Contents - Faperta

256 Biotechnological Approaches for Pest Management and Ecological Sustainability
mortality. Insects that bore into the plant tissue or remain hidden inside the plant structures
are much less susceptible than insects feeding on foliage. All developmental stages
of the insect are not equally susceptible to the microbial agents. As a result, the user must
time the application precisely to prevent crop damage from an insect population exceeding
the economic threshold. Some aspects of insect behavior also infl uence the performance of
biopesticides. If major plant growth occurs after application of the biopesticide, then the
unprotected plant surfaces become prone to insect damage. While proper timing of application
is important, it may also be necessary to formulate the biological insecticide with
feeding attractants such as molasses, which lure the insect to plant surfaces carrying the
biocontrol agent. Contact poisons are superior in that an insect traversing a sprayed plant
surface enroute to its preferred feeding site will contact the toxic residues regardless of its
feeding habits.
Host specifi city, which makes biopesticides ecologically and environmentally attractive,
also constitutes a serious drawback. The narrow host range of biopesticides prevents their
successful use to control a multiplicity of insect pests that feed on crop plants during the
course of the growing season. For example, growers of crops such as sorghum, maize, rice,
sugarcane, pigeonpea, groundnut, and cotton must contend with over a dozen different
pest species, each with a different behavior and numbers that change dramatically over
the crop growing season. As no single biological insecticide has been able to cope with the
diversity of pests in a crop or cropping system, the necessity of multiple applications presents
an economic obstacle to large-scale application of biological insecticides. Potency is
another factor that has limited the use of biological insecticides. For economic reasons, the
user will not apply an insecticide prophylactically in anticipation of an outbreak that might
not occur or might not reach signifi cant proportions. The insecticide must therefore be
capable of controlling an insect infestation once the evidence of a threat is clearly established.
Some biological insecticides, though highly effective under laboratory conditions,
have been found to be ineffective under fi eld conditions. Lack of potency has been ascribed,
in some cases, to poor storage capability, short residual toxicity, slow mortality, requirements
in the fi eld for optimum temperature, relative humidity, and sunlight. Application
of DNA-based technology promises new strains of entomopathogenic microbes with greater
potency, increased host range, and adaptation to the harsh environmental conditions, and
thus render them as an effective weapon for pest management in the future.
Natural versus Engineered Microbes
Microbial pesticides based on natural strains have been used for a long time. Several formulations
based on different strains of B. thuringiensis (Bt) have been used over the past
fi ve decades. Many of these formulations have been improved by selecting or identifying
more potent strains of the bacterium. Improvement of naturally occurring strains has
increased the usefulness of biocontrol agents, and has taken advantage of the natural
recombination based on conjugation, transduction, and transformation (Tortora, Funke,
and Case, 1989; Schnepf et al., 1998). Transformation involves the death and lysis of the
bacterial cell, and release of the DNA, which under certain circumstances can be taken up
by the surrounding bacteria. This allows the transformation of the bacteria under controlled
conditions in the laboratory, and results in new strains suitable for certain purposes
(Stewart, 1992). Transduction is the result of viral infection in bacteria, resulting in the