Leaves of a redcurrant bush (Ribes rubrum) showing blister galls. The picture shows the upper surface of leaves which are host to the redcurrant blister aphid Cryptomyzus ribis. The red "blisters" are galls that have been induced by the presence of aphids on the underside of the leaves. They develop between the main veins of the leaf, producing the fan-shaped pattern visible here. The aphid overwinters as eggs on dormant twigs of the bush. In Spring, the eggs hatch to wingless aphids (aptera) that reproduce parthenogenetically on the underside of the leaves, producing the surface distortion and anomalous colouration visible here. In Summer, winged aphids (alates) are produced; these migrate to other plants, such as hedge woundwort. In Autumn, the aphids return to the redcurrants and lay their eggs. Although a common and striking sight, this blistering has little effect on the crop.