The pine wilt nematode is not something most of us in the Mountain West have heard of before. I personally came to learn about it after witnessing its effects in my landscape and in surrounding communities. You may have noticed mature and established pines decline rapidly and die for no apparent reason. One possible cause of these tree deaths is due to the presence of the pine wilt nematode (PWN) Bursaphelenchus xylophilus. Nematodes like this one are microscopic round worms found in soil, water, plants and animals.
PWN colonizes the vascular tissue of the host pine where its population grows until it clogs the vascular tissue of the tree preventing transpiration. The first signs of infection are often browning of the tips of branches and needles in winter and early spring, followed by an accelerated decline and death of the tree as the season warms up.
The mode of transmission for this often-fatal nematode is the sawyer pine beetle, Monochamus spp. This beetle’s life cycle is dependent on pine species for feeding and breeding, but is more of a nuisance to pines, generally posing no threat to overall survival. When these beetles feed or breed on infected host pines they can then transmit the nematode to neighboring pines.
Are All Pines Susceptible?
The PWN is native to North America and has been slowly spreading westward from the Midwest over the last couple of decades. Common pine species native to the Mountain West including ponderosa pine (Pinus ponderosa) and pinyon pine (Pinus edulis), appear to have greater resistance to the pest but are still vulnerable to stress and death. Non-native species are at much greater risk. Austrian pine (Pinus nigra), Scotch pine (Pinus sylvestris), and mugo pine (Pinus mugo) are common casualties to the nematode. Other introduced species such as limber pine (Pinus flexilis) and Bosnian pine (Pinus heidreichii) are also very vulnerable. From personal observations in the field, I believe that Rocky Mountain juniper (Juniperus scopulorum) and other juniper species are also vulnerable to death caused by PWN.
How to Treat for PWN
Luckily, there are solutions and treatments to the PWN. Unlike common insect pests, nematodes can only be irradicated and controlled with anthelmintic chemicals. Abamectin and emamectin benzoate are the most common chemicals used in the horticultural and forestry industries to control nematodes. The most successful chemical application for pine treatment against PWN is trunk injections. This method requires injection equipment that directly administers the chemicals to the active vascular tissue of the tree. Abamectin and emamectin benzoate can also be absorbed into a plant through a foliar spray or root drench, yet this method for treatment against PWN is unproven.
Unfortunately, by the time visual symptoms indicating pine wilt nematode infestation are observed, it is often too late to counteract, and a tree heavily infested will still likely die, even with treatment. The best defense is preventative treatment and chemical application. Studies show chemical applications every two years are effective in preventing stress and death due to PWN. It is not likely that the chemicals persist within the tree for two years, rather, that the nematode infestation likely takes two years to reach the point of fatal effects. Biannual treatments should properly manage PWN and prevent the death of the tree.
Given that PWN may be more lethal to the non-native species, this offers a good reason why using native pines is advantageous. Nursery professionals should keep an eye out for this pest and keep their customers, as well as the industry at large, informed of its presence, when found. Furthermore, understanding and applying appropriate treatment options can help keep PWN in check.
Derek McGuar is a Colorado native with a degree in Botany from Colorado State University. He has worked many positions in the Colorado nursery industry, leading up to managing a large production farm in Fort Collins. From that experience, he started his own production farm growing perennials and grasses for the Colorado market.