How temperature, plant defenses alter bugs.

Researchers have long tried to thwart insects that devour crops, but the bugs often get the upper hand. Now, a new study may help scientists trying to boost the effectiveness of allelochemicals, natural anti-insect compounds that many plants produce.

Breeders have developed plants with high concentrations of these compounds. By slowing the insects' development, allelochemicals make the bugs more vulnerable to predators and diminish their impact on the plant. Scientists know that temperature alters insects' voracious appetites, but they don't know how temperature and allelochemicals interact, note Nancy E. Stamp of the State University of New York at Binghamton and Yuelong Yang of the University of New Mexico in Albuquerque.

So Stamp and Yang examined how the tomato fruitworm, fall armyworm, and tobacco hornworm-all tomato pests-respond in the laboratory to a diet laced with different combinations of three tomato allelochemicals-chlorogenic acid, rutin, and tomatine. They conducted tests at 21#161#C and 10#161#C, typical springtime day and evening temperatures for Binghamton, and at summer norms of 26#161#C and 15#161#C, they report in the June Ecology. They have since examined four more caterpillar species.

The insects responded in very different ways to the various chemicals and temperatures, the researchers found. The compounds usually slowed the insects' development, but changing the temperature or adding other allelochemicals, or both, sometimes reversed that effect.

"Often the negative effect of an allelochemical was greater at the warm regime than at the cool regime, but not consistently so," Stamp and Yang report. Tomatine and rutin, for example, each harmed the tobacco hornworm more in the spring temperatures than in the summer conditions.

At the summer temperatures, pairs of compounds produced the greatest negative effect, whereas in the cooler air, single chemicals did. But since the chemicals varied in their effects on a particular insect, plants would fare best if armed with all three allelochemicals, they assert.

Stamp and Yang have yet to identify the mechanisms causing these patterns. Their results suggest, however, that testing how insects respond to just one allelochemical at one temperature will prove misleading.

The findings are likely to interest not only plant growers but also researchers who, like Stamp, study the effects of global warming on plant defenses (SN: 4/9/94, p. 230), notes Mark Scriber of Michigan State University in East Lansing. However, he cautions that insects' responses to the chemicals in the wild may differ from their reactions in the lab.