Also known as plant lice, aphids seek out the new leaves and tender stems of garden plants, piercing epidermal tissues and sucking the nitrogen-rich sap. A few aphids rapidly become a herd, standing room only on your favorite herbaceous perennial or shrub.
In the past, I have known otherwise rational gardeners that would bring out the big guns from the chemical shelf of the tool shed when they spotted a single aphid. Some of the most toxic insecticides, including nicotine sulfate, malathion, diazinon, and dimethoate have been used to kill aphids along with beneficial insects and other forms of garden life.
Hopefully those days are gone for good.
I would plead the case that aphids are evidence of a healthy garden ecosystem, playing an enormously important role in transferring solar energy from plants to animals. For starters, entire insect families have become aphid predators, including ladybugs, lacewings, parasitic wasps, and syrphid flies (hoverflies or flower flies). In addition to aphids, these predators also prey on other garden herbivores. In order to maintain populations of these beneficial insects about the garden, there must be a steady supply of aphids.
In the U.S. alone there are over 1400 aphid species, each feeding in summer on one plant species or a group of related plant species. Colonies grow rapidly as females give birth to live young, all female; males enter the picture, if at all, only when it is time to produce eggs for overwintering. In a typical colony, some of the female aphids will be winged, others wingless.
Aphids have complex life cycles that maximize acquisition of nitrogen from their plant hosts. They become so numerous each summer because they are able to use plant resources efficiently, exploiting the plants with few measurable effects, particularly if the plants are not water stressed. I often find aphid colonies literally covering the stems of garden plants, yet the plants continue to grow normally. And typically the aphid colony has has been infiltrated by one or more aphid predators bent on reducing the colony’s numbers.
While the details differ, the typical aphid life cycle begins with tiny eggs laid in autumn on the twigs or branches of deciduous trees. The eggs hatch in spring, releasing a generation of “stem mothers” that move to the emerging leaves of the tree where nitrogen is most abundant. These wingless females are both viviparous and parthenogenic, giving birth to live young without mating. One adult aphid can produce five nymphs per day over a 30 day period, all clones of the stem mothers.
The nymphs go through five instar (molting) stages to become adult females, a process that takes about 30 days. As the nitrogen levels in the tree’s leaves decline, winged females are produced that fly to summer host plants, typically herbaceous perennials with stems and leaves that are high in nitrogen. Colonies grow through the summer, often numbering in the hundreds, even thousands, on a single plant.
At the end of summer, triggered by reduced nitrogen levels in the summer host plants, winged females and winged males are produced. After mating, the females fly to the winter host tree species to lay eggs.
One interesting variation in life cycle can be found in
Aphis nerii, the oleander aphid, also called the milkweed aphid. A bright yellow aphid with black appendages, this Mediterranean species has spread throughout the world. In North America it is an obligate parthenogenic species in which males do not occur. Winged females are produced when colonies become overcrowded and when it is necessary to change host plants. Females overwinter on host trees.
In summer, when I find a ladybug larvae in the midst of an aphid colony, or aphid exoskeletons punctured with the emergence holes of adult wasps, or a chickadee, warbler, robin, or norther flicker plucking aphids from garden plants, I feel that all is right in the garden world. In winter I can stand at the window and watch chickadees pecking aphid eggs off of rhododendron branches.
Entomologists who study aphids agree that aphids rarely do enough damage to warrant intervention by the gardener. And in exceptional situations, when predators cannot keep up with a heavy infestation on young plants, mechanical controls will work. The gardener can dislodge them with a strong stream of water. Once knocked to the ground, aphids have a difficult time climbing back up to the stem tip.
Aphids, Ants, and Sooty Mold: A Lesson in Garden Ecology
I recall one June morning, leaving the porch by the back steps, spotting a cluster of blue aphids on the tip of a red elder stem. At least a hundred aphids completely encircled the stem, tapping the sugar-rich sap just beneath the surface, while several winged females prepared to migrate, leaving to establish new colonies on nearby plants.
A dozen or so black and red ants worked at one end of the colony, milking the aphids for honeydew by stroking them with their antennae. Honeydew, excreted by the feeding aphids and rich in plant sugars, is highly valued by ants as food.
Drops of honeydew missed by the ants had accumulated on leaves below the aphids and started a growing colony of a fungus, commonly called sooty mold, on the upper leaf surfaces. The mold blocks sunlight from the leaf surface, rendering the few affected leaves useless in photosynthesis.