Byers, J.A. 1995a. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell
(eds.). Chemical Ecology of Insects 2. Chapman and
Hall, New York, pp. 154-213.
INTRODUCTION--
Bark beetles (order Coleoptera: family Scolytidae) comprise a
taxonomic group of species that look similar although they differ
widely in their ecology and biochemical adaptations to host trees.
This diversity of bark beetle biology, in which each species is
adapted to only one or a few host tree species, has probably
resulted from natural selection due to the great variety of trees
and their biochemicals. It also is likely that each species of tree
has coevolved various chemicals to defend against the herbivorous
selection pressures of bark beetles and other insects (Erlich and
Raven, 1965; Feeny, 1975; Cates, 1981; Berryman et al., 1985). Host
plant chemicals can be attractive, repellent, toxic, or nutritious
to bark beetles and have affects on: (1) finding and accepting the
host tree (host selection and suitability), (2) feeding stimulation
and deterrence, (3) host resistance, (4) pheromone/allomone
biosynthesis and communication, and (5) attraction of predators,
parasites and competitors of bark beetles.
Bark and ambrosia beetles contain at least 6000 species from
181 genera worldwide (S.L. Wood, 1982). In the United States there
are 477 species, and in North and Central America a total of 1430
species occur from 97 genera. Bark beetles may have originated as
early as the Triassic (over 200 million years ago) on conifer hosts
(S.L. Wood, 1982). Baltic amber dating from the Oligocene (25-30
million years ago) sometimes contains entrapped insects that look
identical to bark beetles from species of present-day genera such
as Tomicus (S.L. Wood, 1982).
Since 1970 there have been over 3800 research papers on bark
and ambrosia beetles (BIOSIS Previews computer database,
Philadelphia, PA, USA). The genus Dendroctonus has been the most
studied with over 1196 papers published, primarily on four pest
species of North America, D. frontalis, D. ponderosae, D.
brevicomis, and D. pseudotsugae. Other genera in order of studies
were: Ips, Scolytus, Xyleborus, Trypodendron, Tomicus=Blastophagus,
Pityogenes, Hypothenemus, Pityophthorus, Hylastes, and
Gnathotrichus. The most studied Ips species (852 papers) also were
pests, I. typographus of Europe, and the three North American
species, I. paraconfusus = confusus, I. pini and I. grandicollis.
Scolytus multistriatus, vector of the Dutch elm disease, made up
the majority of papers from this genus. Thus, it is clear that most
biological knowledge on bark beetles derives from studies on
relatively few pest species (obligate and facultative parasites
comprise about 10% of scolytid species in US and Canada, Raffa et
al., 1993). This focus on pests is appropriate, however, since only
commonly occurring bark beetles that kill living trees or their
parts would be expected to have a significant influence on
evolution of the host tree and its chemistry.
CONCLUSIONS--
Of the 6000 bark beetle species worldwide, in a particular
geographic area there are usually from only a few to some tens of
species that colonize any given species of tree, and then only one
or a few species that can attack and kill the tree. Each host-tree
species has a large variety of chemicals, some of which affect the
success of the bark beetle in finding and colonizing its host tree.
Bark beetles probably orient to attractive semiochemical sources
during flight using odor-modulated anemotaxis, as in moths, but
little is known about this process. It is better understood how
beetles orient upwind during walking to attractive sources. Most
studies have observed chemotaxis in arenas for the purpose of
isolating pheromones rather than from the standpoint of basic
behavior.
Bark beetles find suitable host trees by orienting to host
odors, especially ethanol and monoterpenes, as well as to
aggregation pheromones. However, very few studies could be
characterized as complete or rigorous because the attractive
compounds were discovered usually by screening of semiochemicals
previously known for other species. Other studies have selected
compounds for testing based on their presence in the insect or
related species, but it is likely that many bioactive compounds
have been overlooked. Also, blends of compounds have rarely been
tested for synergism. Tree volatiles that attract predators and
parasites of bark beetles are often the same as those that attract
their host, i.e., most of these chemicals have been discovered by
chance when testing compounds on bark beetles. Feeding stimulants
and deterrents of conifer bark beetles have been isolated in
various solvent fractions but not identified. Several compounds
that elicit or deter feeding in deciduous bark beetles have been
identified, but undoubtedly many behaviorally active compounds
remain to be discovered. It is likely that behavioral responses of
bark beetles within a species to semiochemicals may vary
geographically as well as the semiochemicals produced by the bark
beetles and the host trees.
Fractionation of a biological extract by chromatographic
methods (usually GC) and then recombination of certain fractions
with an additive method has been used to test for synergism among
semiochemicals in a behavioral bioassay. This method was used to
isolate some of the first multi-component pheromones of bark
beetles (Silverstein et al., 1966, 1967; 1968; Pearce et al.,
1975). However, due to the substantial work involved with these
classical isolation methods, most studies have discovered
semiochemicals by screening or comparative methods which are
inherently less rigorous and are dependent on chromatographic
resolution. The subtractive method, where each of the fractions is
subtracted from the blend and tested such that blends with lowered
activity indicate subtracted synergists, should aid in isolation of
synergistic semiochemicals that otherwise have been hard to detect
(Byers et al., 1990a; Byers, 1992b).
Future studies should be careful to report the release rates
of test volatiles, and in many cases these should be adjusted to
coincide with natural rates (Byers, 1988). This means that
measurements of volatile release of semiochemicals must be done in
many bark beetle systems for both host- and beetle-released
semiochemicals (Birgersson and Bergström, 1989). When testing
semiochemicals in the field, the spatial and temporal variation of
responding insect populations with respect to trap placement may
lead to erroneous conclusions. To counter this potential problem,
relatively numerous trap replications have been previously
employed; however, the mechanical slow rotation of a pair of traps
(1-2 rph at 6 m separation) can even this catch variation (Byers et
al., 1990b).
Resistance of trees has been studied for many years and
monoterpenes, such as limonene, are implicated in their resistance
to bark beetles and their symbiotic microorganisms (mostly fungi).
However, there has been little recent toxicological work and the
relative importance of the purported toxins remains to be
established. Also, the monoterpenes have been tested at much higher
vapor concentrations than that in nature, and they have not been
evaluated in diets. Synergism or interactions of various "toxins"
have not been investigated. Also, geographic variation in toxicity
of host compounds has been little studied. A correlation between
high-limonene trees and historical "predation" by bark beetles has
been suggested as an example of host chemical evolution. However,
more studies are needed in stands with ongoing outbreaks of bark
beetle to determine if natural selection can alter the genetic
frequencies in the population of trees, and at the same time if
populations of bark beetle change their tolerance to particular
monoterpenes that were initially most toxic.
Host tree monoterpenes alpha-pinene and myrcene can be converted
by a simple hydroxylation to ipsenol, ipsdienol, cis-verbenol and
trans-verbenol, pheromone components of bark beetles. However, I.
paraconfusus is able to make the same amounts of ipsenol and
ipsdienol regardless of the myrcene titer in the host tree,
suggesting the major pathway is de novo. Recent studies in Ips
species also suggest that pheromonal analogues of myrcene may not
be derived primarily from myrcene but by synthesis from mevalonate.
Although ipsenol/ipsdienol and E-myrcenol biosynthesis in some
species of bark beetle are probably not coevolving with myrcene in
the tree, it is possible that cis- and trans-verbenol biosynthesis
may coevolve with alpha-pinene levels in hosts. Both cis-verbenol and
trans-verbenol appear to be directly produced in bark beetles by
conversion of alpha-pinene enantiomers from the host tree. However,
verbenone, an inhibitor of aggregation in many bark beetle species,
may not be directly converted from alpha-pinene. Other bark beetle
pheromone components are probably biosynthesized from small
molecules into the more complex structures in several or more
different biosynthetic pathways.
Evolution of tree chemistry in response to predation by bark
beetles is best supported in studies of host compounds that are
toxic to bark beetles or that deter feeding. The bark beetle has
also coevolved detoxification mechanisms for the toxic
monoterpenes, some of which have been secondarily utilized as
pheromone components. Volatile host attractants can be termed
kairomones, and there is little evidence that trees evolve these
compounds to repel herbivores not adapted to this potential host,
since the same compounds attract their herbivores. The compounds
probably are beneficial in some way to the tree and can not be
dispensed with even though bark beetles (and some of their
predators and parasites) have evolved to utilize the compounds as
kairomones. Host tree chemistry affects most aspects of bark beetle
biology, moreover, bark beetles probably differentially affect
survival of host trees and alter genotypic frequencies and host
chemistry both at the micro-evolutionary scale (cycling of endemic
and epidemic insect populations) and at the macro-evolutionary
level (host tree selection in response to new species of bark
beetle).
Chemical Ecology