Cycad Plants "Woo" Insects With Heat, Odor

A primitive type of cycad plant from Australia relies on a surprisingly sophisticated system of meting out food to ensure successful pollination.

These plants take an active role in their reproduction by selectively attracting and repelling small insects known as thrips, a new study shows.

Scientists had long thought that cycads were passively wind-pollinated.

But in a push-pull system, male cones of this "living fossil" species heat up and emit strong odors to send pollen-bearing insects fleeing.

Female cones then emit a more attractive perfume to lure the bugs back in.

Pollination accomplished.

"I think the work demonstrates that plants aren't just sitting there looking pretty or just smelling good to attract their pollinators and that there's a lot more dynamics involved," said study co-author Irene Terry, a biologist at the University of Utah.

The mechanics of such primeval systems could provide insight into how pollination occurs in natural environments such as forests, something that Terry says there is still much to learn about.

The study appears this week in the journal Science and was funded in part by the National Geographic Society, which owns National Geographic News.

Nagging Mystery

Cycads are one ancient lineage of seed-producing plants called gymnosperms, which also include firs, spruce, pines, sequoias, and redwoods.

Found in tropical and subtropical areas, all species of cycads have separate male and female parts, often producing cones that look similar to a pinecone.

Tiny cracks between the tightly woven scales of the cones allow thrips to gain access to pollen in male cones and to structures containing eggs in female cones.

Both larval and adult thrips prefer the male cones, because the insects feed only on pollen.

But using some chemical tricks, the plants looked at in the new study ensure that thrips carry the male reproductive cells into the female cones.

"Twenty-five years ago when I began studying cycad pollination, it was still widely believed that cycads, like other gymnosperms, were wind-pollinated," said William Tang, an entomologist with the U.S. Department of Agriculture.

"Although I was involved in some of the early studies on cycad insect behavior, cone physiology, and odor analyses that supported the notion of insect pollination in this group, the precise details of this mutualism remained a nagging mystery," added Tang, who was not involved with the new study.

"This paper presents an elegant use of biochemistry, physiology, and field ecology to reveal the fine details of how this plant and its insect pollinator interact," he said.

"Through the use of heat and volatile chemicals, the plant choreographs a dance with its pollinators."

Stinky Males

Cycads have variable pollination periods, with reproduction occurring once a year to once every several years and lasting up to four weeks.

During that time, male cones use stockpiles of sugars and fats to heat themselves up, sometimes reaching 100 degrees Fahrenheit (38 degrees Celsius).

The cones also spew out huge doses of an odorous chemical called beta-myrcene.

"The odors get so strong that I don't want to be around them," study co-author Terry said.

"Some plants produce a stinky odor like a dead animal or poop, but this odor is just very harsh, hard to describe in terms of something similar."

Beta-myrcene attracts thrips at low concentrations, but in the amounts emitted by male cones, it becomes highly repellant, explained Robert A. Raguso, a chemical ecologist from Cornell University who was not involved in the study.

"This is a rather emphatic invitation for the thrips [to] leave home, and they do so bearing pollen," he said.

The plants do this "without dramatically changing their structural or visual characteristics, or even the chemistry of their scent," he added.

Tricky Females

Female cones emit much less beta-myrcene, so fleeing thrips searching for more pollen are drawn there.

When the thrips get inside the female cone, the ovule—containing the female reproductive cells—releases a little droplet, similar to a nectar droplet.

This draws the insects farther in. If they are carrying pollen, pollination is inevitable.

"The [study] highlights an elegant physiological mechanism by which cycads manipulate the behavior of thrips, transforming them from neutral or perhaps detrimental pollen predators to helpful pollinators," Raguso said.