Pine wilt nematode

Bursaphelenchus xylophilus, commonly known as pine wilt nematode (PWN) or the pine wood nematode, are exotic plant pests that are not present in Australia.

PWN are a serious threat to Australia’s forestry industry. This species:

• infects trees in the Pinus genus of coniferous trees
• causes the disease pine wilt.

Background

PWN has a typical nematode life cycle, with 4 juvenile stages and an adult stage. Both male and female individuals reproduce sexually.

The mycophagous phase of the life cycle takes place in dead or dying wood, where the nematodes live and feed upon fungi, rather than the wood itself. The nematode cannot travel outside of the wood independently; it must be transported by an insect vector.

Vectors

The pine wilt nematode is spread by bark beetles and wood borers, typically associated with the genus Monochamus of pine sawyers.

Pine sawyer beetles lay their eggs in the bark of dead timber. The growing larva feeds on the wood and pupates in the resulting cavity.

Nematodes of the third juvenile stage congregate in the cavity around the pupa, and moult into the fourth juvenile stage, then invade the trachea of the adult beetle. During this dispersive stage, the beetle transports the nematode to other trees.

^{Schematic representation of the interrelationships between the pinewood nematode and its insect vector (adapted from Evans et al.)}

Impacts

Pine wilt nematodes mostly affect Pinus but will also attack:

• Spruce (Picea)
• Douglas fir (Pseudotsuga menziseii)
• Larch (Larix)
• Fir (Abies).

Few pines are known to be killed by this nematode as mature trees, including Maritime pine (P. pinaster), Black pine (P. nigra) and Scots pine (P. sylvestris).

Identification

The first symptoms of a pine wilt nematode infestation include brown and dry needles in the tree branches. Tree death usually progresses from the top of the tree downward.

A serious nematode infestation can result in rapid dieback of the pine tree, sometimes within a few weeks.

Many pine trees normally have dead needles present in the canopy and the odd dead branch – up to 15% of the tree canopy can be affected this way due to environmental factors. Suspected nematode presence needs to be confirmed by a technical expert.