[Read before the Auckland Institute, 4th October, 1927; received by Editor, 5th June, 1998; issued separately, 30th August, 1928.]

Plate 66.

Manuka manna is a white, finely-crystalline, soluble substance occasionally found on Leptospermum scoparium in dry weather in the middle of summer. The cause of its occurrence has apparently not been investigated, nor has its chemical nature been fully determined. Preliminary examination of a small amount collected during the past few years has indicated that, like other mannas, it is a complex sugar. On hydrolysis, it yields only glucose. Further work on its composition is in process, but a larger amount of manna than at present available is required for complete investigation. The difficulty of obtaining the manna is due not only to its rare occurrence, but also to the fact that it is washed away by a slight shower of rain.

After spending considerable time looking for manuka manna without success, I was surprised to find in my own garden a young manuka (L. scoparium with double white blossoms) with many of the leaves in the middle and lower part of the shrub covered with manna. The mode of occurrence is shown in the accompanying figure.

No damage to the leaves could be detected, but on branches directly above the affected leaves numerous white plant-hoppers were observed, apparently feeding on the bark. Some of these were isolated and kept under observation until metamorphosis had occurred. The adult insect was identified by Mr. David Miller, Government Entomologist as Scolypopa australis. Neighbouring plants, including Phormium tenax and a large-leafed privet, were infested with the nymph and adult forms of S. australis, but no manna was present. The leaves of the privet and other shrubs were, however, covered with honey-dew, known to be produced by sap-sucking insects.

The following summer there was no occurrence of manna on the above manuka shrub, and no nymphs or adults of S. australis were observed, though they were abundant on neighbouring shrubs and produced a considerable amount of honey-dew.

Large areas of manuka on the Thames-Coromandel coast were examined, but the only occurrence of manna observed was on a very young tree in the grounds of a house I was renting at Thornton Bay. As in the previous occurrence, numerous nymphs of S. australis were observed on the branchlets above the deposits

of manna. There appears to be little doubt that the manna was produced by S. australis in the nymph form.

The nymphs apparently require for their rapid development and metamorphosis only part of the constituents of the sap they consume, excreting or rejecting the greater part of the carbohydrate. In the case of manuka-sap diet, the excreted carbohydrate material crystallizes to a white solid, but when the food is derived from other trees, the carbohydrate remains on the leaves as a viscous covering of so-called honey-dew.

Manuka manna is sometimes formed in larger masses not distributed over the leaves, being due apparently to injury caused by a boring animal. It is possible that other sap-sucking insects besides S. australis may produce manuka manna, and it would be interesting to ascertain whether the manna produced by different insects has the same composition. It is also desirable to ascertain whether the honey-dew produced by the same insect on different trees differs in composition.

I should be glad of supplies of manuka manna and information of its mode of occurrence. I also wish to express my indebtedness to Mr. David Miller.