A diffuser which "smells like fear" could help keep pests off garden plants, according to a new study.

The special odor is made up of compounds produced by ladybugs, a natural predator of plant-eating insects which ravage gardens and crops.

Pests which catch a whiff of the stuff will change their behavior, thinking predators are nearby.

Plant-eating insects represent a major threat for gardeners and farmers' crops around the world, especially as they can carry diseases and are becoming increasingly resistant to traditional pesticides.

Now, researchers at Pennsylvania State University have come up with a sweet-smelling solution.

Study author Dr Sara Hermann said, "It is not uncommon to use our senses to avoid risky situations. If a building was on fire, we as humans could use our senses of sight or smell to detect the threat.

"There is evidence for such behavioral responses to risk across taxa that suggest prey organisms can detect predation threats, but the mechanisms for detection aren't very well understood, especially with insects."

Aphids are highly destructive, and their ability to transmit plant diseases make them a persistent problem for growers.

They also happen to be a favorite food of ladybugs, which gardeners and farmers welcome as a kind of natural pest-control. That is because aphids and other plant-eating insects will steer clear of fields if they can smell predators nearby, the researchers found.

Smells given off by ladybugs signal aphids to stop reproducing as much and grow larger wings, both behaviors seen to avoid threats.

The research team identified and extracted the ladybugs' "volatile odour" using gas chromatography, a technique which separates the different components of a smell.

Aphids were then exposed to each component individually to see which one got the biggest reaction.

The strength of their response was based on the signal picked up by an electroantennogram machine, which is specifically designed to test insects' reactions to odors.

Of the many compounds emitted by ladybugs, the strongest response was to a class of chemical compounds known as methoxypyrazines.

Specifically these included isopropyl methoxypyrazine, isobutyl methoxypyrazine, and sec-butyl methoxypyrazine.

A special odor blend, which can be placed in an essential oil diffuser to spread the scent over time across a garden or field, was then created.

According to a statement, the researchers are now hoping to test their diffusers outdoors to see whether they produce the same results.

They are also looking to measure the diffusers' dispersal area and see whether they could be applied to other pests, predators, and crops.

Study co-author Dr Jessica Kansman added: "Insects rely on olfactory cues to find food, mates, and places to live, so this is a great opportunity to investigate how to use these smells to manipulate their behaviour."

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