Give pests an unwelcome winter hose-down...
Many thanks for your enquiry regarding winter washes, what they are and how they work. There are many of these products on the market, both chemical-based and organic, and they are designed to kill off overwintering pests and eggs, such as aphids on fruit trees.
The gooseberry sawfly you mention actually overwinters in the soil in a cocoon, so a winter wash would not be suitable for this pest. You can physically remove overwintering pest infestations by jets of water or by brushing off egg clusters (a toothbrush is ideal!) but an insecticide wash should give better results and coverage. Depending on which pest you’d like to evict from your fruit trees and bushes, and if you prefer too garden organically, I’d recommend Growing Success’s Winter Tree Wash – available from all good garden centres.
If you need any further advice on controlling winter pests on fruit bushes, please don’t hesitate to contact me and I hope the information I’ve provided above is of some help.