Tuatara: Volume 3, Issue 1, May 1950
The Biology and Control Of Beetles Attacking Seasoned Timber
The insects which destroy seasoned timber comprise two major groups, termites and beetles. This account is confined to the beetles.
All beetles have indirect development with four distinct stages in the life history, egg, larva, pupa and adult. The adults are free living, with biting mouth parts. The antennae are usually well developed and elongate. Beetles are hard bodied and it is characteristic that the first pair of wings are heavily chitinised to form elytra, which cover the membranous second pair of wings and enclose and protect the upper surface of the abdomen.
One of the pecularities of wood as an environment and substrate is the absence of free water. The relatively small amounts (10-25 per cent.) of bound water are available to the insect only when the wood is digested. Wood is compact and since relatively little is digested by the insect, the energy gained on digestion is small relative to the amount expended in removing pieces from the compact substratum. The nitrogen content of wood is very low (approximately 0.2 per cent.) and much wood must be consumed to obtain sufficient protein for growth from egg to adult. These peculiarities are reflected in the life cycle which is prolonged even under the optimum conditions of humidity and temperature. Presumably it needs a long life cycle to accumulate sufficient water, energy and protein for growth to maturity.
In addition to the lengthy life cycle, there is a common core of biology applicable to all timber beetles. In every case the entire life of the larvae is spent tunnelling within the timber. When the grub is mature it approaches the surface and forms a cell in which pupation occurs. When the pupal period has elapsed, the adult hardens for a few days within the cell and then cuts its way to the wood surface. The holes made in this way are known as exit holes and generally their size and shape are characteristic of the species which makes them. Mating occurs on the surface of the wood and usually occurs several times during adult life. The adults live only a few weeks and during this period females lay eggs either in cracks and crevices on the wood surface, or, in Lyctids, in the pores (vessels) of the wood. The average number of eggs is usually less than 100 and in no case is it exceedingly large.
The eggs hatch in from one to three weeks and special conditions are not required, hatching occurring at most normal conditions of temperature and humidity. The young larvae bore through the egg page 13 capsule and enter the wood. In most cases the tunnels made by the larvae meander throughout the wood and all become larger as the larva increases in size. These tunnels are usually packed with wood which has been passed through the gut, but not digested. This frass may be a fine dust or pelleted or it may be tightly packed into a solid mas completely filling the tunnels. Some or all of the frass may be forced through small openings to the surface of the wood and constitutes the so called “borer-dust.”
The various species are each attacked by one or more species of parasites, but unless the infestation is old, percentage parasitism is so low that it is of no importance in controlling the infestation. Once a piece of timber becomes infested it is reinfested with each succeeding emergence of beetles and the population increases until in extreme cases the timber is destroyed.
The beetles which attack seasoned timber in New Zealand can be divided into four sub-groups within which behaviour and biology is similar. The four sub-groups, in order of economic importance, are the Anobiidis, the longhorns, the Lyctids and the cossonid weevils. Each will be discussed in turn.
The Anobiids (Anobium and related genera). Everyone is familiar with “the borer” better called the common house borer, Anobium punctatum De. G., which is not a native of New Zealand and occurs in most parts of the world. It is the most important and wide-spread of the beetles which attack sound seasoned timber in this country. Only the sapwood is attacked and none of the New Zealand timber species appears to be immune. Infestation can occur as soon as the timber is air dry (i.e., less than 30 per cent. moisture). When timber becomes infested for the first time, the attack is not apparent until exit holes appear, by which time one life cycle has already elapsed. Since the initial infestation is usually slight, two and even three generations may be completed before the infestation is noticed and such observations appear to be the origin of the popular belief that timber is not liable to infestation for 5-15 years after it has been milled.
The beetles are small (3-5 mm.), relatively inactive and may often be found in considerable numbers on the surface of infected timber during the period of emergence, which in Auckland extends from October to the end of January. However, the main emergence covers five weeks from the beginning of December with a maximum emergence around the middle of that month. Although there is an annual flight the life cycle is at least three years, while in some timbers six years may elapse between the time eggs are laid and beetles emerge. Thus the annual flight indicates a series of overlapping generations, not a one year life cycle. The eggs are laid either on rough surfaces of the wood or within the pupal cells at the base of the exit holes and are firmly attached to the wood surface. The number of eggs laid page 14 varies with size of the female but ranges from about 15-90 with average values around 40-60 eggs.
Little is known of optimum temperature or wood moisture content for larval growth, but larvae survive and grow well at a constant temperature of 22.5 deg. C. with relative humidity of 85 per cent. and these conditions are used for most tests.
The common house borer has some allies, less important but occasionally met with. In the South Island, and as far as is known not occurring in the North Island, there occurs a native and as yet undescribed species of Xyletinus which is responsible for a good deal of so called “borer” damage at least in older buildings. It is widespread and has on occasion caused severe damage to buildings. Exit holes are of almost the same size and shape as those of Anobium punctatum, while the flight periods are also roughly coincident.
An endemic and much larger beetle Anobium magnum Dumbl. occurs not infrequently in decayed boards throughout the country. The exit holes are larger than those of Anobium punctatum and usually there are but few on each board. Although damage is both intensive and extensive, the presence of this insect always indicates decay due to free water or abnormally high atmospheric humidity. Elimination of the decay hazard by adequate ventilation and/or removal of the source of free water eliminates this insect.
The introduced pine borer, Ernobius mollis L. has been present for many years, but has not become a serious pest. This species attacks only the sapwood of pine (Pinus sp.) but cannot infest a board unless there is bark adhering to the wood. When bark is present infestation can be severe. Prevention of attack results from removal of all traces of bark.
Two other species are occasionally collected from infested timber and from houses. These are Capnodes griseipilus Broun, and Dorcatoma oblonga Broun, both natives of New Zealand. Nothing definite is known of their biology, but it is thought that they breed in decayed wood. They have no economic significance.
The Longhorns: The two toothed longhorn, Ambeodontus tristis F. is second only to the common house borer in the severity of its attack on building timbers.
The length of the life cycle is unknown and estimates range from two to seven years or more and these are probably correct, larvae reaching maturity within a year or two under optimum conditions, while under adverse conditions the life cycle is prolonged. The emergence period does not appear to be well defined and although it probably reaches a maximum during May and June isolated adults may be found throughout the year. The adults are variable in size, females measuring from 8-22 mm. in length, while the males are less variable and are never as large as the largest females.page 15
Females die rapidly and do not lay eggs under the normally somewhat dry conditions of the laboratory. Under saturated conditions females survive well and deposit all their eggs. However, saturation is not essential and at humidities of 95 per cent. and above all the eggs are laid while at humidities below 92 per cent, few eggs are produced (at 23 deg. C.). Females do not lay eggs on the normal surfaces of wood blocks, but if egglaying sites 1/32 inch wide and 1/16 inch deep and long are made on the relatively smooth block surfaces, females lay usually one, but sometimes two and rarely more eggs in each site. The eggs hatch well under all normal conditions. The number of eggs laid by each female varies according to size and in the few females studied, the smallest produced 28 eggs, the largest 239 eggs. The average length of the females was 14.2 mm. and the average eggs 97.
The Lyctids: (Lyctus and related genera). This group of beetles, commonly termed powder post beetles or borers, are pests in all parts of the world where sapwood of hardwoods (i.e., wood from broad-leaved trees) is used. There are no species native to New Zealand. Of the many species known as pests Lyctus brunneus Steph. is most abundant in New Zealand and is perhaps the only species which has established. Occasionally other species are found infesting furniture and packing boxes, but usually the circumstances indicate that infestation occurred in another country and that the article was infested when imported; as examples, L. linearis (Goeze) has been found infesting an oak table from England; L. planicollis (Lec.) infesting hickory axe handles from the United States of America and Trogoxylon inequale Woll. infesting a wood toy from Trinidad. Likewise packing cases and wooden objects from most parts of the world arrive in New Zealand and are sometimes found to be infested with L. brunneus.
Most of the commercial timbers used in New Zealand (rimu, matai, kahikatea, exotic pinus, Pinus ml, etc.) being soft woods, are immune from attack by powder post beetles. Of the native hardwoods both the beeches (Nothofagus sp.) and ratas (Metrosideros sp.) are little attacked although some beeches are known to be susceptible at times. However, sapwood of tawa (Bielschmieda tawa) is so susceptible to attack that the increased utilisation of this species depends almost entirely on control of this beetle.
Only sapwood is attacked. However, the sapwood of some hardwood species is not attacked and it is also common to find amongst susceptible species, e.g., tawa, that some boards will be attacked while others—perhaps even from the same tree—remain free of infestation. Females lay their eggs in the pores (vessels) of the wood. When these are very small (maximum size less than 0.0035 inch in greatest width) eggs cannot be inserted into the pores. Thus pore size and absence of pores explains why some hardwood species and all soft woods are immune from attack. The major food of Lyctus larvae is page 16 the starch stored in the sapwood. When starch is absent the wood is not attacked.
The length of life-cycle varies considerably according to climate: under optimum conditions it may be as short as three months, but under natural conditions a generation a year is normal.
Eggs are laid as soon as the wood surfaces are air dry and very little susceptible sapwood escapes attack for more than one-two years after milling. Boards frequently become infested in timber yards and stacks and because the initial stages of attack are not obvious, infested wood is often used for making furniture and doors, without the infestation being detected. Up to twelve months later, exit holes, sometimes in great numbers, appear on one or more boards and unless the attack is arrested these boards will be completely destroyed in from two to four years. This rapidity of destruction is an important distinction between Lyctus and other timber beetles. For instance with Anobium much timber is infested for from 15-30 years before the strength is much impaired; with Lyctus complete destruction of susceptible timber seldom takes more than three years.
The cossonid weevils: (Torostoma apicale Br., related genera and species). This is the least important sub-group of the timber insects. Attack is confined to sapwood, and while damage can at times be severe, usually it is associated with or follows decay. Not infrequently these weevils attack wood that has been severely infested with Anobium and it is thought that Anobium attack has opened up the timber, permitting access of water with subsequent decay and weevil attack. In the few cases in which apparently sound timber has been attacked — and this is usually flooring or subfloor—atmospheric humidity was high due to inadequate ventilation.
Although there is a good general knowledge of most aspects of the biology of the timber attacking beetles of major importance, one problem and that perhaps the most important remains untouched. Almost nothing is known of the nutrition of these insects, or of the factors governing susceptibility of various timber species and contributing to susceptibility variation within and between boards of the same species.
It is known that starch is an important factor in determining whether sapwood of hardwoods is attacked by Lyctus, but equally it is realised that this is not the full picture. On occasion boards with heavy starch deposits remain immune to attack or are damaged but lightly. It is often stated—presumably by analogy with Lyctus—that presence or absence of starch and free sugar determines whether or not a board will be attacked by the common borer Anobium punctatum. While it is certain that this is incorrect it is not known which component of the wood is digested by Anobium larvae or indeed how page 17 much of the wood is utilised, although it is likely that about 20 per cent. of the original dry weight of the wood is digested. Of the nutrition of the two-twoothed longhorn nothing is known.
Similarly there is little information on the factors which govern susceptibility of boards to attack, but it is now recognised that some sapwood boards of all species of timbers, including the very susceptible timbers kahikatea and matai are immune from Anobium attack and are therefore to be regarded as non-susceptible. Detailed investigation of the allied problems of larval nutrition and wood susceptibility is very necessary and when this knowledge becomes available most of the remaining and foreseeable problems in the biology of the New Zealand timber beetles will be capable of solution. Perhaps detailed knowledge of these problems will point the way to the possibility or otherwise of preventing attack other than by the use of chemicals. Because of the importance of these problems it is proposed to reorient the timber work at this Division and concentrate on nutrition and susceptibility coming back to other outstanding problems only when this information is available.
It is the damage done by timber insects that has focussed attention upon them and the objective of all the research is prevention of this damage. In the control of timber insects there are two problems, one the treatment of wood to prevent attack, the other treatment of buildings and structural timber to eliminate insects which are already there. It is usual to designate these problems as pretreatment and eradication.
Consider first pre-treatment. Here also are two problems distinct in approach and methods. Firstly to determine what materials are sufficiently toxic to the insect to be worthy of use against them and secondly the experimental operation of pilot treating plants to ascertain how best to treat timber with these materials. This second aspect does not deal with insect biology or with toxicity of materials to insects and will not be dealt with here.
Relatively simple methods are now available for determination of toxicity of materials. Small blocks of wood (approximately one inch cubes) are impregnated with chemicals under conditions which ensure complete penetration and uniform distribution of the material to be tested. The impregnated blocks are allowed to dry and are then placed in cages in the testing room, where sufficient Anobium females are introduced to ensure oviposition of more than 150 eggs. The blocks are stored for about nine months under conditions known to be suitable for development of Anobium larvae. At the end of this period each block is sectioned on a microtome and presence or absence of larvae determined. By employing a series of concentrations of preservative from very low to high it is possible to determine for each page 18 chemical or combination of chemicals, how much is necessary to prevent attack. Some materials, e.g., boric acid, Wolman tanalith, are very toxic (less than 0.10 per cent. of the weight of the wood); some as zinc chloride and sodium fluoride are less so, while yet others as sodium and magnesium sulphates do not prevent development of Anobium larvae even when present to the extent of two per cent. of the weight of the wood.
The figures given by the above method of testing toxicity apply only when the timber is completely penetrated by preservative, a requirement which, although good practice, necessitates the use of relatively expensive treating plants. Because of this, treating cannot be carried out other than on an extensive scale. This means that those wishing to use pre-treated timber, must buy it already treated. There are other methods of treating timber which have been extensively used and widely recommended that do not suffer this disadvantage. Mostly these involve dipping dry timber in solutions of preservatives in petroleum oils. For various reasons these methods are not satisfactory.
For most people, the major interest in timber insects is not insect biology or treatment of timber to prevent attack, but eliminating or reducing an attack in a house or wooden structure. The methods available comprise two operations which may be used separately, but normally are used in conjunction. Treating material is dissolved in a carrier or solvent (kerosene, mineral turpentine, diesel oil, etc.), and the solution injected into the timber with a pressure gun. After all exit holes have been treated the available surfaces of the timber are sprayed or brushed with the solution used for injection. Since it is impossible to obtain complete penetration of treating liquid only population reduction and not eradication is to be expected with these methods unless the treating liquid contains a residual contact insecticide.
When a treating liquid containing a residual contact insecticide is used, a film of insecticide is left which may either kill the beetles when boring their exit holes or after they have emerged and before eggs are laid. Because the life cycle of Anobium is at least three years the well known contact insecticides (e.g., D.D.T., Gammexane) may not be suited to this purpose as their films usually lose effectiveness within a few weeks or months of application. With such materials it would be necessary to spray prior to the flight in each year making the cost prohibitive. If, however, a potent contact insecticide, effective for at least three years can be found, economic control of infestations will be assured.
Wood is a very variable material and when it is infested with insects the extent of infestation is another variable. It is relatively easy to obtain good control (i.e., with kerosine, mineral turpentine or page 19 diesel oil alone) when the boards are heavily infected and the wood virtually honeycombed. Such wood acts as a sponge and readily soaks up preservative so that good penetration and control is achieved. In boards with light infestations and therefore virtually intact, penetration of preservative is poor and many larvae escape its action and survive in pockets of untreated wood. In such cases, control is much less.
Because of this variation no two tests can be expected to give identical results and no material behaves consistently as far as percentage control is concerned. However from these tests, a pattern of effects is gradually built up which helps in understanding and therefore in solving the problem. When this pattern is clear and the types of action of the various materials and the mechanics of the problem are clearly understood, it will cease to be of laboratory interest and must be taken into the field for further trial and evaluation of both methods and materials.
This review is based on work carried out at the Plant Diseases Division during the last ten years. Much of the work is as yet unpublished. Those interested in types of damage and recognition of the insects concerned should refer to “Insects attacking milled timber, poles and posts in New Zealand” by J. M. Kelsey, N.Z. Journ. Sci. & Tech. 28 (Sec. B) 65-100, Recent volumes of the same journal should be consulted for further details on insect biology and control, on preservative pre-treatment of timber, or toxicity of various chemicals to wood destroying fungi. It is anticipated that many papers on these allied problems will be published in that journal during the next few years.