Probable Effects of Manuka Blight on Beekeeping in North Auckland

New Zealand Journal of Agriculture, Volume 95, Issue 3, 16 September 1957, Page 279

Probable Effects of Manuka Blight on Beekeeping in North Auckland

By

D. ROBERTS,

Apiary Instructor, Department of Agriculture, Auckland SINCE first noted in the Canterbury district in 1937 the scale insect friococcus scoparium has by natural spread and artificial distribution become widely established throughout the country. The insect is found associated with both red manuka (Leptospermum scoparium) and white manuka or kanuka [Leptospermum ericoides). After infestation by the insect, plants of both species develop a heavy coating of black fungus. The condition of infestation by both insect and fungus, commonly called manuka blight, though not destroying kanuka has proved lethal to manuka, and its use for biological control of this species has become extremely widespread in North Auckland. This article discusses the importance of manuka and other nectar sources to North Auckland beekeepers and adjustments in apiary management that may be necessary when the main source of nectar disappears as a result of manuka blight.

MANUKA blight, though only of comparatively recent introduction to North Auckland, has spread rapidly in most districts and today gives the impression that it will eliminate the manuka within a few years. In the northern part of Hokianga County a great proportion of the manuka has been destroyed already and in most other counties large areas are either

dead or dying. Because the black fungus which appears on infected manuka plants is often found on other species growing in association with manuka, there have been many reports of the blight affecting other plants. There is to date no direct evidence that the presence of this fungus on other species is due to other than distribution by wind and rain. The

coccid insect responsible for me blight is a highly specialised feeder and it seems , unlikely that any adaptation to other plants, as a source of sustenance, will occur. Increased Land Development Apart from the destruction of manuka by blight, the tremendous increase of land development, both by the State and private enterprise, which has taken place in the post-war years has contributed also to the rapid diminution of manuka. This factor in itself must be of increasing consequence to beekeepers, as it is the more readily accessible and workable areas that are being so rapidly developed. Had the blight not been introduced, it is likely that sufficient

manuka to meet the needs of established apiarists would have remained in the less accessible and broken areas for many years. It is in these areas, however, that the blight has been introduced and has spread unchecked.

Biological control of manuka by blight undoubtedly offers great advantages where immediate development of the land into pasture is undertaken on the death of the manuka, but where this is not done or is impracticable because of the contour of the land, infestation by more undesirable plants and weeds is occurring.

Unfortunately it seems that these secondary infestations will be of plants unlikely to be of economic importance to beekeeping and the existence of a healthy, vigorous beekeeping industry is important to the economic welfare of agriculturists and pastoralists. Intensive land development and the widespread use of modern insecticides have so depleted the numbers of wild pollinating insects that the honey bee is today the only insect of consequence in providing the pollination so essential to the reproduction of clovers and related pasture plants. Manuka as Source of Nectar A vigorous beekeeping industry cannot exist in any area unless that area supports plants yielding nectar sufficient to return to the beekeepers an adequate living from the sale of honey. In the past the honey crops of North Auckland have generally been gathered from manuka and some have been secured wholly from, this source. Hive management and the choice of apiary sites, with few exceptions, have developed around the manuka honey flow. Because of the quality of thixotropicity inherent in manuka honey much time and thought have been devoted to the improvement and utilisation of honey house equipment designed to handle this particular type of honey and over the years a very high degree of efficiency has been attained which has enabled the production of a honey of a very high standard.

Though it may be thought that with the elimination of manuka and the progress of extensive land development sufficient nectar from pasture plants will become available to beekeepers to provide a payable alternative, the presence of nectar-yielding plants does not automatically guarantee a honey crop.

In assessing the possibilities of honey production from the clovers, which are the mainstay of beekeepers in many parts of New Zealand, it must be borne in mind that these plants produce nectar attractive to bees only when soil conditions, temperature, and relative humidity are favourable.

North Auckland is predominantly a dairying area and its development is based on the establishment of permanent pastures and though in the laying down of pastures clover-ryegrass

seed mixtures are commonly sown, a proportion of paspalum is included in most seedings. Though the clovers may be dominant for the first 2 or 3 years after sowing, paspalum establishment is very rapid, with the result that most North Auckland pastures contain a very strong growth of this grass from early summer onward. This vigorous growth of paspalum overgrows the clover at the period when it is most likely to yield nectar.

The high average relative humidity generally experienced throughout the area is another factor militating against good clover honey production. High relative humidity, though not perhaps affecting the amount of nectar secreted by clover, does seriously influence its sugar concentration. The sugar concentration of the nectar under conditions of high relative humidity is often so low that the nectar loses all appeal to honey bees.

Most exotic nectar-yielding trees and plants common to the area are similarly affected. Indigenous trees and plants are also affected, but to a much less degree. Pohutukawa is one common indigenous plant that is even more affected than most exotics, the nectar, which is generally secreted prolifically, being attractive to bees only in warm, dry, windless conditions.

It is apparent that under prevalent farming practice and weather peculiar to the area the clovers alone cannot be relied on to provide a reasonably economic return to beekeepers in North Auckland. The trefoils, buttercups, and various other less common plants yield nectar well in most seasons, but under present management systems the crops from these are as a rule only supplementary. Other Sources of Nectar From its inception up to the present beekeeping in North Auckland has been based mainly on the nectar yielded by trees and shrubs. With the felling of the forests and the initiation of large-scale land development the incidence of many indigenous nectar-producing trees and shrubs has been reduced 1 so greatly that today only manuka and kanuka are sufficiently widespread to be of interest to all beekeepers.

Though it is not generally accepted by beekeepers that kanuka yields nectar consistently, it is known to be worked by bees for nectar occasionally. When growing in association with manuka it does not seem to offer an attraction sufficient to overcome the counter-attraction of the rich manuka nectar, but in the absence of this competition it may possibly provide a substantial source.

Rewarewa, cabbage tree, bush lawyer, and various other nectar-pro-

ducing trees and shrubs are present in bush and scrublands; but with the exception of the few remaining forest areas they are nowhere in sufficient quantity to be of more than secondary importance. Two exotic shrubs, a legume Psoralea pinnata, locally called blue pine, and a member of the Erica family known as pink heath (Erica baccans) have become established in certain limited areas. Blue pine, found mainly in isolated stands, yields nectar prolifically and is the source of a fine, white, mildflavoured honey. Pink heath is confined to one area on the coastal lands west of Dargaville. It, too, yields nectar prolifically and crops of a dark, rather aromatic-flavoured honey can be secured from apiaries located within the larger stands of this shrub. Neither of these shrubs is present in sufficient quantity, however, to be of widespread importance and they are of value only to beekeepers situated within the limited areas of their establishment.

Pohutukawa (Metrosideros tomentosa) is common in coastal areas, but it is generally much more prevalent on the eastern side of the peninsula. Pohutukawa yields nectar very heavily indeed, but because of its particular susceptibility to the effects of high relative humidity, temperature changes, and wind, it is not a reliable source of honey. The sugar concentration of the nectar is so readily depressed by these factors that the secur-

ing of good crops from this source is the exception rather than the rule. Hive Management The systems of hive management in North Auckland have largely developed around the manuka honey flow. Large crops from this source have been secured under the present type of management, but it seems likely much of the later pasture honeys are lost. Bees brought to strength at a time which will enable them to gather a good crop of manuka honey seldom seem able to take the best advantage of subsequent flows unless the hives are moved some distance to. a new site, as occurs in migratory beekeeping practised in other parts of the world.

The manuka honey flow, in varying degrees of intensity, often lasts for some months. Colonies that have built up strength and gathered a crop on it . seem to lose the urge to work intensively and fail to exploit subsequent sources to any great degree. There is no definite reason for this behaviour, but it seems that a combination of the length of the manuka flow, the very high sugar content of the manuka nectar, and its ready accessibility is responsible. Bees working manuka often will be found to neglect and fly over other sources yielding nectar of quite high sugar concentration.

It has been, found that colonies in manuka areas which are slow in building up and fail to reach gathering strength in time for the manuka flow often secure much better crops of the later pasture honeys than those that reach full strength and gather a surplus on manuka.

It is evident that the best results from any one nectar source can be secured only by hive management directed to that particular source with colony development controlled to that end. With the almost total disappearance of manuka in North Auckland, a likely possibility in the near future, and the uncertainty of securing payable yields from other tree sources, it is apparent that if beekeeping is to remain an economic proposition in the area, a widespread adaptation to changing circumstances will be needed.

Beekeepers who have apiaries situated within economic distance of the few remaining larger areas of native forest will no doubt continue to secure payable crops with present practices. To ensure that they continue successfully in commercial beekeeping others less fortunately situated and where manuka is fast disappearing should make a close study of alternative nectar sources with a view, if necessary, to the readjustment of apiary sites and to the developing of appropriate management techniques.