Pine wilt nematode—Bursaphelenchus xylophilus
Pine wilt nematode, also called pine wood nematode or pinewood nematode, is a microscopic roundworm native to North America. In native conifers weakened or damaged by other causes, the nematode feeds harmlessly on fungi that bark- and wood-boring beetles introduce into the trees. But when the nematode infects exotic pines, it feeds within their vascular system and kills the trees.
Identification
Because the adult nematodes are only 1/50 to 1/25 inch (0.5 to 1 mm) long and occur inside conifers, the damage they cause (as described below) is generally the first clue of this nematode’s presence. Because the nematodes are spread by adults of sawyers (Monochamus species of longhorned beetles or roundheaded borers, family Cerambycidae), when these beetles infest a conifer, it is likely that pine wilt nematodes are also present in the tree.
The nematode adults and juveniles are translucent, long and slender, and bluntly rounded at both ends. Their width is about 1/30 of their length. The nematodes superficially resemble those of various other species and an expert examination of microscopic characters is needed to identify them.
Life cycle
The nematode develops through 3 life stages: egg, juvenile, and adult. After hatching from an egg, the juveniles develop through 4 increasingly larger stages before becoming adults. When temperatures are warm, pine wilt nematode can develop from an egg to a reproductive adult in about 4 days.
Pine wilt nematode has two types of life cycle: fungus feeding and plant feeding. The nematode's fungus-feeding cycle occurs in conifers native to North America. The plant-feeding cycle occurs when the nematodes infect exotic pines. In both cases the nematodes are transmitted from one host tree to another by sawyer beetles.
In conifers native to North America, the nematode only feeds on the hyphae of wood-infecting blue stain fungi, primarily Ceratocystis species that are introduced to stressed and dying conifers by various wood-boring beetles. Fungus feeding by the nematode is harmless to trees. It is the blue stain fungi, bark- and wood-boring beetles, and the factors that caused the trees to become susceptible to these (e.g., drought stress) that cause tree decline or death.
Sawyer beetles are attracted to lay their eggs into stressed and dying conifers. As the adult female beetles chew an egg-laying hole in conifer bark and wood, the nematodes move from the beetles into the wood. After the beetles complete their feeding as wood boring larvae, they pupate in the wood. The nematodes are attracted to the beetle pupae. As soon as they change into the next generation of adults, the nematodes move to the beetles and beneath their wing covers and into their trachea (breathing tubes). The new adult beetles then leave the tree and carry the nematodes to another host.
Before adult sawyers lay eggs, they chew and feed on conifer foliage. If the foliage-feeding is on a pine species not native to North America, the nematodes are able to enter the shoots through the beetles' feeding wounds. The nematodes then colonize the tree's vascular system and feed in the resin ducts of sapwood. As the nematodes feed and reproduce in the exotic pine, the tree becomes increasingly unable to produce resin that is important to pine health. Pine shoots die, followed later by the entire tree.
Damage
One indication that pine wilt nematode is infesting an exotic pine's vascular system is the lack of resin exuded after woody parts are cut or otherwise wounded. Resin oozing is characteristic of wounded conifers.
As pine wilt nematodes feed in the vascular tissue of stems, trunks, and twigs of exotic pines, they cause foliage to yellow and wilt then turn brown as the infested plant parts are killed. Symptoms may first appear in one or several branches. Later the entire tree exhibits symptoms. Note that various other pine maladies also cause foliage to discolor and die. See the table "Comparison of Pine Tree Maladies with Some Similar Symptoms" in Pest Notes: Pitch Canker to help diagnose the cause of dying pines.
Cedar, Douglas-fir, fir, larch, and spruce can become colonized by pine wilt nematodes but are rarely damaged by them. Generally only pine species that are not native to North America are damaged or killed by the nematode because it is only in these hosts that the nematode's plant-feeding life cycle occurs. Exotic pines commonly die 30 to 40 days after becoming infected by the nematodes. By then the host can contain millions of nematodes throughout its branches, roots, and trunk. These nematodes are then spread to other hosts by sawyer beetles.
Solutions
Provide trees a good growing environment and proper cultural care to reduce the likelihood they will be attacked by nematode-vectoring beetles. No direct management practices are available except for removal of dying limbs or trees. Replant with nonhost species, such as pines native to North America or non-pine conifers.
Adapted from Bursaphelenchus xylophilus (pine wilt nematode), CAB International. |
Shoots of an exotic pine killed by pine wilt nematode. Other maladies can cause this same damage.
Exotic pines dying from infection with pine wilt nematode.
An adult sawyer beetle, one of several Monochamus species that spread pine wilt nematodes.
Adult pine wilt nematode, which is 1/50 to 1/25 inch (0.5 to 1 mm) long.
Logs infected with blue stain fungi, the food of pine wilt nematode when it feeds in native conifers.
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