4.5 Environmental consequences of Bti persistence
The Ae. aegypti LiTOX strain developed significant resistance to
both Cry and Cyt toxins when using field-collected leaf litter
containing Bti toxins as a selective agent, suggesting that both
Cyt and Cry toxins are persistent in toxic leaf litter. However, the
resistance level of the LiTOX strain to leaf litter was significantly
higher than to the commercial Bti mixture, suggesting that the
relative amount of each Bti toxin in the commercial Bti mixture
compared with toxic leaf litter may differ. This is not surprising,
as the degradation of the different Bti toxins is likely to vary
over time. Indeed, toxic leaf litter was collected several months
after Bti treatment,19 allowing sufficient time for toxins to exhibit
differential persistence and degradation. A modification in toxin
proportions could also lead to a decrease in their synergistic
interactions. Furthermore, as the synergism between Cyt and Cry
toxins is likely to circumvent resistance to Cry toxins,27,46,47 a
decrease in Cyt toxins in the environment may lead to the rapid
emergence of resistance to Cry toxins. Although the emergence
of multiple resistance mechanisms (i.e. several independent
resistance mechanisms to different toxins) is considered likely
to be delayed because all toxins are spread together,14,47,67 the
differential persistence of particular Bti toxins may favour the
sequential selection of resistance to Bti toxins.