Pine processionary development, pine processionary caterpillars are highly irritating to the skin. Contact with the hairs of the caterpillar can cause severe rashes (urticaria) and eye irritation. Some individuals may have an allergic reaction to the caterpillar's hairs. 5th stage larvae can eject hairs when threatened or stressed; the hairs, which have the form of harpoons, then penetrate and irritate all areas of exposed skin nearby with an urticating protein. Allergic reactions may follow in susceptible individuals on subsequent exposure to the hairs. The pine processionary is an economic pest in coniferous forests in southern Europe. It is controlled to some extent
Oak "Lymantria dispar", which can defoliate oak and other broadleaved tree species in North America. A considerable number of galls are found on oak leaves, buds, flowers, roots, etc. Examples are oak artichoke gall, oak marble gall, oak apple gall, knopper gall, and spangle gall. A number of species of fungus cause powdery mildew on oak species. In Europe the species "Erysiphe alphitoides" is the most common cause. A new and yet little understood disease of mature oaks, acute oak decline, has been reported in parts of the UK since 2009. Oak processionary moth ("Thaumetopoea processionea") has become a serious threat