Candy cane strategy sweetens life for goldenrods

Plants haven’t dodged a shoe, but they can duck aerial attackers, says a new study.

Some tall goldenrods temporarily crook the tops of their stems in a candy-cane shape when gall midges are flying at full force, says Michael Wise of the University of Virginia’s Blandy Experimental Farm in Boyce.

Plants that go into candy-cane mode are less likely to end up with deformities caused by a particular galling insect than are plants that stay straight, report Wise and Warren Abrahamson, of Bucknell University in Lewisburg, Penn., in the December Ecology. 

“It helps us view plants less as passive victims,” Wise says.

Studies of plant-form effects are rare, comments ecologist Jennifer Rudgers of Rice University in Houston. She says most research on plant protection has focused on chemical weapons or forbidding structures such as thorns. For the most part, she says, “architecture has definitely been overlooked.”

The architecture of the common and widespread tall goldenrod (Solidago altissima) is easy to spot along North American roadsides, especially in the East, Wise says. To study the forms under standard conditions, he and Abrahamson grew straight and candy-cane lineages in a greenhouse protected from pests. Candy-cane plants dipped their heads in mid-May but righted themselves in mid-June.

To see whether that short nod can benefit the plants, Wise and Abrahamson planted a field with some 2,000 goldenrods, all sprouts from wild lineages. Nearly a fifth candy-caned in late spring.

By the end of the season, these plants proved only half as likely as their straight-stemmed neighbors to develop a leaf rosette gall at the top of the stem. Resembling a road-killed cabbage, the compressed bunch of leaves indicates that the Rhopalomyia solidaginis midge had attacked.

During their few days of adult life, these midges seek goldenrods and inject eggs into the stem tip. There, a stub of rapidly growing tissue, the plant equivalent of animal stem cells, is just beginning to differentiate into cell types. If a midge egg hatches, the larva hijacks tissue that normally would become a stem. Instead, the plant creates the gall, a home for the larva.

Previous studies have shown that galls harm goldenrods by reducing seed production, so lessening the risk of midge attacks would be an advantage for the plant, Wise says.

The threat from the midge passes in a few weeks as the pest’s breeding season ends. That’s just about when the goldenrod straightens up again, Wise and Abrahamson report.

Wise speculates that ducking helps the plant because insects with only a few days to find a nursery for eggs will seize the first suitable stems they encounter, which could be the ones standing straight and tall.

Plenty of other pests attack goldenrods too, including a tephritid fruit fly, Eurosta solidaginis, that makes stems swell. “It looks like the plant has swallowed a Ping-Pong ball,” Wise says.

In this and an earlier study, plants that ducked showed fewer holes from fruit fly visits than straight-stem plants did, Wise says. Yet the early advantage of the candy-cane form faded. The flies that did attack seemed especially likely to induce ball galls.

What Wise is thinking about now, he says, is why so many tall goldenrods stay upright if ducking is such a great idea. Bending down might bring some disadvantages, he says. So far his work hasn’t found any shortfalls in seed production, so he’s looking for other possible trade-offs.

And yes, Wise says, Abrahamson had been calling the stems candy canes years before Ecology decided to publish the paper in the December issue. It was just a seasonal impulse to color the bar graphs red and green.