Because these menacing mini predators are none other than the babies of that scourge of aphids, a ladybug.
In this insect family, it's the adults who are endowed with all the features that make people say, "Awww." They're pudgy. They're round. They come in Gummi-bear colors. They often have polka-dots. They trustingly walk on your arm. Sometimes they forget to completely fold their wings underneath their wing cases and potter around looking like church ladies with too-long petticoats or little boys who've forgotten to tuck in their shirt tails. What could be more charming?
(OK, they also ooze foul-smelling yellow fluid from their joints if alarmed. Nobody's perfect.)
The larvae, however, won't be winning any Beautiful Baby contests anytime soon. Upon hatching, they look a bit like deer ticks (minus one pair of legs). They grow into elongated little black alligators, eventually acquiring bright orange, red, or yellow markings. (Thus far I haven't been able to determine whether lady-beetle larvae are as foul-tasting to birds as the adults are and if these colors serve as fair warning to them.)
Mama lady beetle considerately lays her eggs on a leaf in a veritable smorgasbord of aphids, so when the larvae hatch, they blunder into a feast. I imagine that'd be like waking up in the morning surrounded by chocolate croissants. The babies tuck into the meal about an hour or two after hatching, stabbing aphids right and left with hollow mandibles and slurping them up like milkshakes.
Then they probably enjoy a nice aphid-dinner mint.
A typical larva with a good appetite may gobble up 30 to 40 aphids a day over the course of four or five weeks. (According to many sources, "the convergent lady beetle may eat its weight in aphids every day as a larva." This sounds like an awesome statistic until you realize that you don't know how much a lady-beetle larva or an aphid weighs. Still, that's a lot of chocolate croissants.)
But wait, kids! It's WORD PROBLEM TIME! According to a 2007 scientific paper, "Molecular sabotage of plant defense by aphid saliva," a typical adult aphid weighs approximately 1 milligram. If an aphid weighs 1 mg, and a lady-beetle larva stuffs itself with 40 aphids, how much does the larva weigh?
Adult lady beetles consume a lot of aphids too, but they're not as voracious as the growing youngsters, plus they can fly away if your garden runs out of aphids and other small chitin-clad snacks. The larvae can't.
I once watched a lady-beetle larva approach a cluster of fat aphids on a stem--so delighted with its find that it was nearly rubbing its hands (if it'd had hands) together with anticipation like a cartoon fox in a henhouse--only to be met by a hostile ant that was guarding the aphids; ants tend to little herds of aphids so as to enjoy the honeydew they produce. The ant was ferocious; the larva seemed surprised; the aphids were oblivious. The ant won the gladiatoral event, but sort of by default--the larva did what looked like a pratfall, falling off the stem in astonishment, actually a very effective "I'm outta here" reflexive response. This time the ant was the one to dust off its virtual hands, satisfied with a job well done.
