Introduction to Fungi, Third Edition

CLAVICIPITALES
363
conidia. The association has had a long history. A
worker ant covered with a fungus similar to the
present-day B. bassiana has been discovered
embedded in 25 million year-old amber (Poinar
& Thomas, 1984). Beauveria bassiana grows readily
in culture, forming dry conidia and, occasionally,
synnemata. The conidiophores form densely
clustered whorls of conidiogenous cells which
are swollen at the base and extend into a zigzag
shaped rachis forming small, globose, smooth,
hyaline conidia (Fig. 12.35a).
Several mycotoxins have been obtained
from cultures of B. bassiana including beauvericin,
a cyclic depsipeptide, oosporein and bassianolide,
all of which are toxic to insect larvae
(see Boucias & Pendland, 1998). Numerous
attempts have been made to use commercial
preparations of conidia in the biological control
of insect pests, such as the Colorado beetle
on potatoes or the codling moth on apples.
Conidia are either applied alone or, in integrated
control, in conjunction with a chemical insecticide.
However, dependable success in biocontrol
using B. bassiana is still awaited. Whilst it
is possible to produce large quantities of inoculum,
this cannot be stored for very long, and
there are also problems in maintaining a
reproducibly high level of biocontrol of insect
pathogens in outdoor situations (Boucias &
Pendland, 1998).
Metarhizium anisopliae
This fungus is the cause of green muscardine
disease of insects. There are three varieties,
var. anisopliae, var. acridum and var. major. The
teleomorph of M. anisopliae var. major is C. brittlebankisoides
(Liu et al., 2001, 2002). Metarhizium
anisopliae grows in soil (Domsch et al., 1980), but
it is also one of the most important insect
pathogens. Metarhizium anisopliae var. anisopliae
has a wide host range, attacking members of the
Coleoptera, Orthoptera, Hemiptera and
Hymenoptera as well as Arachnida. Metarhizium
anisopliae var. major is more host-specific, mostly
infecting soil-inhabiting scarabeid beetles
(Boucias & Pendland, 1998). In culture it grows
slowly, forming columns of green, shortly cylindrical,
uninucleate phialoconidia which are rich
in lipid droplets (Fig. 12.35b). The wall of the
conidium is three-layered and the outermost
layer is highly hydrophobic due to impregnation
by a hydrophobin. There have been extensive and
detailed studies of the physiology and enzymology
of germination and penetration by the germ
tubes, mainly carried out by Charnley and St
Leger (1991). The brief account which follows
Fig12.35 Anamorphic states associated with Cordyceps. (a) Beauveria bassiana.(b)Metarhizium anisopliae. (c) Tolypocladium inflatum.
The phialides have a swollen base and a long neck. (a,b) to same scale.