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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8: Mosquitocidal B. sphaericus: Toxins, Genetics, Mode <strong>of</strong> Action, Use, <strong>and</strong> Resistance Mechanisms 301<br />
unpublished data). Further studies are required<br />
to evaluate whether strain IAB-59 is <strong>of</strong> commercial<br />
value as an alternative to B. sphaericus strains 1593,<br />
2362, 101-B, <strong>and</strong> C3-41 (the only strains currently<br />
produced for use in the field). This depends on its<br />
potency <strong>and</strong> field performance. Additionally, several<br />
groups are currently trying to identify the genes<br />
encoding the additional toxic compounds from<br />
strains IAB-59 <strong>and</strong> LP1-G.<br />
The fact that the Bin toxin behaves as a unique<br />
moiety during the binding step indicates that the<br />
probability <strong>of</strong> selecting resistance to B. sphaericus<br />
is much higher than for Bti, which contains at least<br />
four proteins that potentially act on different target<br />
molecules. Despite reports <strong>of</strong> laboratory <strong>and</strong> field<br />
resistance, the use <strong>of</strong> B. sphaericus for the control<br />
<strong>of</strong> mosquito larvae is still <strong>of</strong> interest. Nevertheless, it<br />
is important to collect information about baseline<br />
susceptibility before starting treatment. However, as<br />
resistance to B. sphaericus is recessive, it is only possible<br />
to detect resistance when it has already become<br />
homozygous <strong>and</strong> therefore difficult to h<strong>and</strong>le. Thus, it<br />
may be possible to monitor the risk <strong>of</strong> resistance development<br />
in a given area by use <strong>of</strong> molecular nucleotide<br />
probes based on the mutation <strong>of</strong> resistance genes.<br />
However, resistance to B. sphaericus seems to be very<br />
complex <strong>and</strong> different probes would be needed to<br />
cover all possible mutations, only one <strong>of</strong> which has<br />
been identified so far (Darboux et al., 2002).<br />
Both from a practical <strong>and</strong> an ecological point <strong>of</strong><br />
view, the best <strong>and</strong> most direct way to overcome<br />
<strong>and</strong> to manage B. sphaericus resistance is <strong>of</strong> course<br />
Bti, which is already commercialised. Fortunately,<br />
all Culex populations reported to display field resistance<br />
to B. sphaericus have been tested for<br />
susceptibility to Bti. No cross-resistance has been<br />
observed. There is even some evidence for increased<br />
susceptibility in some colonies (Rao et al., 1995;<br />
Silva-Filha et al., 1995; Yuan et al., 2000). This<br />
was expected, because the multi-toxin complex <strong>of</strong><br />
this bacterium does not share any receptor-binding<br />
sites with the B. sphaericus binary toxin (Nielsen-<br />
LeRoux <strong>and</strong> Charles, 1992). In several situations, it<br />
is recommended that Bti be used in rotation with<br />
B. sphaericus (Regis <strong>and</strong> Nielsen-LeRoux, 2000;<br />
Yuan et al., 2000). Recent results obtained in<br />
Thail<strong>and</strong> have suggested that a mixture <strong>of</strong> Bti <strong>and</strong><br />
B. sphaericus may help reduce the risk <strong>of</strong> appearance<br />
<strong>of</strong> B. sphaericus resistance while preserving<br />
the superior field performance <strong>of</strong> B. sphaericus<br />
compared to Bti, particularly in breeding sites rich<br />
in organic matter (Mulla et al., 2003).<br />
Alternatives to natural Bti <strong>and</strong> B. sphaericus<br />
strains include genetically modified organisms, as<br />
combining different toxins binding to different<br />
receptors in the same organism is also a good way<br />
<strong>of</strong> preventing resistance. In addition to the Bin toxins,<br />
other toxins from either B. sphaericus or other<br />
insecticidal microorganisms could be produced. For<br />
example, the expression <strong>of</strong> Mtx1 genes in B. sphaericus<br />
during sporulation, i.e., under the control <strong>of</strong> a<br />
sporulation promoter, could allow the diversification<br />
<strong>of</strong> toxins. Similarly, Bti mosquitocidal toxin<br />
genes could be combined with B. sphaericus genes;<br />
some groups have already attempted this, for example<br />
by introducing the Bti genes encoding Cry4B,<br />
Cry4D or Cry11A into toxic B. sphaericus strains<br />
(Trisrisook et al., 1990; Bar et al., 1991; Poncet<br />
et al., 1994; Poncet et al., 1997). Conversely,<br />
B. sphaericus crystal toxin genes have been successfully<br />
introduced into toxic Bti (Bourgouin et al.,<br />
1990). Although these studies did not demonstrate<br />
any increase in toxicity against Anopheles <strong>and</strong><br />
Culex, such recombinants may delay the emergence<br />
<strong>of</strong> resistant insects.<br />
Several attempts have been made to introduce or<br />
to combine toxin genes from Bti, B. thuringiensis<br />
serovar jegathesan (Bt jeg) <strong>and</strong> serovar medellin<br />
(Bt med), <strong>and</strong> to evaluate their activity towards<br />
both susceptible <strong>and</strong> B. sphaericus-resistant colonies.<br />
The introduction <strong>of</strong> Cry11A from Bti <strong>and</strong><br />
Cry11Ba from Bt jeg partially restored the susceptibility<br />
<strong>of</strong> resistant colonies (Poncet et al., 1997;<br />
Servant et al., 1999). Nonspecific cytolytic toxins<br />
(Cyt1A <strong>and</strong> Cyt1B) also partially restored its toxicity<br />
towards two resistant colonies (Thiéry et al.,<br />
1998; Wirth et al., 2000a). A Bti strain harboring<br />
genes encoding the binary toxin as well as Cyt1 <strong>and</strong><br />
Cry11B was more toxic than unmodified Bti IPS-82<br />
(Park et al., 2003). Several groups have also<br />
attempted to introduce <strong>and</strong> to overproduce entomopathogenic<br />
toxins into organisms living in the<br />
environment <strong>and</strong> serving as natural foods for mosquitoes,<br />
such as cyanobacteria (Liu et al., 1996b), or<br />
to stably integrate the toxins encoding these genes<br />
into the chromosomes <strong>of</strong> other bacteria such as<br />
Enterobacter amnigenus (Tanapongpipat et al.,<br />
2003). However, given our current knowledge <strong>of</strong><br />
B. sphaericus resistance, the expression <strong>of</strong> Bin<br />
toxin as the only active compound should be<br />
avoided. No recombinant strains have yet proved<br />
to have a better field performance than the natural<br />
strains, <strong>and</strong> no products based on recombinant<br />
strains have yet been registered for use in mosquito<br />
control programs. Therefore there is a continuing<br />
need to identify new strains with novel modes <strong>of</strong><br />
action against the principal mosquito vectors <strong>of</strong><br />
human diseases.