AMNA NAWAZ: It doesn't have sharp teeth or claws, but the bite of a small mosquito kills more people than any other animal on Earth.

And as the climate warms, mosquito season is lasting longer, giving them more time to spread dangerous diseases.

Ben Tracy with Climate Central reports on the surprising way scientists are fighting back.

BEN TRACY: It's delivery day in the San Gabriel Valley... MAN: Oh, good.

Heck yes.

BEN TRACY: ... just east of Los Angeles.

TRISTAN HALLUM, Director of Scientific Programs, San Gabriel Valley Mosquito and Vector Control District: This is completely bonkers.

BEN TRACY: And Tristan Hallum'S latest shipment... TRISTAN HALLUM: All 25 tubes are here.

BEN TRACY: ... is not something you normally find in a box.

TRISTAN HALLUM: What I'm listening for is the sound of 600 adults flying around.

All right, let's head on in.

BEN TRACY: Their blue vests are a pretty good clue as to what's inside.

So what's in the tube?

TRISTAN HALLUM: So inside this tube is actually 600 male Aedes aegypti mosquitoes that we just received overnight from Kentucky.

BEN TRACY: Hallum oversees scientific programs for the San Gabriel Valley Mosquito Control District.

Do you like mosquitoes?

TRISTAN HALLUM: I enjoy working with them, but I like to kill them.

(LAUGHTER) BEN TRACY: He's targeting invasive mosquitoes that can spread diseases, including Zika, West Nile virus, and dengue fever.

TRISTAN HALLUM: San Gabriel valley was one of the first agencies to see local dengue transmission just a few years back.

BEN TRACY: Oh, yikes.

There they come.

They're releasing 30,000 mosquitoes every week containing a bacteria that helps block disease transmission.

TRISTAN HALLUM: So, in total, we're going to be releasing just under half-a-million mosquitoes.

BEN TRACY: And how effective is this?

TRISTAN HALLUM: That's one of the things that we're hoping to find out.

In previous studies, they have seen upwards of 80 to 95 percent control reduction in the areas that they have been releasing.

BEN TRACY: As the climate warms, mosquito seasons are getting longer.

Climate Central found nearly 95 percent of the cities it analyzed now have more days each year suitable for some of the most common species of mosquitoes to spread disease, 18 more days on average than in the early 1970s.

TRISTAN HALLUM: What we would used to refer to as the mosquito season would only be, say, May to September.

But now we have individuals treating for mosquito control activity all season long.

BEN TRACY: For these releases, they don't need bug spray.

I don't see any bites on your arms.

Because they're all male mosquitoes, and males don't bite.

STEPHEN DOBSON, President, MosquitoMate: I tell people it's the opposite of the human species.

It's the females that cause trouble in the mosquito world.

BEN TRACY: Professor Stephen Dobson supplies those mosquitoes.

He's president of MosquitoMate in Lexington, Kentucky.

STEPHEN DOBSON: The common name for this mosquito is the yellow fever mosquito, and yellow fevers are really nasty.

BEN TRACY: His team breeds this specific species of mosquito in their lab and ships them to control districts across the country.

These males carry a naturally occurring bacteria called Wolbachia, which makes them incompatible with females they try to mate with.

STEPHEN DOBSON: And when they do, those females are then sterile for the rest of their lives.

They will lay eggs, but those eggs don't hatch.

If enough of the females are not producing any offspring, then the population goes down.

BEN TRACY: Are these things genetically modified in some way?

STEPHEN DOBSON: They are not.

This is a naturally occurring bacterium.

We're just moving the bacterium from one mosquito to another.

BEN TRACY: This EPA-approved approach can cut certain mosquito populations by 70 to 95 percent.

The World Mosquito Program is using similar methods in 16 countries, including Brazil, where it produces 110 million Wolbachia mosquitoes each week.

Some communities there have seen dengue cases fall by 89 percent.

STEPHEN DOBSON: These are the immature mosquitoes.

BEN TRACY: Dobson says new tools are needed because climate change isn't just making mosquito seasons longer.

It's expanding their range.

STEPHEN DOBSON: The general trend is, they're moving northward into places that historically have been too cold.

BEN TRACY: Overall, temperatures are going up and the atmosphere is becoming wetter.

Is that a good thing for mosquitoes in terms of their ability to survive?

STEPHEN DOBSON: For these mosquitoes, unfortunately, yes.

It's exactly what they like, a nice warm, humid environment.

TRISTAN HALLUM: Let's see what's inside.

BEN TRACY: Tristan Hallum says if his traps catch fewer disease-carrying females this summer, he will know the boy bugs are doing their job.

TRISTAN HALLUM: As long as we're not seeing females recollected in this trap, we know that we're seeing a reduction in the population in this area.

BEN TRACY: For the "PBS News Hour," I'm Ben Tracy with Climate Central.