Biology Reference
In-Depth Information
Many phytophages feed on fungi and plant tissues as both larvae and adults. The degree
of dependence of beetles on fungi ranges from obligate to facultative to opportunistic. Asso-
ciations further range from highly species speciÝc to merely incidental. Unfortunately, for
most species virtually nothing is known regarding fungal associates, speciÝcity, or degree of
obligacy of association.
The mycetophage category includes the ambrosia beetles that live in wood or other plant tissues
but cultivate and feed strictly on associated fungi. These associations are obligate, and, predictably,
mycetophagous beetles possess mycangia, ensuring the continuity of insectÏfungus association
from generation to generation. Most of these beetles colonize nutritionally poor and mostly indi-
gestible substrates including xylem. The fungal associates allow the exploitation of these otherwise
marginal habitats by concentrating nitrogen and providing essential nutrients, including sterols and
vitamins, to the beetle host (Beaver, 1989). Despite the complete dependence of mycetophagous
beetles on their fungal associates, these associations are seldom monophilic (one symbiotic partner
per beetle species) but are more often oligophilic (two to several symbiotic partners per beetle
species) (Batra, 1966).
BARK BEETLE-ASSOCIATED FUNGI
M
F
YCELIAL
UNGI
Bark beetles are associated with Ýlamentous fungi in the Ascomycotina and Basidiomycotina. Most
ascomycete associates are in three genera Ð
Ophiostoma
Syd.
, Ceratocystiopsis
Upadhyay and
Kendrick
included in a group of morphologically similar fungi
collectively referred to as the ophiostomatoid fungi.
,
and
Ceratocystis
Ell. and Halst.
,
Ceratocystis
possess
Chalara
Corda anamorphs
(asexual states), while
Ophiostoma
possess several anamorph states including
Leptographium
Lagerb. and Melin,
Graphium
Corda,
Hyalorhinocladiella
Upadhyay and Kendrick, and
Sporothrix
Hektoen and Perkins.
Ceratocystiopsis
anamorphs include
Hyalorhinocladiella
and
Sporothrix
.
Despite many morphological similarities,
Ophiostoma
and
Ceratocystiopsis
are phylogeneti-
cally distinct from
Ceratocystis
(Hausner et al., 1993a,b).
Ophiostoma
and
Ceratocystiopsis
form
a highly variable but monophyletic group in the Diaporthiales, while
Ceratocystis
are
most similar
to the Microascales (Hausner et al., 1993b; Spatafora and Blackwell, 1994).
species are vectored primarily by subcortical insects, includ-
ing bark beetles, and are often found in temperate forest trees (Harrington, 1993). Most are
competitive saprophytes, and only a few are known plant pathogens (Harrington, 1993).
Ophiostoma
and
Ceratocystiopsis
Ophiostoma
species associated with bark beetles are strongly pleomorphic. The mycelial state is produced when
the fungi grow in phloem and wood; however, when the fungi are growing in mycangia, or adjacent
to beetles in pupal chambers
(Figure 7.1),
growth can become highly concentrated and sporogenous
(Tsuneda, 1988) or even yeast-like (Barras and Perry, 1972; Happ et al., 1976).
grow on a wide variety of herbaceous as well as woody
plants in temperate and tropical zones. While two bark-beetle species vector
Ceratocystis
species, on the other hand,
species
at least occasionally (Furniss et al. 1990; Solheim and Safranyik, 1997; Viiri, 1997), most vectors
are sap-feeding insects (Harrington, 1987). In contrast to
Ceratocystis
Ophiostoma
, many
Ceratocystis
are
capable of causing plant disease.
Other ascomycetes have on occasion been isolated from bark beetles or from their galleries;
however, effects of these apparently incidental fungi on the beetles remain unknown. Some, such
as
species, are common in older galleries (Whitney, 1971). These
fungi may be present as ubiquitous saprophytes, but due to their antagonistic mycoparasitic nature
they may interact in signiÝcant, but unknown, ways with bark beetleÏfungal associates.
While most associations among bark beetles and fungi involve ascomycetes, a few associations
with basidiomycetes have been characterized.
Trichoderma
and
Penicillium
Gloeocystidium ipidophilum
Siemasko was described
from galleries of
Ips
typographus
L. (Siemasko, 1939), and a
Sebacina
-like basidiomycete was
