Speaking of Science Column May 24, 2009

When not biting you, mosquitoes prefer plant nectar

When I was young, I reveled in Edgar Rice Burroughs’ “Tarzan” books. I don’t know if I read all 23 or not, but I read a lot of them. And while I didn’t exactly know what a tsetse fly was then, I knew it was dreaded and carried deadly disease.

So I was excited years later when I found myself in a class studying African sleeping sickness, transmitted by the “dreaded tsetse fly.”

Tsetse flies are large flies, about the size of our western horsefly, which are only found in Africa.

They feed during the day, and both males and females feed exclusively on blood. In feeding, they transmit a microscopic parasite called a trypanosome that causes African sleeping sickness.

Now, there is a great fascination by young boys, of almost any age, with gory things such as blood sucking. It is a recent phenomenon that so many young girls and women have also become interested in blood sucking.

One fascinating subject is how a blood-sucking insect finds its food. It is surprising to me how little is known about the insect food-selection process.

It quickly becomes apparent that the simple act of getting lunch is actually a multistep, multisensory, complex interaction of senses, behaviors and environmental cues for an insect. For example, how does an insect even know when it’s time to eat? It is generally thought that mosquitoes feed at dusk. But how do they know when it is dusk? Is it by day length and light? That certainly seems to be one cue.

However, mosquitoes kept in captivity will become restless, act “hungry” and feed when kept in constant light conditions if presented with other cues. Then there are those species that feed at 2 in the morning; you know the one that wakes you up with that dreaded buzzing in your ear.

Blood-sucking insects can’t really expect dinner to remain in a fixed position until the next meal, like a McDonald’s restaurant. So how do they locate a blood source? It is commonly thought that they follow a carbon dioxide plume, but carbon dioxide is actually only an exciter. The mosquitoes get excited whenever the concentration of carbon dioxide changes, whether it increases or decreases. They don’t follow the carbon dioxide as much as they react to it. Carbon dioxide levels fluctuate continually, so why don’t they react then? When presented with several selections on the menu, why do they always pick me? How do they decide between an arm and a leg, and which position on the arm is most attractive? The questions seem endless.

This is all complicated by the fact that there are more than 3,000 species of mosquito in the world and each has its own peculiar time, place and preferred host for feeding.

Now for the amazing part: Only half of mosquitoes take a blood meal, the females. The males feed entirely on plant sources of sugar. Females only require blood during reproduction. The rest of the time they live on plant nectar also. So various floral and plant sources provide the great bulk of day-to-day mosquito energy needs. Yet our knowledge of when, where, why and how they seek floral nectars is minimal.

Better understanding of the feeding habits of blood-sucking insects would aid in the development of better control strategies and improved disease prevention because most blood-sucking insects are capable of transmitting disease.

Answering such questions as “How do tsetse flies know when it is dinnertime?” and “What flowers do mosquitoes prefer?” are what biologists do.

Gary McCallister is professor of biology at Mesa State College and CEO of Flaming Moth Productions.

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