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A Bioengineered Solution to Crop-Killing Fruit Flies: Turn Them All into Dudes

The all-out war between human farmers and Mediterranean fruit flies comes to a head.

by Michael Byrne
Aug 13 2014, 2:40pm

Image: USDA

Fruit flies are a small wonder of agricultural pestilence, but new research provides a potential answer: By engineering flies to only produce males, entire populations can be wiped out within a few generations.

The species known as the Mediterranean fruit fly (Ceratitis captitata), or medfly for short, is almost supernatural in its ability to persist and migrate over very large distances. They hatch and grow their young inside of actual fruit, protected in a cocoon of sugar, one that might possibly be about to travel a very large distance via the modern-day globalized produce market. Pesticides are a weak defense against the medfly, and they've even persisted in the face of sterilization programs.

Now, according to a paper to be published this week in the Proceedings of the Royal Society B: Biology, researchers have gone beyond sterilization into a sort of reproductive fine tuning: medflies are being genetically crafted to sterilize themselves at the colony level, limited by an inability to produce female offspring. Dudes only.

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The new research should be of special interest to California governor Jerry Brown. In 1981, during his first term leading the state, medflies, which had appeared two years earlier for the very first time on the West Coast, waged all-out war on California's massive agricultural industry.

A relatively green-valued or at least green-influenced politician, Brown insisted on starting the state's resistance small, rejecting recommendations from both the industry and the US Department of Agriculture's Animal and Plant Health Inspection service to respond with an airborne pesticide assault, and instead spray from ground-level only.

An unsuccessful month later, and fleets of helicopters were dumping the pesticide malathion over the state by night, while National Guard troops confiscated fruit at checkpoints. Responding to a growing uproar over the pesticide's alleged toxicity to humans ( it's not so much, per the EPA), the governor's press guy famously downed a glass of the stuff at a news conference. (Unsurprisingly, the episode has left a long-lasting mark on a certain conspiracy-minded set of people.)

The aim of this technology is localised population reduction and not elimination.

Nonetheless, it took the introduction of radiation-sterilized medflies into still-exploding populations of the insects to stem the spread and, thus, the damage. Still, eight years later, the flies hit back hard, blanketing the state's farm fields once again, dominating and destroying for a full year before, once again, sterilized flies (known as SITs) came to the rescue.

A human group calling themselves the Breeders took credit for the 1989 swarms. In a letter, the group said that it had, "decided to make the Medfly 'problem' unmanageable and aerial spraying politically and financially intolerable," according to the Los Angeles Times.

While it was quite the attention-grabbing plot, it's unlikely to have been actually carried out, mainly because medflies are perfectly capable of extreme acts of persistence on their own, and had hardly been in hiding between the two 80s outbreaks. Taking credit for the 1981 outbreak then would have been a bit more believable than the 1989 version.

In any case, state and federal authorities took the Breeders quite seriously, with the US Department of Agriculture even taking out an ad in the Times saying, "Breeders, If you're for real send one of your little friends. We want to talk. Call John at USDA."

Not much came of it, but medflies kept up and continued to spread around the planet. In 2011, Japan blocked the importation of blueberries from Australia out of medfly concerns. As the name suggests, the flies are native to the Mediterranean, but by now they've spread across North and South American, Australia, and parts of Asia.

Eradication efforts are focused and aggressive—the costs of an outbreak are massive, ranging from overseas trade bans to increased pesticide use to actual lost fruit—and populations have remained relatively under control in many US farm states. SIT is a key part of that, and when sterilized flies are released in still relatively small populations they can make a big difference.

This graph from the paper shows how, within 15 weeks, test populations plummeted to nothing, while control populations held steady.

SIT is self-limiting, however. Irradiated males don't compete very well for female flies, particularly as populations increase, and so populations continue to grow. The proposal put forth in the new paper, which comes courtesy of researchers at the University of Anglia, uses genetic modification to not sterilize but restrict the reproductive potential of male flies such that they can only produce male offspring.

They still get to do their fly business (and good for them), but in a couple of generations, which amounts to a couple of months, the threatening colony of medflies has become de facto sterile because all the females have disappeared. Everyone wins, save for evolutionary imperative.

"The genetically engineered flies are not sterile, but they are only capable of producing male offspring after mating with local pest females," said Philip Leftwich, the study's lead investigator, in a statement. "This rapidly reduces the number of crop-damaging females in the population. Using this method means that the males do not have to be sterilized by radiation before release, and we have shown they are healthier than the flies traditionally used for SIT."

"We simulated a wild environment within secure eight-meter greenhouses containing lemon trees at the University of Crete," Leftwich continued. "When we tested the release of the genetically modified male flies, we found that they were capable of producing rapid population collapse in our closed system."

He noted that, compared to radiation or pesticides, the method is "environmentally friendly and effective," not to mention quite a bit cheaper.

But does the spread of ineffective reproducers mean the decimation of the medfly species? Leftwich is confident the answer is no.

"Unlike concerns with GM crops, this provides no natural advantage to the GM flies, rather the opposite," he told me. "[It reduces] their fertility, [which] makes the technology self-limiting. If active release of the flies was stopped, the gene system would be rapidly removed from a target population by natural selection, and population numbers would rapidly recover. The aim of this technology is localised population reduction and not elimination."

The next step is gaining regulatory approval for moving the research from closed environments to an open-field, and seeing how all-bro populations of medflies handle the real world.

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