The remarkable little Aphid

The ubiquitous little insects often called plant lice but more correctly known as aphids have probably been encountered by everyone who has ever attempted to stick anything green into the ground. Although the bane of gardeners everywhere they are remarkable little creatures with interesting and very complex lifecycles.

Aphids are members of the insect Order Hemiptera which is the Order in which all true bugs are placed. Although the word "bug" is often generally used to refer to virtually anything with six or more legs this is a misnomer and only insects that are members of Hemiptera are scientifically classified as bugs. The aphid furthermore belongs to the Sub-order Homoptera and then more specifically to the Family Aphididae (pronounced: ay-phid-di-dee).

Aphids are capable of breeding prolifically.

They are tiny pear-shaped soft-bodied insects, most measure between 1mm and 5mm in length and can vary greatly in colour depending on the specific species, of which there are a great number. Some are black with white markings others green, yellow or even reddish. One thing, however, that they do have in common with all other bugs is that they are equipped with a piercing, sucking mouthpart known as a proboscis. Aphids are usually found clinging to soft shoots or the undersides of leaves where they are able to insert their proboscis into the plant tissue and in this way suck up the plant juices on which they feed. Although their small size may lead one to believe that they can’t do much damage their ability to breed in great numbers can cause plants to lose vigour, wilt and even die as valuable plant sap is drained from the plant while some aphids can also serve as vectors of numerous plant viruses.

The aphids in fact extract much greater volumes of plant sap than they require, surplus sap is voided as waste from two tubes on their back known as cornicles and careful study of an aphid infested plant will show minute drops of plant sap on the surface of the underside of leaves. This liquid can in turn serve as a medium for saprophytic fungi. These fungi grow on the leaf surface and reduce the photosynthetic properties and overall vigour of the plant.

Ants herd, protect and milk aphids for honeydew.

 

This voided plant sap, termed honeydew, also attracts ants that feed on it. In fact, the mutual relationship between ants and aphids is so well developed that ants actually "herd" the insects, aggressively protecting them from predators as well as "milking" them to obtain honeydew. Where aphids are found ants will consequently also invariably be present, the ants will often even physically carry aphids from one location on the plant to another as well as from one plant to another to ensure a constant supply of honeydew. By gently stroking the aphids with their feelers and front legs the ants are able to induce the aphid to extrude a drop of golden honeydew which will then eagerly be lapped up. The scientifically minded gardener may wish as an experiment, to take one or two hairs from a fine paintbrush and with these gently stroke over one or more aphids while observing them with a hand lens. Soon it will be seen that a drop of honeydew begins forming on one of the cornicles, this will slowly enlarge until it drops off onto the plant surface.

A winged aphid showing it's long delicately veined wings.

 

 

 

 

 

 

A group of aphids with one extruding honeydew. Mouse over to see honeydew drop.

 

The complex and rather unusual life-cycles of aphids are worth a closer look as they facilitate prolific breeding and aphids are capable of building up tremendous populations in a very short time. They are especially unusual in that they are able to breed parthenogenetically, that is without mating, as well as by mating. They are furthermore capable of giving birth to live young as well as being able to produce eggs, while some are able to fly and others not. The aphids’ life-cycle may be said to commence with the hatching of fertilised eggs which have overwintered attached to plants where they were placed the previous season. The young aphid hatchlings, called nymphs, are all female and start feeding immediately by inserting their proboscis into the soft tissue of the plant. They grow rapidly, in the process shedding their skins, which can often be seen adhering to the underside of leaves, a number of times before reaching an adult stage known as "stem mothers". Stem mothers are plump little insects able to reproduce without mating and as their (unfertilised) eggs develop within their bodies they produce live young, usually dropping them while quite happily continuing to feed. These nymphs in turn again develop into another generation of stem mothers. An aphid population will at anytime be comprised of adults (stem mothers) and numerous nymphs in various stages of development. The nymphs are similar in general appearance to the adults except that they are smaller and usually differ in colour. This process of parthenogenetic reproduction continues for a number of generations, all are females and most are wingless. From time to time a few winged individuals may be found, the development of wings appears as if it may be associated with the suitability and health of the host plant, when the plant is no longer capable of supporting the aphids some develop wings and fly to another plant. The winged individuals are thinner and slightly longer in general appearance than the stem mothers with fine delicately veined, transparent wings which are much longer than their body length. Despite their fragile appearance they are fast to launch themselves if disturbed and are probably able to fly considerable distances as they will easily be carried by the wind. In some cases and with certain species they may migrate to a similar or specific host plant, in other any suitable plant seems to suffice. One authority states that some may use different host plants for different generations.

Ladybirds are natural enemies of aphids and do much to control their numbers.

 

In the autumn as cooler weather approaches a generation suddenly emerges consisting of both males and females capable of mating. After mating the female will place the now fertilised eggs on a suitable plant where they will again overwinter to hatch into stem mothers in the spring when the cycle will be repeated. Indeed, they do have a complex life-cycle.

Left uncontrolled an infestation of aphids can do considerable damage to both ornamental plants as well as crops and they have the potential of developing into serious agricultural pests. Unfortunately many commercial insecticides while effectively controlling aphids also decimate numerous other beneficial insects and careful selection of control methods are thus advised. Ladybirds in both adult and larval stages are voracious predicators of aphids as are the larvae of the beautiful little lacewings and some hoverflies, these insects, amongst others, greatly assist in controlling aphid populations and need to be protected rather than indiscriminately killed together with the pests.

 

 

Text and photographs copyright Lambert Smith.

 

© COPYRIGHT Lambert Smith, 2010