INSECT PESTS

Dominion, Volume 31, Issue 70, 16 December 1937, Page 16

INSECT PESTS

Precautions Needed To

Protect Trees

EUCALYPTS AND PINES

In recent years there has been a remarkable development of afforestation in different parts of New Zealand, particularly on pumice lands of the Central North Island plateau. Comencing in 1898 with an area of 54 acres of exotic timbers planted by the State, the acreage has increased rapidly during the past 15 years until in 1936 the total area of plantations owned by the State, commercial and private interests reached the figure of 750,000 acres. Conifers, particularly pinus insignis, constitute the bulk of the plantations, eucalyptus occupying a very subsidiary position, says Mr. D. Miller, asistant director and chief entomologist, Oawthron Institute, in a report on insect pests. As a natural accompaniment of these afforestation activities, one might expect to find insect depredations increasing on a proportionate scale as has happened in the case of othercrops introduced into New Zealand. Although, for the time being, we may congratulate ourselves on the comparative absence of serious insect pests—at least among the exotic conifers, the possibility of great damage, resulting from the casual introduction of several overseas insect pests; should hot be forgotten. Even under the best of quarantine regulations, it is difficult to maintain absolute freedom from chance introduction of destructive insects, while in the case of New Zealand the absence of a quarantine service greatly increases the risk of such introductions.

Even in New Zealand, illustrations are not lacking of the great damage resulting from insect attack on exotic timbers. The case of some species of eucalypts in certain parts of the country may be quoted in this respect. In other countries, outstanding instances of damage wrought by insect pests may be cited in the case of the bark-beetles in the United States of America, where these insects over a period of 50 years are said to have destroyed trees to the value of £200,000,000, and of the European larchsawfly which destroyed vast areas of larch in the Eastern States before it was effectively controlled by the introduction of a parasite from Scotland. In view of these instances of the great destruction caused by introduced insects, it is imperative that every precaution be taken to prevent importation of harmful insects and to maintain adequate surveys of the exotic plantations in order to control as quickly as possible any harmful insects that may become established. Insect Pests of the Eucalypts. Eucalypts either singly, in groves or in plantations are widely distributed throughout New Zealand. Most of the different species established in New Zealand are free from serious insect damage but the blue-gum is an outstanding exception.

The primary factor inhibiting the health of the blue-gum and sometimes causing its death, is the attack of the blue-gum gall-cbalcid. Infested trees can readily be detected by the absence of foliage on the terminal branches and, in bad cases, even on the large limbs. An examination of the affected branches shows that they have a gnarled appearance and that

they are punctured by innumerable minute holes through which the insects have emerged. The gall-chalcid attacks by laying its eggs in the soft bark of new growth, the resultant grubs forming cells aud swellings in the growing tissues. Where infestation is heavy the surface of the bark takes on a gnarled appearance and death of shoot or branch finally ensues. New shoots from healthy wood lower on the branch are attacked in turn by the gall-chalcid, leading eventually to the death of the tree. Unfortunately there is as yet no known method for controlling this insect. The gum-tree scale is a native of Australia and was first noted in New Zealand at Timaru about 30 years ago. Owing to prevailing winds it was carried north aud infested Marlborough within a few years. Its spread south of Timaru has not been so rapid, largely owing to adverse winds. The Southern Alps have prevented the spread of the scale to Westland. Until a few years ago the North Island was free from attack but recently the scale has become established on a wide front extending from Marton to Wanganui. From this district, prevailing winds have carried infection to the main valleys in the North and through the Manawatu Gorge in the east. Considerable damage has been done by the scale insect but it is now being held in check by a ladybird beetle introduced some years ago from Australia. Another important euealypt pest in New Zealand is the gum-tree weevil. The weevil destroys the growing tips of young shoots. Its presence can be detected by the serrated nature of the leaf-margins when the beetle has been feeding and by the linear stripes eaten from the surface of the leaves by the grubs of the weevil. During the winter, colonies of the beetle may be easily found under the loose bark of the tree trunks. The control of the weevil has been an outstanding success in some parts of the country as a result of the introduction of a parasite secured from Australia. The parasite attacks the eggs and has proved successful in controlling the weevi] in South Africa and the Argentine also. Insect Pests of Pine Trees. On the whole exotic conifers in New Zealand are remarkably healthy in so far as insect infestation is concerned, but there are two insect pests of major importance. The most outstanding of these is the European bark-beetle which is likely to play a leading part, if left uueontrollc in seriously hindering the re-establishment of coniferous plantations once the present crop of trees has been cut.

The bark-beetle is a small insect which, with its grubs, attacks under the bark of growing pine trees. Seedlings are particularly liable to damage and considerable mortality up to 30 per cent, may result. 'The bark-beetle has been found on older trees in the trunk 10 feet from the ground and the roots likewise may be attacked.

Owing to the fadt that stumps and dead trees offer a favourable breeding ground there is great danger of serious damage to seedlings planted- on cutover pine lands. An attempt is being made to control this insect through the introduction of three species of predaceous beetles from England. A more common aud conspicuous pine-tree insect is the European steelblue horntail borer. Its presence throughout New Zealand is readily detected by the holes with a diameter of a pencil which are so common in dead pine trees. The insect itself, is conspicuous by reason of its size, the largest specimens measuring over one inch in length. A parasite has now been successfully established in New Zealand for the control of the horntail-borer. Last sea-

son both at Hanmer and on the Moutere Hills, the parasite was recovered in plantations where it had been previously liberated. The parasite is also a conspicuous insect and has a remarkable hair-like egg laying apparatus by means of which it bores through the bark depositing the eggs in the burrow of the horntail borer. Another enemy of pine trees is an aphis which forms white woolly masses ou the bark. A natural enemy of the aphis has been introduced and' liberated but as yet no statement can be made concerning its value under New Zealand conditions for the control of the aphis.