Decoding Chemical Communications to Control Insects

Jocelyn MillarBuild a better mouse trap, they say, and the world will beat a path to your door.

But build a Lygus bug trap, and not only will farmers beat that path to your door, they’ll pave it, light it and landscape it for you, too. Lygus are that serious of a problem in dozens of crops, and they’re hard to control.

“What I’d love to see is a tower where all the Lygus fly in and come out the bottom as garbage,” said Washington alfalfa seed grower Mark Wagoner, expressing the desire of countless growers around the West. “Lygus are hard to kill.”

If that Lygus Tower comes to pass, Jocelyn Millar at University of California, Riverside and his longtime collaborator Kent Daane at UC Berkeley just might be two of its architects.

A chemical ecologist, Millar identifies the chemical signals insects use to communicate, then synthesizes versions of them to help monitor, trap or disrupt their activities. One of the insects his lab is working on is Lygus.

“We first started working on Lygus in the 1990s and identified a lot of compounds and did a lot of testing,” he said. “But we never really got anywhere and didn’t see any light at the end of the tunnel, so we put it aside.”

Millar turned his attention to longhorn beetles with great success, but was recently drawn back to the Lygus effort when a fellow researcher in England, David Hall at Greenwich University, made a breakthrough. He discovered that a defensive chemical Lygus emit when threatened also has attractant qualities when emitted at lower levels. Hall is now a key member of the team trying to crack the North American Lygus pheromone.

“If the rate is too low, they just ignore it and if it’s too high it probably acts as a repellant,” Millar explained. “We’re trying now to identify the optimal blend and rate.”

Lygus is just one of dozens of species Millar and his team are working on, including some non-insect species. The common thread is that they all communicate chemically, and decoding those chemical signals can create new ways to control those species where they are pests.

Preparing for Invasive Species

A big part of Millar’s work has been developing chemical attractants for cerambycid beetles, which are commonly known as longhorn beetles.

“There are about 35,000 identified species, and many have the potential to become invasive,” he explained. “Their larvae develop in wood and when trees are cut and turned into lumber or pallets or furniture it’s easy to move them through international commerce.”

In fact, the U.S. Department of Agriculture developed a list a high-risk species, mostly beetles from Asia, and Millar’s lab has been working down that list to identify and synthesize chemicals that can be used to detect and eradicate them if they are ever introduced to U.S. shores.

“They are all cerambycid species and there are 15 or so on the list now,” he said. “We’ve gotten through at least a half-dozen of them, so we’re making good progress.”

Prionus californicus

One early success in the cerambycid effort was identifying a mating pheromone used by Prionus californicus, the California root borer, whose thumb-sized larvae can cause serious damage feeding on the roots of hops and cherry trees. The Western IPM Center helped fund the early research.

“Back in 2003, almost nothing was known about pheromones in cerambycid beetles, and a 2004 literature review article said they were unlikely to use pheromones,” Millar said. “It turns out almost exactly the opposite is true.”

The pheromone blend Millar’s lab created to attract Prionus californicus also appears to work on the more than 20 other North American Prionus species, and efforts are under way to commercialize a product for several crops where the beetle is a problem, including hops, cherries, pecans and apples.

“Because their life cycle is so long – two to three years – they are slow to replace,” Millar explained. “If you put pressure on the population through mass trapping, you can really push down their numbers quite effectively.”

Monitoring for Endangered Species  

An interesting twist to the use of chemical attractants is using them to monitor rare and endangered species instead of just pests.

“When you’re looking for invasive pests, you’re dealing with small populations that may be found over a large area,” Millar explained. “It can be exactly the same with an endangered species, which makes them difficult to find if you’re just out there with a sweep net.”

Developing and using chemical attractants can help wildlife managers determine if a species is present in an area and get a better sense of the population size, Millar said, adding that so far there’s been more interest shown for this approach in Europe than the United States.

To identify particular chemicals that insects use to communicate, Millar’s lab uses a technique developed in the 1970s called coupled gas chromatography - electroantennogram detection, which is just as sci-fi as it sounds.

Because insects absorb oxygen directly, their appendages can survive up to several hours after being cut from the insect. So researchers will cut off a segment of an insect’s antenna and wire it to a detector that records when the antenna reacts to a stimulant. Coupled with that, they run chemicals emitted by insects or other crude extracts through a gas chromatograph to separate the volatile chemicals into their individual components.

They then expose the antenna segment to each of those component chemicals, monitoring the response to see which chemicals it is sensitive to. Those are the most likely compounds to be attractants.

It’s Not All Insects

The technique works wonders, unless you’re working on slugs and snails. You can’t detach their antennae, so you just have to watch them slither over to stuff (or not) to test baits one by one.

“We’re working with Rory Mc Donnell, a slug and snail expert at Oregon State University, to try and develop an attractant to use for baiting invasive slug and snail species,” Millar said. “We’ve tested a number of possible attractants, and nothing worked as well as sliced cucumber. We thought why not juice a cucumber, because that’d be easy, but it didn’t work anywhere near as well as sliced cucumber.”

So now the team has to isolate the particular chemical that makes sliced cucumber so attractive to snails because once they’ve identified it, they can synthesize a version that can be used to bait traps. But different snails need different traps.

“One species of particular concern is the giant African land snail,” Millar said. “It not only causes direct damage to plants, but it also vectors a nasty disease of humans called rat lungworm.” 

That particular snail has invaded Florida and the Hawaiian islands, and for this tropical species, Millar’s team is working with USDA scientist Amy Roda on optimizing an attractant based on papaya odor.  It’s delicate, technical work, often involving nanogram quantities of chemicals, and Millar loves it.

“I never get tired of the science,” he said. “What’s really interesting is the new chemistries and figuring out exactly how these chemical communication systems work, then developing practical applications for some of the stuff that we do.  Nothing gives me more satisfaction than to see something that we developed being used by growers and other end users.”

Like a Lygus Tower, for instance….