Punching holes in a sticky defense.

Punching holes in a sticky defense

Some insects have evolved a method of carefully draining a plant's latex-carrying vein system as a way to avoid contact with the gummy substance, say researchers studying "vein-cutting behavior' among insects.

Intrigued by a series of observations begun a century ago that suggest that plants use the sticky, toxic latex to protect themselves from insect attack, David E. Dussourd and Thomas Eisner of Cornell University exposed some of these so-called laticiferous plants to a variety of insects in their laboratory. As reported in the Aug. 21 SCIENCE, they confirmed that the clipping of leaf veins among certain groups of insects is not serendipitous eating, but a calculated attempt to render a plant defenseless by eliminating its latex.

Better known as a tropical source of natural rubber, latex also flows through more common plants like milkweed, dandelion and lettuce. Dussourd, now studying the chemical composition of latex at the University of Maryland in College Park, told SCIENCE NEWS that each of these plants is associated with specific veincutting insects. The insects themselves seem to display customized cutting behavior. Some insects studied at Cornell make simple cuts across the midrib of a leaf, while others cut circular trenches and then eat the latex-less center, say the authors. One type of caterpillar handles the intricate, looping vein system of the papaya leaf by chewing across the length of the leaf.

Dussourd, who believes latex contains a chemical that stimulates vein-cutting behavior, says the cutting may take an hour to complete. But the process apparently is efficient in emptying leaves of their latex, making them more palatable to insects normally repelled by latex's defense. While some insects included in the study avoided the sticky milk by deliberately draining a leaf's supply with a well-placed cut, the authors found that other freeloading insects apparently took advantage of leaves with latex supply-lines already severed by the vein-cutters.

Photo: A katydid called Amblycorypha rotundifolia enjoys a lunch of Indian hemp after draining globules of unappetizing latex away from the leaf tip.