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INTERRUPTION OF BARK BEETLE AGGREGATION BY A
VIGOR-DEPENDENT PINUS HOST COMPOUND
KENNETH R. HOBSON
Utah State University, Department of Forest Resources and USDA Forest Service, Forestry Sciences Laboratroy,
Logan, Utah 84322, USA
INTRODUCTION
The attraction of bark beetles to pheromones has been abundantly demonstrated in the last quarter century (Wood
1982, Borden 1985). Much less is known about the response of bark beetles to host volatiles. This paper reports preliminary
evidence that one common host compound of North American conifers is a strong repellent or interruptant of aggregation for
several species of bark beetles. It also reviews work that suggests that the level of this compound may be an indicator of host
tree stress typically associated with bark beetle infestation in several species of pines. This compound, methyl chavicol,
fulfills the two essential requirements for a kairomonal cue that can explain the selective infestation of susceptible trees by
bark beetles, lit is correlated to tree susceptibility, and bark beetles respond to it.
The search for a biochemical indicator of host tree stress that might serve as an olfactory cue for bark beetles is not
new (Rudinsky 1962 and references therein, Cobb et al. 1968). Miller et al. (1968) examined the resin from smog-damaged
ponderosa pines, Pinus ponderosa, in the San Bernardino Mountains east of Los Angeles. These trees were being selectively
infested by mountain pine beetle, Dendroctonus ponderosae, and western pine beetle, Dendroctonus brevicomis. Of the six
monoterpenes examined, none were shown to be significantly different between damaged and healthy trees. However, Cobb
et al. (1972) found a strong, significant difference in the level of one compound, methyl chavicol, in the foliage of smogdamaged
ponderosa pines and healthy trees. Smog-damaged trees had 71% less methyl chavicol in their foliage - by far the
sharpest phytochemical difference found.
More recently Nebeker et al. (1995) measured 18 host volatiles from healthy lodgepole, Pinus contorta, and those
diseased by dwarf mistletoe, Arceuthobium americanum; armillaria root disease, Armillaria mellea; or comandra blister rust,
Cronartium comandrae. The latter two diseases are reported to be the most common predisposing agents of lodgepole to
mountain pine beetle attack in the Intermountain West (Tkacz and Schmitz 1986). In the case of both predisposing diseases
of lodgepole, there was a large and significant difference in the level of 4-allylanisole (a synonym for methyl chavicol)
between healthy and diseased trees. This was not true for dwarf mistletoe, which was not shown to be associated with bark
beetle infestation (Tkacz and Schmitz 1986). For both comandra and armillaria, the difference in the level of methyl chavicol
was one of the greatest phytochemical differences found between diseased and healthy trees. Trees infected with comandra
had 43.6% less methyl chavicol than healthy trees; trees infected with armillaria had 63% less methyl chavicol than healthy
trees. Cobb et al. and Nebeker et al. both found the strongest biochemical differences between diseased and healthy trees in
one compound.
These results begged the question, "What is the response of bark beetles to methyl chavicol?" Electroantennograms
(EAG) show a strong response to methyl chavicol. In 1989, Peter White found that estragole (methyl chavicol) produced the
third highest response of all 11 host compounds tested on the red turpentine beetle, Dendroctonus valens (White and Hobson
1993). In 1993 Armand Whitehead showed a strong EAG response of mountain pine beetle to methyl chavicol (Hobson et al.
in prep). A large EAG response does not indicate that a compound will be a strong attractant or repellent. However, ecologically
relevant compounds that produce a large EAG are likely to have behavioral significance (Masson and Mustaparta
1990). These data encouraged us to conduct behavioral field studies.
Mattson, W.J., Niemel_i, R, and Rousi, M., eds. 1996. Dynamics of forest herbivory: quest for pattern and principle. USDA
For. Serv. Gen. Tech. Rep. NC-183, N.C. For. Exp. Sta., St. Paul, MN 55108.
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