Hywel Roberts
HYWEL ROBERTS is with the Forestry Branch of Papua New Guinea. Under the auspices of the Ministry for Overseas Development, London, he spent six months (1973) in Fiji studying the ambrosia beetle problem.
Because of the absence from Fiji of the "shoot-borer" of Meliaceae Hypsipyla robusta Moore (Lep., Pyralidae) the Fiji Forestry Department, after trials, established plantations of "big-leaf mahogany" (Swietenia macrophylla) in the southeastern, high rainfall area of the largest island of the group, Viti Levu. Trees were planted out after 1965 as base-rooted striplings at 11 x 3 m espacement in logged forest and managed on a 40-year rotation. By 1971, 8 294 ha had been established. The mahogany was grown to produce high-quality furniture timber and veneer. When the first detailed peeling trials were made in 1971 evidence of ambrosia beetle attack (shoot-hole borer) was found in the living tree. Infestation was high enough to suggest the problem was serious, and planting immediately stopped. A survey carried out in 1972 showed attack was present in all the main plantation areas (Naboutini, Ngaloa, Nukurua, Yarawa), but levels of infestation apparently varied widely within plantations (Mang and Vincent, 1972). Two non-selective ambrosia beetles, Crossotarsus externe-dentatus Fairmaire and Platypus gerstaeckeri Chapuis, both belonging to the family Platypodidae, were found to be responsible. The smaller species, C. externe-dentatus, is widely distributed in the tropics, being recorded in east Africa, most of tropical Asia, and east to Hawaii (Schedl, 1972). In contrast P. gerstaeckeri is endemic to Fiji. Fiji both species are common throughout the high-rainfall areas of the two main islands, Viti and Vanua Levu.
SHOOT-HOLE BORER ATTACK IN A MAHOGANY LOG - also on islands
In the natural rain forest, both Platypodidae, as do most of this insect family, normally attack and breed within only newly felled, injured, or badly diseased trees. Where breeding is successful a gallery system, termed the nest, is made within the host tree, where the young are reared. During all developmental stages they feed on an ambrosia fungus that grows on the gallery walls, not on the wood. In felled trees the life cycle, from initial attack to emergence of young adults, in C. externe-dentatus takes about 50 days and in P. gerstaeckeri twice as long, about 100 days. The smaller species also differs from the larger in that the host is attacked within a short time-a few days or of felling, or injury. On felled trees attack by P. gerstaeckeri does not normally occur for some three to four weeks after felling. While both species are apparently non-selective with regard to plant families attacked (in Fiji C. externe-dentatus is recorded from 69 tree species belonging to 42 plant families, and P. gerstaeckeri from 40 tree species belonging to 30 plant families (Roberts, 1976)), the larger platypodid rarely attacks main stems below about 15 cm diameter; C. externe-dentatus is not size selective attacking stems of from 5 to 50 cm diameter. Naturally P. gerstaeckeri, unlike the smaller species, is commonly found attacking felled trees in very large numbers, entrance holes being closely grouped together. When this occurs C. externe-dentatus is usually present only in small numbers, or completely absent. This suggests the larger platypodid produces a repellent pheromone, which has the effect of reducing competition for hosts, and nest space, by other ambrosia beetles. Attack by both species on living trees always apparently fails, beetles being killed by gum exudation produced by the mahogany, and breeding never occurs. Though holes are occluded by subsequent cambial growth, the mainly short galleries made remain within the stem, and on conversion are seen as numerous holes on the cut surface. This causes heavy degrade of the timber.
In plantations, attack by C. externe-dentatus is much more widespread than by P. gerstaeckeri. This results partly from the size selection shown by the large species; but also the greater aggressiveness of the smaller platypodid must contribute toward the difference. Trees under five years of age are not usually attacked, but older trees of all ages may suffer from C. externe-dentatus. Possibly late in the rotation, when mean stem diameter will be much greater (the oldest plantation in 1973 was only 11 years old), P. gerstaeckeri will be more important than it is now.
Evidence of fresh, active attack is not easy to find. That seen was always associated with interference, or actual destruction of the plantations' vegetation matrix. Most common was attack on marginal trees of compartments.
Where this occurred invariably roadwork had been carried out nearby, though trees were never obviously damaged.
Inside plantations attack nearly always related to some forest operation such as thinning, pruning, cleaning, or the local removal of sample trees for experimental purposes. When it appeared, attack occurred within one to three weeks of the beginning of the operation and fresh attack would continue if the vegetation matrix was drastically changed, as with heavy cleaning, for up to five months after the operation ceased. The number of trees attacked, and the abundance of P. gerstaeckeri in particular, appeared to relate directly to the intensity of cleaning. It was found that attack could easily be induced by intensively cleaning half-acre plots of plantation. When this was done it was noticeable that large indigenous trees left standing among the mahogany were never attacked. Though heavy attack was associated with forest operations, very light attack was seen well inside plantations, where no operations had occurred during the previous six months. A monthly survey of four such sample blocks (each of 200 trees) chosen in each of three of the main plantation areas (Ngaloa, Colo-i-Suva, Nukurua) revealed light attack in every month of the year (May 1973-April 1974) by C. externe-dentatus. Except at Nukurua no more than 5 percent of the total number of trees sampled in each plantation were attacked in any one month.
Within plantations attack is apparently related to site. Among trees from twelve sites chosen in 1973 to represent variations in conditions in the 1965 Ngaloa Plantation, high mean attack levels were obtained from trees located where drainage was obviously bad (1.653 holes/board super foot), where vegetation indicators, e.g. giant ferns, suggested soil conditions were poor (1.523 holes/board super foot) and where a hurricane (1972) had caused extensive damage (1.240 holes/board super foot). These figures are much higher than those obtained for large trees from well-drained sites, which had comparatively fertile soils (0.130 holes/board super foot). On valley slopes trees located well up the slope were not as heavily attacked as those below them, though (because growth rates were lower) all were smaller in diameter than those in the valley bottom. At the base of slopes where growth conditions were apparently good it was not unusual to find trees of over 100 board super feet completely free of attack. At each site attack levels were obtained by felling trees at ground level, converting stems to half-inch boards, and counting the shot-holes on the cut surface. Figures quoted therefore represent old, subsurface attack.
Small areas of mahogany, together with a hybrid (S. macrophylla x S. mahogani) have been grown at high altitude (900-1 100 m), near Nandarivatu, in the centre of Viti Levu. Examination of old attack in selected large stems from these plantations showed that the two beetles had attacked both S. macrophylla and the hybrid in the past but, compared with low altitudes, infestation was much lower.
Seasonal variation in attack is generally to be expected in high-rainfall tropical rain forest conditions, but there was some evidence to suggest that attack was heavier during the months from October to March than at other times of the year. The surveys of current attack on the aforementioned blocks of 200 trees showed for Nukurua, where large plantations are grown under the lowest rainfall regime (including a weak dry season, according to Berry and Howard, 1973), a greater mean monthly level of attack than in the wetter plantations, with a peak in attack in February 1974. Though rainfall in all areas is high (3 050-3 800 mm+) all plantations experience short periods of up to a week without rain, and the chances of such short breaks in rainfall occurring are greater between August and December than at other times of the year. The flowering and fruiting of mahogany between October and December also are indirect evidence that the last three months are annually among the driest in Fiji. In the wild in South and Central America flowering and fruiting are said to be linked closely with the driest months (Lamb, 1966).
In Fiji S. macrophylla suffers infestation by three kinds of butt- or stem-rot one termed "marginated butt-rot" and not easy to recognize, and two others, Armillaria and Fomes. In plantations obviously diseased trees were always seen to be heavily attacked, but, based on examination of freshly felled stumps, it would seem that in some localities the prevalence of rot may be greater than evidence from tree condition alone suggests. If incipient rot occurs, attack by ambrosia beetles may be expected.
The practice when establishing new plantations was to precede planting by poisoning indigenous trees. In many instances poisoning was extensive, but at the same time it was only partially effective, many trees being still alive one or two years after treatment. Three common trees - Dillenia biflora (Dilleniaceae), local name, kuluva, Myristica castaneifolia (Myristicaceae), local name, kaudamu, and Pagiantha thurstonii (Apocynaceae), local name, tadalo - are all very resistant to poisoning, and therefore difficult to kill. It was common to find stems of these still alive five to seven years after treatment. Poisoned trees are heavily attacked by a wide variety of ambrosia beetles, including in Fiji the two nonselective Platypodidae which attack mahogany. Where trees are properly killed the abnormal rise in ambrosia beetle population which follows poisoning will fall after four years, as all suitable breeding material disappears. Where, however, trees die very slowly they remain active breeding sites for much longer. Young beetles emerging from these trees will attack any planted mahogany surrounding them, provided it is not less than five years old; also any older plantations that may occur nearby will be vulnerable. Of the three trees resistant to poisoning, Myristica in particular was often seen with heavy infestations of ambrosia beetle within mahogany plantations. Two other sources of breeding material exist in the plantations. Thinnings and prunings are commonly left on the plantation floor. This mahogany is quickly attacked, and both Platypodidae readily breed successfully in it. Hurricanes, not uncommon in Fiji, also leave large quantities of material suitable for breeding. In strong winds it is not only the mahogany that suffers; often they in fact escape complete uprooting. More vulnerable are the much taller indigenous trees left in the plantations. Large populations of ambrosia beetles quickly develop in the latter.
Natural controls for both C. externe-dentatus and P. gerstaeckeri exist in the rain forest of Fiji. Gummation is commonly responsible for heavy mortality among males of the smaller species in felled, fallen, or injured trees. A limited variety of predators and nest parasites attack both species. The wood-swallow (Artamus leucorhynchus) was seen taking swarming adult C. externe-dentatus in flight, and on arrival on the host tree the same platypodid was taken by a skink (Emoia samoensis), and the larger species by a gekko (Cyrtodactylus pelagicus) and more commonly by an insect predator, Omadius lividipes (fem. Cleridae). Inside nests of C. externe-dentatus the brenthid nest-parasite Cyphagogus fijianus was sometimes found breeding, and in nests of P. gerstaeckeri larvae and adults of a colydiid beetle, Nematidium sp., were commonly found feeding on both larvae and young adults of the platypodid. These various mortality factors are however ineffective in limiting numbers of Platypodidae in the artificial situation represented by the mahogany plantation, which so favours the ambrosia beetles.
Attack by Platypodidae on living, apparently healthy, trees is unusual. Although three species (Austroplatypus confertus in Australia; Bendroplatypus impar in Malaysia; Trachyostus ghanaensis in Ghana and Ivory Coast) are known which attack only living trees, the intensity of attack is always low, and all breed successfully within the living tree. One species only, Doliopygus dubius in Nigeria, is known to attack regularly in large numbers and to breed within standing trees host - Terminalia superba (Combretaceae) and at the same time to breed within a wide variety of felled trees. More common among Platypodidae is unsuccessful attack in large numbers on standing trees of reduced vigour (Browne, 1965). This habit has been recorded however only from areas which undergo a regular dry season, attack coinciding with the arrival of the dry weather. The pattern of attack shown by C. externe-dentatus and P. gerstueckeri suggests attacked trees are of reduced vigour, but the loss in vigour does not coincide with any obvious period of dry weather. Perhaps S. macrophylla in some way is not fully adapted to the environment found in the area of Fiji in which it is grown. Trees in such a condition will show stress symptoms in response to temporary changes of the environment, and under stress produce chemicals attractive to ambrosia beetles. Non-selective species, such as C. externe-dentatus and P. gerstaeckeri, are the Platypodidae most likely to respond to attractants produced by an exotic tree. Evidence that S. macrophylla is poorly adapted is suggested by the absence of attack on indigenous trees compared with the heavy attack on some mahogany in areas of intensively cleaned plantation. It is known that very slight, temporary changes in climate, above or below the ground, can lead to stress symptoms in pines, which are followed by bark beetle attack (Barlow, 1966). In the virtually nonseasonal climate where mahogany is grown perhaps the very short periods without rain are in some way associated with production of stress symptoms. It was noticeable that, provided the tree retained a partial root system, windblown mahogany was often not attacked, possibly because its foliage remained in the comparatively stable climatic zone near the ground, and so was not exposed to the climatic variations crowns normally suffer from. The root systems of many eight-year-old trees were exposed and though they were apparently adequate, most lacked the taproot found in indigenous rain forest tree species. Where butt-rot occurs poorly adapted trees will be more likely to show stress symptoms, and so be more attractive to ambrosia beetles. Presumably one reason C. externe-dentatus is the most widespread pest is that it is more sensitive to attractants put out by a stressed tree, and therefore trees under only little stress are unlikely to be attacked. The very early arrival of this platypodid at felled trees indicates it is very sensitive to attractants.
Economically, the possible depredations and effects on quality by ambrosia beetle attack are potentially very serious. Based on 1973 values, the present mahogany plantations should produce a revenue of about F$23 million, but if beetle attack is extensive the financial return could be reduced by some two thirds. At the moment one of the problems is lack of knowledge on the extent of attack within the area at present under mahogany. Platypodid attack on living exotics would appear to be an expanding problem in the west Pacific, and perhaps elsewhere in the tropics as well. In Fiji the same insects are also known to attack living Eucalyptus species (Myrtaceae), Khaya anthotheca (Meliaceae), Nauclea diderrichii (Rutaceae), and Terminalia superba (Combretaceae), but luckily none of these as yet occupy large areas (Alston, pers. comm.). C. externe-dentatus is also reported to attack S. macrophylla, and two species of Eucalyptus in Samoa (Beaver, 1976). Surprisingly, neither felled nor standing plantations of pines are attacked, though felled indigenous conifers of the same plant family are heavily attacked by both platypodids, for example Agathis vitiensis (fam. Pinaceae).
MAHOGANY LOGS - worth research
In Fiji the long-term solution to the problem demands more research into the exact requirements of S. macrophylla. Also, a detailed examination of the extent and levels of infestation by the different butt-rots in plantations needs to be made. Immediate remedies, all of them silvicultural, which can be taken must aim to greatly reduce breeding sites available to both Platypodidae, and at the same time to ensure a healthier crop of trees. Where timber contractors have been working, an interval of five years should elapse before the area is planted up. This would give sufficient time for the inevitable build-up in ambrosia beetle population to subside. The present practice of wholesale poisoning of logged forest as one of the initial stages of plantation establishment should be more effectively carried out. Fewer trees need be poisoned, and these inspected at six-monthly intervals over two years to see that they die. Species difficult to kill, particularly kaudamu (Myristica sp.), could be felled rather than poisoned. It is also important that poisoning not be carried out in areas where plantations more than five years old already exist. In the management of plantations it is most important that thinnings and prunings not be left lying on the floor. In the long term it would be better to collect and sell them, at a loss if necessary; there is a local demand for firewood and poles. Likewise, where hurricane damage occurs efforts should be made to remove windblown stems by burning, clearing, or sale of timber. After plantations have been established it is important to disturb trees as little as possible by forest operations. Attack on compartment margins may well have to be accepted, but within compartments cleaning and thinning operations should be reduced to a minimum, and, when necessary, vegetation at the base of individual trees should always be left or not reduced below a metre in height.
BARLOW A.R. 1966. The relationship between resin pressure and Scolytid beetle activity. Forestry Commission, Forestry Record No. 57, Her Majesty's Stationery Office, London.
BEAVER, R.A. 1976. The biology of Samoan bark and ambrosia beetles (Coleoptera: Scolytidae and Platypodidae). Bull. ent. Res., 65: 172-184.
BERRY, M.J. & HOWARD, W.J. 1973. Fiji Forestry Inventory. Land Resource Study No. 12, Land Resources Division, England. 98 P.
BROWN, F.G. 1965. Types of ambrosia beetle at tack on living trees in tropical forests. Proc. XIIth Int. Cong. Ent. p. 680.
GRAY, B. 1974. Forest insect problems in the South Pacific Islands. Commonw. For. Rev., 53: 39-48.
LAMB, F.B. 1966. Mahogany of tropical America. Univ. of Michigan Press. 220 p.
MANG, M. & VINCENT, A.J. 1972. A report of findings from an extent of damage survey in standing mahogany (Swietenia macrophylla) attacked by ambrosia beetle in Fiji. Fiji Forest Survey and Inventory Rep. No 5. 17 p.
ROBERTS, H. 1976. Ambrosia beetle attack (Coleoptera: Platypodidae) on established living mahogany (Swietenia macrophylla) in Fiji (with descriptions of two new Platypodidae). Bull. ent. Res., 66.
SCHEDL, K.E. 1972. Monographie Familie Platypodidae (Coleoptera). 322 P. Den Haag.
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