Vedalia ladybird feeding on the cottony cushion scale. (Photo: USDA/Public domain)

When it comes to killing invasive or otherwise bothersome plants, humans aren’t always the ablest assassins. Despite our best attempts at slashing, hacking, uprooting, and using chemical weapons on them, some of these plant invaders will never fall to humans. That’s when we have to call in help. That’s when we need the insects.

European water chestnut, for example, has thwarted human controls, and with its “nearly impenetrable floating mats of vegetation” taken over waterways from Virginia to Maine. Soon, Bernd Blossey, a biological invasion expert at Cornell University, will make a four-day trip to China, and if all goes as planned, he’ll bring back hundreds of leaf beetles that could take care of these plants.

Unlike hiring an assassin to kill a person, enlisting a fleet of bugs to kill plants—or other bugs—is perfectly legal. If, that is, the bugs make the grade. You can’t just send any insect out to do the job. They have to be clean, destructive killers, with one-track minds. You don’t want your insect assassin going rogue. Which is why they have to be quarantined first.

Biological control programs, in which scientists release natural enemies of invasive species to control them, have been going on for more than a century. In early experiments, plans sometimes went horribly wrong (see: cane toads). Sometimes, though, biological control works as desired: in 1888, the USDA began bringing in the vedalia ladybird to deal with the cottony cushion scale, a pest that threatened California’s citrus industry. The ladybird did its job.

Klamathweed beetle did its job. (Photo: Martinlagerwey/CC BY-SA 4.0)

Now, the process of importing an insect to control a pest is much safer and more highly regulated. As a species, we’ve gotten good at enlisting others to do our dirty work, but not letting it get out of hand. We’ve brought in the Russian knapweed gall midge, the Dalmatian toadflax stem weevil, the Klamathweed beetle, the cinnabar moth, and other highly specialized goons, who quietly worked to eliminate the threats they were brought in to deal with.

People who fear biological control, Blossey says, “don’t understand how sophisticated we have become over the last 100 years.”

To get to the point where Blossey is in this process, where he will actually be bringing insects from one country to another, requires years. Another project he’s been working on, to control spindly phragmites, has been going on for almost two decades. First, a team of scientists has to narrow down the field of potential control agents to the few most promising candidates. What bugs attack the plant you want to control? Which parts of the plant do they attack? How much damage do they do—not just cosmetic damage, but potentially fatal damage?

“You want the maximum damage,” says Blossey. 

A potential insect agent. (Photo: CSIRO/Public domain)

Once the best candidates have been identified, they need to be tested for their commitment to the mission. Will they stay focused on the plant that needs to be eliminated? Or will they target other, unsuspecting plants? Will they accidentally assassinate an endangered plant that happens to live in the same place as the target plant?

Insects that are brought into the country are whisked to specially constructed and approved containment facilities. Some are run by the government. Some are run by universities and other research organizations. They’re designed to keep bugs in. The doors are usually locked, and only authorized personnel are allowed in. There are at least two sets of doors separating the quarantine facility from the rest of the world. The lab is calibrated so that its air pressure is negative—so that no insects get sucked out to where they’re not supposed to be. Everything is sealed.

At Cornell, for instance, the quarantine greenhouse is specially constructed so that if a tree falls on it, no insects could escape. Anything that leaves the lab needs to be autoclaved—sterilized with steam in a special compartment. There are no backpacks and no computers. Everyone runs around in white lab coats, which is not normal in an ecology lab.

In these labs, the insects are tested. The scientists raise plants native to the area where the insects will be living, and they wait to see what the insects will do to them. Will the imported insects eat the natives? Will they use the plants to lay their eggs? If they do, that’s a strike against them. They need to be focused on the job at hand: breaking the hold of the invasive plant on the environment.

If the insects pass all of these tests, they’re released into the field (with the proper state and federal permits). Their work is just beginning. It can take decades for insect assassins to get their target species under control—if they ever do. (Around a third of biological control projects are successful.)

If they succeed, though, they may be celebrated and immortalized. In 1968, the Humboldt County Wool Growers Association was so grateful for the Klamathweed beetle’s work dealing with a weed that had been plaguing them, that they erected a bronze and granite monument to the beetle in a local redwood forest.