The Centers for Disease Control and Prevention, the Environmental Protection Agency, and anyone concerned with mosquito control in general, or Zika in particular, have a big problem: Standard mosquito control measures are failing. Mosquitoes are developing resistance to common pesticides, and people are increasingly wary of the chemicals being used to replace them.

In Florida, for example, residents are calling for a two-week moratorium on aerial spraying of the insecticide Naled over Miami Beach, even as the number of locally acquired Zika infections tops 70. Protesters are urging officials to consider other options, but the CDC and others say that effective alternatives are in short supply.

As the crisis persists, scientists are turning to an unlikely ally: other mosquitoes. Some researchers are developing genetically modified (GM) mosquitoes that can decimate wild populations by interfering with the mosquitoes’ ability to reproduce. Others are infecting mosquitoes with a bacteria that inhibits Zika transmission or releasing into the wild a cannibalistic species that devours the disease-carrying ones.

Each of those options holds promise, but each also comes with safety concerns and regulatory challenges. Here's a quick primer on the non-chemical options for battling the disease-carrying Aedes mosquito:

Alter Their Genes

There are several ways to turn a mosquito's genome against itself, but the technology that’s furthest along comes from the British company Oxitec. The company has conducted five field trials of its GM mosquitoes—in Brazil, Panama, and the Cayman Islands—and its insects are actively being used in the Cayman Islands and in Piracicaba, Brazil, a city of 60,000 people.

In the U.S., the Food and Drug Administration has approved a field test of Oxitec mosquitoes in Key Haven, Fla., but that trial can't proceed until the Florida Keys Mosquito Control District gives its approval. The agency has scheduled a nonbinding referendum for citizens in November and is expected to make a decision soon after. If it proceeds, the field trial will take several months, and will be only one of several trials needed for the technology to obtain federal approval for commercial use.

The Oxitec strategy involves inserting an engineered gene into lab-bred mosquitoes. That gene produces a protein that gums up the cellular machinery involved in the mosquito’s transition from larva to adult. In the lab, the mosquitoes are exposed to tetracycline, an antibiotic that neutralizes the protein so that the insects can reach adulthood. The adult mosquitoes are then released into the wild to mate with normal ones. The offspring of those unions inherit the engineered gene and -- because they have no access to tetracycline in the wild -- die before they can reproduce.

When enough engineered mosquitoes are deployed, the wild population begins to die off: Oxitec has reported a more than 90 percent reduction of Aedes aegypti mosquitoes at each of its test sites.

But the relationship between mosquito population and disease transmission is complicated; it depends on a range of factors including human population density, human immunity, and characteristics of the virus itself. And it remains to be seen whether a 90 percent reduction in Aedes aegypti is enough to stop the spread of Zika.

Opponents to Oxitec's plan in Florida (and elsewhere) have detailed a number of safety and environmental concerns. One is that the protein from the engineered gene could make its way into humans (the company plans to only release male mosquitoes, which minimizes this risk in theory, because male mosquitoe don't bite humans. But critics say that some females are bound to be released as well, even if by accident).

Another concern is that the tetracycline in the mosquitoes could harm the environment. Another still is that the gene will somehow be deactivated in the wild, allowing the engineered mosquitoes to thrive, and exacerbating the very problem the technology was designed to resolve. The FDA says it has evaluated those concerns and determined that the risk of any coming to pass is neglibible. 

Because the Oxitec mosquitoes would have to be relased multiple times, as mosquito populations inevitably rebound, critics also say the approach will be cost-prohibitive, especially for developing countries most often plagued by mosquito-borne viruses. 

Another way to alter a mosquito’s genome involves a relatively new scientific concept known as a gene drive. As with the Oxitec approach, a gene drive against Aedes would involve altering the mosquito’s genome in a way that prevents its offspring from surviving in the wild.

But unlike the Oxitec approach, a gene drive can potentially push altered genes through an entire wild population in just a few generations—a capacity that is unprecedented, based on brand-new technology, and the source of much hand-wringing within the scientific community. In June, a panel of experts convened by the National Academies of Science announced that although gene drive technology holds great promise and should continue to be developed and invested in, it is not yet ready to be employed in the real world.

Infect Them With Bacteria

Another non-chemical mosquito control strategy being developed involves infecting mosquitoes with Wolbachia bacteria; these microbes exist naturally in most insects but are absent in the Aedes aegypti. The bacteria can interfere with the Aedes’ ability to spread viruses like Zika in two ways.

The strategy being developed by MosquitoMate, a company based in Kentucky, involves infecting lab-bred male Aedes mosquitoes with the bacteria and releasing them into the wild. When those males mate with Wolbachia-free females, the resulting eggs won’t hatch. As more infected males are released and mate with wild Wolbachia-free females, the wild Aedes population is reduced.

MosquitoMate has already applied for insecticide registration of its Wolbachia-infected Aedes allbopictus mosquitoes, based on field trials in New York, California, and Kentucky, which the company says showed greater than 80 percent reductions in local Aedes populations. It expects to be granted final approval sometime in September 2016, after which it would work with state and local governments to deploy its mosquitoes.

As with Oxitec’s mosquitoes, it’s difficult to know for certain whether reducing the mosquito population by 80 percent will be enough to stop the Zika virus from spreading. And because this approach also requires releasing multiple batches of mosquitoes, critics say that cost may be an issue here as well.

Eliminate Dengue, a nonprofit based in Australia, has a slightly different Wolbachia-based strategy: The group wants to release infected female Aedes mosquitoes. Infected females can still produce viable offspring, so the mosquito population wouldn't be suppressed—but the bacteria make it more difficult for them to pass viruses like Zika and Dengue on to humans. 

Eliminate Dengue has been conducting field trials of its technology since 2011, in Australia, Vietnam, Brazil, Colombia, and elsewhere. Based on those trials, the company says it believes its strategy would result in a self-sustaining, Wolbachia-infected Aedes population—i.e., the mosquitoes would not have to be redeployed indefinitely, making them a potentially more cost-effective option. 

Neither the MosquitoMate nor the Eliminate Dengue group has met much resistance to its technology, especially compared with Oxitec, presumably because their approaches involve naturally occurring bacteria, not engineered genes. Eliminate Dengue reports more than 90 percent acceptance of its work in communities where field trials have been launched. And in Florida, some opponents to GM technology are actively lobbying for MosquitoMate's Wolbachia approach.

Introduce Them to Cannibals

Several traits make Toxorhynchites mosquitoes (also known as elephant mosquitoes because they are among the largest known) an appealing weapon in the battle against Zika.

They feed on the larvae of other mosquitoes, including those of the disease-carrying Aedes. They don't feed on human blood, so there's little risk that they could spread disease themselves. And the females prefer to lay their eggs in the same types of pools and puddles that the Aedes females use (including some hiding places that aren't accessible to chemical insecticides), a similarity that allows them to readily cannibalize the Aedes larvae.

Experts have tried to use these mosquitoes to control other mosquito populations since the 1920s, when their potential was first discovered. The approach has been tested in Africa, American Samoa, Fiji, Florida, Manilla, Panama, and elsewhere across the globe. The results have been mixed.

But though scientists have a good understanding of what went wrong in the failed trials, they don’t necessarily have solutions for those glitches. Sometimes the elephant mosquitoes failed to gain a toehold, other times they simply didn’t succeed in controlling the “bad mosquitoes.”

One problem is that elephant mosquitoes take three times as long to breed as Aedes do, making it tough for them to keep up. Another is that when the elephant mosquitoes claim the same breeding sites as the Aedes, the Aedes sometimes retreat into the forests.

This strategy has also fallen victim to the same problem that other straightforward control measures now face—too many breeding sites to treat and not enough people or time to treat them. So, as with the other options, the biggest risk here is failure.

At least one mosquito abatement district, in Harris County, Texas (where local climate, including tropical storms and an established Aedes aegypti population, makes pesticides a seasonal reality), is working with an insect lab to grow elephant mosquitoes, though the county has no immediate plans to launch a field trial. The bugs are expensive to grow, says Mustapha Debboun, director of Harris County's Public Health and Environmental Services, and it's not yet clear how well they'll work. But he sees no reason not to at least try them out. "I believe it could be a viable tool," he says. "Why use a pesticide if you can use a mosquito to kill another mosquito?"