Backyard scientists have trapped them with a turkey baster and a screen.
Although the traps that University of North Texas students set around Denton are more sophisticated, they have a little of that backyard-scientist look. Residents from Philadelphia to Colorado Springs to Coppell have called 911, not knowing what to make of the netted contraption in their city with a little light bulb, a tiny stream of gas and a battery pack.
It’s a mosquito trap.
UNT students set out two kinds of traps each week during mosquito season, part of a city program that monitors for West Nile virus and other mosquito-borne illnesses. They put the traps out in the late afternoon and gather them up early the next morning.
One kind of trap lures the insects with light and carbon dioxide — the signals a mosquito follows for its blood meal. The other trap uses a bucket of stagnant water to lure gravid mosquitoes seeking a place to lay eggs.
The scientists make the “stinky water” using things like manure and grass clippings, said James H. Kennedy, the UNT biological sciences professor who supervises the sampling. “That’s a scientific term,” he said.
The mosquitoes must be caught alive, which means they need to survive being vacuumed through a tiny plastic fan. Hence, nothing stronger than a 6-volt battery drives it.
“Otherwise the fan is too fast,” Kennedy said.
The bugs also must be chilled and packed for transport to protect the sample.
“The DNA of the virus breaks down quickly,” Kennedy said.
Once the samples are at a state health department lab in Austin, they are analyzed and both the city and UNT are notified of the test results.
By design, the traps lure mosquitoes, but occasionally other bugs get in the traps. The students must sort through the samples to make sure they are sending only mosquitoes.
Undergraduate students Sarah Hammontree and Colleen VonEhr have found other flying insects, such as moths, green lacewings, ant lions and parasitic wasps, in the traps.
The students must also care not to let too many Chironomidae mix among the Culex modestus and other kinds of mosquitoes. Too many midge, which look like mosquitoes, in the sample and they get a note from the lab, according to Jeff Mabe, a doctoral candidate.
Once in the traps, the disease-carrying mosquitoes don’t look any different than the others. Trapping this year has been different, however.
Kennedy said he told Mabe that he thought the virus would emerge early this year, but he was surprised at how early.
“It came with our first traps in May,” Kennedy said.
He wonders if they had set traps in April whether those mosquitoes, too, would have been positive for West Nile. Since May, mosquitoes caught in 16 traps around the city have tested positive, with several traps ensnaring more than one disease-carrying mosquito so far this year.
UNT first started working with state health officials and the city on mosquito monitoring after another graduate student, Bethany Bolling, set traps as part of a research project 10 years ago. West Nile virus had been reported on the East Coast of the U.S. in 1999 and it spread west quickly, Kennedy said.
He had extra funding to help her pay for traps, which they set up along the Greenbelt Corridor from Ray Roberts Lake to U.S. Highway 380 in the summer of 2002.
“We knew we were going to find it,” Kennedy said. “In August, we got our first positive.”
For West Nile, the virus amplifies by going back and forth between one kind of mosquito and local birds. In the first years of the virus in North Texas, a lot of birds died, Kennedy said.
Since then, area birds appear to be building up antigens, Kennedy said.
That virus amplification may explain why the traps set in the city’s yard at Texas Street and Mingo Road frequently test positive, even in years when no human West Nile cases are reported. A creek runs through the yard. Large trees line the creek bed to the north and south of the yard, making good habitat for both birds and mosquitoes, Kennedy said.
Since then, Denton and UNT have worked together to monitor mosquitoes, a sampling program that costs the city less than $15,000 per year. Because spraying is expensive and sometimes controversial, Denton’s response to mosquito control is graduated and based on documented presence of the disease — both in the traps and in humans.
This week, the students hung traps in three different areas along Hinkle Drive, all on private property. The traps are close to where human cases have been reported and where the city has sprayed for adult mosquitoes. They may get enough data to see whether spraying has an effect, Kennedy said.
The city’s spray is based on pyrethrum, an insecticide derived from chrysanthemums and generally considered safe for humans and other mammals. It breaks down in sunlight. But its effectiveness in actually controlling mosquito-borne disease is not understood, Kennedy said.
“It may give a little relief in the number of adult mosquitoes,” Kennedy said.
Such spraying has no effect on the larva ready to hatch and continue amplifying the virus.
Mosquitoes don’t need much stagnant water to breed. A child could play with a toy outside and leave it in the yard to collect a bit of water from a sprinkler or thunderstorm, and that’s enough for another brood of mosquitoes, Kennedy said.
Residents can do a lot to eliminate standing water in their yards. Briquettes containing the bacteria Bacillus thuringiensis can control mosquito larvae in stagnant water.
The need to trap and test mosquitoes for disease may not wane, Kennedy said. About 50 of the 80 types of mosquitoes found in Texas have been found in Denton, he said. Warmer temperatures have expanded the range of many insects in Texas, including mosquitoes.
The state lab monitors for not only the West Nile virus but also other diseases mosquitoes carry, which, like West Nile, can be debilitating or deadly.
Some diseases, such as malaria, could be more of a problem in Texas, but haven’t been so far because of good habitat control and other public health measures, Kennedy said.
But other diseases, such as dengue fever, which has emerged in Florida, can be more difficult.
“The students we are training today, we are teaching how to communicate what they are doing,” Kennedy said. “When the public asks questions, they need to know.”
PEGGY HEINKEL-WOLFE can be reached at 940-566-6881. Her e-mail address is firstname.lastname@example.org .