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Thursday, March 18, 2010

Candy-cane plants avoid gall flies

Over the eons, plants have developed a variety of tactics for dealing with herbivores, ranging from impenetrable bark, sharp thorns, to the production of toxins. Michael Wise, Warren Abrahamson and Julia Cole have discovered a novel mechanism employed by some plants for fending off insect attacks.

The team examined goldenrods, a common prey of the gall-inducing fly Eurosta solidaginis. This fly lays its eggs in apical leaf buds (the growing tip of a leaf), preventing the plants from flowering.

Some members of one species of goldenrod (Solidago altissima) bend their apices downward into a candy-cane shape. This dip occurs slowly, beginning in early spring. This corresponds with the reproductive cycle of the flies, so that during the peak egg-laying times, the plants are at their most downward-angled. By late summer, the plants have straightened out and are flowering.

A gall fly sits on a leaf near the top of the plant, seemingly oblivious to the nodding apical-leaf bud just centimeters below.

Photo credit: Courtesy of Michael Wise

Wise and his colleagues wondered whether the candy-cane shape offered any protection from the flies. They noticed that the same S. altissima plants that normally bent their stems would keep their stems straight if grown in the shade. Thus, the researchers set up a greenhouse experiment comparing bending goldenrods grown in sun (where they bent over) and in shade (where they stayed straight), with non-bending goldenrods grown in sun and in shade.

They found that the flies laid their eggs equally in all straight-stemmed plants, but not at all in the candy-cane shaped plants. This indicates that it is indeed the shape of the plant that is preventing egg-laying, rather than a distaste for the type of plants that can sometimes duck their stems or for growth conditions.

Surprisingly, bending specimens make up a minority within goldenrod populations. Clearly, there are advantages to staying upright that supercede the disadvantage of greater predation.