If they've been feeding on a nearby oak that's infected with wilt, they can carry the disease to another tree. In the plant world as in the animal world, disease occurs when a plant is exposed to a virus or a bacteria .
Exposure can occur in a variety of ways as described below.
Since its introduction form Europe about 1930, it has destroyed millions of elm trees across North America. It has probably been here at undetected level few some years now - in my opinion. The fungus responsible for Dutch elm disease is thought to have originated in Asia. This site of origin is proposed because Asian species of elm demonstrate at least a moderate level of resistance to the fungus which causes the disease. The leaves on one or more branches in an affected tree will be light almost lime-green and appear wilted.