CASE STUDY: THE APHID
Commonly known as the greenfly, the aphid is a member of Class Insecta and exhibits what is known as ‘incomplete metamorphosis’ where the young are miniature versions of the adult known as nymphs. This is in contrast to ‘complete metamorphosis’ where distinct stages in the life cycle exist ie egg, larva, pupa and adult. For most of the year, the aphids present are females which produce live young without fertilisation.
Both winged and wingless females exist. The winged aphids fly to new plants. In late autumn, males and females are produced. These mate and the females lay eggs which are the over wintering stage for many species of aphid. The life cycle may be summarised as follows:
Aphids are one of the most destructive insect pests, feeding on the sap of plant tissue. As they feed, they transmit plant viruses to many crops such as potatoes, cereals and sugar-beet. Plants subject to aphid damage may exhibit a variety of symptoms including discolouration, stunted growth, low yields and death. Aphids have also been known to contribute to other plant diseases such as the spread of blight in Ireland in the late 1840’s resulting in the great Irish potato famine.
Various insecticides can be used to control populations and there is also an increase in the use of ecofriendly plant extracts and products effective against aphids, limiting the use of chemicals. Biological control can also be used with the introduction of natural enemies including the ladybird.
Other insect pests include the leatherjacket ie the larval stage of the crane-fly, which eats the roots of grasses, cereals and vegetables and the wireworm ie the larval stage of the click beetle which also damages the roots of grasses.