tarsonemid mites is the introduction of the predator Amblyseius cucumeris. When temperatures are high, however, the predator cannot keep pace with the pest's multiplication on everbearers and that is generally when a curative spray of Dynamec plus Silwet would be necessary.

June-bearers are usually sprayed in the autumn so they go into the winter mite-free.

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