Intrinsic Resistance Management
The Hidden Benefits of Bacillus thuringiensis subsp. israelensis (Bti)
For more than 30 years, Bti has played a pivotal role in public health programs by helping to control vector and nuisance insects around the world. Discovered and isolated in the 1970s, Bti is proven as an effective larvicide in the fight against mosquitoes and black flies (Simulium) while avoiding harm to non-target populations and the environment. A fundamental challenge in public health — as in any situation where an active ingredient (AI) is used to control diverse populations of living organisms — is the concern about the onset of insect resistance. Resistance carries with it a number of logistical and financial complications, including the need to:
Increase application rates
Accelerating the resistance process
Increase application frequency
Accelerating resistance and increasing costs
Seek new solutions
Which require additional investments of time and funding for long-term implementation
What makes Bti significant in this regard? Consider that in the more than 30 years since it was introduced to the public health domain, no commercially available Bti formulation has ever demonstrated operational-level resistance. Not a single case. How can this be?
Intrinsic Resistance Management: The Science Behind Bti
The reason Bti has remained effective since its introduction is the synergistic nature of four protein toxins that give Bti its efficacy. These four “protoxins” belong to three distinct toxin classes, each of which Bti releases when ingested by target larvae:
Cyt1A (27 kDa)
Cry4A (134 kDa)
Cry4B (128 kDa)
Cry11A (66 kDa)
While it’s true that studies in a laboratory setting have shown resistance potential when individual toxins were isolated1 from a particular strain of Bti 2, no empirical evidence of resistance has ever been substantiated when using whole Bti — a fact that has been documented by many of the foremost public health scientists in the world.3 For this reason, it is not uncommon to hear Bti referred to as the single most important active ingredient available for public health larviciding programs.
To put it simply, Bti has resistance management qualities “built in.” Unlike other chemical or biochemical AIs used in public health pesticide products, Bti products provide not only superior efficacy, but also offer intrinsic resistance management benefits that have strong implications as part of a larger resistance management effort.
Building on the Benefits of Intrinsic Resistance Management
Biologists know there is no such thing as a silver bullet. While Bti has built-in resistance management properties that make it an indispensable tool for sustainable mosquito control programs, Bti products as “stand-alone” solutions do have some limitations:
- The same qualities that make Bti a low-impact product for the environment also limit its residual activity. Bti breaks down quickly, meaning that most Bti applications made to open water bodies provide control of mosquito larvae for a limited amount of time.
- Bti is proven to be extremely effective in “clean” water habitats, but higher rates are needed in habitats with more organic content.
- Bti is only effective against mosquito larvae (1st through early 4th instar). As such, the use of Bti requires dedicated site surveillance to ensure that applications are made within the appropriate treatment window.
These limitations can be overcome, however, by formulating Bti in combination with other AIs that have greater residual efficacy, such as Bacillus sphaericus (Bsph). Although Bsph carries the potential for resistance development, the IRM properties of Bti can prevent that resistance from developing. The result is an elegant, synergistic solution for longer lasting, broader spectrum mosquito control while addressing resistance concerns.4
In fact, combinations with Bti have been shown to restore or “bring back” susceptibility to Bsph in cases where resistance has developed.5 This synergistic effect occurred only when the AIs were combined, as opposed to being used separately in rotation.5
Bti: The Bottom Line
With intrinsic resistance management provided by Bti, mosquito abatement programs are able to standardize their operations with fewer products while at the same time maintaining best practices for resistance management. That means simplified operations: fewer decisions, fewer calibrations/characterizations for application equipment, and increased opportunities to maximize efficiency, cost-effectiveness, and expertise.
It’s a welcome combination that has made Bti the number-one biological mosquito larvicide around the globe. And for an industry that has traditionally had a limited number of tools, Bti’s intrinsic resistance management qualities show tremendous promise as a sustainable solution not only today, but for the long term.
It’s a welcome combination that has made Bti the number-one biological mosquito larvicide around the globe. And for an industry that has traditionally had a limited number of tools, Bti’s intrinsic resistance management qualities show tremendous promise as a sustainable solution not only today, but for the long term.
1 Wirth. “Mosquito resistance to bacterial larvicidal toxins.” The Open Toxinology Journal, 2010, 3:126–140.
2 Paul A, et al. “Insecticide resistance in Culex pipiens from New York.” Journal of the American Mosquito Control Association, 2005, 21(3):305–309.
3 Becker N, Ludwig M. “Investigations on possible resistance in Aedes vexans field populations after a 10-year application of Bacillus thuringiensis israelensis.” Journal of the American Mosquito Control Association, June 1993, 9(2):221–224.
4 Zahiri NS, Mulla MS. “Susceptibility profile of Culex quinquefasciatus (Diptera: Culicidae) to Bacillus sphaericus on selection with rotation and mixture of B. sphaericus and B. thuringiensis israelensis.” 2003, J Med Entomol 40:672–677.
5 Zahiri NS, Su TY, Mulla MS. “Strategies for the management of resistance in mosquitoes to the microbial control agent Bacillus sphaericus.” 2002, J Med Entomol 39:513–520.
