January 2004 Bti - A Biological Weapon Against Mosquitoes It’s not Bacillus anthracis (anthrax), Yersinia pestis (plague), or Vibrio cholerae (cholera), but Bacillus thuringiensis istraelensis (Bti), a biological agent that we use to combat mosquitoes. The definition of a biological weapon is an organism or toxin found in nature that can be used to incapacitate, kill, or otherwise impede an adversary. Biological weapons are characterized by low visibility, high potency, substantial accessibility, and relatively easy delivery - the use of Bti for mosquito control is all of these things.
In the pest control business Bti is considered a “biological pesticide.” An important aspect of microbial control agents is that they can be used to replace, at least in part, some of the more hazardous, less discriminating, chemical pesticides. Presently, chemical pesticides are used much more commonly worldwide than microbial agents. It is unclear whether or not all chemical pesticides are environmentally harmful, especially if good stewardship of their application is practiced. Replacing them with microbial agents does not necessarily guarantee fewer environmental risks. Nonetheless, for the numerous chemical pesticides known to have toxic effects on non-target organisms, the opportunity to substitute safer, more selective, biodegradable, biocontrol agents can provide important ecological benefits.
Insect pest control with microbial agents has a long history. In 1834 Bassi recognized that microorganisms were associated with disease in silkworms and in 1964 Louis Pasteur confirmed that various diseases in silkworms were caused by microorganisms and suggested the use of microbes to control insect pests.
In 1975 the World Health Organization (WHO) sponsored a project in Israel to examine mosquitoes for the presence of pathogens and parasites. In 1978, Doctors Goldberg and Margalit, the researchers heading the project, discovered a new strain of Bacillus thuringiensis (Bt). This microbe, a gram-positive, rod-shaped, aerobic, spore-forming bacterium, was identified and designated Bacillus thuringiensis israelensis, a subspecies to Bt. This new strain was found in the Negev desert of Israel and showed significantly more toxicity to mosquitoes than any other known bacterial strains being used at the time. Although several Bt strains were known to be effective against many species of flies, Bti was found to be much more toxic and specific to mosquito and blackfly species. This specificity is what gives Bti a huge advantage over chemical pesticides, killing just mosquito and blackfly larva and having very little to no effect on non-target organisms. Because of this, Bti was considered to have commercial potential as a biological pesticide for control of mosquitoes and blackflies. At the time of Bti’s discovery fuel oil was used extensively as a larviciding agent and the incidence of resistance to other chemical larvicides was increasing. The need for a more environmentally benign mosquito control agent, plus the development of resistance to chemical pesticides, stimulated the rapid development of Bti as an effective alternative in the 1980's. Bti has been used as a larviciding agent, extensively, throughout the world, for both mosquito and blackfly control programs for over 20 years now with excellent results.
Bti is grown commercially in large fermentation vessels using sophisticated techniques to control environmental variables such as temperature, moisture, oxygen, pH, and nutrients. The process is similar to the production of beer, except that the Bti bacteria are grown on high protein substrates such as fish meal or soy flour. At the end of the fermentation process, when the bacteria have exhausted the nutrients, they produce spores then die. The spores contain delta (50 endotoxins, the biological pesticide. The spores/endotoxins are concentrated by centrifugation and micro-filtration of the slurry to become the end product of the process. Once separated from the slurry the product can be dried for processing into granules, pellets, powder, briquets or processed further to produce concentrated liquid formulations. These different forms of the product allow it to be used to treat the various environmental situations that are encountered.
The delta endotoxins are highly toxic and act synergistically when released from the spore. The fact that the toxins are contained within the spore makes Bti safe for the applicator, non-targets, and the environment. The fact that the endotoxins act synergistically and each has a different mode of action, has kept genetic resistance from developing in the mosquitoes and blackflies it is used on.
So how does it kill just blackfly and mosquito larva? When applied at label rates Bti has virtually no adverse effects on applicators, livestock, plants, or wildlife, including beneficial insects, annelid worms, flatworms, crustaceans, mollusks, fish, amphibians, reptiles, birds, or mammals. Bti kills mosquito and blackfly larva when the spores are ingested and the endotoxins are released. However, in order to be released from the spore, the spore cover must be dissolved, this occurs in the presence of an alkaline environment. It just so happens that the digestive system of larval mosquitoes and blackflies have a highly alkaline pH level. This pH level breaks down the spore and releases the toxins which bind to the midgut destroying the cells and killing the insect. It takes about 4 to 6 hours after ingestion for the larva to die. Bti will remain effective for 24 to 72 hours in the natural environment, depending on water conditions, eventually breaking down from exposure to UV radiation (from sunlight) and biodegradation. Bti is most effective when applied during the early larval stages of development. Therefore, when using Bti, timing of application is extremely important. The only real disadvantage of using Bti is that it has a limited treatment window, the early larval stages, about 3 to 4 days. Bti has no effect on the non-feeding pupal stage or the adults.
In summary, Bti is an extremely safe and effective biological weapon in the control of mosquitoes and blackflies when properly applied. It is ideally suited for Integrated Pest Management (IPM) programs because the active ingredient does not interrupt the activities of beneficial insects and predators. It is safe for applicators, domestic animals, wildlife, plants, and the environment. Its mode of action in target species is too complicated for them to develop a genetic resistance. Operationally, Bti is inexpensive, easy to store, easy to formulate, and easy to apply. Its only drawback is its limited treatment window. But, as with any pesticide application, biological or chemical timing is the key to a successful application. |