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DOUGLAS-FIR AND WESTERN LARCH DEFENSIVE REACTIONS TO
LEPTOGRAPHIUM ABIETINUM AND OPHIOSTOMA PSEUDOTSUGAE
DARRELL W. ROSS 1and HALVOR SOLHEIM 2
_Department of Forest Science, Oregon State University, Corvallis, Oregon, 97331-7501, USA
2Norwegian Forest Research Institute, Section of Forest Ecology, Division of Forest Pathology, N-1423 As-NLH, Norway
INTRODUCTION
The Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, is one of the most important insects associated with
Douglas-fir, Psuedotsugae menziesii (Mirb.) Franco, in western North America (Furniss and Carolin 1977). At high population
densities, the beetle is capable of causing significant tree mortality and impacting forest resource values (Cornelius 1955,
Rudinsky 1966, Johnson and Belluschi 1969, Furniss and Orr 1978, Furniss et al. 1979). The Douglas-fir beetle usually
breeds in recently dead or live Douglas-fir trees. The only other tree species in which the beetle has been reported to successfully
breed is western larch (Ross 1967). However, the beetle is apparently able to breed only in dead larch (Furniss et al.
1981). Live larch may be attacked, but brood production from live trees has never been observed. Reed et al. (1986)
compared various chemical and physical properties of Douglas-fir and western larch to identify possible reasons that brood
fail to survive in live larch trees. Compared with Douglas-fir, larch had a higher content of 3-carene, thinner phloem, higher
phloem moisture content, larger diameter vertical resin ducts, and lower oleoresin exudation pressure. The authors speculated
that the high content of 3-carene in live larch may be responsible for the failure of Douglas-fir beetle brood in those
trees.
One or more species of ophiostomatoid fungi are invariably associated with each species of conifer-infesting bark
beetle (Whitney 1982, Harrington 1988). Although the ecological relationships among the fungi and bark beetles are not
completely understood in many cases, there is evidence that some of these fungi help the beetles to overcome the natural
defenses of mass attacked trees. Conifer response to invasion by bark beetles and their associated fungi involves two
different systems: (1) the flow of preformed resin from severed resin ducts and (2) the induced wound response or resistant
reaction (Reid et al. 1967, Berryman 1972, Christiansen and Horntvedt 1983). The dimensions of the induced response in
host tree tissues are generally correlated to the growth of the fungi or the extent of mechanical damage associated with beetle
tunneling (Ross et al. 1992, Lieutier 1993, Solheim 1993). If the fungi associated with the Douglas-fir beetle are involved in
overcoming tree resistance, then a possible reason for the failure of brood development in larch may be the inability of the
fungi to survive and grow in larch. A significant variation in the pathogenicity of the fungi to Douglas-fir and western larch
should he reflected by differences in the induced response following artificial inoculations.
Despite a considerable amount of past research related to the biology and management of the Douglas-fir beetle, little
is known about its funsal associates. The two species of ophiostomatoid fungi most commonly associated with the Douglasfir
beetle are Ophiostoma pseudotsugae (Rumb.) yon Arx and Leptographium abietinum (Peck) Wingf. (Rumbold 1936,
Harrington 1988, Solheim unpubl.). There are no published reports on the pathogenicity of these fungi or their ecological
relationships to the beetle. In 1993, we installed a study to evaluate the pathogenicity of these fungi to Douglas-fir. The final
results of that study will be published elsewhere in the near future. At the same time, we installed a small test to assess the
response of Douglas-fir and western larch, Larix occidentalis Nutt., to artificial inoculations with these fungi. The objective
of our study was to measure the length of the induced response in the phloem of Douglas-fir and western larch following
artificial inoculation with O. pseudotsugae and L. abietinum. This paper reports the results of this test.
Mattson, W.J., Niemela, E, and Rouse, 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. i
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