The milkweed in our Vermont meadow droops, yellowing, as hordes of orange aphids huddle on the wilted undersides of its leaves, delicate stiletto mouth parts sucking plant juices as the blades dry and twist. If I touch a single aphid, the entire population reacts, so tightly are they in contact with one another.
An aphid sucks sap with a sharp stylet thin enough to pass between plant cells. As juices are withdrawn, secretions prevent the sticky fluid from jamming up the insect’s system. Digestive processes within an aphid are helped by resident yeasts and bacteria living in fat cells alongside the intestines, and these microorganisms produce necessary enzymes as waves of food pass through the digestive tract.
Plant sap is watery and high in sugar content but low in protein and other nutrients, so only moderate nourishment is derived from large volumes of fluid. To make room inside themselves for ever more sap, aphids constantly excrete a sugary solution known as honeydew. As it accumulates on an aphid’s abdomen, the insect periodically flicks drops of honeydew to other leaves and the ground below, where the “sweet rain” dries to a shiny glaze that nourishes rich growths of fungi, some found nowhere else.
Honeydew often attracts ants that corral and milk their aphid cattle and protect them against intruders. When winter draws near, these ants may herd their aphid flocks to the protection of underground shelters.
An aphid’s anatomy is unimpressive and vulnerable. Its swollen body bears a tiny head and thorax, weak spindly legs, and slender antennae. A pair of tubes known as cornicles project from the abdomen, and from them some species secrete waxy strands, resulting in a dense mass of white protective threads.
You can tell if a particular newborn aphid nymph will fly one day from the presence of small wing pads which will grow into uncomplicated yet functional wings with each molt. Those with wing pads will soon be flying off in search of greener pastures, but aphid flight is so feeble that it works best when assisted by wind.
Farmers and gardeners sometimes believe they are dealing with two different species of aphids—winged and wingless—when in reality it is often two versions of the same species. Ample food, long daylight hours, and high temperatures suppress wing formation in aphids. They don’t waste energy growing wings when there is no need for them. But when aphid-infected plants dry up and wilt, wings appear on subsequent generations to enable the insects to move in search of fresh vegetation. Once a new source of food is located, successive offspring are again wingless.
Aphids achieve staggering populations using a reproduction short-cut from which males are excluded. In spring, a special “earth mother” female, the fundatrix, cuts through her protective egg membrane. She quickly bears living young—tiny pear-shaped females—and from then on, females busily produce more females, generation after generation, by parthenogenesis, or virgin birth, bypassing both fertilization and egg laying. Production of perhaps a dozen generations continues, until autumn’s chill induces the last females in the virgin series to bear both males and females. Following mating, the females lay winter eggs in preparation for next year’s emergence of fundatrix mothers—and off they go again.
Without predators, aphid populations would surely overcome our feeble and costly efforts at chemical control. Colorful, banded syrphid flies and delicate lacewings feed upon larval and adult aphids. Small parasitoid wasps insert their eggs into aphids, and a wasp larva munching away inside kills its host, after which the aphid’s body swells while the wasp pupates safely within. The victor finally emerges from the dry husk and flies away to mate and seek more aphids.
Larval and adult ladybird beetles are fierce predators upon weak, defenseless, and abundant aphids. Domestic ladybirds are collected in the Rocky Mountains and sold to farmers, but the most valuable beetle is a ladybird species introduced to Vermont from Australia, the vedalia. An insatiable ladybird larva, feeding upon one aphid after the next, exhibits compulsive eating at its most extreme.
Tiny wizards, aphids have evolved unparalleled reproductive powers and a knack for changing shape in response to changing food availability. They take the world by storm, and, like it or not, it’s an astonishing story.