With their cheery red shells and cheeky black polka dots, ladybugs are bugs even non-bug lovers can get behind. They fascinate children, bring good luck and inspire countless Halloween costumes.

On a jog up Napa’s Dry Creek Road recently, I saw a magnificent sight: thousands of ladybugs clumped together on fences (even barbed wire) and on old, mossy tree stumps. The most unusual ladybug ...