Lord of the Gnats

Seducing a mushroom pest with scent

Graduate student Kevin Cloonan uses an aspirator to collect fungus gnats for his odor experiments. Credit: © Brittany DodsonAll Rights Reserved.

Drive up to a mushroom farm, open the car door, and you’ll understand why facilities like this one operate in rural areas. An overwhelming odor of manure emanates from compost piles scattered around the farm and from inside mushroom houses — the long, squat, wood and concrete structures where mushrooms are grown.

“If there was a steak in that house, we wouldn’t be able to smell it, because all we would smell is poop,” says Kevin Cloonan, a doctoral student in Tom Baker’s lab. “But the fungus gnat’s sense of smell is so precise they can sort it all out.” 

Fungus gnats, Lycoriella ingenua, are small enough to squeeze into a mushroom house through the tiniest of crevices. Although adult gnats don’t damage the crop directly, they can transfer a devastating fungal pathogen that lays waste to the tender young mushrooms. Even if a female isn’t carrying this pathogen, she spawns about 200 larvae that devour belowground mushroom parts and gobble up compost nutrients that the mushrooms need to grow. An infestation of fungus gnats can cut a mushroom harvest by 70 percent — a huge hit to an industry that contributes $2 billion to the Pennsylvania economy each year.

Insecticides that easily controlled these pests have been banned one by one over the years. “Now we’re starting to see a reemergence of these gnats, but we don’t know much about them,” says Cloonan. He is looking for a way to turn their acute sense of smell against them — to prevent their entry into mushroom houses or capture them if they do sneak in.


Luring male gnats

Cloonan, along with postdoctoral researcher Stefanos Andreadis, is working to identify chemicals that attract male gnats to female gnats. These chemicals, called pheromones, are produced and released by the female so the males can find her for mating. “If the female is producing something the male likes the smell of, we can use it to trap males and control the gnats,” he says. This tactic has been used successfully to control Mediterranean flour moths in flour mills and olive flies in olive groves.

In the lab, Cloonan uses a colony of fungus gnats collected from mushroom houses in Chester County, Pennsylvania. From that colony, he selects mature females and douses them in hexane, a solvent that washes off any pheromones clinging to them. Next, Cloonan feeds the pheromone-containing hexane into a gas chromatograph (GC) that separates the chemical blend into individual compounds.

The GC uses half of each compound for identification and loads the other half into an instrument called an electroantennograph that tests which compounds attract males. For this test, larger insects are commonly restrained with a straitjacket-like apparatus, but gnats are too tiny for that method. Instead, Cloonan sticks four male gnats to a base electrode with ultrasound gel that keeps them in place and helps current flow. Then, the antennae of the restrained males are gently placed across a recording electrode. One at a time, potential pheromones are puffed from the chromatograph across the males’ antennae. The recording electrode registers responses of the antennae in sharp peaks, like a heart monitor.

“We’re fairly confident that we’re about to identify the pheromone, but it’s a bit tricky,” says Cloonan. Each compound has potentially dozens of forms and he needs to find out which one attracts males. If he can identify the exact structure of this pheromone, it can be made in the lab in a pure, concentrated form.

“You can use a pheromone to trap all the males so they don’t mate with females,” Cloonan says. “Or, you can use mating disruption: You saturate an environment with the pheromone and the males get confused. They spend too much energy flying around and die before they mate.”