Both the backyard gardener and commercial farmer must contend with aphids, the tiny green bugs with a voracious appetite. Small but mighty, aphids are good at two things: developing resistance to once-effective insecticides and transmitting plant pathogens that can quickly devastate an entire crop. 

“They’re like the mosquitos of the plant world,” said Dr. Sara Hermann, an entomologist at Penn State University. Now, Herman and her fellow scientists say they have found a way to deter aphids naturally, without the need for harsh chemicals or more potent pesticides. Instead, they’re trying to “scare bad bugs into behaving better,” by using “the smell of fear.”

What’s the scariest thing to an aphid? A ladybug. 

A ladybug can eat up to 50 aphids a day. Ladybugs are so good at controlling aphid infestations, many nurseries sell them by the bag for growers to release into their gardens.

But science is only now beginning to understand how effective this strategy really is. Recent research has shown that the mere smell of ladybugs can keep aphids at bay. It can also, surprisingly, cause aphids to slow their reproductive rates and increase their ability to grow wings, two behaviors that help the green bugs avoid threats. The way you and I wouldn’t walk into a building that smells like smoke, aphids are much less likely to hang out in a field that reeks of ladybugs. 

With that research in mind, Hermann’s team set out to see if they could isolate the olfactory cues given off by ladybugs and use scent alone to prevent aphid infestations. First, they identified and extracted the volatile odor profile from live ladybugs using gas chromatography. Then, they hooked up aphids’ antennae (what the insects use as a nose) to a recording electrode and, one by one, puffed each compound in that profile over the aphid to record its response.

“We call it their ‘electrophysiological response.’ It’s an innate reaction,” says Jessica Kansman, an entomology postdoc who works in Hermann’s lab and collaborated on the project.

In order for these “innate reactions” to prove useful, the scientists needed to determine if they signified attraction or repulsion. “I have a strong reaction to the smell of skunks and the smell of Indian food,” explains Kansman. “But one I’m going toward, and one I’m going away from.” 

So the team ran simple experiments that allowed aphids to choose among plants marked with the scents of these arousing compounds, noting which ones they were attracted to and which ones they avoided. In doing so, Hermann and her crew were able to isolate the compound that most repelled the aphids: methoxypyrazines, AKA “the smell of fear.” 

The team defused this smell—using a household essential oil diffuser—over a small plot for one growing season and proved that it did in fact ward off aphids. They presented their findings at last summer’s meeting of the American Chemical Society.

“So far it does seem like there’s an influence on how many aphids you would expect to see coming into the field in a way that’s beneficial to the plants,” says Hermann. “It’s not quite as effective as actual, live ladybugs. But we’re still refining the smell.” 

Next summer, they will set out to test whether the smell of fear can protect larger agricultural systems over longer periods of time. If it works, it will be a major breakthrough: a long-lasting, non-toxic, and sustainable way to protect food crops—no pesticides needed. 

According to Hermann, that the compounds that make up the “smell of fear” are commercially available and affordable, and the researchers are already collaborating with industry partners to devise the most effective diffuser. She thinks when all is said and done, that this method might prove best for smaller operations. 

“It might not be for the huge, Midwestern cornfield,” she says, but instead be more appropriate for small farms with several acres of diverse plantings, CSAs, and backyard gardeners—and especially for anyone who’s trying to grow organically. “A lot of systems that are organic or biologically-based in their practices, they really need a way to manage some of these pest problems.”

Most of all, Hermann is excited about the global implications of this system of pest management. 

“We’re studying this interaction on one crop right now, but because aphids are a global pest of hundreds of plant species, if this does work, it could be a useful tactic across crop types,” she says. “Aphids are everywhere, ladybugs are everywhere. This could be broadly applicable.”