Insect Control: Biological and Synthetic Agents - Index of

has been used to control Aedes vexans and Culex
pipiens pipiens mosquitoes. Each year, approximately
300 km of river and 600 km 2 inundation
areas are treated. Since 1981, more than 170 000 ha
of mosquito breeding sites have been treated with
different B. thuringiensis subsp. israelensis formulations.
This program (KABS) has resulted in the
reduction of mosquito populations by over 90%
each year (Becker, 2000). These results are comparable
to the use of B. thuringiensis subsp. israelensis for
the control of A. vexans in the United States and
in southern Switzerland (Becker, 2000). Again, no
resistance to has been observed even after more than
20 years of the launch of the program (Becker, 2000).
High incidences of malaria in Hubei Province in
China were reduced by more than 90% by the
application of B. thuringiensis subsp. israelensis
and B. sphaericus into the Yangtze River. The vector
mosquitoes Anopheles sinensis and A. anthropophagus
populations were reduced 90% by weekly
applications of fluid formulations of two B. thuringiensis
subsp. israelensis and B. sphaericus strains
(Becker, 2000). In Brazil the outbreak of A. aegypti
resistant populations to chemical insecticides in Rio
de Janeiro and São Paulo led to the application of
B. thuringiensis subsp. israelensis formulations for
mosquito control (Regis et al., 2000).
The success of vector control using B. thuringiensis
subsp. israelensis will certainly spread the use of
this B. thuringiensis subspecies around the world.
However, the low activity of B. thuringiensis subsp.
israelensis against certain vector mosquitoes, particular
Anopheles spp., will require the isolation of
strains with novel cry genes that are effective against
different disease vectors.
7.8.3. Transgenic Crops
Bacillus thuringiensis is the leading biopesticide
used in agriculture. However, only 2% of the insecticidal
market consists of B. thuringiensis sprayable
products; the rest consists mostly of chemical insecticides
(Navon, 2000). Since the discovery of the
HD-1 strain, formulations for lepidopteran pests
control were released and products for the control of
coleopteran insects are also available. However, the
use of sprayable B. thuringiensis for insect control
has been limited. Many reasons have contributed to
this low acceptance of such commercial B. thuringiensis
products. These include: the higher costs
of commercial B. thuringiensis products; the lower
efficacy of B. thuringiensis as compared to chemical
insecticides; the limited persistence of B. thuringiensis
products; the narrow insect spectrum of B. thuringiensis;
and the fact that sucking and borer insects
are poorly controlled (Navon, 2000).
7: Bacillus thuringiensis: Mechanisms and Use 269
However, the creation of transgenic crops that
produce crystal B. thuringiensis proteins has been a
major breakthrough, and substitution of chemical
insecticides provides environmental friendly alternatives.
In transgenic plants Cry toxin is produced
continuously protecting the toxin from degradation
and making it available to chewing and boring
insects. Cry protein production has been improved
by engineering cry genes with a codon usage compatible
with that of plants, by removal of putative
splicing signal sequences and by deletion of the
C-terminal region of the protoxin (Schuler et al.,
1998). The first B. thuringiensis crop (Bt-potato)
was commercialized in 1995. These transgenic
plants expressed the Cry3A protein for protection
against Colorado potato beetles resulting in 40%
reduction in insecticidal use in 1997 (Schuler et al.,
1998). In 1996 Bt-cotton, producing the Cry1Ac
toxin, was released to protect cotton from tobacco
budworm, cotton bollworm, and pink bollworm.
Also in 1996 Bt-maize, producing Cry1Ab toxin,
was released for the control of European corn
borer (Ostrimia nubilalis) larvae (Schuler et al.,
1998). By 2000, transgenic crops were grown on
44.3 million ha globally. Of this, 23% were insect
resistant Bt-maize and 74% were herbicide resistant
crops (James, 2001). Yield effects of insect resistant
crops in the United States are less than 10% on
average, but nevertheless use of insect resistant
crops has considerably reduced the use of chemical
pesticides (Qaim and Zilberman, 2003). However,
the use of Bt-cotton in countries like China, Mexico,
and India has showed that the use of this Bt-crop has
a significant effect on yields and also in the use
of chemical pesticides, since in underdeveloped
countries yield loss is mainly due to technical and
economical constraints that are overcome in part
by the use of insect resistant crops (Qaim and
Zilberman, 2003; Toenniessen et al., 2003).
7.9. Public Concerns on the Use of
B. thuringiensis Crops
Three major concerns have been raised on the use
of Bt-crops: insect resistance, a direct effect on
nontarget insects, and transgene flow to native
landraces.
Widescale introduction of Bt-crops endangers the
durability of B. thuringiensis as an insecticide in
crops and in sprays since continuous exposure to
Bt-plants will lead to selection for Cry toxin resistant
insect populations. In response to this threat,
resistance management strategies associated with
the release of Bt-crops have been developed. The
predominant approach involves the combination of