WASHINGTON (AP) _ When a voodoo lily gets into a reproductive mood, it takes some aspirin and calls up a beetle in the morning.

That's the finding of a group of scientists who investigated the chemical basis for a rise in temperature detected in the flowering organ of the voodoo lily.

In research conducted at Modesto, Calif., the researchers found that salicylic acid, the chemical in common aspirin, will trigger a temperature increase in an organ called the appendix, a part of the voodoo lily flower.

The temperature rise, said Ilya Raskin of the E.I. du Pont de Nemours Co., causes the flower to give off a strong aroma which is attractive to the carrion beetle, the primary pollinating insect for the voodoo lily.

Raskin said that in the experiment, he and his collegues applied salicylic acid to the appendix just before the flower was to open.

The next day, after receiving about 4 1/2 hours of light, said Raskin, the appendix temperature rose quickly by about 15 degrees.

He said the heat accelerates the volatilization of chemicals to produce a ''strong putrescent odor'' which is attractive to the carrion beetle.

Once the beetle is inside the flower, said Raskin, the plant even works to keep it there.

After sunset, he said, the flower closes up and a second episode of heating occurs in a lower portion of the flower, deep inside the pollination chamber.

''It heats up at night to keep the pollinators happy inside the flower,'' said Raskin.

Pollen moved by insects from the male to the female portion of flowers cause plants to produce seeds.

The temperature rise of the voodoo lily appendix was first described by researchers in the 18th century, but the chemical basis for it was not understood. A scientist in 1937 suggested that the temperature increase was triggered by a substance called ''calorigen'' produced in the male part of the flower.

Experiments by Raskin and his team of Axel Ehmann, Wayne R. Melander and Bastiaan J.D. Meeusez confirmed this and used mass spectral analysis to identify salicylic acid, the principal ingredient of aspirin, as the active chemical in calorigen.

To test their findings, the researchers excised portions of the appendix from voodoo lilies just before the blooms were to open. Salicylic acid was applied to some samples and ordinary water to others.

The appendix tissue then was incubated overnight in a growth chamber. The next morning, lights were turned on in the chamber to mimic sunrise. A thermal detection system measured the temperature of the tissue every half hour thereafter.

About 90 minutes after ''sunrise,'' the temperature of the appendix tissue treated with salicylic acid began to increase. After 4 1/2 hours, the aspirin- treated tissue typically was about 15 degrees hotter than the untreated tissue. The heat slowly declined in the appendix, but Raskin said a second temperature increase occurs during the night in another portion of the flower.

Raskin said their findings show that the voodoo lily apparently generates salicylic acid as part of its reproduction scheme and the chemical may have functions other than just triggering the heat rise.

''There are indications that it (salicylic acid) is involved in the flower unfolding,'' he said.

Raskin said salicylic acid may also serve as a natural thermogenic, or heat generation, trigger in other plants and is known to stimulate flowering of plants such as the duckweed.

In a paper on the experiment published this week in Science magazine, Raskin and his group note, ''All this raises the possibility that salicylic acid plays a much broader regulatory role in plants than previously appreciated.''