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Science

Of Millers And Monarchs (And Milkweed)

This spring, miller moths seemed to swarm like a miniaturized version of Alfred Hitchcock's "The Birds." Just as we opened a door or turned on a light, there they were, bombarding us. But their numbers were actually average this year.

Entomology expert Dr. Deane Bowers is a professor at Colorado University in Boulder and the curator of the entomology section of CU's museum. She said there seemed to be a lot of moths this year because we've had about four years of below average numbers. Additionally, a dry spring and a freeze in April reduced the nectar flow, the food that moths need. This led them to congregating in irrigated areas, like our backyards.

The army cutworm develops into the miller moth on the Eastern Plains. Like the pioneers of the gold rush, the moths go west in search of their prized golden bounty: nectar. They feed on spring flowers along the foothills, their last stop at the blooms of the Russian olive, before heading to higher elevations to find later blooming varieties.

The moths spend the summer in the mountains before returning to the plains to lay their eggs in the fall. While they're a nuisance to us, they do provide an ample food source for birds and even bears.

As the moths leave, Dr. Bowers has her sights set on another insect migration: butterflies.

"It could be a very interesting year biologically. I just saw a butterfly in my garden over the weekend that normally doesn't occur around here," she said. She's talking about the queen butterfly, a relative of the well-known monarch butterfly.

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Credit bvi4092 / CC BY 2.0
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CC BY 2.0
The "Arizona queen" is similar to the monarch but black stripes are only seen on the ventral side of the wing.

The queen usually lives in the tropics and is more common in southern parts of the U.S. It's rarely seen in Colorado. Monarchs are more familiar to us as they move through Colorado on their annual migration to cooler northern latitudes.

Both the queen and the monarch use the milkweed plant to fend off predators. Milkweed contains a chemical that is toxic to birds, but not to these butterflies. The butterflies lay their eggs on the underside of milkweed leaves. The eggs develop into caterpillars that eat the milkweed and store this chemical. The stored chemical makes the caterpillars — and the butterflies that they turn into — "taste disgusting to potential predators," Bowers explained.

These compounds do not produce any smell. The bird must eat the disgusting butterfly before it knows the butterfly contains a toxin. "The predators can learn, but they have to taste one to learn that," Bowers said. "That's why a lot of insects that are unpalatable or toxic are very brightly colored, so that potentially it's easier for predators to learn to avoid them."

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Credit Seré Williams / KUNC
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KUNC
A honeybee and grasshopper on milkweed flowers.

As Bowers attests, the milkweed is not much of a "weed." Milkweed have wide leaves and can get up to five feet tall. Light pink flowers bloom in a ball at the top of their stalk. They're called "milkweed" because if you break off a leaf, white, milky sap flows out. They grow in disturbed areas along ditches and can do well in a garden.

Because of the monarch's reliance on milkweed, the plant helps dictate the path of the monarch's migration. If you find milkweed, you may be getting closer to seeing a monarch, or this year, even a queen.

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