The "smell of fear" emitted by predator insects may help farmers and gardeners seeking to protect what they grow from plant-eating bugs resistant to traditional pesticides, research presented Wednesday during the American Chemical Society meeting found.

Herbivorous insects present a major threat to plants and crops but the predator insects that feed on these bugs emit odors that pests can sense, the researchers said.

Smelling this odor causes pests to change their behavior and, in some cases, their physiology -- body structure -- to avoid being eaten, they said.

However, the researchers, from the Pennsylvania State University, have developed a way to bottle this smell to repel and disrupt destructive insects naturally, without the need for harsh chemicals that may also harm other, beneficial insects.

"Insects rely on olfactory cues to find food, mates and places to live," one of the researchers, Jessica Kansman, said in a press release.

"This is a great opportunity to investigate how to use these smells to manipulate their behavior," said Kansman, a post-doctoral researcher at Penn State in State College.

Insects called aphids are a highly destructive pest to an array of crops, and their large numbers, ability to transmit plant pathogens and increased resistance to insecticides make them a persistent problem for growers, according to the researchers.

These insects also happen to be a favorite food of the ladybug, which gardeners welcome as a source of sustainable pest management, the researchers said.

Earlier research by the same team found that aphids and other herbivorous insects avoid fields and gardens if they can smell predators nearby.

In addition, exposure to the odor cues given off by ladybugs can cause aphids to slow their rates of reproduction and increase their ability to grow wings, both of which are behaviors designed to avoid threats, according to the researchers.

To see whether the olfactory cues given off by ladybugs could, by themselves, control pests, the researchers identified and extracted the volatile odor profile from live ladybugs using gas chromatography-mass spectrometry.

This process allowed them to identify and separate the individual components of the ladybugs' smell, the researchers said.

To see which compounds the herbivorous insects would respond to, the researchers hooked up the antennae of live bugs to an electroantennogram machine and exposed them to each individual odor the ladybug emitted to see which compounds were detected.

The strength of their reactions was measured based on the signal picked up by the machine, according to the researchers.

Of the many compounds emitted by ladybugs, herbivorous insects had the strongest response to chemicals called methoxypyrazines, including isopropyl methoxypyrazine, isobutyl methoxypyrazine and sec-butyl methoxypyrazine.

The researchers then used the chemicals produced by the ladybugs to create a special odor blend that can be used in an essential oil diffuser for spreading across a garden or field.

Next, the team plans to conduct field tests of their scent diffusers to see if the effects on herbivorous insects and ladybugs are similar to what was observed in the lab.

They also want to determine the dispersal area of the diffusers, and whether they could be applied to other pests and predators, as well as various types of crops, the researchers said.

In addition, they are collaborating with a manufacturing company to design special diffusers for eventual commercial use by both farmers and gardeners, they 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," researcher Sara Hermann said in a press release.

"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," she said.

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