Pine wilt is the result of the pine wood nematode (Bursaphelebchus xylophilus) invading xylem tissue. The nematode is usually transported from one tree to another via insects. The nematode breeds fairly rapidly and sometimes in association with bacteria quickly causes the vascular tissue to plugkilling the tree. It has been proven that the nematode alone can cause the tree to die. The nematode feeds on plant tissue or fungal mycelium such as blue stain canker mycelium. Scots pine is one of the most susceptible and eastern white pine is one of the most resistant. Other pines fall somewhere in between. It has also been found in other types of needle evergreens.
Most pines infected in the spring are often dead by late summer early fall. Large trees may take two years to die. Vigor of plant does not seem to have a bearing on which plants become infected. Infected plants quickly become stunted. Foliage begins to fade to and off green or slightly yellowish color. Dead needles hang on to the branches for a long time. On large trees that take two years to die, the older needles turn yellow first and fall off before the younger needles turns brown. Sawyer beetles, which are native long horns, are known to transmit this nematode. The beetles that nest in dead trees and feed on "healthy" trees are the most likely to transmit the nematode. One long horn can carry several thousand beetles on and in their bodies. The nematode can alter its development based on whether it is in a dead or drying out tree or in a tree still somewhat viable. The nematode can take between four and twelve days to go from egg to adult depending on temperature. Females lay dozen of eggs over several weeks before dying themselves.
Dying or dead trees are then not only attacked by longhorns but also bark beetles and other saprophytic insects. In addition saprophytic nematodes and predator nematodes can be found on the dead pines. Proper identification of the nematodes is important. The nematode presence does not necessarily prove that they killed the tree. Research has shown that under certain growing conditions the pines do not die. Pines under stress are more likely to be killed by the pine wood nematode.
Have suspected plants tested. If results are positive, remove diseased and dead evergreens as soon as possible to reduce breeding sites for the insects that transmit the pine wood nematode.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Bruce Paulsrud, Extension Specialist, Pesticide Applicator Training and Plant Pathology, University of Illinois at Urbana-Champaign.