Only a limited number of the more than 1000 species of parasitic plants have a reputation as weeds in forestry and agriculture, causing unacceptable yield depressions. In forestry the dwarf mistletoes ( Arceuthobium spp.) cause, mainly in large area losses of timber of Coniferous trees (Pinaceae). The losses in the forestry regions of the western part of North America are estimated to be of the order of 20 million cubic meters of wood annually. The problems with these species increase as new afforested areas favour the spreading of the parasitic plants. Control is, until now, only possible by expensive cultural methods.