A leafy home for one insect hormone.

A leafy home for one insect hormone

A Malaysian plant called grasshopper's Cyperus apparently uses a built-in pesticide for insect control by producing its own supply of a common insect growth hormone, scientists reported this week. the hormone, called juvenile hormone III (JH III), is one of the so-called juvenoid hormones important in the molting process, a stepwise shedding of external skeletons as an insect grows.

When fed the plant, grasshopper juveniles -- which look like adults but are not fully developed -- grew at the same rate as those fed wheat seedlings lacking the hormone. By the time they reached maturity, however, nearly all of the Cyperus-fed insects had some abnormality, including underdeveloped egg formation in females and twisted wings.

The discovery is the first time scientists have isolated JH III -- the most widely occurring juvenoid hormone among insects -- from a plant, according to the authors of the May 12 NATURE report. Coauthor Yock C. Toong at the Universiti Sains Malaysia in Penang was seeking plant extracts with drug potential when he found what resembled an insect hormone in the grass-like plant. David A. Schooley and Fred C. Baker of Sandoz's Zoecon Research Institute in Palo Alto, Calif., purified and characterized the substance, which they then found in large amounts in greenhouse-grown Cyperus.

"It is believed that all insects need [juvenoids] to control the nature of their molt," Schooley said in an interview. Proper control requirs a feast-or-famine approach, an ebb and flow of hormones. When these hormones are present, for example, a larva will molt into a larger larva. For a larva to molt into the preadult stage called a pupa, however, juvenoids must be absent. And for insects without a pupal stage, such as cockroaches and grasshoppers, there still must be an absence of the hormones before the adult stage can develop normally. But then the juvenoid hormones must reappear in the adult female to trigger ovarian development. Their role in male adults has not been determined, Schooley says.

For about 20 years, scientists have periodically isolated compounds found both in insects and plants. Among those compounds have been a few juvenoids, yet those have been very limited in terms of the plants involved or the number of insects affected, Schooley says.

While thoughs about an evolution-based strategy that places juvenoids in plants are enticing, Schooley says he is reluctant to claim that evolutionary pressure guided the appearance of JH III in plants. "Plants make a wide variety of chemicals, and [the presence of juvenoids] could be completely accidental," he says.

The latest finding is primarily of scientific interest, without obvious commercial applications, according to Schooley. "It's very trendy these days to speak of transferring these types of genes into plants to make them resistant to insects, but these hormones are a complicated multi-gened trait not easily transferred," he says.