B. xylophilius plate
In North America, where B. xylophilus
is native, there is very little wilt expression or tree mortality and the nematode persists primarily through transfer during oviposition (egg-laying) by female Monochamus spp.
Under these circumstances, B. xylophilus
breeds on any residual intact cells in the dying or dead tree and, particularly, on fungi that exploit the dead tree. This saprophytic cycle is seen at all locations where PWN exists, even when trees are not killed by the nematode as in North America, including Canada.
A critical factor in the cycle illustrated is the fact that Monochamus spp
. females are unable to lay eggs in living host trees and consequently the transfer of PWN during oviposition is the primary co-evolved mechanism that ensures survival of the nematode and retains it in close proximity to the vector breeding site enabling further transfer to new hosts.
It is also significant that virtually all conifer hosts, with the exception of Thuja
, are suitable for the saprophytic phase of the nematode cycle, which extends the availability of suitable host trees in Europe significantly.
Pine wilt, leading to tree mortality, is a phenomenon that is only noted under particular combinations of susceptible tree species and eco-climatic conditions, especially high temperatures and low soil moisture contents.