Biopesticides - Pesticides: Evaluation of Environmental Pollution - page 592

Because fungal biopesticides are so diverse in nature, their means of affecting the
target pests can be equally diverse. The most common modes of action are through
competitive exclusion, mycoparasitism, and production of metabolites. Some fungi
can exhibit all of these modes of action. Two of the most common commercial fungal
biopesticides are Trichoderma spp. and Beauveria bassiana . Each are frequently used in
the nursery, ornamental, vegetable, field crop, and forestry industries to control a vari-
ety of pests.
Trichoderma spp. are some of the most common fungi in nature. Many beneficial
Trichoderma have the ability to readily colonize plant roots, without harming the plants.
It is this close relationship with the plants that makes these species excellent biocontrol
agents. These microbial biofungicides can outcompete pathogenic fungi for food and space
and, in the process, can stimulate plant host defenses and affect root growth. In addition,
they have the ability to attack and parasitize plant pathogens under certain environmental
conditions.
Beauveria bassiana is a fungus that acts as a parasite on many insect species. B. bassiana
has a broad host range, although individual strains may be restricted in the number of
insects it can attack (Inglis et al. 2001). B. bassiana spores adhere directly to the host cuticle,
where they will germinate, produce enzymes that attack and dissolve the cuticle, pen-
etrate, and grow into the insect's body, feeding on internal tissues and releasing an insect
toxin (Feng et al. 1994). As the insect dies, it changes color to pink or brown and eventually
the entire body cavity is filled with fungal mass. B. bassiana has proved effective in control-
ling troublesome crop pests such as aphids, thrips and whitefly—even chemical pesticide-
resistant strains such as Q-Biotype Whitefly (Pell et al. 2001).
19.5 Protozoa Biopesticides
Protozoa are single-celled eukaryotic organisms that exist in both water and soil. While
most protozoa feed on bacteria and decaying organic matter, a wide range of protozoan
species are insect parasites (Jehle et al. 2006).
For example, Nosema locustae are known to be natural biocontrol agents of many grass-
hopper species. Nosema infects at least 90 species of grasshoppers. It is nontoxic to humans
and other mammals, as well as to over 250 natural predators of grasshoppers. Nosema is
particularly important in areas where environmental issues preclude the use of traditional
insecticides.
Grasshoppers cause millions of dollars of damage to forage crops each year. The proto-
zoan Nosema locustae has been developed into a commercially available, grasshopper con-
trol agent that infects and weakens young grasshoppers and adversely affects the female
grasshoppers' ability to reproduce.
After consuming Nosema locustae , grasshopper feeding is typically curtailed within a
week. Within 2 weeks or longer, as many as 50% of the infected insects die and approxi-
mately half of the surviving population remain weak, consuming 75% less forage than a
healthy insect.
Grasshoppers are notorious scavengers. An important function in the transmission of
Nosema spores to healthy grasshoppers occurs as the insects scavenge and cannibalize
infected cadavers. Since infected grasshoppers develop a large number of the spores within