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Remember the thousands of butterflies that passed through Utah a month or so ago? They're back again; this time in the form of caterpillars.

In May the parents of these caterpillars, called Painted Lady butterflies, passed through the state in one of the largest migrations since 1973. They left behind their legacy in the form of eggs. In recent weeks, the eggs have begun to hatch out, said Jay Karren, Utah State University Extension entomology specialist.He said agriculturalists and home gardeners need not be concerned because most of the caterpillars are feeding on their normal preferred host plant, the thistle. Their love for the plant is one reason the butterflies are also known as the Thistle butterfly.

In some cases when thistles are scarce, Karren said the insects will feed on other plants. The damage to these plants, however, is usually minimal.

"We're now seeing quite a population of them," he said.

"This means we can expect to see a new surge of butterflies over the next two to three weeks."

The butterfly's scientific name is Vanessa cardui. As an adult, it is multihued with two-inch wide wings that appear like blotches of orange, black, yellow and white paint with white underbodies. The tip of its antennae is characteristically white, he said.

As caterpillars, they can be spotted anywhere thistles grow, normally along Utah ditch banks or in grain, corn and alfalfa fields, he said.

The larvae don spikes along their backs and are black with yellow stripes and have faint blue and red spots. Before entering the pupa stage, devouring the thistle plants they feed on, they turn from green to black.

Karren said the insects leave the root system of the thistles intact, allowing them to regenerate the following spring.

Unlike their migrating cousin, the monarch butterfly, Karren said Painted Lady butterflies do not return south for the winter.

When the cold hits, they die.

The butterflies always originate from the warmer southern climates of Mexico, southern Arizona, New Mexico and even southern Utah's Dixie area. He said both adults and pupae spend winter in warm regions. They are reintroduced annually in this area through migration - a one-way mass exodus.