Biology Reference
In-Depth Information
adults and probably on yeasts throughout development. In many cases, mycelium, yeast cells, and
conidia are ingested; however,
I
.
calligraphus
have been observed to seek out and
ingest entire
O. ips
perithecia during maturation feeding as young adults (Yearian et al., 1972).
The two mycangial fungi of
D. frontalis
impart positive impacts on brood development and
survival that likely occur through nutrition gained during feeding on fungi by larvae and adult
beetles (Barras, 1973; Bridges, 1983; Goldhammer et al., 1990; Ayres et al., 2000). However, the
two fungi are not equal in their effects (Bridges, 1983; Goldhammer et al., 1990). Female
D.
frontalis
carrying
Entomocorticium
sp. A or
C. ranaculosus
and
Entomocorticium
sp. A are larger
and possess higher lipid content than those carrying only
C. ranaculosus
(Coppedge et al., 1995).
Such differences can greatly affect the Ýtness of host beetles. Greater body size and weight positively
affect survival (Safranyik, 1976; Botterweg, 1982, 1983; Anderbrandt, 1988), dispersal ability
(Atkins, 1969; Thompson and Bennett, 1971), and reproductive capacity of female beetles (Reid,
1962; Amman, 1972; Clarke et al., 1979). Furthermore, a large proportion of
Dendroctonus
eggs
is composed of lipid (Barras and Hodges, 1974), indicating that the lipid content, and consequently
the nutrition, of parental female beetles is important for optimal egg production.
The two mycangial fungi of
D. ponderosae
also differentially affect the reproductive potential
of their host in a way that indicates nutritional effects (Six and Paine, 1998). The production of
progeny
D. ponderosae
is signiÝcantly higher, and emergence occurs signiÝcantly earlier (indicating
a more nutritious food source) for brood developing with
O. clavigerum
than with
O. montium
.
For this beetle, only one mycangial fungus appears to beneÝt larval development.
Nonmycangial beetles appear to be less speciÝc in their associations and are likely to be less
dependent on their fungal associates than mycangial beetles. For example,
I. avulsus
(Eichoff)
is considered nonmycangial. It is associated with two fungi,
Entomocorticium
sp. and
O. ips
(Klepzig et al., 2001). Wild beetles associated with their full fungal complement (including
Entomocorticium
sp.) are larger, produce more brood, and exhibit higher brood survival than
beetles associated only with
O. ips
, a potential antagonist, or beetles that are fungus free (Yearian
et al., 1972). Association with fungi, however, does not appear to be obligate with
I. avulsus
, as
the beetles can be reared without fungi, although with a reduced level of success (Yearian et al.,
1972). In contrast, fungus-free
I. calligraphus
(Germar) and
I. grandicollis
(Eichoff) laid more
eggs and produced more brood than when reared with fungi (Yearian et al., 1972). Fungus-free
I. paraconfusus
were also observed to develop successfully to adult, but size of brood and
developmental rate of larvae were reduced in fungus-free beetles compared with beetles devel-
oping with fungi (Fox et al., 1993). Caution, however, must be exercised in interpreting effects
of so-called nonmycangial fungi because many ÑnonmycangialÒ beetles have not been formally
investigated for the presence of mycangia.
Many of the observed positive effects of fungal association on phytophagous bark beetles
are likely to be directly related to nutrition, either through modiÝcation of nutrient form or
availability, production of essential nutrients not found in phloem or bark (or present only in
inadequate amounts), or concentration of nutrients by the fungal mycelium. Wood is a poor
source of vitamins, sterols, and other growth factors (Norris and Baker, 1967), and fungi may
provide alternative sources. Recently, evidence was found supporting the role of nitrogen con-
centration by fungi associated with
D. frontalis
(Ayres et al., 2000). In that study, feeding by
D. frontalis
, a mycangium-bearing species, was compared with that of
I. calligraphus
, a nonmy-
cangium-bearing species. Results indicate that the mycangial fungi of
D. frontalis
concentrate
nitrogen better than an antagonistic nonmycangial associate,
O. minus
, reducing the amount of
phloem required for development.
I
.
calligraphus,
which is associated primarily with
O. minus
,
had to consume additional phloem to obtain its nitrogen requirements. Similarly, other nonmy-
cangial beetles may also need to feed more extensively in phloem to compensate for low nutrient
content in the absence of nutritionally beneÝcial fungi. For example,
D. ruÝpennis
, an apparently
nonmycangial beetle, feeds extensively in the phloem and in many populations has a 2-year life
cycle (Furniss and Carolin, 1977). This feeding pattern is in strong contrast with sympatric
I. avulsus
and
