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novel control of nematode
From the International Ecology Congress
IE 2002 - Day 1 - Tuesday 13 August 2002
Vaccine design branches out
Investigator: Hajime Kosaka
Tuesday Aug 13th, 2002
by John Bonner
Damage caused by pine
Image courtesy of
Japanese researchers are using a form of vaccination to control the
spread of a disease that is destroying the country's pine forests.
Injecting healthy trees with a non-virulent strain of the nematode
parasite responsible induces resistance to later attack with the wild
strain, the scientists report.
Pine wilt disease was introduced into Japan from North America at the
beginning of the last century. Over the next decades, it spread
throughout the Japanese archipelago with the exception of the most
More recently, it was accidentally exported to several neighboring
states, including South Korea in 1988, and to Taiwan and China. In 1999,
it also established a foothold in Europe, and damaged Portuguese pine
At the height of the Japanese epidemic in 1979, the disease destroyed an
estimated 2.4 million cubic metres of pine timber. Through the 1990s,
even with a reduced population of mature trees, the infection destroyed
nearly 1 million cubic metres each year.
Apart from being economically disastrous for the logging industry, the
disease causes both ecological and aesthetic damage - by killing the
salt resistant coastal pine forests that prevent coastal erosion, and
attacking trees in parks and gardens.
The pine wilt nematode (Buraphelenchus xylophilus) is transmitted by the
Japanese pine sawyer beetle, Monochamus alternatus. It is carried in the
beetle's tracheae and is transferred on to the tree when young adult
beetles land on its bark to feed.
The nematode enters the resin channels and multiplies quickly as it
spreads through the tree, feeding on its sapwood and phloem vessels.
Within a few weeks of infection in the early summer the trees begin to
wilt and eventually die.
Nematodes living within the dead trees seek out beetle larva and
overwinter within the immature insect's respiratory vessels before
hitching a ride to their next meal when the new adult emerges the
As its name suggests, the beetle vector is a native species, formerly
found in relatively small numbers across its range. But the arrival of
the alien parasite has hugely increased the numbers of dead trees on
which it lays its eggs, triggering a population increase that helps to
fuel the disease epidemic.
Because pesticide sprays can harm the environment, Japanese forestry
managers have tried to control the disease by burning dead trees to
destroy the beetle vector before it can emerge, but the fires sometimes
rage out of control creating even more damage.
Hajime Kosaka, a researcher at the Japanese Forestry and Forest Products
Research Institute, Ibarake, described a new approach to the control of
pine wilt disease.
Kosaka and his colleagues found that different strains of the nematode
vary in their effect on the host tree. By repeatedly infecting pine
seedlings in the laboratory, the researchers isolated a benign strain,
which can be cultured artificially on barley to produce large numbers of
In controlled trials conducted at the beginning of the 1990s, the
scientists inoculated a dose of about 30,000 of the non-virulent
nematodes into the bark of pine trees.
The benign nematodes protected against the damage caused by subsequent
exposure to a pathogenic nematode strain, the researchers found.
Compared with only 10% of control trees, all treated trees survived to
the following growing season.
Kosaka described the results of the latest trial in which the scientists
treated trees in an area of commercial forest with epidemic disease.
Over three years, annual treatment of a proportion of the trees
increased the survival of trees throughout the area, but did not
eliminate further transmission.
"The level of induced resistance is not sufficient to prevent the spread
of disease in the field situation but it does slow it down," Kosaka told
Drawing a parallel with vaccinating against influenza, Kosaka said that
while it is not possible to inoculate everybody, scientists may be able
to save vulnerable groups or particularly valuable trees. "It may also
be possible to ring vaccinate around a new outbreak to stop the disease
spreading," he said.