THE GALL CHALCID OF BLUE-GUM.

New Zealand Journal of Agriculture, Volume XXIII, Issue 5, 21 November 1921, Page 282

THE GALL CHALCID OF BLUE-GUM.

A PRELIMINARY NOTE.

DAVID MILLER,

F.E.S., Entomologist, Biology Section.

A conspicuous feature of many parts of the countryside is . the dead or dying blue-gum (Eucalyptus globulus). The injury is commonly due to a gall insect, which Dr. L. O. Howard, Chief of the United States Bureau of Entomology, to whom specimens were sent, has kindly identified, and is describing as a new species of chalcid of the genus Rhicnopeltella. This insect occurs in most parts of New Zealand, but, as far as observations to date show, is most prevalent in the west-coast districts of the North Island, its depredations extending from Wellington to as far north as Awanui in the North Auckland Peninsula. In the South Island severe infestations occur in the east-coast districts throughout Canterbury to as far south as Oamaru.

The adult is a minute black insect which emerges during the summer months, when it may be found in large numbers on the bark of infested trees. Each of the small white grubs which cause all the damage lives in a cell on the sap-wood immediately beneath the bark, causing the latter to develop abnormally into a pimple-like swelling on the surface. When present in large numbers the bark assumes a gnarled appearance , (Fig. 3), and the infested twig or branch soon dies. The tree endeavours to recover by throwing out new growth, but this is attacked by the grubs from the next brood of adults to emerge, and so on until the tree is completely killed. When fully grown the larva transforms to the pupa in the cell beneath the bark, from which the adult eventually esc£p>es through a minute hole.

The 'complete life-history of this insect is being worked out, and an endeavour is being made to locate some parasite by which the gall chalcid might be controlled. Inquiries made in Tasmania and Australia, whence the insect probably comes,.show that so far it has not been recorded from those countries. - It is therefore hoped that it does exist there, since, confining itself solely to Eucalyptus, globulus, it may be so limited by the agency of some natural enemy as to be very inconspicuous, its presence on slightly infested trees being readily overlooked.

The gnarled appearance of the bark of blue-gums caused by infestation of the gall chalcid has been frequently mistaken for the gum-tree scale insect Eriococcus coriaceous Mask., but a comparison of Figs. 3 and 4 will show the distinction.