THE voracious gypsy moth caterpillar, which strips trees bare across much of
North America and Europe, may have met its match. An American entomologist has
identified chemicals in the green ash, a common American tree, which make it so
repellent to the gypsy moth larvae that they steer well clear. And farmers and
gardeners may be able to use the chemicals to protect other trees.
“Caterpillars coming into contact with any part of the tree move in the
opposite direction,” says Dale Norris of the University of Wisconsin, “and those
who do nibble on the foliage will be killed if they ingest more than a tiny
Norris and graduate student Ingrid Markovic are the first to identify all the
constituents of the chemical defences of green ash (Fraxinus
pennsylvanica). “People have known for many decades that green ash is one
of the few deciduous trees that gypsy moth larvae avoid,” Norris says. Although
oaks are their favourite hosts, gypsy moths (Lymantria dispar)
are exceptionally catholic in their tastes. They will attack over 300 species of
trees and shrubs, and will even eat conifer needles.
After an exhaustive search of the literature to ensure that no one else had
investigated the ash’s chemical defences, the researchers patented the tree’s
unique blend of chemicals.
“The ash releases several volatile compounds into the air around it,
including linalool and methyl salicylate,” says Norris. “Gypsy moth larvae can
detect their odour several metres from the tree.” He found that the recipe has
to be just right to make the combination of compounds repellent. The
concentration of the brew is critical. “More is not always better. All the
compounds are found in other plants, and some are even used by insects as
pheromones,” says Norris.
If a caterpillar is foolish enough to eat the ash leaves, it encounters the
next layer of the tree’s defences, namely the chemicals’ toxic effects. “The
antifeedants found in ash tissue act so quickly that maximum defoliation is just
2 to 4 per cent—not enough to be noticed by casual observation,” says
Finally, chemicals that disrupt the caterpillar’s hormones prevent any larvae
that survive from maturing into adult moths. “These chemicals disrupt digestion,
and then interfere with moulting and metamorphosis—we never found normal
pupae among the larvae which had fed even on diluted ash chemicals,” he
Although most of the active chemicals in the ash are made by chemical
companies, Norris believes that if his work with Markovic is exploited
commercially, it will prove easier to extract them from the trees. “The ash tree
probably knows the recipe better than humans will,” he says.
He suggests that strips of cloth or other material, impregnated with the
chemicals, could be wrapped around the trunks of vulnerable trees. The
caterpillars regularly travel up and down the trunks of the trees they feed on,
and so a repellent ring of ash chemicals would deter them. “The chemicals could
also be made into a powder or liquid and applied to selected trees or even
larger areas,” he says.