The mosquitoes must dye, even if by photodynamic means

My wife thinks that mosquitoes have a contract out on me for killing their babies. For many years I worked in mosquito control, and we mostly eliminated the larval, or baby, stages. I suppose one shouldn’t be proud of being a paid baby-killer. In the case of mosquitoes though, I am rather proud of having served my community.

There is more than one way to kill a mosquito. The old way of swatting them is effective in the short term, but can sometimes be messy. Then there are circumstances where one simply can’t swat fast enough to avoid exsanguination. So, in the quest by humankind to find better ways to commit mosquito murder, some Mesa State College students have come up with a unique method of killing mosquitoes. They readily admit, however, that it is somewhat impractical. What are they doing? They’re using photodynamic dyes.

Some dyes exhibit a property called a “photodynamic effect” that works like this. When these compounds are exposed to light, the added energy kicks an electron loose from its orbit and into a higher energy level. But the electron quickly falls back to its original position, releasing the energy again. This released energy is seen as light, and we call the phenomenon fluorescence.

Now, when electrons start flying around, weird things can happen. This series of chemical changes sometimes causes the dyes to release oxygen, but the oxygen isn’t released in one of its normal configurations. Instead of being bonded together as two oxygen’s, the gas, or with two hydrogen’s, water, it is sometimes released as a single oxygen atom. Single oxygen atoms, called oxidants, are very reactive and can attack other molecules in the vicinity, breaking bonds and causing mischief. Oxidants can damage cell membranes and other structures in the cell. You and I make these oxidants sometimes when we use oxygen. That is why we need to have anti-oxidants in our diet, to counteract the destructive oxidants.

Scientists have taken advantage of photodynamic fluorescence in many positive ways. For example, these dyes can have a localized, destructive effect on cancer cells if they are injected into a tumor and then illuminated. The procedure has been used on some inoperable tumors. Dr. Richard Heckmann, from Brigham Young University and I were able to kill a type of parasite found on fish gills using this technique. By exposing the fish to the dye in the dark, the parasites also absorbed the dye, and when the light was turned on the parasites were killed.

MSC students used the technique to expose different stages in the life cycle of the mosquito Culex tarsalis to a dye called Rose Bengal. They accomplished this by exposing the mosquitoes to the dye in a dark box and then later exposing the mosquitoes to light. The results were intriguing. They found that the dye could not penetrate the eggs, so it had no effect. The larvae however, ingested the dye in the water as they fed and when exposed to light died very quickly. The pupae stage of mosquitoes does not feed and so could not ingest the dye. So if the pupae weren’t exposed before they pupated, they were not killed. But the students fed adult mosquitos’ sugar water laced with Rose Bengal, and when the adults were exposed to light, they also succumbed.

Now this is entirely useless information. We can’t put Rose Bengal in the water supply because it could damage all living things. But these dyes could be used in closed systems to purify water. A MSC student is working on that idea now. I have to confess that I have considered how entertaining it might be to drink some Rose Bengal, go out at night and let some mosquitoes take a blood meal from me. Then I could shine a flashlight on them and watch them explode. Things could get a little messy when the sun came up though.

Gary McCallister is professor of biology at Mesa State College.


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