Helping spuds defend themselves.

Helping spuds defend themselves

Each year, managing insect predation costs potato producers an estimated $120 million, according to the U.S. Department of Agriculture (USDA). And because of growing insect resistance to them, these chemicals are becoming increasingly less effective. Now plant breeders are helping potatoes fight back naturally -- with an indigenous pesticide and a bug-catching glue.

At Cornell Unversity in Ithaca, N.Y., scientists are crossing a wild Bolivian potato known as Solanum berthaultii with is cultivated North American kin. Explains entomologist Ward M. Tingey, the Bolivian plant bears glandular hairs on its leaves that, when triggered by touch, release a clear chemical. Upon exposure to air, this chemical quickly begins darkening and turning sticky, owing to the presence of the enzyme polyphenoloxidase. Tingey and his colleagues have shown that this chemical system, acting much like flypaper, traps or otherwise seriously glues up the pests that feed on potatoes.

Their data indicate that this defense system works with differing degrees of success not only against the Colorado potato beetle -- the most serious U.S. potato pest -- but also against many other of the plant's foes. And after seven years of crossbreeding S. berthaultii stock, Cornell's Robert Plaisted has armed a number of hybrids with the glandular-hair trait.

Compared to conventional U.S. potatoes, the more successful hybrids suffer 80 percent less from aphids, 90 percent less from leafhoppers and 60 to 80 percent less from flea beetles. Ironically, Tingey notes, although the potato's glue isn't sticky enough to actually entrap the Colorado potato beetle, it does lead them to feed less and rest more -- contributing to a reduction in their growth, delay in their maturation and decrease in their reproductive capacity. In fact, he told SCIENCE NEWS, this sticky-chemical system appears to provide some hybrids with about the same protection from Colorado potato beetles as do three or four insecticide applications.

However, while the nontoxic glue works, the hybrids in which it appears are far from commercially acceptable, according to Plaisted: They set their tubers (potatoes) too far from the plant roots and do it too late in the growing season to produce an acceptable yield. While he's optimistic these traits can be corrected, he suspects it may take almost another two decades before an insect-gluing potato makes its commercial debut.

Crossing U.S. cultivars with another wild species, this one from Argentian and Peru, results in a potato that produces its own repellent against Colorado potato beetles--and probably against leafhoppers too. Researchers at USDA's Beltsville (Md.) Agricultural Research Center (BARC) found that S. chacoense, an Andean potato, produces the glcoalkaloid leptine in its leaves. Unlike a potato's most common glycoalkaloids -- solanine and choconine -- this one is acetylated (contains an extra CH.sub.3.COOH group on its molecule). The result, explains BARC's Stephen Sinden, is that this chemical is a 10-times more potent inhibitor of insect feeding than the usual glycoalkaloids. Though hybrids are undergoing field tests to quantify the effect of their leaves' unpalatable taste on insect dining (this glycoalkaloid does not appear in the edible tubers), Sinden says that "we're not looking to release this variety." Lacking many attributes demanded by commercial growers, it will instead be used for breeding.