Biology Reference
In-Depth Information
mycangial
Dendroctonus
species, which typically produce short feeding galleries and exhibit
one or more generations per year (Furniss and Carolin, 1977).
It is likely that nutritionally beneÝcial fungi not only concentrate nitrogen but also provide a
source of sterols to their host beetles. Insects depend on dietary sources of sterols for growth,
molting, and reproduction (Clayton, 1964; Svoboda et al., 1978). Sterols are important elements
of cellular structure, provide necessary precursors for hormone synthesis (including juvenile hor-
mone), and are critical for the production of viable eggs (Clayton, 1964). Sterol concentrations in
plant tissues like phloem and xylem are typically low. A lack of fungal-produced sterols may
account for observations that fungus-free larvae produce longer mines, are smaller than normal,
and do not pupate (Webb and Franklin, 1978; Strongman, 1982). Furthermore, beetles developing
with some fungal associates pupate successfully, while those developing with other fungi either
fail to pupate or have reduced rates of pupation (Strongman, 1982).
Such differential effects on beetle development may relate to the quantity and quality of sterols
present in the beetleÔs diet. It is clear that not all sterols are equally suitable for use by insects. Ergosterol,
a major, and often the only, sterol produced by many fungi, has been found to be one of the most
suitable sterols for insect nutrition, whereas many plant-produced sterols are not usable (Clayton, 1964).
In several insectÏfungus symbioses, the insect associate has been found to be dependent on
sterols provided by the fungal associate (Norris et al., 1969; Kok et al., 1970; Norris, 1972;
Maurer et al., 1992; Morales-Ramos et al., 2000; Mondy and Corio-Costet, 2000). For the
ambrosia beetle,
Xyleborus ferrigineus
(Fabricius), the presence of an associated fungus in its
diet is required for pupation (Kok, 1979) and oviposition of viable eggs (Norris and Baker,
1967) and for oocyte development, oviposition, larval development, and pupation in a related
species,
X. dispar
(French and Roeper, 1975). For both beetles, ergosterol produced by their
associated fungi is responsible for these effects (Norris et al., 1969). Similar results were also
found for another scolytid,
Hypothenemus hampei
(Ferrari). This beetle bores into, and feeds
on, coffee berries but cannot molt or reproduce without ergosterol from its symbiotic fungus,
Fusarium solani
(Morales-Ramos et al., 2000). For several phytophage beetles, reduced levels
of pupation or lack of oviposition have been noted in fungus-free beetles (Strongman, 1982;
Fox et al., 1993; Six and Paine, 1998), indicating that for at least some beetles in this group,
fungus-derived compounds may also be important.
Ergosterol production varies considerably by fungal species (Kok and Norris, 1973), and this
differential production may account for different fungal effects on the beetle host. Kok et al. (1970)
and Kok and Norris (1973) found that the species of fungi associated with
Xyleborus
ambrosia
beetles each produced different amounts of ergosterol, the only sterol detected in any of the fungi.
The fungi most beneÝcial for beetle success were found to contain the greatest concentration of
ergosterol. Similar investigations being conducted with phytophagous scolytids may help elucidate
the potential roles of fungal-produced sterols with these insects (D.L. Six, B.J. Bentz, K. Wallin,
and K. Bleiker, unpublished data).
Effects, if any, of yeast associates on host beetles are largely uninvestigated. Grosmann (1930),
after rearing a single larva successfully to adulthood without yeasts, concluded that yeasts were not
nutritional mutualists. Holst (1937) reached a similar conclusion after successfully rearing beetles to
the adult stage under sterile
in vitro
conditions. However, the limited scope of these studies, combined
with a failure to extend the experiment into a second generation to test whether yeast feeding is critical
for reproduction, makes it difÝcult to accept their conclusions without reservation.
Fungi may also beneÝt nutritionally from their associations with bark beetles. The glands
associated with mycangia may provide a source of nutrition for fungi during transit between
host trees. For example, small amounts of fungi are acquired in the mycangia of teneral adult
D. brevicomis
during maturation feeding in the pupal chamber. The fungi then grow within
and Ýll the mycangium by the time the beetle matures and colonizes a new tree (Paine and
Birch, 1983). Such extensive growth within the mycangium would be unlikely in the absence
of nutrients.
