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Nosema locustae
Case Study of a Microsporidium
Developed as a Microbial Pesticide
The only microsporidium that has been extensively studied for use as a microbial
insecticide is Nosema locustae,
which was originally isolated from the African migratory locust (Canning,
1953;
1962). This species also occurs naturally
in North American Great Plains and prairie populations of grasshoppers and
has been used as a biological insecticide against a variety of grasshopper
and locust species. N. locustae is primarily a fat body parasite;
it reproduces slowly in the fat body tissues, eventually depleting the host
of energy reserves
(Johnson, 1997). Fecundity, behavior, and survival of infected hosts is affected.
N. locustae is
produced commercially in living grasshoppers. The mature infective spores
are harvested from the hosts between 20 and 35 days post inoculation
(Henry, 1985; Menely
and Sluss, 1988), The spores are typically
formulated
as a bait
in wheat bran with hydroxymethyl cellulose sticker and applied with Buffalo® turbine
equipment mounted on a truck. Ultra low volume (ULV) aqueous sprays were not
as successful as the application of wheat bran bait, requiring much higher
concentrations of spores and resulting in less population reduction (Henry
et al., 1978). The
standard formulation adopted was 2.46 x 109 spores/ha. This application rate
produced 50-60% reductions in grasshopper populations and 35-50% infection
among survivors (Henry et al, 1973).
Lange
and Lantari (2000) recently documented persistence in the field of released N. locustae for
up to 13 years after inoculation. N. locustae did not meet
the requirements of a typical microbial insecticide for rapid control at
the time
of application. Nevertheless, field research on occurrence and host range
of the pathogen, in addition to known effects on survival, fecundity, and
behavior
of affected grasshoppers, indicate that the pathogen can be efficacious over
a period of years. At least 13 species of grasshoppers were shown to be affected
by N. locustae in the western Pampas, and prevalences were higher than those
recorded in areas where it is native (North America and India). Susceptible
grasshopper species appear to be less abundant in areas where epizootics
were previously
observed.
An excellent review of
three decades of research on N. locustae as a biological control agent of
grasshopper and locust species was recently published (Goettel
and Johnson, 1997) and documents various strategies for control of acridoid
pests and the use of N. locustae within an IPM program. |
