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Disease Management

In the midwestern United States, the disease has the greatest impact on homeowners and planted natural areas, wherever susceptible, non-native, poorly adapted pines are planted. Large plantings of Scots pine as windbreaks and landscape planting occurred especially in the Midwest, where this tree species was considered well adapted to the environmental conditions. Stands of single tree species (monocultures) provide an excellent breeding site for the insects and associated nematodes. Because the trees become more susceptible after they reach 10 years of age, the affected and dying trees are well-established trees and their removal has a big impact on the landscape.

Scots pine is the tree of choice for Christmas tree plantations in the Midwest. Most Christmas trees are harvested before they reach 10 years of age unless they are kept for display in large commercial buildings. The biggest threats to existing Scots pines are abandoned Christmas tree plantations, homeowners who do not remove the dead trees, and public areas where dead trees are left standing.

Genetic resistance

Homeowners should be discouraged from planting susceptible tree species, including Scots pine, Japanese red, or black pine as ornamentals. Within these susceptible tree species, there is no genetic resistance to pine wilt disease although when trees die, not all die at the same time. At this time it is not known whether the longevity of certain trees in a grouping, such as a windbreak, is due to a form of resistance, avoidance, or other factors. It is not uncommon, however, to see an entire windbreak die from pine wilt disease over the course of ten years. This would suggest that resistance was not present, but that for whatever reasons the beetles feed differentially on the trees in the windbreak.

Justin Evertson from the Nebraska Statewide Arboretum has suggested the following as better Midwestern-adapted evergreen tree species [the "(N)" indicates trees native to the Midwest or Great Plains]: balsam fir, concolor fir, nikko fir, Korean fir, Chinese juniper, Rocky Mountain juniper, eastern red cedar (N), Norway spruce, white spruce, Serbian spruce, Colorado spruce, Chinese white pine, jack pine, lacebark pine, Swiss stone pine, pinyon pine, lodgepole pine, limber pine (N), Bosnian pine, Korean pine, mugo pine, Japanese white pine, ponderosa pine (N), Balkan pine, Virginia pine, Douglas fir, Japanese yew, anglojap yew, eastern arborvitae, western arborvitae, and eastern hemlock. There are thus many tree species homeowners can plant that will not succumb to pine wilt.

Cultural practices

Pine wilt management at the current time in the United States is best achieved by removal of symptomatic trees. Adult beetles are attracted to recently dead, dying, or freshly cut wood (Figure 11). The beetles fly from dying trees to healthy trees in the spring until the fall and therefore spread of the disease from tree to tree does not occur during the winter months. Depending on the temperature from spring through fall, the beetle can have several generations during a growing season, so trees can be infected anytime until the onset of winter. Dead trees should be burned or buried as soon as possible. It is important that the stumps also be removed or buried, as the stumps are attractive to the sawyer beetles.

Figure 11.

In areas of the world where susceptible trees are common and the disease is not present (Europe and Scandinavia), avoidance is the primary means of control. There are strict quarantines on import of wood into these countries. Wood can be heat-treated or debarked before export, in some cases (Figure 12).

Figure 12.

Chemical control

In the United States, nematicide and insecticide treatment of trees is impractical, very expensive, and ineffective. In Japan, where the disease is much more devastating, insecticide and nematicide treatments, biological control, and induced resistance through use of less virulent strains of Bursaphelenchus have been investigated.