Garden enemy Number One: Aphids

We think of aphids as more pesky than dangerous in the garden but they do untold billions of dollars’ worth of damage to commercial food crops. Learning more about them will help you cope in your own home garden.

Aphids are enemy Number One in the temperate zone garden. If you never have another pest, you can almost rest assured that aphids will infiltrate no matter what your defences. Understanding them and how they roll is one of the keys to keeping these suckers to a minimum.

You probably know that they can reproduce both sexually and asexually, meaning the females, born of over-wintered eggs in cold climates, can produce offspring without the aid of a male in springtime. They just push out little clones of themselves, also female; in some cases the body of the newly emerged nymph is already bearing an incipient offspring. Come fall, triggered by light and temperature, the females will begin to produce sexual nymphs, both male and female, sometimes born without eyes or wings but with reproductive organs that allow them to produce eggs that will overwinter in the nooks and crannies of trees and shrubs.

It’s not that aphids are that short-lived – a generation can live for 20 to 24 days. In one season, some species can produce as many as 41 generations. Under ideal conditions, it is estimated that a single cabbage aphid could start a dynasty that will grow to 1,560,000,000,000,000,000,000,000 offspring in a single season. Fortunately those perfect conditions seldom occur.

Suck and flow

We think of aphids as sucking insects and they do act as suckers on occasion, but mostly they simply stick their needle-like mouth parts into the phloem of a plant and let the sap flow – well, actually, it veritably gushes into their mouth parts and through their digestive tract, coming out the other end as honey dew.

The sucking part happens when they tap into the xylem sap of the plant. The xylem cells transport water and minerals upward on the plant. This sap contains only 10 per cent as much carbohydrate and amino acids as the phloem sap, which transports recently manufactured sugars to parts of the plant, including the roots that need them. So why do the aphids go to all this trouble? Some think it is to balance their moisture content, but others believe that it has something to do with their ability to reproduce. Still others say it reduces the osmotic pressures caused by the phloem sap gushing into their stomachs.

Among the 4,400 species in 10 families of aphids, there is an aphid for every plant. Sometimes aphids specialize with just one plant on their menu; sometimes they alternate between plants, often starting off on trees and shrubs in springtime and moving on to herbaceous plants as they grow. Sometimes, as with the green peach aphid, they are not fussy at all and will latch onto just about any plant.

Aphids come in sizes ranging from one millimetre long to 10 millimetres long, and in colours that include green, brown, black, pink, grey, red and transparent.

Ant herding black aphids on an iris seed head in fall.

Close up, you can see the cornicles on the rear end of the aphid. This is where the wax is secreted.
A little help from their friends

It is commonly known that some ants herd and farm aphids for their honeydew. They stroke the aphid with their antennae to squeeze the syrupy excrement from the aphid’s digestive tract.

Some ants even have a relationship with a caterpillar that eats aphids. Rather than attacking the caterpillar though, they carry the caterpillar larvae off to their nests and actually feed them aphids. Why? Because the caterpillar is a highly efficient producer of excreted honeydew itself – saves the ants a lot of energy that would otherwise be spent milking the tiny aphids. And there is no residual cleanup, because when the caterpillars have finished feeding, they make a cocoon at the exit to the ants’ nest and fly away after metamorphosis.

Honeydew, which we abhor when it gets all over our possessions and hands, is also highly prized by a certain coniferous forest bee, which collects honeydew to make forest honey.

Aphid defences

Aphids may look soft but they are not without their own defences. They will put up a fight before being eaten, kicking their attackers with their hind feet. Some have spiny backs that make them unpleasant to eat and others have thick skins that make them tough.

They can also send out pheromone alarm signals to warn nearby colonies that there is an enemy at hand. Unfortunately for the aphid, some ladybugs have figured this out and use the signal to their own advantage.

Aphids have two little venting organs called cornicles in their third segment that they use in their defence to emit a waxy substance to clog attacker mouth parts. The wooly aphid covers itself in this substance as a long term strategy. For a long time, people thought that the cornicles were where the honeydew came from, but we now know that honeydew is just plain aphid poop.

Some aphids create leaf galls as a defence. They hide inside these waxy houses and venture out, sending out burly soldier aphids produced by the females to sacrifice themselves in protection of the clan.

Finally, although aphids are generally wingless, they will produce a generation of “alates”, aphids with wings, if the food runs out.

Vulnerabilities, enemies and your defences

Aphids are vulnerable to late frosts, excessive heat, and pounding rain (and your garden hose). The rain or hose knocks them off their perch and stops them from flying.

Their enemies include ladybugs, hoverfly larvae, parasitic wasps, certain caterpillars, aphid midge larvae, crab spiders, lace wings, certain types of fungus on certain types of aphids – and, of course, hose-wielding humans.

They are sensitive to neem oil and lantana products and to garlic mixtures as well as insecticidal soaps. Some gardeners swear by diatomaceous earth as a cure and a prevention (you can safely mix it with water and spray the leaves, top and bottom).

Spraying trees and shrubs with dormant oil in spring suffocates overwintering eggs. Banding trees with Tanglefoot and insulation will help keep aphid-herding ants away. Washing trees in early spring and summer can also help dislodge first the eggs and then the nymphs and adults before they move on to your plants.

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