The fungi are the most varied, common, and important plant pathogens, but the great majority of fungal pathogens do not require mobile vectors such as insects or mites. Instead fungal pathogens disperse to plants mainly in wind, rain, or soil. A large variety of fungi colonize plant wounds, including those made by arthropod feeding. However, some fungi are specialized for transmission by insect vectors. Dutch elm disease is the best-known example of a fungal disease of plants transmitted by an insect vector. The causal fungus, Ophiostoma ulmi, grows into a spore-bearing fungal mass (mycelium) under the bark and into the water-conducting woody tissues of elms. Adult bark beetles, such as the European elm bark beetle (Scolytus multistriatus), are especially attracted to distressed elms or freshly cut elm logs. The adult beetles excavate a tunnel by feeding beneath the bark and deposit eggs along the tunnel. Beetle larvae hatch from the eggs, tunnel further under the bark, pupate, and then emerge as adults the following year. The Dutch elm disease fungus grows throughout brood chambers excavated by the beetles and produces sticky spores that attach to body and mouthparts of the adult beetles that bore out of the bark to exit the tree. The beetles transmit the fungal spores to wounds they inflict while feeding on elm twigs. The fungus gradually spreads from the point of infection into the larger branches of the tree, then to the tree’s trunk, where its action on the woody tissues eventually kills the tree. In very cold climates of North America, the native elm bark beetle (Hylurgopinus rufipes) is the main Dutch elm disease vector. Its transmission of fungal spores to elms leads to more rapid development of disease because it feeds principally on the trunk and large branches of the elm tree rather than small branches. Oak wilt disease, caused by the fungus Ceratocystis fagacearum, is spread by sap beetles (family Nitidulidae) that vector the spores from oozing cankers on diseased trees to fresh wounds on trees to which these beetles are attracted. Some insects can create wounds which fungi can then colonize without transport by the insects. Yet even though insects in these cases are not vectors of the fungi, they can be important in determining how severe fungal infestation becomes. An example is a variety of fungi that can colonize the feeding wounds of caterpillars that feed on maize or peanuts. Some of these fungi (notably Aspergillus species) can produce powerful toxins, called aflatoxins that sicken or even kill animals that are sensitive to the toxins. Preventing insect damage to grain in the field or storage is an important step in preventing high levels of aflatoxins.