Genetic modification of insects as pest control – Part 1

posted in: Science | 10

Vector borne diseases (VBDs) are generally pretty bad. Yellow fever, the disease which stopped the Panama canal dead in it’s tracks, makes your liver fail and turns you yellow… hence the name. Malaria is caused by a parasite which ruptures blood cells in unison. African trypanosomiasis makes you go to sleep and then die. Dengue fever lays you up in bed for six months in some of the worst agony imaginable. Hell, even veterinary VBDs are horrifying… outbreaks of rift valley fever usually present with random farm animal abortions.

In 2008, malaria alone killed 708,000 to over a million people, most of them young children in sub-Saharan Africa, and an estimated 190 to 311 million cases of malaria occurred worldwide, according to the CDC. Diseases like malaria, may actually work to keep poor regions poor. Control methods often do not reach the poorest people due to high cost… vector borne diseases tend to be diseases of poverty.

Check out this video made as a public service announcement in 1943 by Disney. Some of the control methods have changed, such as specific chemicals, but not that much.

So… what can be done to help save lives in regions with VBD problems? We can vaccinate, in those few cases where vaccines have been developed, and we can kill the vectors, but vector control efforts can be expensive. Education and awareness is great, too… let the locals know what the potential harm is and send them on search and destroy missions to remove insect breeding grounds.

But… and this is the cool part… we’ve got a new tool in our arsenal.

Genetically modified mosquitoes.

First, a history lesson for part 1, which will continue in part 2. We’ll get around to the whole mosquito thing in part 3.

Screw-worm larva.

A lot of techniques used in vector control actually originated in agriculture. Pesticides and degree-day systems that help predict growth of insects were both developed to help fight crop pests. Prior to the 1950s, one of the biggest pests of cattle in the US was a critter called Cochliomyia homnivorax, the New World Screw-worm. For cattle (and rarely… humans) this disease is even more horrifying than anything I’ve mentioned yet. This fly lays eggs in major or minor wounds of cattle, and the maggots proceeded to eat the animal from the outside in. Once one female lays her eggs, the smell of the infested wound would draw more and more females.

There were some rather fortunate quirks to the biology of this pest, though. It couldn’t overwinter in most areas of the US and the females only mated once while the males tended to be really ugly six-legged Casanovas. This meant that if we could eliminate them from their overwintering grounds, they wouldn’t spread over the US over the following year. They’d be gone.

Enter Raymond Bushland and Edward Knipling. They hypothesized that if we could sterilize male flies, each male would each prevent multiple females from laying eggs. If you kept this up, eventually the population would drop. If you kept it up a bit longer, eventually the sterile males would outnumber the fertile males.

This pretty little blue fly can mess up a cow pretty good. The number on the back is a sticker used to track the flies in recapture programs. We want to make sure they go out and get laid instead of hanging around their release sites.

So, this is what they did. They raised thousands upon thousands of screw-worms, bombarded them with radioactive cobalt which shredded the DNA in their quickly reproducing gametes, rendering them unable to bear children, and released them. The screw-worm was quickly eliminated and remains only in South America to this day, held at bay by release programs. Drs. Bushland and Knipling were the 1992 World Food Prize Laureates in recognition of their work.

There are some problems with this technique, though. Sometimes, the males which have gone through the radiation aren’t as competitive as the fertile males which results in the females favoring the wrong males from our point of view (you know… the one which matters) and sometimes the females mate multiple times which means that there’s only a decrease in reproductive output instead of elimination. There’s also the problem of working around radiation… we’re not talking about chernobyl grade material here, but whenever you can you’d ideally like to eliminate the risk to the people who work around these things.

Of course, science is by no means a rigid process. We try to refine our techniques whenever we can, and this will be the focus of Genetic modification of insects as pest control Part 2.

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Joe Ballenger is an entomologist who works in the biotech industry as a contractor. In his spare time, he helps answer questions about bugs at Ask an Entomologist.