untitled

environments, but it has a relatively high temperature

optimum for growth, so its use as a biological control

agent is restricted to glasshouse environments in the

cooler parts of the world. In addition to these conidial

fungi, several species of Zygomycota commonly cause

insect diseases in natural environments and also in field

crops. These fungi belong to a subgroup of Zygomycota

termed the Entomophthorales(see Fig. 2.11). They

include genera such as Entomophthoraand Pandora(pre-

viously called Erynia). They produce large sporangia at

the tips of their hyphae, and the sporangia are released

intact, functioning as dispersal spores (Fig. 15.1d). The

Entomophthorales often cause natural epizootics in

humid conditions, and attempts are being made to

develop them as commercial biocontrol agents.

The infection cycle

The general infection cycle of insect-pathogenic fungi

is summarized in Fig. 15.3. In almost all cases these fungi

initiate infection from spores that land on, and adhere

to, the insect cuticle. If the relative humidity is high

enough, the spore then germinates and usually forms

an appressorium, equivalent to the appressoria produced

by many plant-pathogenic fungi (see Fig. 5.2). The

appressorium often develops over an intersegmental

region of the cuticle, and in one of the common insect

pathogens, Metarhizium anisopliae, this behavior closely

parallels the behavior of the rust fungi (Chapter 5),

because the germ-tube recognizes the host surface

topography, or artificial surfaces with appropriate

FUNGAL PARASITES OF INSECTS AND NEMATODES 311

Fig. 15.2Beauveria bassianaand ex-
amples of the diseases that it causes.
(a) Spore-bearing structures of Beauveria
bassiana in laboratory culture. The
conidia typically develop in zigzag-like
chains on long conidiophores. (b) An
adult cicada beetle densely covered
with white sporulating pustules of B.
bassianathat have emerged through the
intersegmental plates of the insect
cuticle. (c) Sporulation of Beauveria
from between the cuticular plates of a
naturally infected green cockchafer
beetle. (d) Heavy infection of an adult
pecan weevil by Beauveria.(e) Grubs
of pecan weevil at different stages of
infection by Beauveria; healthy grubs are
shown at left. ((a),(b) Courtesy of G.L.
Barron. (c) Courtesy of Shirley Kerr;
http://www.kaimaibush.co.nz/. (d),(e)
Courtesy of Louis Tedders (photogra-
pher) and USDA, Agricultural Research
Service.)