10

he may have to pay more for than he really careful field investigations of similar stands— likes. the prospective yield of the particular forest Climatic conditions are often given little dealt with. consideration in some working plans, because When using the term yield in the usual some people don't care to read about what sense of the word, the yield in volume—and kind of weather they are having in that parti­ as a rule in cubic feet—is implied. cular locality, but only the amount of timber Quite often however an attempt is made obtainable. When the time sheet comes round to convert this future volume yield into an however, and the loss of time owing to wet equivalent money value. Such a conversion weather is shown, the influence of the climate must necessarily be based on so many assumed on the purse-strings becomes only too evident. data, however that the value of the result is Having arrived at a fair inventory with doubtful. sufficient details to carry one past the inquisi­ The main factor which makes the pre­ tive contractor and a careful accountant, the diction of money yield from a forest difficult, second part of the forest working plan, which is the steady decrease of the buying power of is also the main part of the working plan, money, with a consequent apparent increase is the description of the works and improve­ in the value of most commodities. ments which are suggested in order to main­ Again, the demand for the produce from tain the area on an economic basis. the forest will influence the value in the In this part of the working plan, details future beyond calculations. Special local con­ are again necessary, and the suggestion of an ditions may create an abnormal demand, with operation or an improvement without specify­ high prices for the produce, or the reverse ing the estimated time it will take with a may be the case owing to a glut in that parti­ certain amount of labour, and the cost of the cular locality. various stages of the work, together with the The general deforestation carried on in cost of necessary equipment, will kill the pro­ most parts of the world, may enhance the posal at once. value of wood beyond present day expecta­ We all like to see an article priced when tions, while on the other hand it is possible to exhibited in a shop window. If the price suits imagine that some fungus or insect attacking us we may buy that article, if the price does wood may become so common and so numer­ not suit us we don't buy, and it is the same ous as to practically make the use of wood with the Boss. He wants to know the price, impossible. and what is more, he wants to know definitely It seems safe to assume however, that that the cost given will not be exceeded. wood will always hold its own place in the This means, that the compilation of a list of necessities in the world—not perhaps forest working plan without actual practical in its natural state, but more and more in experience in the various operations calls for manufactured products such as fibre board, the utmost care and the repeated verification and the multitude of derived products, such of the cost data collected from various sources. as paper, celluloid, silk, explosives, alcohol, sugar, adhesives, lacquer, paint, non-splinter With experience, one will naturally ac­ glass, aeroplane dope, etc., etc. quire an amount of private cost data which will be used for the purposes mentioned and Keeping the depreciation of the value which will be a direct reflex of the ability of of money in mind, it is safe to estimate the the collector of the facts. future money yield on an increasing scale as regards the value of the forest produce—up The last stage of the working plan, to a certain point, when the time arrives when namely the estimate or prediction of the timber will be considered as much a crop as future yield, is probably the part which is any other produce of the soil, and the supply most often abused. and demand for wood will be balanced by the Statements within a certain reasonable management of the world's forests according limit as regards yields from a forest, cannot to carefully prepared forest working plans. well be disputed by the average man, as he lacks the necessary data on which to carry . an argument. With the compilation of an THE INFESTATION OF increased number of yield tables however, the prediction of yield becomes more and more SIREX JUVENCUS liable to questioning, and no statements should IN CANTERBURY be made before all the factors affecting the (A. F. Clark) yield have been taken into consideration. Introduction When on the other hand the question of The establishment of exotic plantations yield is thoroughly understood, it becomes an in the Province of Canterbury dates back in easy matter for a technician to predict—after many cases to the 'eighties or even earlier. As ll soon as the early settlers found that they large specimens of the female will measure were unable to secure the reproduction of the 1 inch to, 1^ inches; usually smaller sizes are native trees for timber purposes, many, with found, however; the male in most cases is highly commendable foresight, commenced smaller. The abdomen terminates in a spine the planting of introduced species; again, the which, in the male is short and triangular but protecting of farm lands from the high parch­ which is longer and lanceolate in the female. ing winds was the motive that actuated many, This appendage, in the female, forms an ex­ while the aesthetic value of homestead plan­ ceedingly strong serviceable ovipositor and is tations was the object in other cases. The the source from which the insect derives its result is that the province possesses exotic common name. When at rest the ovipositor plantations to the extent of some 32,000 acres, projects in a horizontal plane giving the ap­ which are being added to yearly. A forest pearance of a powerful sting. The wings are asset is thus being built up the value of which long and narrow with distinct venation; the is enormous, and which is almost irreplace­ mouthparts are strong and well formed for able. biting. It was therefore with no little .consterna­ Life History of Sirex tion that the public received the news that the The development of Sirex juvencus is plantations were seriously affected by an in­ holometabolic, that is, the insect has complete sect infestation which threatened to encom­ metamorphosis. The insect goes through four pass their destruction. Conflicting reports ap­ phases of development commencing with the pearing in the daily press contributed in no egg stage, to the larval stage, to the pupal small way to the general feeling of uneasiness. stage, and finally emerging as an adult, when Most of these reports appear to have been it has the characters mentioned in the pre­ based upon the flimsiest of evidence and are vious paragraph. to be strongly deprecated. It was in order to The eggs are whitish with a hard shell or arrive at the true position and to draw some chorion which encloses the embryo. The em­ conclusions therefrom that this investigation bryo is supplied with an amount of food was conducted. material in the form of yolk or vitellus from Systematic Description of the Pest which it obtains the nourishment necessary The insect has been variously described for its full development. On reaching matur­ under the names of 'The Giant Horntail/' the ity the small larva emerges and commences "Steel Blue Saw Fly" and the "Wood Wasp." feeding. The grubs are whitish or yellowish The first step undertaken was therefore the in appearance with an upturned spine at the securing of the insect and its positive identi­ end of the abdomen. They are equipped with fication. The plantation at Bottle Lake was strong biting mouthparts with which they bite visited and here, after some searching, an their way through the woody tissue of the infested tree was found, the tree was felled tree. The digestive system extracts the nutri­ and split open and specimens of the adult of ment from the wood, the fras or excreta being both sexes, which were about to emerge, were left behind as the grub moves forward. The readily obtained therefrom. larval stage is essentially the feeding and growing stage; the grub, developing in size, The insect is undoubtedly Sirex juven­ finds that its skin is now too tight so by the cus (Linn) the common name for which is process of ecdysis or moulting the skin is cast. "The Horntail." It belongs to the order A series of such moults are gone through and Hymenoptera which order is characterised by the third or pupal stage is reached. This third the possession of stiff membranous wings stage is superficially a resting stage in that which are coupled together in flight by minute the insect has little power of movement, but hooks; by the close union of the first abdomi­ in reality the most intense physiological nal segment with the thorax, and by the pre­ changes are taking place. The breaking down sence of a complete ovipositor in the female. of the larval tissues and the building up of The mouthparts are primarily adapted for new forms is a gigantic task, the completion biting, but also often for lapping or sucking. of which sees the production of the imago, or The order contains two sub-orders. The perfect insect. The imago is concerned main­ sub-order Chalastrogastra has several super ly with the reproduction of the species after families, one of which, the Tenthredinoidea, the carrying out of which the imago dies. contains the family Siricdae which in turn em­ An attempt was made to commence the braces the genus Sirex. The genus has the study of an individual specimen through all general characters of the order together with these stages with a view to ascertaining ex­ the following special characteristics:— actly how long the insect takes to pass from The coloration is striking, in the female one stage to another. As will be seen, the it is blue with a high lustre but in the male larval stage is the one in which damage by the abdomen is marked with a broad yellow this insect is caused, while the length of the band. The size of the insect is variable. Some life cycle should be known with reasonable 12 accuracy if control by an insect parasite is emerged. The aim of the adult is to repro­ attempted. It is therefore to be regretted that duce the species and to do this it must reach time and opportunity would not allow of much the surface, it therefore bores only to emerge progress being made in this work, but it js after pupation. Adults have been found in the hoped that this side of the investigation will tree which have found the task of boring their be carried to a successful conclusion else­ way out too large a one and have succumbed where. From specimens collected it is safe to before reaching the surface. It is seen that say that the life cycle of the insect extends the larvae bore directly into the woody tissue at least two years, and possibly longer. The so that when they reach the pith they are in origin of Sirex is Europe where it was first dead non-conductive tissue and from the point described by Linnaeus. The length of the life of view of the well-being of the tree are in cycle in Europe is variously stated as from the safest place. The meristematic and conduc­ two to three years, but apparently it has not tive tissues are situated towards the periphery been definitely investigated, one authority of the trunk or limb so that to encompass stating that the larval life alone extends over the immediate death of the tree these tissues two years, the whole life cycle taking three must be destroyed. Sirex does not appear to years. While it must be stressed that with do this. " In some places a habit has been different climatic* conditions the life cycle of noted by which the larvae bore first inwards an insect and even its mode of reproduction and then outwards again towards the cam­ can be radically changed, it would appear that bium several times forming a number of "W" at present Sirex is still following the same shaped tunnels. In the bends of these tunnels habits as it has in its country of origin. the excreta of the grub is desposited and by bacterial action which this deposit encourages Mode of Attack by Sirex the cambium layer is attacked and the tree The . adult male emerges from pupation ring-barked. However, as far as this investi­ slightly before the female, but does not appear gation is concerned this habit has not yet been to move far from the tree from which it has observed in Canterbury. emerged. The male is strong, virile and ex­ Damage Caused in Canterbury hibits considerable ardour. Copulation takes place soon after the female emerges. The eggs The product of the exotic plantations are deposited by the female in the trunk or finds a market in and around the City of branches of the tree. After selecting a suit­ Christchurch. About ten per cent of the tim­ able spot the female pushes its body back­ ber is cut into dimensional stock and the re­ wards by extending the legs to the fullest mainder is sawn to special sizes for the manu­ extent. The abdomen is then curled up and facture of fruit cases, crates and boxes, while the ovipositor driven into the tree approxi­ a third product of the mills is firewood. It is mately at right angles so that the ovipositing the practice of the millers to stack their sawn insect forms, roughly, the shape of the letter timber in the plantation to season. No reports "C." The stout sheath of the ovipositor is have been received of attack being made upon moved with a plunging motion back and forth timber so stacked or from the city timber until sufficient penetration has been obtained yards in which large quantities of timber, when the egg passes down the egg-laying- k mostly from indigenous species is being sea­ tube and is deposited in the tree just beneath soned. So that Sirex does not at the moment the cambium layer. One egg at a time is so attack sawn timber. It is the practice of the desposited. The object of the insect is to firewood dealers to buy Pinus logs which are place the egg actually in the woody tissue of unfit for saw-timber purposes, and cut them the tree and for this reason it selects the thin into 8-inch lengths; slabs and limbs are also barked portions of the tree as a rule. The sent in from the mills and find their way to eggs hatch and produce the larvae which tun­ the yards of the firewood dealers. The wood nel vigorously usually in a downward direc­ is sometimes bagged up immediately, but tion, working towards the pith in many cases. more often it is merely thrown into a heap. The tunnel is cleanly cut, and is often J The presence of Sirex in these heaps has been inch in diameter. The distance tunnelled recorded. Many of these trees were of small varies considerably from 2 to 3 inches to size or were defective, and no doubt the insect several inches. Pupation takes place within was present when the tree was felled. It the tunnel; the pupa is of the libera type, that emerges in the usual way and the adult causes is, the parts are plainly visible beneath the considerable comment. If the female ovi­ pupal covering. While many insects which posits again in the firewood and it is quickly are good borers during their larval life come sold and burnt then little damage results. close to the surface to pupate Sirex does not In the plantations in various parts of Can­ always do this; pupation takes place some terbury, the presence of the insect has occa­ distance inside the wood, and the adult.bores sioned the greatest alarm. Its presence in its way out. It must not be inferred that the logs and fallen trees has been noted for some adult will commence boring again after it has time past. It is the practice of the millers, in THE FORESTRY CLUB, 1927 Back.—G. H. Hocking, R. J. McLaren, W. Te A. Haig, D. Turnbull, W. S. Tannock M. R. Skipworth, H. Roche Front.—D. Kennedy, Mr. Hutchinson, Mr. Foweraker, A. F. Clark, C. S. Barker

FIG I XS. xl5

Anomalous Tissue in Stem of Rimu (See Text page 19) 13 some cases to allow considerable numbers of Sirex can cause fatal damage to a thrifty, logs to accumulate in the plantations. If the well growing tree. At the time the in­ infestation becomes acute it will necessitate spection was made the presence of tufts a slight change in milling methods so that as of brown needles was frequently quoted as soon as the tree is fallen and cross cut it is evidence of the presence of Sirex. As at Spye hauled directly into the mill and converted to this is not wholly correct. A tree which sawn timber. In order to estimate the extent showed these symptoms was marked and later and type of damage done in the plantations in the year when a considerable quantity of the City Council's Reserve at Bottle Lake was brown needles were present .this tree was the first visited. These plantations, which are felled and examined. The tree was perfectly situated on the sand dunes, are composed of free from insect attack and some other cause Pinus radiata, Pinus ponderosa, Pinus pinas­ for the wilting of the foliage must be looked ter, etc. All are closely spaced and in many for. The geologic formation is a light layer cases would be considerably improved by a of loam over great depths of river and glacier thinning, which would remove at least the deposited greywacke shingle. suppressed members. However, it was only In carrying out an inspection to ascertain after some time and trouble that the insect the extent of an insect infestation, provided was discovered in some dead Pinus radiata. the infestation is extensive in nature the No living trees were attacked. At Spye in method employed is that familiar to all timber North Canterbury, Pinus radiata forms a cruisers. A periphery survey of the planta­ closely planted plantation; here numerous tion is made with chain and compass and a cases of dead and suppressed trees attacked number of strips of one chain width, accord­ by Sirex were found. Some of these were ing to the percentage of the area required, are felled and the presence of the larvae definitely run through the plantation. On these strips proved. Some of the trees were bearing clus­ the trees are examined and those showing ters of dead needles which gave to the tree signs of attack are noted. Thus a representa­ a rather unusual bronzed appearance. It ap­ tive percentage of the trees are examined and peared that the prevalent idea was that this serve as a reasonably accurate basis upon bronzed appearance was due to the attack of which to place definite figures calculated to Sirex so a number of these trees were felled show the seriousness or otherwise of the and split but no trace of the larvae was dis­ damage. covered. At the time the investigation com­ menced in April, it is no uncommon thing to In the plantations inspected it was at once find clusters of dead needles on pines: a cer­ seen that the amount of damage caused was tain amount of leaf cast occurs every year. not large and the strip method, therefore, was A further reason for the browning of the not necessary. Indeed, considering the size of needles will be discussed later, but it is cer­ the plantations, especially those at Hanmer tainly incorrect to assume that the browning Springs, it can be said that at the moment the of the needles is wholly due to the attack of damage caused by Sirex .is not really very great. Sirex. In this plantation suppressed and dead trees were all too prevalent, and it was in The areas were chosen because the geo­ these that the attack had been successful. logic formation is different in each case and Trees of the dominant and co-dominant classes also because it appears that Sirex is causing showed no sign of attack. The soil upon which damage in North and North-east Canterbury, the plantation is established is a light shallow but not in the Southern portion of Canterbury. loam which overlies substrata of sandy marl. Except for its presence in logs in the planta­ Other North Canterbury plantations fur­ tions which are being milled on the Plains, actual damage to standing trees in the Plains nished evidence of attack upon P. ponderosa, Plantations is not known so that the spread P. muricata, P. radiata and other pines. Many of this insect has not been great and appears of these plantations are also closely spaced and to be more or less confined to that area around contain many suppressed and unhealthy mem­ Christchurch, and to the North and East of the bers. Again it was in these members of the City. stand that the evidence of attack by Sirex was apparent. No case of a healthy, well-growing Environmental and other Factors tree being successfully attacked was recorded. As has been mentioned it would seem that An interesting and valuable experiment is Sirex is, at the moment, limited to some being carried out by allowing a fertilised fe­ extent in its geographical distribution. It is male Sirex to ovisposit in a perfectly healthy not intended to state that Sirex does not tree; the tree will be closely observed and the exist in South and South-western Canter­ result of the attack will be carefully noted. It bury and maybe Otago, but at the moment is far too early and rather unwise to the North and North-east. This is more re­ hazard the probable result, but in the markable when it is considered that the meantime it has yet to be seen whether greater proportion of the exotic plantations 14 of the Province are situated to the south of exotic species is concerned comparatively the Waimakariri River, including those admin­ little data is available at the moment mainly istered by the Selwyn Plantation Board, the because the stands are of no great age and in Ashburton and Mackenzie County Councils, those older plantations the use of sample plots and many which are privately owned. It has was not known until recently. It is not pro­ long been recognised that climatic conditions posed to discuss the various methods of deter­ exercise a large measure of control over the mining and classifying sites, but it would geographical distribution of insect life. Cli­ appear that upon areas which are of low site matic conditions are effective in two ways, quality we can expect greater damage from an firstly, in what may be termed direct effects insect pest such as Sirex, than upon areas of and secondly, in indirect effects. Dealing with good site quality. This fact is brought out the direct effect of climate on distribution, into sharp relief in Canterbury mainly in con­ various and ingenious attempts have been nection with the pests in our Eucalypt Plan­ made to fix a standard which has one single tations. It was found in the plantations in­ climatic factor such as, Mean daily tempera­ spected that attack by Sirex had been success­ ture above a certain point, Total temperatures ful in the cases of suppressed or partly domi­ or Atmospheric moisture. It cannot be said, nated trees, or trees unhealthy for any other however, that any one single factor can be reason. The factors which render a plant considered quite by itself, but the various fac­ immune or able to withstand insect attack are tors, those of air temperature, air humidity, complicated. The presence of gums, resins, precipitation and air movements, should be essential oils or repellent juices will help to taken together. In the same way that plants render the plant immune and to these may be and animals are limited in distribution to some added vigour of growth, precocity and the extent by the climatic conditions—insects also ability to recover from set-backs. These are show the same limitations. The climatic effects the factors all or some of which determine are most noticeable in the distribution, rate the ability of the species to resist attack by of metabolism, life cycle and even coloration Sirex. A suppressed tree is an unhealthy tree, and mode of reproduction of insects. For each it is robbed of its share of light and soil mois- m individual species there exist definite boundar­ ture, its rate of growth slackens and its ability ies of humidity and temperature which limit to resist attack by Sirex is definitely lowered. metabolism, activity and development. There The condition of the root system of the tree is of course a temperature below which life is affects to a large extent its rate of growth. impossible; similarly there is one above which Of the three plantations examined, in two life is not possible and again there is a degree cases, at Spye and at Hanmer, the trees were. of dryness which is fatal and a degree of shallow rooted, the geologic formation render­ humidity which is fatal. These factors can ing root penetration difficult, and in these be correlated and humidity and tempera­ plantations the attack was most marked. At ture considered together fix limits within Bottle Lake the root system developed is a which the activities of an insect can deep one allowing a more regular supply of move towards a maximum efficiency or soil moisture to be obtained: attack by Sirex decline from it, so that an optimum- in these plantations is negligible. No living set of conditions can be defined under trees were attacked. From these few cases which the activities of the insect is at its high­ it is not wise to draw definite conclu­ est. Outside these limits it is not possible for sions but it must be noted that root penetra­ the insect to exist. When factors relating to tions and the condition of the absorptive areas climate are so important it is to be regretted of the root system has a profound effect upon that more care is not taken by all foresters to the well-being of the tree and therefore upon keep accurate records. Although Sirex has its ability to withstand insect attack. The been known in the Province for many years its ability of the tree to obtain an adequate sup­ appearance in such numbers this year is pro­ ply of soil moisture particularly in the dry bably due to optimum climatic conditions season undoubtedly raises its resistance obtaining. It would also appear that these* con­ ability. ditions existed in the North and North-east Since anything which lowers the power of the Province while they were not so of the tree to resist attack by Sirex must be marked in the South. It does not appear that considered, attention must here be drawn to Sirex is able to withstand a wide range of the possibility of the insect attack following temperatures; fairly low temperatures with the successful attack of a fungus. As has been low relative humidity are likely to prove fatal. stated the browning of needles was frequently The second or indirect effect of climate ia that quoted as evidence of successful attack by of its effect upon tree growth, and together Sirex, but this was found to be incorrect and with this may be considered the soil factor. the explanation, apart from natural leaf cast, With the consideration of soil and climate the lies possibly in the presence of the fungus forester is inevitably brought back to the term Botryodiplodia pinea. A description of this "site quality." So far as the growth of our fungus has been given by Dr. Curtis (Trans. 15

N.Z. Institute, Vol. 56 pp. 52-57), and it ap­ insects is of common occurrence and through pears probable that this fungus is more pre­ its agency the balance of the insect world is valent in Pinus than is generally credited. largely maintained. There is no doubt that Specimens obtained from trees showing the much has been accomplished by the introduc­ excessive browning of the needles showed un­ tion of parasites to control insect pests, but doubted evidence of fungoid attack, the fruc- the matter is not at all easy of accomplish­ • tifications (pyenidia) of the fungus being pre­ ment. The parasite must be able to adapt sent upon the cones. Though time has not itself to the climatic conditions of the count- permitted the writer to go very deeply into try into which it is introduced; it must be this aspect it appears to be one of the very more numerous than its host; that is, its life greatest importance. Dr. Curtis records the cycle must be shorter or its power of repro­ presence of Sirex larvae in P. radiata affected duction greater. Again the parasite itself by Botryodiplodia and points out that the should be free from parasites and it should death of the tree was caused not by the larvae be specific in its action. The natural enemy but by the fungus as trees killed by the fun­ of the genus Sirex exists in Europe in the gus may show no signs of insect attack. It form of Rhyssa persuasoria. This insect be­ has been shown that Sirex will attack un­ longs to the order Hymenoptera, family Ich­ healthy or dead trees, and trees rendered neumonidae. The family is a large one con­ unhealthy or killed by the fungus are liable to taining many insects which are parasitic in attack. nature upon Lepidoptera, Hymenoptera and Methods of Control other orders; mainly however they are para­ It is proposed to consider two methods of sitic upon the two orders named. The writer control, silvicultural and biological. As regards claims no credit for having made a new dis­ the former method much may be accomplished covery as the parasitic ability of Rhyssa has by the practice of sound silviculture. In none been known in Europe for many years. Some of the plantations visited can it be said that confusion appears to have existed at first as to proper silvicultural methods Were being prac­ the exact manner in which Rhyssa parasitised tised. It must be obvious to all engaged in Sirex, it being thought that the eggs were plantation practice that with a close planting laid actually in the body of the Sirex larvae. distance such as 8 feet by 8 feet, not all the It is now apparent that this is not so. The 680 trees forming the original stocking will Rhyssa is possessed of a lengthy ovipositor reach maturity. In an unmanaged state the with which the female penetrates the bark and more vigorous and thrifty members dominate, wood and attempts to lay its egg in the bur­ suppress and eventually kill the weaker, or row of the Sirex larva. The egg on hatching- ' again, the struggle for existence may be so produces a legless grub which seeks the Sirex evenly balanced that the whole stand becomes larva and attaches itself to it. It remains as an stagnant. It is the aim of the forester to ectoparasite throughout its life and emerges apply his knowledge of silviculture in such a later as an adult. So far as New Zealand is way that throughout the life of the stand a concerned similar species occur, the most regular series of thinnings is carried out re­ striking being Megarhyssa fractinervis which moving those trees which, through lack of is found in the native bush of the West Coast. form or height growth, it is unprofitable to The ovipositon by the Rhyssa is a lengthy pro­ leave. cess and, as in the case of the adult Sirex, Full use can be made of sample plot data withdrawal of the ovipositor is in some cases to determine the time at which a thinning impossible and the insect dies. Whether this should be carried out. In the early stages of parasite is the factor which controls Sirex in the life of a plantation little in the way of Europe, or whether it is only one of many thinning is necessary . As soon as the crowns factors can only be discovered by studying the become definitely interlocked a careful thin­ insect in Europe. "Whether the same control­ ning is indicated. ling influence would be exercised out here also It has been seen that Sirex is attacking remains to be seen, the acclimatisation of an at the moment the unhealthy members of the introduced insect is always problematical: it stands. In a properly managed plantation may also fall an easy victim to natural these members should be reduced to a mini­ enemies such as birds or an insect enemy. mum or better still should be non-existent and Another factor in the control of insects their retention in the stand is defeating the which cannot be ignored is that of birds. ends of scientific management. Every encouragement should be given to bird The second method of control is biological life; plantation practice will not permit of the and supposes that since in Europe Sirex is not leaving of wind-throws and brush which form a great menace it is kept in check by some the natural nesting places for birds but this natural enemy. It is hoped that with the dis­ difficulty may be got over by planting suitable covery of this enemy and its introduction into shrubs around the plantations. The study of this country it will seek its host and exercise bird life particularly of their feeding habits a like controlling influence. Parasitism among is well worth the attention of the forester, 16

Conclusion establishment and maintenance of the exotic It has been stated that the damage caused plantations is one of the most striking fea­ by Sirex is at the moment really quite small. tures of present day economic development in This is so but given favourable conditions the the Dominion. These plantations in a healthy insect will increase its numbers and the state, such as sound silvicultural methods damage caused will be greater. ensure, represent an asset of enormous value; It is as well for the plantation owner to in an uncared-for state with large numbers keep in view the difficulties in connection with of dead, unhealthy and suppressed trees much biological control; for it is slow to establish, of this valuable asset is transformed into a and requires much patient investigation. To definite danger. Neglected plantations of merely say "If an infestation commences, forest trees merely form in many cases breed­ leave it to the entomologist!" is to shuffle ing grounds for harmful insects. Eucalypt out of a large part of his duties and such a plantations are to be seen to-day in Canter­ policy is fraught with grave danger. The pre­ bury which have failed through neglect or through the unsuitability of the site and these vention of a serious infestation particularly still remain a deal loss from the forester's such a one as that threatened by Sirex lies in point of view, that is financially; while more the carrying out of sound silvicultural serious, they have become a mere breeding methods and the practising of the elementary ground for our many Eucalypt pests, the con­ rules of forest sanitation. Many individuals trol of which is becoming an increasingly and private companies have shown their long­ complex problem. With other exotic planta­ sightedness by following the lead of the tions the same will apply and all those who Government and planting exotics thereby in­ have established or helped to establish exotic creasing the national as well as their own plantations should co-operate in the keeping resources. But while with the initial effort of the stands in a healthy, properly thinned, the movement has been given an excellent and clean condition, thereby doing a very start, more is required. The present planting large share in combating the present danger spacing of 8 feet by 8 feet is no doubt the and also ensuring a quicker, greater and most suitable distance, but not all will form better quality yield of forest produce from the final crop. A series of regular thinnings their stands at maturity. throughout the life of the stand is necessary. These thinnings represent an intermediate Literature Consulted yield and should be removed. To thin the Dr. R. J. Tillyard, F.R.S.—Insects of Aus­ stand and leave the thinnings lying is to de­ tralia and New Zealand. feat the main objects of the operation, those Dr. K. M. Curtis.—Trans. New Zealand of securing an intermediate return and of Institute, Vol: 56, 1926, pp. 52-57. keeping the stand healthy. It has been shown R. A. Wardle and P. Buckle.—Principles that Sirex will readily attack dead timber; of Insect Control. high stumps, wind-fallen trees, and thinnings Dr. Imms.—Textbook of Entomology. that are left to lie are favourite places for the A. Howard.—Annals of Applied Biology, female to oviposit. Forest sanitation demands 1921. primarily that the forest floor should be kept C. V. Riley.—Insect Life, Vol. I. clear, as far as possible, of all dead and fallen F. E. Hutchinson.—A Forest Survey of matter. Trees which have become infected Canterbury {unpublished). should be removed without delay, piled in a D. Miller.—Forest and Timber Insects in convenient place and burnt, The practice of New Zealand. removing the infected trees, sawing them for A. T. Gillanders.—Forest Entomology. firewood and leaving them stacked for some time, possibly years, must be condemned. The larvae will live and develop just as well MEASUREMENTS IN A whether the tree is in 4 foot lengths or is SAMPLE PLOT IN entire and in due course will issue from the firewood pile as adults ready to play their YOUNG RIMU part in the reproductive processes and so increase to no small extent the numerical strength of this insect pest. (M. R. Skipworth) With the large number of private planta­ The data set out below are the result of tions established which are in the younger the remeasurement in September, 1927, by the stages of growth little attention in the way students of the School of Forestry as a field of thinnings will probably be necessary for exercise in forest mensuration, of a permanent some time but so soon as complete canopy sample plot installed in January, 1921, by the is formed and definite suppression of some State Forest Service in a stand of pole rimu members takes place the commencement of a near Hokitika in the tract known as Perry's regular series of thinnings is indicated. The Bush.