Brief Summary of Bti:

• The bacterium Bacillus thuringiensis serovar israelensis (Bti) was isolated from a stagnant pond located in the Negev in south Israel in 1976, and was proven in laboratory experiments and field research to be a highly selective control agent for biting insects (Margalit and Dean, 1985).
• Bti produces crystals containing multiple toxins; after consumption these toxins are activated by specific conditions in the mosquito’s gut and disrupts the gut wall.
• Today, Bti is commercially produced in various formulations for use all over the world and several environmental safety reviews have concluded that Bti is both effective and ecologically safe (e.g. Lawler, 2017).
• Bti typically causes 90–100% mortality to target organism larvae, with generally limited or no documented direct impacts on other aquatic and terrestrial species, especially compared with alternative chemical controls (Land et al., 2019).
• In Florida following a Zika virus outbreak, an evaluation of urban mosquito suppression methods concluded: “Stop spraying permethrin and naled to suppress [mosquitoes] in urban areas. Permethrin had no effect. Naled failed to depress mosquitoes on Miami Beach and lost its efficacy in Wynwood. Both insecticides inflict documented harm on wildlife, and cause health problems for a small but vulnerable subset of the human population… Continue using Bti until new guidance is provided concerning its efficacy…” (Stoddard, 2018). 
• Treatment of urban water sources in Kenya resulted in 99.9% to 100% mortality depending on mosquito species (Mwangangi et al., 2010).
• Use of a mist to disperse Bti around high rise buildings in Malaysia resulted in a reduction in mosquito population of 93.8% (Zaki et al, 2020)
• In Sweden, Bti has been applied since 2002 in the lower Dalälven River area, to control mass outbreaks of floodplain mosquitos (Land et al., 2019).
• Full EPA documentation, “Bacillus thuringiensis subspecies israelensis strain EG2215 (006476) Fact Sheet”

What about commercial lawn sprays?

We’re glad you asked! More info here, because it’s a whole lot more science so it gets its own summary.

What about animals like bats that eat mosquitoes?

As Lawler (2017) writes: 

“Mosquito biologists are sometimes asked what might happen to food webs due to reducing mosquito abundances… the prevailing view is that few if any predators are dependent on mosquitoes (Fang, 2010)… and even mosquitofish consume many other types of prey…”
In short, the ecological effect of reducing the mosquito population is not widely seen as negatively impactful, whereas the morbidity, mortality, and other harm caused by mosquitos is well documented.