INJURIOUS INSECTS

Hawke's Bay Tribune, Volume XIX, Issue 64, 2 March 1929, Page 12

INJURIOUS INSECTS

SOME NATURAL PARASITES THE BALANCE OF POWER. (Walter W. Froggart in the Sydney “Morning Herald.”) An economic entomologist recognises and deals with two groat groups at insects; injurious pest insects and useful insects. Injurious insects are those best known to tho general public, and while a certain number of them are always in evidence, their ebb and flow is regulated by many factors. Climatic conditions certainly play an important part; food supplies; the increase of their many enemies, such as small mammals, birds, spiders, centipedes, and predatory insects.

We must bear in mind that while most insects prefer a special food plant, upon which they may general' ly be found, they instinctively recognise a cultivated plant belonging to the same or a close allied genius, and they will either desert their native food plant for the more succulent cultivated one, or feed impartially upon both. If' such a cultivated, allied food plant is grown over large areas previously infested with insects feeding upon the native plants, theywill naturally turn to the new host plant. Therefore, as the cultivated land increases, so do the numbers of the insect pest. A few examples might be recorded of such expansion of previously harm less insects. The sugar planters cleared off the virgin forest which clothed the rich black soil of northern Queensland. There was a certain amount of grass land, and on the grass roots lived the large white grub of a lamellicorn beetle. When this land was planted with sugarcane the grubs missed the indigenous grass, but found in its place "the fine fibrous roots of a gian grass. Within less than 20 years these bottle grubs had increased a million-fold. The canogrowers found their plants turning yellow, and the sugar contents uecreasing, because the grubs were feed ing upon the roots. In consequence, many thousands of pounds have been paid out to cheek the increase of the post. THE COLORADO BEETLE. A similar instance in the United States is found in the spread of the potato pest, popularly known as tho Colorado beetle. This beetle ted upon the foliage of a hcardy “sandburr,” a solaneaceous plant growing on the slopes of the Rocky Mountains. Pioneer settlers from the eastern States occupied the islands, and among other crops planted the potato, also a solanium. The Colorado beetles recognised the affinity of the two plants and deposited their eggs upon the foliage, on which the resultant larvae and adult beetles both feasted. They they turned eastward and followed the potato crops right back to the Atlantic seaboard. So scared were the British authorities that a special Colorado Beetle Act was passed to protect the English and Irish potato fields from an invasion. Another instance. In Australia coastal scrubs is a small shrub (Notalaea longifolia), closely related to the cultivated olive tree. It is infested with a tiny laco bug. When the .olive trees, were well grown in the Hawkesbury College orchard, they began to shed their leaves. On examination it was found that thg under surfaces were covered with these tiny plant bugs, w-hich xere sucking up all the moisture in tho leaves, and causing them to turn yellow and fall to the ground. NATURAL PARASITES . Under normal conditions in Australia, before the disturbing advent ot civilised man, the balance of power between insect and host plant regulated itself. Neither plant nor insect died out, for the latter seldom devoured all tho foliage or sucked up all the sap, before it was discovered by its parasite, which, once started, was very prolific and frequently ate up all the jicst insects. Tho economic entomologist engaged upon field and forest investigations meets with many examples of tho natural parasitism of injurious insects. A genus of Coccid beetles (Eriococcus) cover tho branchlets of the young gum trees growing in the scrub with their oval white ovisacs. Each one sucks up the sap with its sharp-pointed, tubular beak, and at the same time discharges a sugary secretion, known as “honeydew.” This coats the surface of the leaves with a fine, sugary varnish, which, as soon as damp weather comes, attracts fungus spores, which generate black mould on tho leaves of an infested tree. In the srub the coccids are discovered by a small black ladybird beetle (Rhizobius), which devours them. The gum tree is thus freed of its enemies. It frequently happens that when young gum trees are planted in avenues or gardens, they become badly infested with tlio Eriococcus scale insect. Away from the protection of the lady bird beetles, they become so badly smothored with the scale and mould that the small lateral branches die back, and tho gardener has to spray the trees to destroy the scale insects. THE APHID PEST. Several years ago tho cypress pines in the vicinity of Buddo were very seriously attacked by an aphijl pest. The tiny, brown-wdngod creatures swarmed on tho branchlets and slender stems in countless millions, all sucking up the sap with their slend-er-pointed beaks. So serious was this infostation that the dark green foliage began to turn yellow, and hundreds of trees appeared to bo dying. Within a few weeks, tho red-Spotted ladybird (Leis conformis) was noticed on the cypress pine trunks, and commenced depositing its eggs upon the bark. They, in turn, multiplied so rapidly that they could be counted in thousands, and as both larvae and adult beetles were carnivores they soon cleared off all the pine aphis. THE “CUTWORM” PLAGUE. Everyone has heard of the “Bugnng Moth,” mother of tho “cutworm” plague. Tho smooth, olive

green caterpillars swarm over the grass lands and cultivation paddocks, and cat off everything before them. They have two active enemies—one a large Ichneumon wasp, which lays its egg in the body of tho caterpillar; the other large shining green Carab beetle, which eats tho caterpillar. Usually these beetles are only found singly, under clods in the cultivation paddocks. But one summer some years ago. after we had suffered from a serious cutworm invasion, tho beetles appeared in thousands in tho streets of Sydney. They clustereu along the stone pillars round Hyde Park, and hundreds were trodden under foot about the electric lights at tho Central Railway Station. All through tho bush land we find a very largo percentage of tho moth cocoons infested by the larvae of parastitic wasps and flies, and it is difficult to obtain the perfect specimens of .<bme moths on this account. tVhen one is breeding out gall-mak-ing insects, one finds So many varieties emerging from the tissue of the woody galls, that .one is often in doubt as to which is the true gullmaker, and which the parsite. Nearly every native shrub is attacked by some scale insect or “mealy bug’’; but these in turn are usually heavily parasitised by minute wasps or native ladybird beetle larvno. If it were not for this balance of power between pest, and pnrsitc, there would hardly be a green leaf let in tho bush. Ti is natuie regulates tho law of supply and demand.