Pests and natural enemies

Press, Volume CX, Issue 32459, 20 November 1970, Page 7

Pests and natural enemies

The belief that nearly all Insects are harmful is quite commonly held. Actually beneficial insects outnumber the injurious ones and it can be said that such few as are injurious have become so as a result of i human activity. Today the stage has been reached where plant protec-j tion has become imperative, or crops will be destroyed by ; insect pests. There are perhaps two. principal reasons why crops! (and other plants for that | matter) suffer so much from insect attack. One is that the[ growing of the plants on the intensive scales practised today provides more food, shelter and suitable conditions generally for the insects which feed on them. The other is that many pests have their natural enemies and the absence or presence of these may influence whether an insect becomes a pest. This particularly applies where the parasite and/or predator of a particular pest does not abound or exist in an area or region of accidental introduction of the pest. The limitation of one insect by another, whether as a parasite or a predator, isj called biological control. Parasites in this sense arei usually deposited as eggs on

lor in the egg, larvae or pupal stage of the host. There they hatch out and the larvae feed on the body contents of the insect attacked, and finally kill it. Predators, on the other hand, are insects, ranging from juvenile to adult stages, i which catch and devour other insects.

A walk in the garden is bound to bring to notice the 1 lady bird. This is a very ! familiar insect, of which there are a number of predacious species identifiable by colour and spots on the back. Both the larvae and adult forms feed on certain mites, aphides and scales. For instance there is a black one which feeds on the gum tree scale (witness the holes in the scales); or the steel blue ladybird which is particularly active in citrus orchards where the olive scale and red scale may be more prominent. You may perhaps be more likely to come across the two spotted one, which attacks aphides.

Another voracious predator is the larva of the lace wing, a small-bodied insect with large transparent veined wings. The adult lays her eggs amongst a colony of ! aphides and when these i hatch, the aphis lions, as they are called, soon begin to catch and feed on their prey. Apart from these two predators there are hoverflies which lay their eggs among colonies of aphides, assassin bugs which feed on caterpillars, predatory thrips

which feed on plant-feeding thrips and various others. In addition there are numerous other insects such as dragon flies and mantids which are not particular as to the menu at any time they feed on what they can catch. . The most important group of parasites in the insect world is wasps. They range in size from an inch or so in length to all but invisible species. Parasitic wasps lay their eggs inside other insects where the larva hatch and steadily devour the host. If you have a problem with woolly aphides on your apple trees go and take a closer look at a colony of them during the summer. Those which are comparatively free of the characteristic fluff and are black and brittle looking are but eaten out shells which have satisfied the appetite of hungry wasp larva. Close examination of the cabbage patch later on in the year may reveal a number of hole-riddled pupas. These are all that remain from a cabbage white butterfly chrysalis after the larva of anomer wasp have completed part of their life cycle. At present entomologists are working on various aspects of biological control in their attempts to discover ways and means of extending knowledge and methods of this form of overcoming the very expensive business of defeating insect pests. May be one day the release of a packet of insects will supersede spraying.