Why do suicidal beetles invade my basement each spring?

From The Mercury News
June 20, 2018

DEAR JOAN: Every year around this time, a herd of suicidal beetles appears in our basement, which is built into a hill. There is sandy ground in some areas, and one wall faces the outside. The beetles appear out of nowhere and I cannot find any opening to the outside they could have used to enter the basement.

Most of them are dead when I find them, but sometimes they are still alive. When I capture them to bring them outside, they make a deep humming sound. They are about 1 to 1½ inches long. I find them until about August or September, then they are gone again.

How can I prevent these beautiful beetles from continuing to commit suicide, and what are they?  Help please.

Annette Scheibner, Scotts Valley

DEAR ANNETTE: Your suicidal beetles are ten-lined June beetles (Polyphylla decemlineata), also known as watermelon or scarab beetles. How or why they’re in your basement is a mystery, although I’m pretty certain they aren’t going there deliberately to die.

Steve Schutz, scientific programs manager for Contra Costa County Mosquito and Vector Control, identified the beetles from your photos, but he’s just as perplexed as you are about why they’re in your basement.

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