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