Pest management - Ecological Applications: Toward a Sustainable World - page 159

the material in 'mass-emergence' devices, consisting of large tubs with openings
covered by fi ne mesh that provided easy passage for the small adult parasitoids to
emerge, but not the larger moths. Tubs containing 10 kg of leaf litter, and hung in
the crown of a horsechestnut tree, increased percent parasitism of C. ohridella
there.

6.3.4 Inundation
biological control -
using fungi, viruses,
bacteria and
nematodes

Inundation often involves the use of insect pathogens (fungi, viruses, bacteria) to
control insect pests. Each of these is generally present in the natural environment
but only at low densities. Their use as 'biological pesticides' requires commercial
production and application to swamp the target insect populations. Fungal agents
include the insect pathogenic fungus Beauveria bassiana , which is used in a com-
mercial spray against locusts and grasshoppers. The fungal spores germinate when
they contact the insect's cuticle, penetrating its body cavity where fungal hyphae
fi ll the body. The insects appear mummifi ed and fuzzy because of the fungal growth.
Viruses are even more specifi c in their action, but the expense of production and
application means they are not used extensively. One example is ' Lymantr ia dispar
nuclear polyhedrosis virus', which acts only against the gypsy moth ( L. dispar ), a
signifi cant forest pest.
But by far the most signifi cant insect pathogenic agent is the bacterium Bacillus
thuringiensis (Bt), which can easily be produced on artifi cial media. After being
ingested by an insect, gut juices release powerful toxins and death occurs 30
minutes to 3 days later. Signifi cantly, there is a range of strains of Bt, some specifi c
to caterpillars (many agricultural pests), others to two-winged fl ies, especially
mosquitoes and blackfl ies (vectors of malaria and onchocerciasis), and yet others to
beetles and their grubs (many agricultural and stored product pests). The advantages
of Bt are its powerful toxicity against target insects but a lack of toxicity against
organisms outside this narrow group (including most of the pest's natural
enemies).
The control action of Bt has been extended by genetically modifying crops, includ-
ing potato, corn and cotton, to express the Bt toxin (crystal protein Cry1Ac); the
survivorship of pink bollworm larvae ( Pectinaphora gossypiella ) on genetically modi-
fi ed cotton was 46-100% lower than on nonmodifi ed cotton (Lui et al., 2001).
However, concern has arisen about the widespread insertion of Bt into commercial
genetically modifi ed crops. This upscaling of the killing power of Bt poses such a
constant and high selection pressure that the probability of evolution of resistance
is enhanced for what is currently one of the most effective 'natural' insecticides
available (Section 6.4).
Finally, the so-called entomopathogenic nematode worms deserve mention. These
hold promise as biological control agents of insect pests by infecting, rapidly multi-
plying and quickly killing their hosts. Worms in the families Steinernematidae and
Heterorhabditidae infect insects as juveniles, gaining access through natural open-
ings (spiracles, mouth, anus). The infective juveniles carry in their guts a symbiotic
bacterium that is released, multiplies and kills the host, leaving the body contents
as a soup used by three or four generations of nematodes. When the soup runs out,
many thousands of infective juveniles are released into the environment. Fenton
et al. (2000) explored the population dynamics of the nematodes and concluded
that they are probably incapable of regulating a host population to a persistent stable
equilibrium, but because they are easy to mass-culture they are well equipped for