Biology Reference
In-Depth Information
comprehensive body of literature on host-tree defense reactions to invasion by aggressive beetles
and their associated fungi as well as a more limited literature characterizing interactions between
beetles and their associates. Ultimately, however, if we are to gain a fuller understanding of these
systems, it will be necessary to develop a broader conceptual framework that considers the full
range of diverse bark beetle hosts and their associated fungi.
BARK BEETLES
Bark beetles are in the Scolytidae (alt. Scolytinae), a family that includes the cone beetles and
many ambrosia beetles. Bark beetles typically invade the bark and phloem of plants but not the
woody tissues (Wood, 1982). These beetles exhibit great variation in life histories, ranging from
highly aggressive (tree-killing) to facultative (colonizing weak or recently killed trees) to parasitic
(using living trees) to saprophagous (using dead hosts) (Paine et al., 1997). Aggressive beetles have
attracted the greatest interest due to their eruptive population dynamics and subsequent economic
impact. However, relative to the large number of bark-beetle species that have been described, the
number of aggressive species is very small. For example, less than 2% of the nearly 500 species
of bark beetles described in North America commonly kill trees.
Scolytids typically feed in nutritionally poor substrates. They colonize a great variety of plant
tissues including woody tissues, bark and phloem, fruits, and the pith of twigs (Wood, 1982). Only
the ambrosia beetles do not feed on plant tissues, feeding solely on their associated fungi. Other
scolytids feed on plant tissues or, more often, on a combination of plant and fungal tissues. Berryman
(1989) grouped the Scolytidae into three broad categories based on feeding strategy: saprophages,
phytophages, and mycetophages. Saprophages include scolytids that feed exclusively on dead or
decaying tree tissues. While numerous, this group of beetles is not well studied, and little infor-
mation exists on their associations with fungi. Because these insects arrive late in the sequence of
colonization of plant host material, they are more likely to encounter competition from other insect
groups (Wood, 1982). Likewise, any fungi carried by these late-arriving beetles would face resource
depletion or strong competition with earlier arriving saprophytic species and, in some cases, with
fungi introduced by earlier arriving scolytids. Berryman (1989) predicted that strict associations
with speciÝc fungi were unlikely in the saprophagous beetles. However, it is highly plausible that
these beetles ingest fungal tissues along with wood. Rather than transporting speciÝc fungi into
wood, these beetles may instead rely upon fungi already present, although the species composition
of the fungal community encountered is likely to vary.
Some insects that feed on dead wood show marked attraction to decayed wood and, in particular,
prefer decay fungi (French et al., 1981). Further, feeding rate and survivorship increase on wood
containing some fungi but decrease on wood containing others (Moein and Rust, 1992). Similar
relationships may also exist with saprophagous bark beetles and decay fungi and may even account
for the differential attraction of beetles to various substrates. Alternatively, these beetles may have
no attraction to, or dependence on, decay fungi and may merely feed on fungi incidentally as an
inevitable and unavoidable component of their food.
The phytophages feed on living or recently killed plant tissues. This category includes bark
beetles that feed primarily on the phloem and cambial tissues of trees, although some species that
feed on these tissues may spend a portion of their developmental period in the outer bark (Wood,
1982). Associations with fungi are apparently universal within this group of scolytids, and many
possess specialized structures, termed mycangia (treated below), for the transport of fungi. These
beetles use living or freshly killed plant tissues and consequently experience less interspeciÝc
competition for resources. The fungi they carry are able to exploit uncolonized tissues well in
advance of highly competitive saprophytes. Indeed, Harrington (1993) hypothesized that the weak
pathogenicity exhibited by some bark beetleÏassociated fungi may allow these fungi a competitive
advantage over strictly saprophytic species.
