During some University of Georgia research on a new insecticide to control aphids on tobacco, scientists found a surprising side effect: hornworms on the plants turned white.
"We thought there would be some effect on the worms because of the nature of the chemical," said Robert McPherson, a UGA entomologist at the Coastal Plain Experiment Station in Tifton, Ga. "But we didn't expect them to change color. At all."
Scientists found the new insecticide, pyriproxyfen, did an "OK" job controlling aphids, McPherson said. But the overall effect on the worms is more significant. It keeps them from changing from caterpillars into adults.
The hornworm caterpillar stage lasts about three weeks. "And that's long enough to do a lot of damage to plants," McPherson said. In 1996, hornworms caused $2.6 million worth of damage in tobacco. They also caused some damage in tomatoes.
So waiting until the caterpillars pupate to die wasn't fast enough for many farmers. They still saw the growing worms hungrily eating their tobacco and tomatoes.
"But, what we've found is with white worms, the birds see them more easily and quickly pick them off the plants," he said.
That's the fascinating part of the trial, McPherson said. "The hornworms the birds don't get will die when they pupate," he said.
Later work in the lab showed the chemical binds to the chlorophyll in the plants. Chlorophyll is what gives the worms their green skin color. The bound chemicals prevented the green from moving to the caterpillars' skin. The worms turned white within two days of the insecticide application.
Most worms that eat plants are really caterpillars. Caterpillars are young moths or butterflies. They'll grow for a while, eating just about every plant leaf they can, then pupate. During their pupal stage, they mature into adult moths or butterflies.
This new pesticide disrupts their life cycle. Normally, McPherson said, when the worms begin to pupate, their hormones cause them to mature into adults. "But the chemical prevents that from happening. So they get stuck between juvenile and adult and die," he said.
"The good thing about this chemical is that it can't affect any other insects that might be beneficial," he said. "It works only on the worms' hormones. It doesn't even affect the birds that eat the hornworms."
The chemical does have similar effects on other types of worms, McPherson said. "Soybean loopers and velvet bean caterpillars both lose color and die during their pupal stage," he said. "So this insecticide may have some promise for the crops they eat, too."
During the 1997 tests, the first of the series, McPherson and the technicians who work with him found that birds picked off nearly 40 percent of the worms before they began to pupate. Though the worms lived about three days longer than normal, they still died before they could mature.
"The generation that's treated in the field does continue to cause damage," he said. "But there isn't another generation to feed on the plants and cause more damage."