From Granaries to Insectaries:
Research leader Floyd Dowell
examines adult tsetse fly
specimens to determine
It all started at a 1998 meeting of entomologists in Gainesville, Florida.
James Throne, an expert on stored-products insects with the Agricultural
Research Service, stood at the podium sharing his lab's latest research
on detecting tiny insects that lurk inside grains. By using an automated
single-kernel handling system coupled with a near-infrared spectrometer,
he and colleagues found they could pinpoint grain kernels that looked
ordinary on the outside but harbored growing insects inside.
Many in the audience tuned in to hear more. Insect-infested grains
are a problem for the milling and processing industriesand for
consumers who typically cringe at the sight of a weevil in their bag
of flour or box of cornflakes.
The ARS technology relies partly on near-infrared (NIR) light, that part of the light spectrum that's invisible to the human eye. Each living organism absorbs NIR radiation uniquely, depending on its particular chemistry. The energy reflected back can be analyzed by a spectrometer that researchers then interpret.
Female tsetse fly specimen
James Baker, an ARS entomologist and colleague of Throne's, first thought
to use NIR spectroscopy for singling out bug-infested kernels.
"Insects inside of grains are one of the hardest defects to detect,"
he says. "For instance, a female grain weevil will bore into a
wheat kernel to lay an egg. She'll seal the opening with a tiny, gelatinous
plug. The mark left behind is hard to see with the naked eye."
NIR spectroscopy seemed a natural tool for tackling this job, since its light zeroes in on molecules made up of nitrogen, carbon, hydrogen, and oxygen within biological materials. Grains and insects have different amounts and combinations of these molecules.
Research leader James
Throne and engineer
Elizabeth Maghirang place
tsetse fly pupae in an
automated scanning and
"We knew NIR spectroscopy was useful for analyzing the protein
inside grains, so I wondered if it couldn't detect developing insects,
too," Baker says. He and Throne both work at the ARS Grain Marketing
and Production Research Center in Manhattan, Kansas.
Back at the meeting, someone elsewith a different set of research objectiveswas listening to Throne's talk. But his interest wasn't in grains or their larval stowaways. Robert Wirtz, who heads the Entomological Branch of the Division of Parasitic Diseases at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, had his sights on another pestone that leaves a more direct mark on human health and well-being.
Identifying Malaria Carriers
Wirtz wondered if the NIR-based technology could help identify mosquitoes
infected with the malaria parasite.
"Researchers used to have to dissect the insects," he says,
"and remove and analyze the salivary glands to see whether they
The ARS technology could possibly speed this critical identification
process. He thought it might also be able to distinguish between different
species of the genus Anopheles that carry the malaria-causing
"There are species of Anopheles mosquitoes with different
levels of significance in disease transmission," Wirtz explains.
"We hoped the NIR technology could tell apart these look-alike
According to CDC entomologist Mark Benedict, another challenge is determining
how old the mosquitoes are, since age plays an important role in disease
transmission. "It takes time for mosquitoes to become infected,"
he says. "With older populations, there's more potential for transmission."
Floyd Dowell, ARS engineer and chief architect of the NIR sorting instrument,
was able to find extra time to pursue this unexpected offshoot of his
research. But now he would be working with living, breathing insectsinstead
of the usual, easy-to-handle kernels of grain.
"Since adult insects are about the same size as kernels, we didn't
have to change the design settings used by the instrument for measuring
grains," he says. "But we did have to hand-place the insects
into it." Then the instrument accurately identified the species
of mosquito in just seconds.
Attempts to identify malaria-infected mosquitoes weren't as successful,
but Wirtz says he hasn't given up on that possibility yet.
"Our goal," he says, "is to use the technology being
developed by ARS to someday determine the species, age, and infection
status of mosquitoes. This would be a huge benefit for controlling malaria,
West Nile virus, and other diseases transmitted by mosquitoes."
Energized by the mosquito work, Benedict and Wirtz wanted to see whether
NIR could help distinguish between male and female pupae of the tsetse
flyan insect wreaking havoc in 37 countries in Africa. Coincidentally,
a tsetse fly pupa is about the same size as a grain kernel and could
be fed automatically through the NIR sorting system.
The tsetse fly carries the trypanosome parasite, which causes sleeping
sickness in humans and nagana in livestock. Without medical treatment,
sleeping sicknessor trypanosomiasisis always fatal. According
to the World Health Organization, more than 60 million people, mainly
in rural and agricultural areas of afflicted African regions, are at
Benedict thought NIR spectroscopy could help refine the sterile insect technique (SIT) that he was helping researchers with the United Nations Food and Agriculture Organization (FAO) and the International Atomic Energy Agency (IAEA) pursue to suppress tsetse fly numbers in ravaged parts of Africa.
From Screwworms to Tsetse Flies
SIT was pioneered by another ARS entomologist, the late Edward F. Knipling,
in the 1950s as an alternative to conventional pesticides. Often described
as "birth control for insects," this approach knocks down
problem insect populations by releasing large numbers of sterilized
males, which behave and mate like other males, but aren't able to fertilize
eggs. Over time and with continual releases, the targeted insect population
dwindles and ultimately crashes. The technique was first used to rid
North America of livestock-parasitizing screwworm flies.
But the SIT for tsetse needed a more rapid way to separate male and
female flies. Males must be pulled aside to be irradiated, and females
are needed to spawn future generations. Researchers also want to avoid
accidentally mixing females with the males to be released, since females
have a much greater potential to carry disease.
"Initially, when telling the sexes apart, we had to look at every
adult by hand after it emerged," says Benedict. "It was very
To gain access to thousands of tsetse fly pupae, Dowell traveled to
Seibersdorf, Austria, in January 2004 to the FAO-IAEA Agriculture and
Biotechnology Laboratory. He and the others wanted to see how soon in
the insect's development NIR could distinguish between males and females.
The earlier the better, as researchers need ample time to irradiate
the males and transport them to strategic release sites.
"We thought that we could tell them apart at 1 day before their
emergence as adultsbut probably not at 20 days before emergence,
when the pupae are less developed," Dowell says.
As it turned out, they could sex them best at 5 days before the pupae
emerge as adults. At that point, the differences in reflected light,
or spectra, of female and male pupae were most apparent.
"These differences might be explained by changes in cuticle thickness
or by ovarian development in the females," Dowell speculates. "We
can show without a doubt that there are differences between males and
females, but unfortunately we lack the science to explain why. This
will be a topic of future research if we can continue working in this
According to ARS scientists, the NIR technology may work better for
tsetse fly sex separation than for its original, intended use in grains.
FAO-IAEA has purchased an automated NIR sorting instrument and will
begin routinely sorting tsetse fly pupae in 2005.
"This is exciting technology and has lots of potential impact
in the grain industry and the field of entomologyin addition to
the potential human impact, particularly in underdeveloped countries,"
says Dowell.By Erin
K. Peabody, Agricultural Research Service Information Staff.
This research is part of Quality and Utilization of Agricultural
Products (#306) and Veterinary, Medical, and Urban Entomology (#104),
two ARS National Programs described on the World Wide Web at www.nps.ars.usda.gov.
"From Granaries to Insectaries: NIR Technology Helps Human Health" was published in the March 2005 issue of Agricultural Research magazine.