Bump in ozone bewilders bugs

PENN STATE (US) — Increases in ground-level ozone, especially in rural areas, not only interfere with the ability of predator insects to find host plants but also with pollinators to find flowers.

For a new study, researchers tested the striped cucumber beetle, a predator of cucurbits—cucumber, squash, pumpkin, and melons. These insects dine on the plants from the moment they emerge from the ground and when fruit forms, they eat that, too.

“Insects detect odor with olfactory receptors located on their antennae,” says Jose D. Fuentes, professor of meteorology at Penn State. “These receptors sense plant-emitted volatile organic compounds in very small amounts—as low as six molecules hitting an antenna.”


But ozone, which is a very reactive substance, degrades the volatile organic compounds when they mix to the point where they no longer stimulate the olfactory system.

For the study published in Environmental Research Letters, scientists collected insects from pumpkin and squash plants and tested them using buffalo gourd plants, in an enclosed Y-tube apparatus so that the insect could choose which branch to take.

Separate air streams flowed into the two branches of the Y-tube. Choices of air in each tube were ambient filtered air, ambient filtered air plus up to 120 parts per million ozone, ambient filtered air plus volatile organic compounds, or air plus up to 120 parts per billion ozone and volatile organic compounds from the plant.

To obtain the mix, or only ozone or volatile organic compounds, that branch flowed either to a plant chamber or ozone generator or both.

The researchers tested the insects with all ambient air, with ambient air and ozone, with ambient air and volatile organic compounds, and with ambient air and a mix of ozone and volatile organic compounds. When presented with an ambient air or volatile organic compound airstream, the beetles chose the volatile organic compound tube 80 percent of the time.

“However, as the ozone levels increased, they chose the path to the flower less frequently,” Fuentes says. “By the time the mix contained 80 parts per billion ozone, the beetles showed no preference for either tube.”

The researchers also tested the beetles with volatile organic compounds and a mix of volatile organic compounds and ozone. At low ozone levels, the insects showed no preference, but as ozone levels increased, the insects increasingly preferred the ozone-free path. At 80 parts per billion, the beetles chose the volatile organic compounds without ozone significantly more often than the ozonized mixture.

While one might think that higher ozone levels in the lower atmosphere would improve crops because predator insects would be unable to find their hosts, the additional ozone would also interfere with mutualistic insect plant responses such as pollination.

Researchers from the University of Virginia contributed to the study, which was funded by the National Science Foundation.

Source: Penn State