SOME OBSERVATIONS UPON THE “RED SPIDER”, TETRANYCHUS TELARIUS L., ON HOPS AND ITS CONTROL, WITH NOTES ON SOME PREDATORY INSECTS

Abstract

Summary. Treatment of the soil around hops in November, with crude naphthalene at the rate of 300 lb. per acre, did not prevent infestation by Hop Red Spider in the following summer. Examination of hop poles showed that large numbers of mites may hibernate in deep cracks in the poles. h emulsion containing 5 per cent. of a high‐boiling neutral tar oil, sprayed with force into the cracks of poles, killed the great majority of mites. It was1 more effective in this respect than an emulsion containing 5 per cent. of a semi‐refined petroleum oil. Of a number of washes applied in the summer to infested hops, emulsions of a highly refined (water‐white) petroleum oil at 1 and 2 per cent. oil concentrations, were effective in killing the mites. Lime sulphur a t concentrations of 1 in 30 and 1 in 60 also gave a complete control. A Derris wash of 0.0056 per cent. rotenone content appeared to give a complete control at the second applicabion whereas it failed to do so at the first. 'There is evidence that the toxicity of a Derris wash of the type used increasies up to a point with time after water is added to the powder. Liver of sulphur and colloidal sulphur a t the concentrations used showed no marked toxicity and the two spreaders used were also non‐toxic. The petroleum‐oil emulsions and lime sulphur caused some injury to foliage when applied in late June and the first three weeks in July. Lime sulphur caused no injury to foliage or “burr” on the varieties Fuggles, Cobbs and Early Birds, when applied during late July and early August, a t concentrations ranging from 1 in 60 to 1 in 150. Concentrations smaller than 1 in 100 were not markedly toxic to the mites, but at 1 in 60 and 1 in 80 lime‐sulphur wash was completely effective in killing all forms other than eggs. Notes are added on three insect predators of the Hop Red Spider.