IN TOUCH WITH NATURE

Otago Witness, Issue 4033, 30 June 1931, Page 74

IN TOUCH WITH NATURE

THE GALL-MAKERS.

By J.

Drummond, F.L.S.. F.Z.S.

In spite of the fact that gall-making insects are very well known, their life histories, in many cases, having been disclosed by close .observation and patient experiments, naturalists still discuss the immediate cause of the formation of the unsightly excrescenses on plants, often indirect evidence that trees may wither a ?d . They result from the activities of cells in the plants under a stimulus, attributed to mechanical irritation caused by an insect laying an egg, or to chemical stimulation from fluid injected by a female when laying. The general belief now is that the grub, when newly born, or even as an embryo, sets up the irritation that causes the gall. Around the place where a female lays an egg a gall is formed. In it the grub, when it hatches from the egg, feeds and shelters, it is presumed that the production of galls supplies the wants of the young insect, with little damage to the plant.

About only three-fiftieths of an inch long and black in colour, the gall-chalcid is black death to blue-gum trees. It ranks as one of the most destructive ioiest tree pests in the Dominion, although it attacks no trees except the blue-gums. Dead or dying trees that stand gauntly on many New Zealand landscapes were successfully attacked by this insect, which is high -up on the list ot pestilent uninvited immigrants. It is present in most parts of the Dominion, particularly Canterbury, North Otago, and °f the North Island from Wellington to North Auckland. Its galls give the bark a rough appearance, much tougher than the scaly appearance caused b.v the gum-tree scale insect. All the damage by the gall-chalcid is done by .small white grubs. Each of them lives m a cell in the sap-wood close-Beneath the bark, which develops a pimply swelling. When many grubs are present, the intested twig or branch soon dies. The tree seems to realise its peril and to fight the insect that threatens its life. By throwing out new growth, it tries to recover, but its doom is sealed. The new 1S ? by the grubs from the next brood of adults to emerge and is c ’® sr °- ved ™ turn. The deadly attack is continued .in this way until the tree is killed

tlle eSce Ptionally capable entomologist at the Cawthron Institute Nelson, states that the perfect little miarv “thl® “ p P e ? r in January and Febi • fe . males msert their needlelike egg-laying instruments in young bark W th A* r eggs si ? gly > ot in Masses, but place them together. Fully grown, a grub that hatches from an egg transforms itself into a chrysalis in a cell b c ar x’- Through a pin hole in this the perfect insect escapes. Perfect insects may be seen in numbers on the bark of an infested tree. Entomologically. this destroyer is Rhicnopeltella eucaipti. Efiorts have been made to discover a parasite that will control it.

The stems of leaves of poplars sometimes have excrescences about an inch long, and shaped like miniature sacks or balls. On cutting these open, Dr Miller found that the walls were thick and tough. In each gall there was a mass of white woolly secretion exuded by wingless female aphides, present in the gafi. This gall-aphis has a somewhat complicated life-cycle. During-summer .nd until the end of autumn winged females appear amongst the wingless forms and escape through an opening in the gall. These aphides have black bodies and legs, yellowish abdomens, and grey wings. They are so small, about one-twentieth of an inch long, that a lens is necessary to realise that they are perfect insects, with limbs and organs complete.

Leaving their cosy little nursery, they migrate on to the leaves of rape, cabbage, mustard, wild turnips, and other cruciferous vegetables, and to allied weeds, where they give birth, to living wingless females. These move from the leaves to the roots of different kinds of plants, establishing there colonies of yellowish wingless forms. The subterranean colonies may be detected by patches of woolly secretion exuded by the aphides on the roots. Although they live on the roots of cruciferous crop-plants and weeds, they do no appreciable damage normally, but Dr Miller believes that if they were in great numbers much loss would result.

From the colonies on the roots, in the spring, there come winged females, similar to the winged females that develop from the popular-galls. These winged females return to the popular trees. There they give birth to living males and females, both sexes wingless. This is the only stage in the aphides life-cycle in which males appear. The females lay minute yellowish-white eggs in a crevice in the bark. Young wingless forms hatch out of the eggs, move on to the stems of the leaves, and make the galls from which the winged females escape in the summer and autumn, to migrate back to the cruciferous plants. This insect, commonly known as the popular-leaf stemgall, has an official name out of all proportion to its size—Pemphigus populitransversus.

On the leaves of elms near Auckland there was found for the first time in New Zealand a European aphis that attacks young leaves of the elm. They become deformed and form a pocket, in which an, aphis lies. The deformity grows and absorbs the whole leaf, which becomes an unsightly, crumpled yellowish mass, in which a colony of the insects live. In the Old. Country this gall-maker, Ediosoma ulmi, seems to be known as the currant root-louse. '

The Veronicas are more highly developed in New Zealand than in any other country. In many places they are a conspicuous part of the plant life. They are the largest group of flowering plants in the Dominion. Most of them, including those known by their Maori name, koro-

miko, are beautiful in form, foliage, and flower, especially Veronica speciosa, whose deep purple or crimson flowers are seen in many a New Zealand gardep. Contrasting with it are many Veronicas whose flowers are the purest white. Others have white flowers with a roseate or pale pink blusfi. There are flowers lilac-purple, pale lilac, pale pink, pale rose, white with purple lines, pale blue, lavender blue and purpleblue, but only one New Zealand species, winch grows on the Auckland and Campbell Islands and nowhere else, has flowers that are bright blue, true blue. To compensate for this lack of blue in the Dominion’s Veronicas, there grows abundantly in fields and waste places in many parts of New Zealand Veronica agrestis, the English speedwell, emblem of good wishes, whose salver-shaped flowers were described by Tennyson as “ the speedwell’s darling blue.”

Belonging to the same family as the Veronicas is a weed better known than any of them. This is the foxglove, whico favours bushburns, riverbeds, and slips on hillsides, but sometimes takes a fancy to gardens that are not diligently weeded. Twenty-five years ago Mr T. F. Cheeseman reported it as merely an occasional escape from gardens, not uncommon. Its powers of increase are shown by the fact that. 10 years later, it was listed as one of the worst Weeds in no fewer than 70 North Island districts. Its popular name seems to be a corruption of “folk's glove”-—that is.“fairies’ glove.” Botanically, it is Digitalis purpurea, the first word referring to the fingers of a glove. Dr F. W. Hilgendorf finds that the foxglove will kill itself out in from five to eight years, and that in the poorest localities where pulling or. cutting cannot be carried out the self-poisoning must be resorted to.

Another member of the., family, the flannel-leaf, shakes out its seeds into the backs of sheep that graze near the plants and ensures their distribution far and wide. As it occurs usually in poor land, it is not an important weed. In England it is known as the mullein, goldenrod, and Aaron’s rod.