Gooseberry sawflies lay eggs on your gooseberry plants. When the caterpillars hatch out they can devastate the plant leaves so that you end up with a bush that looks like this -
Gooseberry sawfly damage
They also attack red and white currant bushes. The fly arrives anytime from April onward. The female sawflies lay eggs on the underside of leaves, low down in the center of the bush, so the young larvae go unnoticed until they have eaten their way upwards and outwards, devouring the leaves as they go - often defoliating a bush completely.