MANUKA BLIGHT SURVEY

New Zealand Journal of Agriculture, Volume 79, Issue 2, 15 August 1949, Page 101

MANUKA BLIGHT SURVEY

By

T. G. SEWELL,

Instructor in Agriculture, Department of Agriculture,

Christchurch.

DURING the past year considerable interest has been taken in what is commonly termed “manuka blight,” which has been associated with the death of manuka in Canterbury. Where the blight occurs dead and dying plants covered with a sooty-like growth are to be seen. Because of the numerous inquiries concerning the dispersal of blight ‘ as a means of controlling manuka on hill-country areas, the Department of Agriculture in conjunction with the Entomology Division of the Department of Scientific and Industrial Research undertook a survey of the distribution and nature of manuka blight. The author made a survey of areas of manuka in both the North and South Islands and entomologists studied the blight and checked the identification of specimens forwarded.

A BLIGHT is described as any disease affecting plants, and the fact that manuka trees are dying suggests a blight. However, the death of manuka trees is associated with a sooty mould which has been called manuka blight. The so-called blight on the manuka is in fact a fungus growth which develops on an excretion from a species of scale insect or mealy bug belonging to the genus Eriococcus. The first observations on scale insects in New Zealand were published in 1887 by Maskell (1), who recorded the taxonomy and habits of scale insects and their association with sooty mould. He stated that the New Zealand climate was particularly congenial to scale insects because of the humidity and absence of anything like severe winters. Maskell recorded 6 species of scale insects on — Mytilaspis leptospermi, Inglisia leptospermi, I. ornate,, Ctenochiton flavus, Planchonia epacridis, and Coelostomidia wairoensis, but not the species of Eriococcus which is now assocated with manuka blight in

Canterbury. He pointed out that these insects have 4 stages of development: The egg, first and second larval stages, and the adult stage, in which the insect, does not change its position. The mature insect lays eggs inside its mealy scale or protective covering. When the larvae emerge they are active in locating crevices in or under the bark or on the undersides of leaves of the plant and, having found a suitable position, they begin feeding. They make a slight indentation in the bark and in addition form a protective mealy covering or scale; it is from the latter that this class of insects derives the name “scale” or “mealy bug.” When feeding, the scale excretes from a tube-like, protuberance a clear, sticky solution or honey-dew, which drops on to the lower leaves or branches of the plant. As soon as the honey-dew is produced spores of a native fungus germinate in it and grow, producing the characteristic black appearance with which the insect is always . associated.

Maskell mentioned a “black blight,” which he stated was often referred to as a destructive pest of a number of plants, , but pointed out that there were two possible ways in which the effects of the blight might be produced. First, the activities of the scale insects might drain the “life blood” of the plant; second, the honey-dew and fungus might choke the breathing pores or stomata of the plant and so tend to interfere with growth. In discussing scale insects generally Maskell considered that birds did not favour the insects, and the usual position of the insects on the underside of the leaves or in the crevices of the bark was a great shelter and protection for them . against birds. He. states: — ~ . . . They are in many cases effectively covered by waxy -or fibrous shields or by the masses of cotton with which they surround themselves. In the greater part of New Zealand the winters do not seem to be sufficiently severe to injure . coccids [scale insects] and many of them breed as much in winter as they do in summer. It would therefore seem that everything combines to assist these insects in their career and in their propagation. . Since 1935 and in particular from 1946 onward considerable interest has been taken in the prevalence of blight on manuka. It is stated to have been spreading during the past 5 to 6 years. The species of the genus Eriococcus responsible for damage to and death of manuka occurs also on kanuka, but only the former is susceptible and dies. An article on manuka blight appearing in 1946 in a New Zealand paper (2) having a wide circulation and further articles on the subject in other periodicals during 1948 (3) promoted considerable interest. Opinion has

been divided on the question of whether definite action should be taken to spread the blight. The article published in 1946 stated that “. . . it is hard to find a live manuka bush or tree in South Canterbury.” The article went on to say that to sportsmen manuka scrub is the home of wild pigs, deer, and pheasants, but to farmers it is a pest and a continual drain on farm expenditure because of the cost of controlling it and the loss of grazing involved. Following the recent publicity, many opinions have been expressed concerning manuka blight. Some suggest that it would be a panacea for hill-country farmers; others state that, because manuka is a cover crop for regeneration of native bush and for forestry purposes, it should not be destroyed. Again, manuka grows on sour soils and on much reverting hill country. When it is removed the land is subject to erosion or invasion by other weeds. Further, in places manuka provides one of the main sources of firewood for the domestic, requirements of neighbouring towns.

, The whole question of the eradication or preservation of manuka is complex. To farmers, particularly those on second- or thirdclass hill country, it is a nuisance and a source of financial worry and as such should be destroyed in the cheapest way possible, but the soil-erosion, weed-invasion, and firewood-supply aspects suggest that caution should be observed when considering the deliberate spreading of blight for eradicating manuka.

Though all manuka scrub is similar in general appearance, close examination shows that ...two species of the genus Leptospermum occur: Manuka or red manuka (.Leptospermum scoparium) and kanuka or white manuka (Leptospermum ericoides).

Both species vary in size from short, whippy saplings .to trees, but the following descriptions. set out their broad botanical differences: Manuka has a varied growth form, from stunted bushes 3 to 4ft. high to frees of 20 to 25ft. high. It can be distinguished most easily by the seed capsules, which are about Jin. to l/3in. j n diameter, and by the leaves, which have sharp points, so that the foliage is &

harsh to the touch. It should be noted that the term “red” refers to the wood, not to the colour of the flowers, as both red and white flowers occur within the species.

Kanuka often grows into trees of up to 20 to 60ft. high. It has a small elongated capsule of l/16in to -gin. in diameter; the leaves are not sharply pointed and are soft to the touch. The leaves are usually narrower than those of manuka.

Broadly, manuka is more widespread than kanuka and occurs at higher altitudes or where the soil is infertile, sour, and wet or where the country is swept by strong winds. Kanuka favours the more-fertile, sheltered, warmer areas of lower altitude. Thus in the coastal areas of North Auckland, the Bay of Plenty, and Gisborne kanuka tends to be the dominant species, but in the Central Plateau, in the back country of Taranaki, and in inland Hawke’s Bay manuka is dominant. In the South Island the position is similar. Kanuka tends to be dominant in the warmer areas of coastal Nelson and Marlborough and in sheltered areas further south such as at Peel Forest, while manuka is commoner at higher altitudes, such as in the Awatere River Valley or in colder areas such as the foothills of the Southern Alps in Canterbury and Otago and on infertile or swamp country in Central Otago and Southland.

Types of Scale Insects Found

During the survey to ascertain the distribution of manuka blight 5 species of scale insects. were found on both manuka and kanuka; they were a species of Eriococcus, Coelostomidia wairoensis, Ctenochiton flavus, Inglisia leptospermi, and I. ornata. The first 2 are soft scales dr mealy bugs and the last 3 . hard scales (because of their tough coatings).

The insect of the Eriococcus genus associated with the death of manuka is a. mealy bug which forms tiny white mealy scales up to l/32in. long and l/64in. wide. The insect itself is pinkish and the mature female, which is sluggish or non-motile, remains in the mealy scale. The life history and depredations of this insect are being studied by J. Kelsey and J. M. Hoy, of the Entomology Division, Ashburton, who state (4): . . . the life cycle of the insects is not known for certain and it is possible that there may be more'than - one generation each year. Eggs are ' present from March to July. They are laid in clusters by the parent while still beneath the protective covering; the parent then dies. The small active insects emerging from the eggs are known as “crawlers” and this is the dispersal stage to new areas. Since they are wingless they must rely for dispersal on their 3 pairs of legs. On finding areas where bark is relatively thin on young manuka growth the crawlers squeeze beneath the flaky bark, insert their rostrums or sucking mouth parts into the sap stream, and commence to feed. Once this position is taken up they apparently only move if forced to do so by removal of the bark or their own protective cases.

The term mealy bug is derived from the waxy covering possessed by these insects and in some species this becomes a thick shell which forms an effective protection to the insect beneath. The male is the only winged form among the mealy bug group .of insects and it is the fact that females are wingless that prevents more rapid spread of the blight. During feeding the mealy bugs secrete a clear, sweet, sticky liquid known as honey-dew which falls on to leaves and twigs below feeding points. On the honey-dew a black sooty mould grows and it is this that gives the characteristic black appearance to infected trees and shrubs. On infected plants in the field Eriococcus insects can be seen, most easily on the underside of twigs where there is little accumulation of honeydew and mould. The other scale insects found on manuka and kanuka had no apparent effect on plant growth. The hard scales Ctenochiton flavus, Inglisia leptospermi, and I. ornata were sparsely distributed, occurring in limited colonies on the west coast of the South Island and in the North Island. The other soft scale, Coelostomidia wairoensis, has a wide distribution and occurs principally on kanuka and to a less extent on manuka in the warmer, sheltered, and humid areas in the North Island and in isolated areas in the South Island as far south as Peel Forest. It is a larger insect than Eriococcus and forms mealy scales up to jin. or l/16in. long and about l/16in. to jin. wide. The insect inside the mealy scale is red and forms a long filamentous protuberance which projects through the sooty mould. This mould is like fine moss and differs from that associated with Eriococcus, which is shapeless and to the casual observer resembles black flour. Very often masses of “cotton,” which are the male cocoons of Coelostomidia wairoensis, can be found under the loose bark of the trunks of trees., C. ‘ wairoensis should not be confused with Eriococcus. Though quantities of 'sooty mould festoon the branches and twigs of trees infested with C. wairoensis and make them unsightly, they appear to grow normally and no widespread dying occurs.

Sooty Mould on Other Plants

The characteristic blackening of the leaves, twigs, and branches of plants by sooty mould is not confined to manuka and kanuka. It has been recorded on many native trees and shrubs, including mingimingi and karamu (Coprosma spp.), matipo (Pittosporum spp.), pohuehue (Muehlenbeckia spp.), mountain beech (Nothofagus cliffortioides) , rangiora achy glottis spp.), tutu (Coriaria spp.), and tauhinu (Cassinia spp.). Though in many cases the mould is unsightly, no widespread . mortality among trees and shrubs other than manuka has been recorded.

Distribution of Blight

Between June and September, 1948, the author made a survey through areas infested with manuka in the North and South Islands to ascertain the distribution of blight and its effect on plant growth. The accompanying

map indicates the distribution of Eriococcus in Canterbury.

Eriococcus appears to be confined to Canterbury. It was located in the foothills from Fairlie to the Okuku Range, at Amberley, on the Canterbury Plains on the north bank of the Waimakariri River from Kaiapoi to Burnt Hill, on rolling downs behind Annat, and at Te Pirita, Maronan, and Westerfield. Further centres of infestation have been reported from Waihaorunga (to the west of Waimate), from Cheviot, and from Kai tuna Valley on Banks Peninsula. At Mt. Somers Eriococcus occurred at 1550 ft., the highest altitude recorded.

Almost wherever manuka occurs between Fairlie and the Okuku Range below an altitude of 1500 ft. and in other areas already mentioned dead and dying plants infested with Eriococcus are to be seen.

The occurrence of Eriococcus does not seem to be related to particular soil or climatic conditions, since infestations were found on light, stony, windswept • areas at Te Pirita and Maronan and on heavy, wet, clay land of the foothills at Whitecliffs.

Trees up to 10ft. high have been killed, but the most susceptible plants are those 3ft. high. The exact cause of the death of the manuka is not known, though many suggestions have been made. Investigations are being carried out by the Entomology Division.

Eriococcus also attacks kanuka, but it was noticed that in only a few instances had plants died. In most cases some kanuka bushes were , growing in association with manuka and these were found as living but blackened trees amid dead and dying manuka. No Eriococcus was found in the North Island. C. wairoensis occurred' on ' kanuka and to a less extent on manuka in sheltered areas in North Auckland, the Bay of Plenty, Gisborne, Rotorua, and south-western Wairarapa. Isolated infestations occurred in the South Island in Nelson, Marlborough, and at Peel Forest. Practically all trees infested with C. wairoensis appeared to be growing quite normally; in two places only had infested manuka and kanuka trees died —at Tolaga Bay and south of Whakatane. In each instance only two trees had died, which was of no significance, as adjacent infested trees appeared to be growing normally. Dispersal of Blight Several farmers who had endeavoured to establish Eriococcus with a view to eradicating manuka on their properties were interviewed in the course of the survey. In none of the places inspected had it spread from introduced infested material. In one instance only, according to report, introduction apparently had been sue-

cessful, though this will have to be confirmed by the identification of infected material. The staff of the Entomology Division at Ashburton state that there are two methods of transference for the introduction of Eriococcus (4). “The period over which transfer can be carried out most effectively is during the ‘crawler’ stage of mealy bug development,” says their report. “This stage is present in Canterbury from May to September. During this period transfer may be brought about by sending closelywrapped . parcels of the infested branches and on arrival placing these among branches of uninfested manuka or by sticking infested twigs into ground at the base of and in contact with the trees. The other method that could be adopted at any time of the year is to shift whole infested shrubs with a good ball of soil round the roots. In this method it is necessary to ensure that the shrubs do not receive a setback either during transplanting or after replanting. If the trees wither, the chances of the mealy bug establishing are extremely remote.”

Conclusions ; Manuka is attacked and killed in all stages of growth by Eriococcus, and since seedling growth is killed, the scale may be a possible means of eradicating manuka. . The life history of the insect is being studied at Ashburton, but asyet knowledge of it is incomplete. Until the life history and habitat of Eriococcus are known and introduction trials in new areas are conducted, recommendations concerning the effectiveness of the insect as a means of controlling manuka in other areas cannot be given. However, throughout the areas of manuka in Canterbury already infested Eriococcus appears to kill manuka. Acknowledgments

The writer wishes to express his thanks to J. Kelsey and J. M. Hoy, of the Entomology Division of the Department of Scientific and Industrial Research, Ashburton, for the identification of specimens and for information concerning the life history of Eriococcus. References 1. W. M. Maskell, 1887 — Account of New Zealand Scale Insects,” Government Printer, Wellington. 2. “New Zealand Free Lance,” July 10, 1946. 3. “The Weekly News,” June 9, 1948, and correspondents in subsequent issues. 4. J. Kelsey, 1948. (Private communication.)

New Contract Prices for Meat, Butter, and Cheese

AN increase of 7| per cent, on present contract prices for meat supplied to the United Kingdom Government was announced recently by the Minister of Marketing, Mr. E. L. Cullen.

Mr. Cullen said that there’ had been negotiations between the United Kingdom Ministry of Food and Mr. F. P. Walsh, representing the New Zealand Government, and Mr. J. D. . Ormond, representing the New Zealand Meat Producers’ Board. The long-term contract entered into last year between the two Governments provided for annual discussions on price and for price adjustments not exceeding 7 J per cent, upward or downward in any one year.

“It is very gratifying indeed,” said Mr. Cullen, “that it has been possible to reach agreement on prices for the coming season at the maximum level allowed by the contract. The longterm contract has given New Zealand farmers an assurance that they can plan for increased meat production with the knowledge of a secure market. This latest increase in prices will, I am sure, be an added stimulus to farmers.to produce the additional meat which the United Kingdom needs urgently. The Government’s recent announcement regarding the fertiliser subsidy shows that it stands behind the farmers in their drive to increase production.” He added that he was pleased to learn that the . Dairy Products Marketing Commission had also negotiated with the Ministry of Food an increase of 7g per ’cent, in the butter price and an equivalent increase in the cheese price, and had also reached agreement with the Ministry concerning long-term contracts for the sale and purchase of the Dominion’s exportable • surplus of milk powder.

The dairy produce prices agreed upon are 2525. 6d. sterling a cwt. f.o.b. for butter and 141 s. 6d. sterling a cwt. f.o.b. for cheese. The present prices are 2355. sterling a cwt. f.o.b. for butter and 1335. sterling a cwt. for cheese, and the new prices therefore represent an increase of 17s. 6d. sterling a cwt. for butter and Bs. 6d. sterling a cwt. for cheese. Other terms and conditions under the long-term contract remain unchanged.

Journal of the British Grassland Society

The Journal has been instituted as a medium for the publication of the results of research and practical experience in the realm of grassland husbandry. \ ' CONTENTS OF VOL. 3, No. 4, 1948: 1. Pasture herbage as. a causal factor in animal diseases: A Review. W. R. Muir. 2. Studies on the biochemistry of pasture plants. 2. The pharmocological properties of certain pasture plant juices and their possible significance in the aetiology of some disorders of the grazing animal. E. T. Rees Evans and W. Charles Evans. 3. An experiment to test the influence of date and depth of sowing on four of the chief grasses and on white clover. Sir R. George Stapledon and Dorothy E. Wheeler. 4. Grassland in the East of Scotland. R. G. Heddle. • 5. The control of rushes on newly reseeded land in Yorkshire. H. I. Moore and S. Burr. 6. British Grassland Present Problems.. 7. Research and Problems Overseas. ' , 8. Reports and Notices of Meetings. 9. Book Reviews. This Journal is published quarterly. Subscription: Vol. 1,10 s.; annual subscription thereafter 20s. Orders to Secretary, British Grassland Society, Agricultural Research Building, Penglais, Aberystwyth, Wales.