R.A. Bray
Introduction
Life Cycle
Damage
Techniques for Assessing Psyllid Numbers and Damage
What Options are there for Combatting the Psyllid?
References
In the past, Leucaena leucocephala (leucaena) has been notable for the general absence of problems due to major diseases or insect pests. The successful spread of the common leucaena to much of the tropical world may have been partly due to leaving its main diseases and pests behind. However, in recent years, the leucaena psyllid has proved to be a significant factor in the continued use and expansion of this valuable multipurpose plant.
The leucaena psyllid (Heteropsylla cubana, sometimes called 'jumping plant lice') is a small yellow-green insect about 1-2 mm long (Figure 6.1.1). It is native to Central America and the Caribbean, where it has presumably co-existed with leucaena for thousands of years. Although it has been reported to occur on a few other leguminous shrubs and trees, these are not damaged to any great extent, and it is probable that the psyllid can only complete its life cycle on plants in the genus Leucaena.
The psyllid first became a problem on experimental plantings in Florida (USA) in 1983. From there it spread rapidly: April 1984, Hawaii; February 1985, Western Samoa and Fiji; October 1985, Philippines; March 1986, Papua New Guinea and Indonesia; April 1986, Australia; November 1986, Thailand; 1988, India and Sri Lanka; 1991, Mauritius; 1992, Reunion and coastal East Africa. It is reasonable to assume that, sooner or later, all areas where leucaena is grown will be affected. In Australia, following the first recording at Bowen in north Queensland in April 1986, the insects had within 3 months spread 800 km to Gympie, and by mid-October had reached Brisbane.
The extremely rapid rate of spread suggests that air currents (including high-level winds and cyclone/typhoon activity) are largely responsible for its dispersal, together with movement by aircraft and other man-made transportation. However, it is not uncommon to find psyllids on very isolated stands of leucaena, suggesting that the influence of man is not of great importance.
Fig. 6.1.1. Adult leucaena psyllid (Heteropsylla cubana) (x40).
The female psyllid lays its eggs on very young shoots where they are lodged between the folds of the developing leaflets. The eggs are oval, 0.3 mm long and 0.1 mm wide. Newly laid eggs are white, but turn orange or reddish brown after a day or two. Individual females can lay up to 400 eggs. Eggs hatch in 2-4 days, and there are five nymphal stages. The nymphs rapidly become quite mobile, and can congregate in large numbers on the growing points of young shoots. The total time from egg to adult may vary somewhat with the environment, but is generally between 10 and 16 days. Thus there is the potential for extremely rapid population build-up, and many generations per year.
In the field, psyllid populations normally fluctuate quite widely over time. There have been a number of attempts to assess the effect of environmental factors on psyllid development (see, for example several papers in Napompeth and MacDicken 1989). These usually involve trying to relate environmental factors such as rainfall and temperature to various estimates of psyllid abundance. We have carried out such studies at Samford, southeast Queensland, and Figure 6.1.2 illustrates the type of data obtained. In our work, we monitored numbers of adult psyllids, egg numbers and nymph numbers. Egg-laying (which requires young developing leaves) was rather cyclic, but large numbers of eggs did not always result in large numbers of adults. The peak numbers of nymphs tended to occur soon after rain. A feature of the psyllid seems to be the variability that often exists between adjacent sites. Moisture and nutrient status also influence these variations, and stand density, humidity and exposure to wind could also be important. Management for forage greatly increases psyllid numbers.
Fig. 6.1.2. Numbers of adults and estimated changes in abundance of eggs and nymphs of the leucaena psyllid at Samford during 1987/88. Temperatures shown are mean maximum and minimum for the week preceding the observation and rainfall is the total for that week.
There is no consensus concerning the effect of rainfall and temperature, as in some cases psyllid numbers are reported to be at a maximum during the wet season, and in others, during the dry season. However, it is fairly certain that neither hot dry conditions nor sustained periods of intense rain are favourable for the build-up of psyllid populations. It is clear that when conditions are good for the growth of leucaena, they are usually also good for the multiplication of the psyllid. Large populations of the psyllid cannot develop when the leucaena itself is suffering from moisture stress, or has its growth limited by cool temperatures. Frost will effectively kill psyllids.
One of the difficulties involved in this type of work is that because of the short time involved in the various stages of psyllid development, environmental variables must be measured frequently, ideally on a daily basis. For example, in the Samford work, we frequently found that adult psyllids would appear within a day or two of rain, although they were not previously apparent. Also, one or two days of dry, 35°C temperatures has been sufficient to reduce markedly existing populations. A more detailed discussion of psyllid biology may be found in Waterhouse and Norris (1987).
The leucaena psyllid damages the plant by its sucking action, although it is possible that there is also some phytotoxic principle involved, as occurs with some other psyllids. Both the nymphs and adults feed by sucking from the phloem of the developing shoots and young foliage. The insects exude drops of sticky fluid on the leaves causing the leaflets to stick together. The overall effect is to prevent the growth of new leaves. Since older leaves are not damaged to any great extent, casual observation may suggest that a particular leucaena stand has little or no damage. It is essential to examine the growing points and young foliage for the presence of psyllids, and subsequent leaf loss, to assess adequately the extent of any damage. Frequently, where psyllids have been active, there will be no new leaves for a distance of up to 30 cm from the 'growing point', representing a loss of up to 10-12 leaves, or several months' growth. This can be readily seen from a distance.
Quantification of the damage caused by the psyllid is difficult. Within the first 2 years of invasion by psyllids in the Philippines and Indonesia, defoliation was sufficiently severe to reduce cattle weight gains and stocking rates dramatically. Recent reports would suggest that psyllid damage is sometimes reduced and stocking rates have in places returned to normal. In experiments in north Queensland and Indonesia the effect of psyllids on dry matter yield was examined by comparing the yield from plots sprayed with insecticide and unsprayed plots over a period of 1 year (Palmer et al. 1989). In north Queensland total production when psyllids were not controlled was reduced to about 45% of that in the sprayed treatment. At one site in Indonesia, total yield was reduced by about one-third, while at the other there was no effect of psyllids on yield. In southern Queensland, annual losses of leaf production of over 50% have been recorded (Bray and Woodroffe 1991). Stem (wood) yield was even more severely affected. However, it is likely that losses in the drier areas of central Queensland are less, probably of the order of 20%.
There have been several attempts at assessing the numbers of psyllids present in their different life cycle stages (see papers in NFTA (1987) and Napompeth and MacDicken (1989)). Rating systems have been developed (e.g. Bray and Woodroffe 1988a, Wheeler 1988) based on the numbers of adults and nymphs observed. However, careful counting (Elder and Mayer 1990) suggests that these methods give gross underestimates of the numbers of eggs present, and are unable to distinguish between different levels of nymph infestation. Accurate estimation of egg and nymph numbers must apparently involve time-consuming laboratory counts. For most non-entomological purposes, it is probably sufficient to assess psyllid presence on the basis of 'none, few or many', rating a large number of plants in any one field.
Damage ratings have also been made with arbitrary rating schemes, commonly using a scale ranging from 1 (slight puckering of leaflets) to 6 or 7 (total defoliation of the shoot) or in some cases to 9. These have been quite effective but give no indication of the real loss due to psyllid infestation. The ultimate test of resistance is, of course, to establish the reduction in yield of a particular genotype due to the presence of the psyllid. This is very difficult and time consuming.
The ideal method of assessing resistance must account for a number of factors, including variation in plant phenology (e.g. height, branching, foliage density, shading, plant age), and availability of water and nutrients, which may confound any observed 'resistance'. Accurate experiments to assess resistance must be designed to provide sufficient replication of each entry, susceptible controls and a 'saturated' psyllid environment, such as the high populations generated by stands of susceptible varieties. The plants being compared should be similar in age and phenology and be exposed to the same environmental conditions. Only when these conditions are fulfilled can there be confidence in the assessment. We have characterised the resistance of a number of species using potted seedlings exposed to psyllids in the field (Bray and Woodroffe 1988a).
Do nothing
In any situation, there is a balance between a whole range of biological and abiotic entities. The arrival of the leucaena psyllid where it has not occurred before represents a significant disturbance of the existing ecosystem. Given time, a balance has been reached in most areas invaded by psyllids. This laissez faire strategy assumes that the effects of predators, parasites and diseases already present, together with climatic influences, will serve to keep the psyllid to acceptably low levels. Obviously, the psyllid has been controlled very effectively in its native habitat in this manner.
Use of insecticide
An initial reaction to the psyllid infestation in Indonesia was to spray with insecticides. Although the psyllid is readily killed by low doses of several insecticides, including dimethoate (0.03%), this treatment is generally ineffective. Not only is it difficult to be sure that all trees (including those not in gardens or pastures) are sprayed, but it is hard to ensure complete coverage of large plants. In addition, the extreme mobility of the psyllid means that new recruits may arrive on the wind soon after spraying. At Samford, we have found that it is impossible to keep small plots free of psyllids even by spraying every 2 weeks, when significant areas of unsprayed plants exist in the vicinity. Spraying could perhaps be worthwhile to try and save seedling plants, or to protect valuable plants in nursery situations, such as for seed production. Other important considerations with the use of insecticide are that beneficial native insects will also be killed, and that pesticide residues may be present in the leucaena when fed to animals.
Management options
There are varying reports of the extent of psyllid damage in leucaena grown under various management systems. In central Queensland, it is reported that leucaena grown as large trees seemed to suffer more damage than when grown as hedgerows. However, in other situations, trees that are grown for wood and not harvested for foliage show minimal damage after psyllid populations stabilise. Where moisture stress does not occur and forage harvest or grazing is continuous, psyllid populations are likely to remain high with significant levels of damage. Drought leads to leaf drop and great reduction in psyllid populations.
Biological control
In any environment there will exist some insects that feed on one or more stages of the life cycle of the psyllid. The larvae of the common ladybird beetles are particularly good in this respect, but do not seem to be able to keep the populations under control. However, it may be worth observing existing predator populations. Work in Hawaii has identified two useful predators, Curinus coeruleus and Psyllaephagus yaseeni. Curinus, a beetle that attacks psyllid larvae, is a general predator and has been widely distributed to several countries in southeast Asia It has yet to be established how effectively it is contributing to reduction in psyllid populations in the field. Psyllaephagus (a wasp that attacks the eggs of the psyllid) is a more specific parasite restricted to the genus Heteropsylla, but has not been widely released. The use of entomogenous fungi, now being studied in Taiwan, the Philippines and Papua New Guinea (Hollingsworth et al. 1991) may offer possibilities for psyllid control.
The introduction of biological control agents to a country or region is not a decision that can be taken lightly, or independently from neighbouring countries, due to the complexity of the interactions in any biological system. As an example, one Australian programme is seeking to control the tropical weed Mimosa through the introduction of a psyllid of the genus Heteropsylla. Any attempt to control the leucaena psyllid by the introduction of Psyllaephagus could negate this programme. There is also concern in India that the introduction of a general predator such as Curinus could have a bad effect on populations of the lac insect.
The exploration of the psyllid's native area for further predators and parasites should continue. It is not an easy task, and any new organism will need to be carefully tested before release.
Selection and breeding
There are three possible approaches:
· Uses of psyllid-tolerant genotypes of L. leucocephala. Variation in tolerance of psyllids exists in this species, with arboreal cultivars like K584 and K636 (University of Hawaii) showing major increases in yield under psyllid pressure in several countries. These cultivars appear to have long-lived leaves and the capacity to produce many new axial branches when growing tips are damaged.· Use of other species of Leucaena. Of the available species, L. diversifolia and L. pallida are the most promising, having performed well in the NFTA trials. However, there is considerable variation within these species, and all collections do not have the same degree of resistance. Both Hawaiian (K376, K784) and Australian (CPI46568) lines show promise. However, although these species may be valuable for wood production or soil conservation purposes, they have not yet been adequately assessed for animal feed. Preliminary experiments indicate that, although not as digestible as L. leucocephala, they can still provide a useful source of supplementary feed (B. Palmer, personal communication). More information is needed on their establishment requirements, yield, coppicing ability (regrowth), palatibility and feeding value.
Some form of breeding programme using interspecific hybridisation. Most species of the genus Leucaena are resistant to psyllids, and many can be hybridised with L. leucocephala (Brewbaker and Sorensson 1990). After four cycles of selection, hybrids of L. leucocephala and L. pallida are being tendered for release to forage growers in Hawaii by the University of Hawaii. Selection has led rapidly to high levels of psyllid resistance, maintaining productivity and with some erosion of nutritive quality. The species L. leucocephala is recognised for its superiority in fodder quality (intake, digestibility), and long-term breeding of forage varieties will probably profit by back-crossing generations. In any event more information is needed concerning the nature and stability of resistance, and about possible variation among psyllid populations.
Use of other genera
There are a number of other species of tree and shrub legumes which can be used as alternatives to leucaena, although none can match all its virtues at this time. Gliricidia, Sesbania, Calliandra and Codariocalyx all offer good prospects. However, there remain many unanswered questions with these genera in regard to their perenniality, palatability and nutritional quality. What is clear is that when any species is grown on a large scale, it becomes a potential target for damaging pests and diseases. Thus it is unwise to rely too much on any one species, and future plantings of tree legumes should ensure that a range of species is used.
Bray, R.A. and Woodroffe, T.D. (1988a) Resistance of some Leucaena species to the leucaena psyllid. Tropical Grasslands 22, 11-16.
Bray, R.A. and Woodroffe, T.D. (1988b) Some observations on the population dynamics of the leucaena psyllid in southeast Queensland. Leucaena Research Reports 9, 8-10.
Bray, R.A. and Woodroffe, T.D. (1991) Effect of the leucaena psyllid on yield of Leucaena leucocephala cv. Cunningham in south-east Queensland. Tropical Grasslands 25, 356-357.
Brewbaker, J.L. and Sorensson, C.T. (1990) New tree crops from interspecific Leucaena hybrids. In: Janick, J. and Simon, J.E. (eds), Advances in New Crops, Timber Press, Portland, Oregon, pp. 283-289.
Elder, R.J. and Mayer, D.G. (1990) An improved sampling method for Heteropsylla cubana Crawford (Hemiptera:Psyllidae) on Leucaena leucocephala. Journal of the Australian Entomological Society 29, 131-137.
Hollingsworth, R.G., Hela, F. and Moxon, J.E. (1991) Natural control of leucaena psyllid by fungal pathogens. Leucaena Research Reports 12, 89-90.
Napompeth, B. and MacDicken, K.G. (eds), (1989) Leucaena Psyllid: Problems and Management. Proceedings of an international workshop held in Bogor, Indonesia. NFTA, Hawaii, 208 pp.
NFTA (1987) Proceedings of a Workshop on Biological and Genetic Control Strategies for the Leucaena Psyllid. A special edition of Leucaena Research Reports Volume 7 (2), NFTA, Hawaii, 109 pp.
Palmer, B., Bray, R.A., Ibrahim, T.M. and Fulloon, M.G. (1989) The effect of the leucaena psyllid on the yield of Leucaena leucocephala cv. Cunningham at four sites in the tropics. Tropical Grasslands 23, 105-107.
Waterhouse, D.F. and Norris, K.R. (1987) Biological Control Pacific Prospect. Inkata Press, Melbourne.
Wheeler, R.H. (1988) Leucaena psyllid trial at Waimanalo, Hawaii. Leucaena Research Reports 8, 25-29.
