Sex odor of male fruit flies causes plants to up chemical defenses Adult female fruit fly, family Tephritidae. Credit: Bruce Marlin, Wikipedia Creative Commons UNIVERSITY PARK, Pa., Dec. 18 (UPI) -- A common plant has shown the ability to use odor to marshal its defenses by catching a whiff of a male fly searching for a mate, Penn State entomologists say. Tall goldenrod plants sensing a sex attractant emitted by true fruit fly males appear to prepare chemical defenses making them less appealing to female flies that could damage the plants by laying eggs on them, the researchers reported. "It's become increasingly clear in recent years that plants are responsive to odors," entomology Professor Mark Mescher said. But previous examples of this are all plant-to-plant. For example, some plants have been shown to respond to the odor of insect-damaged neighbors by priming their own defenses. What's new about this is that it seems that plants may sometimes be able to smell the insects themselves. In an experiment the researchers exposed some plants to the odor of the male fly and then counted the number of plants on which female flies laid eggs. Females were significantly less likely to lay eggs on plants exposed to the male emission and about four times more likely to lay eggs on plants in a control group of plants not exposed to this odor cue, they said. "It would seem that the plant senses the odor of the fly," Mescher said. Then, it primes its defenses so that it can respond faster to the threat. Just how tall goldenrod plants detect the odor of the fly is not known, researchers said. "Our understanding of plant olfaction in general remains quite limited," Mescher said.