IN TOUCH WITH NATURE

Otago Daily Times, Issue 22518, 12 March 1935, Page 2

IN TOUCH WITH NATURE

TORTOISE-BEETLE. • DEATH TO GUM TREES. By J. Drummond, F.L.S., P.Z.S. Eighteen years ago a Christchurch resident, spending an afternoon with Mr H. G. Ell, walked along the Summit road to Cooper's Knob, which looks down upon Lyttelton Harbour. They sat down there to rest. On the bare grey rock they saw a reddish insect about, half an inch long, its back arched to a height of three-six-teenths of an inch, distinctly convex. In general appearance it was like a, toy miniature tortoise. Although not brilliant, its peculiar shape and hue made it conspicuous. Taken to Christchurch in a matchbox, it was identified as a member of an Australian species of beetle, belonging to an enormous family called leafbeetles. Living on leaves, they do much damage.

The Australian species that has found its way to New Zealand is believed to feed on the leaves of eucalypts only. In Australia it is known popularly a s the eucalyptus-beetle, occasionally as the tor-toise-beetle. Its presence in New Zealand illustrates the way in which insects are spread far and wide by human agency. A female tortoise-beetle lays tiny egga in clusters on eucalypts. Eggs, doubtless, were unintentionally imported into New Zealand with timber or living plants. From some of the eggs, plump grubs were hatched. They fed on eucalyptus leaves, descended to the ground to be metamorphosed into the chrysalis stage, and climbed the trees again as adults, to continue their destructive work and to establish the species in the new land.

The species was in New Zealand before the individual was found at Cooper's Knob. How long before is not known. The important point is that it has made a place for itself in New Zealand's insect life. This summer, individuals were sent from Lyttelton, Timaru, Oamaru. and Palmerston South. The Selwyn Plantations Board complained that it suffered severely from the beetle's activities in its eucalyptus plantations in North Canterbury. A few years ago a report was received from Quail Island, in Lyttelton Harbour, that the beetle was not uncommon there. Individuals have been found in places where there are no eucalypts. Experiments might be conducted to discover if it turns its undesirable attention to other plants.

Observations have shown that some species of insects change from one foodplant to another. A species that feeds on any or many plants is polyphagous; a species like the tortoise-beetle, favouring only one food-plant, is monophagous. A species of insect may adopt a new foodplant. It may become sufficiently attached to it as partially or completely to abandon the original food-plant. This has occurred in captivity. It is concluded that the same change of habit may take place in natural conditions, especially if the original food-plant is scarce or is lacking. There are many records of insects adopting food-plants they previously were unknown to touch. Several changes in this respect have arisen in recent years. Grubs of a species of fly in North America lived on the blueberry, and, apparently, on no other plant until a few years ago, when, in north-eastern States, the species spread to apple trees.

A leaf-bug, belonging to a group that sucks plants juices and is very mischievous, makes willows the usual food-plant. Until 34 years ago this species was not/an apple pest in England. Many individuals of the species now prefer apple trees to willows. The apple pests have changed slightly in character from the original form of the species. They have advanced the term of life-cycle, from egg to the appearance of the perfect insect, by one month, to coincide with the new foodplant. The tortoise-beetle haa been reported tfrom only the South Island, and from only the east coast of that part of the Dominion. Another Australian lover of eucalypts, the eucalyptus leaf-weevil, or gum-tree weevil, had a good start of the tortoise-beetle in New Zealand. It was here 45 years ago. So congenial is New Zealand climate to this weevil that it flourishes on eucalypts wherever they grow. Like the tortoise-beetle, but unlike most wood-boring beetles and mischievous moths, it is destructive in both the grub stage and the adult stage. Sketching its life history in " Gardon Pests," Dr D. Miller, chief entomologist at the Cawthron Institute, begins with yelkfwish eggs packed in little, hard, black chests on eucalyptus leaves. Grubs of all weevils are legless. Eucalyptus leaf-weevil grubs, when they break out of their eggs, are yellowish, striped with a dark line on each side. They are studded with small, black dots, and have black heads. Fullygrown grubs are yellowish-green. Their skins are wrinkled, but they are plump and well-liking, and look like prosperous slugs.

As soon as they come out of the eggs, they begin to eat the leaves on which they were cradled, making furrows, which are enlarged into holes. Having attained full grubhood, and feeling the drowsiness that precedes the chrysalis stage, they eat no more, fall to the ground, dig in, and become chrysalids. From these, about six or eight weeks after the eggs are laid, emerge perfect adult leafweevils, varying in colour from tawny to brownish-black, all handsomely arrayed in a garment of yellowish-white or golden hairs. Adults eat the leaves and tender shoots, stunting the trees' growth. Adults eat voraciously. This leaf-feeder at least, apparently, does' not favour, a monotonous diet. Mr W. W. Smith reports that in Taranaki it occurs on several different sorts of plants. New Zealand is grateful to Australia for its eucalypts. Amonst them are some of the loftiest, the biggest, the most valuable trees, and the handsomest woods on the face of the earth. Devoid of members of the great group in its native plant-life, the Dominion now shares with the Commonwealth possession of the best of them. On their part, they find New Zealand a pleasant place. Here they grow quickly and vigorously. Australian insects in >iew Zealand create a very different feeling. New Zealand could have done well without them, taking them all in all. This may seem unfair to Austraian ladybirds that destroy scale-insects, but the most injurious scale-insects themselves came from Australia. Dr Miller computes that 71 per cent, of destructive species of insects in New Zealand are foreign and 29 per cent, native. Most of the foreigners, however they may have come directly, are of European origin. The most pronounced feature of the rest is an Australian element, and a large number of injurious Australian insects are associated with trees, almost exclusively eucalypts. The Australian gum-scale is looked upon by Dr Miller as one of the most spectacularly destructive scale-insects in New Zealand. Several species of eucalpyts are its common food-plants, but it is not averse to willows and apricot trees. The tortoise beetle also may have turned its attention to other plants than eucalypts. One evening recently, at the State Forest at Waiotapu, Mr W. G. Sanders caught a moth that hovered close to flowers. It did not alight on them, but sipped nectar from them by thrusting in its proboscis, about three inches long. It probed accurately into the nectary every time but did not stay at any flower for more than a second. The fore-wings were bronzy-grey, the head was grey, and the grev bodv was striped on the sides with white, black, and pink. Mr Sanders has observed insects for many years, .but this is the first hawk-moth hg has seen. The specie's has an interesting history in New Zealand. The family of hawk-moths is a brilliant assemblage, well represented m the Old Country, in Europe, Asia, Africa, Australia, and America, but poorly in New Zealand. Tt is doubtful if New Zealand has a single species of its own. Mr G. V. Hudson does not regard the species Mr Sanders saw as a member of the Dominion's original insect life, but as an immigrant that has established itself in this corner of its wide realm. It is fairly plentiful in North Auckland, is rarer in the south of the North Island, and is seen only occasionally in the South Island as far south as Hokitika and Christchurch. In the caterpillar stage it is big • and fat, bright green, with diagonal yellow lines on its sides, or yellowish-brown. A caterpillar always has a curved red horn that springs from the end of its body. This character marks hawk-moth caterpillars all over the world.