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Ask An Entomologist @BugQuestions
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just released @wildwildcountry, a documentary which explores the first confirmed bioterror attack in US history.

However there's one biosecurity incident that still perplexes entomologists.

For this week's #DeepDive, let's talk about The 1989 California Medfly Incident
Medflies are one of the most damaging pests in the world because they feed directly on the most valuable parts of plants, rendering fruit inedible before harvest.

Conservatively, an introduction would cost tens of billions of dollars in damage.
Medflies are found all over the world, and are well established in Hawaii. However, they haven't gotten to the mainland yet...and @USDA_APHIS works hard to keep it that way with a number of tools that we'll explore a little bit later.
Beginning in 1975, California had a series of Medfly outbreaks. These were generally small, and were assumed to come from Hawaii.

People go on vacation, fruit ends up in luggage, and California is a major travel hub.

Consequently, California is always watching out for medflies
The 1989 California Medfly Incident began as a pretty standard infestation. One single medfly was captured at Dodger Stadium in August of 1989.

I mean, if you're monitoring for pests...why not set up a station at a ballpark and catch the game?
The standard response at the time was to spray a pesticide called Malathion in infested areas from helicopters.

Malathion is relatively safe, and fairly effective, but this is over downtown LA.

Officials already knew that this was unpopular.
As they set up the spray campaigns, there was a lot of public pushback which stemmed mostly from questions about the safety of malathion.

Malathion smells really bad, and sprays can leave residue...but it is fairly safe.

Best tool at the time.…
As the public pushback continued, officials began to notice that this was not a normal infestation.

1.) The flies seemed to always appear just outside the quarantine/spray areas
2.) The flies were mostly gravid females
3.) They weren't finding any larvae
On December 3rd, 1989 a group calling itself The Breeders sent letters to local government officials, as well as the LA Times, claiming responsibility for the infestation.

Specifically, they said they were breeding and releasing the flies to render Malathion spraying unfeasable.
Unfortunately, I have never seen a copy of this letter posted online. However, it's contents caused a stir among local officials and the local media.

Nobody knew quite what to make of it.…
At the time, this was the largest and widest ranging medfly infestation recorded in California.

The flies appeared to be susceptible to pesticides, but were spreading too quickly for agricultural workers to really contain.

This was a major problem.
At this point, the infestation became national news. The combination of spraying in urban areas, as well as the possibility of bioterrorism made national headlines.…
However, the people in charge of the spraying weren't really convinced. This was just before DNA testing was well enough established to allow scientists to trace infestations.

At the time, some scientists even thought that the medfly was established at levels too low to detect.
As the spraying continued, and the flies spread, The Breeders kept making threats.

Finally, in February, the USDA placed classified ads in the LA Times to seek out contact with The Breeders.

Again, I can't find the exact ad...but the text is in here:…
This ad worked, and resulted in a very short and terse conversation with a representative of The Breeders.

These details are recounted in the book Six Legged Soldiers by Jeffery Lockwood, but aren't online AFAIK.
The ad in the LA times asked The Breeders to send them a fly in the mail, and one of The Breeders called and said they'd send them one of their specimens in the mail.

That specimen, unfortunately, never arrived.
There were some leads which were investigated, and a few disgruntled city employees were looked into. There was even a local science teacher who was questioned for releasing flies, but these turned out to be houseflies and he was cleared as a suspect.
As time went on, the infestation continued to spread. Nobody was sure whether it was movement of infested produce, or bioterrorism.

The Breeders stopped contact, and have never been identified to this day.
At this point, the entomologists were in trouble. They weren't sure if it was bioterrorism, but the infestation threatened to reach California's agricultural hub...and they needed to turn to new tools.

So they started releasing their own medflies.…
This isn't as dangerous as it seems.

The medflies the USDA was releasing were mass-reared males which had been sterilized with radiation, to ensure the females would lay eggs that wouldn't develop.
I couldn't find much about how this was done in 1989, and the technology has changed a lot since then.

Nowadays the larvae are reared on a diet containing antibiotics (I think streptamycin), and are sorted by color. Males are bred to be heat tolerant; females die when hot.
They're transported as pupae, and then released as adults from planes after they emerge.

Again, this is a simplified version of the modern technique...but the basics were known back then.
Today, after all these years, nobody is sure whether The Breeders ever really existed. Even if they did exist, we'd never be able to know whether their techniques were effective...or if this terrorist group's releases were overshadowed by natural spread.

We just don't know.
This is why we need to continuously study and monitor invasive species.

Invasive species can cause a lot of damage, and we're still vulnerable to attack.

If The Breeders ever truly existed, their tactics were disturbingly clever...and we may not be so lucky next time.
It looks like the threading got a bit messed up.

The final chapter of this story is here:

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