Interesting Aphid Facts
What are Aphids?
Aphids are small insects with soft pear-shaped bodies. There are over 100 different aphid species in New Zealand. Many are green (often known as greenfly), but some are brown, black, yellow, red, purple and other shades. Aphids can vary in size from less than 1 mm to the large Giant Willow Aphid, which can be 10 mm. Only about 12 of the species are New Zealand natives, the rest have been accidentally introduced with imported plant material.
Almost all plants can be attacked by aphids. Some aphid species are specific to particular plants, but many have a wide host range feeding on a variety of different plants.
Aphids are true bugs of the order Hemiptera which also includes cicadas, planthoppers, shield bugs, scale insects and mealybugs. These all have sucking mouthparts that can be inserted like a hypodermic syringe into the phloem* of plants, so they can feed on the plant sap. They are often called sap-sucking insects, but this is usually a misnomer as the sap is under pressure and the insects do not have to suck; the sap is pumped into them by the pressure.
After extracting the nutrients they need, the aphids have to release the pressure of the sap by excreting much of it as honeydew from their rear end. This sugary sap drips down onto lower foliage and a sooty mould will often grow on it. The honeydew is also attractive to wasps and ants. Ants will farm the aphids, protecting them from predators and stimulating the aphids to produce a drop of honeydew for them to feed on; akin to milking a cow.
Not all aphids feed on the foliage or stems of plants. Root aphids live in the soil and feed on sap from roots.
Aphid infestations can appear very quickly. This is often because they have been ‘hiding’ as tiny aphids in leaf and flower buds and have gone unseen until warmer weather encourages rapid multiplication. Aphids have many methods of reproduction; in favourable conditions, many species can reproduce without males; females give birth asexually to tiny live nymphs that are clones of their ‘mother.’ In some cases, the nymphs are born already pregnant with their own asexual clone developing within. Omitting the egg stage allows shorter life-cycles and populations can grow very rapidly. Males often do not appear until late in the year and the eggs are then produced and hidden to overwinter in buds and crevices in the bark. They will hatch in spring and start the cycle over again.
When populations become crowded, adult aphids may grow wings and disperse to other suitable plants so that infestations can quickly spread from plant to plant and garden to garden.
The natural enemies of aphids include ladybugs, hoverflies, spiders and wasps. Some aphids produce foul-smelling insect repellents to protect themselves from their predators.
*The phloem is one of two systems of vessels within vascular plants; the xylem is the system that mostly carries water from the roots to the rest of the plant and the phloem carries sugars, organic compounds, and minerals from the leaves to the other parts of the plant.