The effect ofNysius huttoniWhite feeding on immature plant seeds was examined by caging the insect with the seed heads of the following plants: wireweed(Polygonum aviculare),twin cress(Coronopus didymus),yarrow(Achillea mille‐folium),fathen(Chenopodium album),and native wheat grass. The largest seeds (wheat grass and wireweed) were the most physically damaged with shrivelling and colour changes. The amount of N.huttoniproteinases in infested seeds varied widely from 48 enzyme units/g wireweed seed to 8075 enzyme units/g yarrow seed, but there appeared to be no relationship between proteinase content and the amount of physical damage to the seeds. Sodium dodecyl sulphate (SDS)‐polyacrylamide gel electro‐phoresis showed that high levels of proteinase in twin cress and yarrow seeds could hydrolyse protein subunits of a wide range of molecular weights. In contrast, incubation of undamaged twin cress and yarrow seeds with proteinase purified fromN.Wtom‐damaged wheat grain, did not hydrolyse any protein. The purified enzyme only hydrolysed high molecular weight (HMW) protein subunits in wheat‐grass seeds and wheat grain(Triticum aestivum),and possibly protein macropolymers in giant Spaniard(Aciphylla scott‐thomsonii)and koromiko{Hebe salicifolia)seeds. These HMW proteins in wheat grass, giant Spaniard, and koromiko seeds may have similar structure to the HMW glutenin subunits of wheat grain. We conclude that whenN. huttonifeeds on some seeds it can inject proteinases of a broader specificity range than the highly specific proteinase found inN. huttoni‐damagsdwheat, and we suggest that particularly the latter enzyme has a role in softening the outer layers of seeds to facilitate bug stylus penetration and maintain the flow of sap.