6.3 Insect Pests of Forage Tree Legumes: Biology and Non-chemical Control

G.H. Walter and W.H. Parry

Introduction
Insect Pests of Forage Tree Legumes in the Tropics
Non-insecticidal Techniques in Insect Control
Conclusions
References

Introduction

Native and exotic tree legume species are grown as multipurpose trees in agroforestry systems throughout the tropics. Species in both categories harbour insects that defoliate, produce galls or leaf mines, eat seeds, suck sap or bore in stems, roots or branches. Some insects, mainly sap suckers, also transmit pathogens. The debilitating impact of insects on forage quality and quantity or on seed production may be so great that remedial activity is desirable (Sen-Sarma 1987).

Introduced plants usually 'accumulate' a range of associated herbivorous insect species. Some invade from elsewhere, whereas other insects may have been pre-adapted to the introduced plants, perhaps through their adaptation to a close relative. Other insects may simply be present in low numbers and use the trees as alternative hosts at times when their usual host is unsuitable or unavailable. The list of insects that attack an introduced plant species in most cases increases rapidly at first and then gradually with time (Strong et al. 1984), presumably as the distribution of the plant increases and as the rate of encounter of insect species pre-adapted to the host are slows down.

Usually it is insects that have been accidentally introduced or that have invaded on their own (e.g. entry of the leucaena psyllid into Australia (Bray and Sands 1987)) that are the major pests. Since their natural enemies (predators and parasitoids) usually do not accompany them when they are introduced, the high densities achieved are seen to be a consequence of their 'escape' from natural enemies. There are, however, other variables to consider; the introduced plants may be present at much higher densities than in their native habitat, and selected cultivars that are planted may be more susceptible to insect attack.

Most of the serious damage to trees is caused when the insects reach high densities, although some species cause considerable damage even when relatively few are present. Few stem borers, and sometimes just one, may be sufficient to kill a tree (Conway 1978). Such aspects of an insect's life history may determine appropriate methods of control. Species that cause serious damage only at high densities may be reduced to acceptably low levels by biological control or the breeding of resistant cultivars, whereas others may need to be dealt with in other ways (Conway 1978).

In the following section we summarise the information available on insect pests relevant to several important tropical forage tree legumes, and make the few generalisations that are possible. Finally, we deal with practical control tactics (besides insecticidal control) that may be used to prevent insect damage.

Insect Pests of Forage Tree Legumes in the Tropics

Table 6.3.1 summarises the data available on known pests of several important forage tree legumes. Several general papers contain additional information (e.g. Sen-Sarma 1987, Kumar 1990). Undoubtedly the list will grow with time. However, it is significant that none of the insects mentioned has been comprehensively studied to unravel their host plant relationships and other aspects of their ecology. For some, even their pest status is unclear. This lack of understanding impedes development of acceptable control measures. Our knowledge of the biology and ecology of the most studied pest of forage tree legumes, the leucaena psyllid (Heteropsylla cubana), is still insufficient to achieve acceptable control (Napompeth and MacDicken 1990).

The leucaena psyllid does, however, demonstrate the devastation that insect herbivores can inflict. Even in more natural situations than Agroforestry, insects may consume more than other, larger herbivores. For example, over 60% of total leaf material consumed by herbivores in a South African savannah was eaten by insects, mostly lepidopteran larvae (Gander 1982). Such data must be considered as being only indicative as, in nature, damage levels are likely to vary among systems and among situations. In any case, the extent to which herbivorous insects have reduced the impact of weed species is a good measure of their capabilities against plant species, legume trees included (Hoffmann 1988, Moran and Hoffmann 1989, Hoffmann et al. 1990, Dennill and Donnelly 1991).

In dealing with specific examples of insect pests, their adaptations and ecology must be considered. Each species presents us with its own challenge, which may vary with changing environmental conditions from area to area and from time to time (Walter 1994). For example, trees may vary in their physiological status and this may in turn influence insect behaviour and abundance. Some herbivorous insects 'perform' better on stressed trees and the environmental cause of the stress thus induces herbivore outbreaks (White 1969, 1970a,b). Other insects (e.g. Acizzia russellae on Acacia karroo) may be primarily adapted to flushing growth, the production of which may enhance 'performance' (Webb and Moran 1978). Little is known about such influences on tropical forage tree legumes, but Singh (1986) found that the amount of damage to Acacia nilotica, Dalbergia sissoo and Prosopis cineraria varied with soil type. This presumably influenced the physiological status of the tree in relation to the pests, which were bagworms (Cryptothelea cramerii) and several species of chafer beetles (Holotricha consanguinea, H. serrata, Adorata spp. and Anomala spp.).

Although virtually all insect species exhibit some degree of host specificity and many are indeed species-specific in their host requirements, some herbivores can feed and reproduce on several host species, sometimes even on ones that are not closely related. Such polyphagous species present problems when populations transfer from one host species to a cultivated species. For instance, many borers have a wide host range. The teak sapling borer, Sahyadrassus malabaricus, feeds on many trees besides teak, including Eucalyptus spp. and the legumes Paraserianthes falcataria and Calliandra calothyrsus. Forage tree legumes planted in close proximity to teak plantations, for example, may be subjected to rates of colonisation by this pest that are higher than on trees further from teak plantations. Care should therefore be taken when planting tree legumes to ensure that such situations are avoided. However, in researching and interpreting the population biology of polyphagous species it is imperative to eliminate the possibility that one is combining species from different host plants, simply because they look alike. Mahon et al. (1982) provide an example involving such sibling species of leaf miners on jarrah (Eucalyptus marginata), flooded gum (E. rudis) and prickly bark (E. todtiana). The principles and appropriate techniques applied in the unravelling of sibling species complexes have been outlined by Paterson (1991).

Also of possible relevance to the cultivation of tree legumes is the phenomenon of 'induced resistance'. This arises when herbivory induces chemical changes in the plant, which may cause negative feedback on insects of the same species and sometimes even on other species (Haukioja 1991). The effect is so strong that it may influence population numbers of insects, even to the point of playing a role in population cycles (Haukioja 1991). The extent to which insect herbivory affects the nutritive value of tree legumes grown for forage is not known and requires evaluation (Bryant et al. 1991).

Non-insecticidal Techniques in Insect Control

Insecticides have been recommended for the control of a number of forage tree legume pests (Sen-Sarma 1987). For example, Monocrotophos was used to control the bark-eating caterpillar (Indarbela quadrinotata) on Albizia lebbeck in the Punjab (Sandhu et al. 1987). Although insecticides can be effective in increasing forage yield (e.g. Palmer et al. 1989) they are generally too costly for this purpose and less expensive alternatives should be sought.

Table 6.3.1. Insects that damage some important tropical forage tree legumes. The insect fauna associated with each plant in its area of origin is not included, and neither is Heteropsylla cubana.

Tree	Insect	Damage	Impact and comments	Place	Ref.*
Acacia nilotica	Celosterna scabrator (Cerambycidae)	stem and root borer	devastated trees, and taungyas had to be abandoned. May make establishment difficult.	India	1, 2, 3
	Cryptothelea cramerii (Psychidae)	leaf feeder	defoliation	India	4
	Ophiusa lanata (Noctuidae)	defoliator	under taungya situations	India	1
	Oxyrachis tarandus (Membracidae)	sap sucker	polyphagous pest	India	37
	Sphenoptera chalcichroa arenosa (Buprestidae)	bark tunnelling	dieback and gradual tree mortality	The Sudan	5
	Caryedon serratus (Bruchidae)	destruction to stored seeds	unknown	India	6
	Batocera rufomaculate (Lamiidae)	stem borer	not stated	India	37
Albizia lebbeck	Bruchidius spp. (Bruchidae)	seed borer	unknown	India	7
	Bruchidius sparsemaculatus (Bruchidae)	seed borer	a new pest	India	8
	Ferrisia virgata (Pseudococcidae)	sap sucker	attacked in screenhouse	Nigeria	9
	Glophodes pyloalis	leaf feeder	serious defoliation	India	4
	Indarbela quadrinotata (Metarbelidae)	bark-eating caterpillar	considered a pest	India	10
	Oxyrachis tarandus (Membracidae)	sap sucker	polyphagous pest	India	37
	Psylla hyalina (Psyllidae)	sap sucker	sometimes damages seedlings and saplings	India	11, 12, 13
	Spirama retorta (Noctuidae)	leaf feeder	defoliation	India	4
Calliandra calothyrsus	Leucopholis irrorata (Scarabaeidae)	leaf feeder	foliar damage of ornamentals	Philippines	14
	Myllocerus viridanus (Curculionidae)	polyphagous leaf feeder	considerable defoliation	India	15
	Pachnoda ephippiata (Cetoniinae)	feeds on fruits, flowers, and leaves	often total loss of seeds	Kenya	16
	Sahyadrassus malabaricus (Hepialidae)	borer	unknown	India	17
	"Similar to Hypsipyla robusta" (Pyralidae)	stem borer	about 40% of trees infested	Philippines	18
	Unidentified lymantriid	flower feeder	unknown	Philippines	14
Gliricidia sepium	Azeta versicolor (Noctuidae)	leaf feeder	defoliation; decreased yield of vanilla crop it was shading	Costa Rica	19
	Ferrisia virgata (Pseudococcidae)	sap sucker	attacked in screenhouse	Nigeria	9
	Sahyadrassus malabaricus (Hepialidae)	borer	is a pest	India	20
Leucaena collinsii	Acanthoscelides macrophythalmus (Bruchidae)	seed eater	unknown	Mexico	21
Leucaena diversifolia	Acanthoscelides macrophythalmus (Bruchidae)	seed eater	unknown	Mexico	21
	Spatularia mimosae (Tineidae)	seed feeder	high proportion of seeds damaged	Philippines	22, 23
Leucaena leucocephala	Acanthoscelides macrophythalmus (Bruchidae)	seed eater	unknown	Mexico	21
	Araecerus fasciculatus (Anthribidae)	seed boring beetle	major pest	Philippines	24, 25
	Asterolecanium pustulans (Asterolecaniidae)	sap sucker on branches and trunks	can kill trees	Taiwan	26
Leucaena leucocephala	Coccus elongatus (Coccidae)	sap sucker	occasional damage to stem, leaves and seedlings	Taiwan	27
	Coccus longulus (Coccidae)	sap sucker on branches and trunks	can kill trees	Taiwan	26
	Cerambycidae (undetermined sp.)	bores into stems and branches	minor pest	Philippines	25
	Cossus sp. (Cossidae)	bores into stems and branches	minor pest	Philippines	26
	Diaspididae (undetermined sp.)	sucks sap from stems and leaves	major pest, defoliate seedlings	Philippines	28
	Ferrisia virgata (Pseudococcidae)	sucks sap from stems, branches, leaves and fruits	major pest, causes wilting and defoliation	Philippines, Nigeria, India	28, 25, 9
	Gryllotalpa africana (Gryllotalpidae)	feeds on the roots	minor pest	Philippines	25
	Hemiberlesia implicate (Diaspididae)	sap sucker on branches and trunks	can kill trees	Taiwan	26
	Ithome lassula (Cosmopterigidae)	feeds on flower heads	affects commercial seed production	Australia	29
	Oncideres pustulata (Cerambycidae)	severs branches	serious pest	New World	30
	Spatularia mimosae (Tineidae)	seed feeder	major pest	Philippines, Taiwan	27, 25, 23
	Zeuzera coffeae (Cossidae)	bores into stems and branches	minor pest	Philippines	25
Leucaena pulverulenta	Oncideres pustulata (Cerambycidae)	severs branches	serious pest	New World	30
Leucaena sp.	Apogonia rouca (Scarabaeidae)	leaf-feeder	damages newly-emerged leaves and seedlings	India	31
	Oncideres sp. (Cerambycidae)	branch ring girdler	unknown	New World	30
Prosopis alba	Oncideres pustulata (Cerambycidae)	severs branches	serious pest	New World	30
Prosopis chilensis	Oncideres pustulata (Cerambycidae)	severs branches	serious pest	New World	30
Prosopis cineraria	Celosterna scabrator (Cerambycidae)	root and stem borer	has started attacking this host	India	37
	Oxyrachis tarandus (Membracidae)	sap sucker	polyphagous pest	India	37
Prosopis juliflora	Oxyrachis tarandus (Membracidae)	sap sucker	polyphagous pest	India	37
Sesbania aculeata	Bruchophagus mellipes (Eurytomidae)	seed feeder	requires control in stored seeds	India	34, 32
	Dasychira mendosa (Lymantriidae)	polyphagous leaf feeder	pest	Bangladesh	33
Sesbania esculenta	Bruchophagus mellipes (Eurytomidae)	seed feeder	unknown	India	34
Sesbania grandiflora	Ceroplastodes sp. (Coccidae)	sap sucker	rapid range extension and establishment	Andaman & Nicobar Islands	35
Sesbania rostrata	Bruchophagus mellipes (Eurytomidae)	seed feeder	unknown	India	34
Sesbania sesban	Bruchophagus mellipes (Eurytomidae)	seed feeder	unknown	India	34
	Ceroplastodes sp. (Coccidae)	sap sucker	rapid range extension and establishment	Andaman and Nicobar Islands	35
Sesbania speciosa	Bruchophagus mellipes (Eurytomidae)	seed feeder	unknown	India	34

* References: 1. Singh and Singh (1987); 2. Anon. (1981); 3. Ralph (1990); 4. Singh (1986); 5. El-Atta (1988); 6. Singal and Toki (1990); 7. Tewari and Arora (1984); 8. Verma et al. (1987); 9. Kadiata et al. (1992); 10. Das et al. (1985); 11. Hegde and Relwani (1986); 12. Singh et al. (1989); 13. Peter et al. (1990); 14. Braza (1991); 15. Ahmed (1989); 16. Kaudia (1990); 17. Nair (1982); 18. Luego (1989); 19. Young (1988); 20. Devasahayam et al. (1987); 21. Oakes (1981); 22. Braza (1988); 23. Braza (1989); 24. Braza (1987); 25. Braza and Salise (1988); 26. Chang et al. (1982); 27. Chang (1980); 28. Balakrishnan et al. (1991); 29. Beattie (1981); 30. Felker et al. (1983); 31. Pawar (1986); 32. Verma (1991); 33. Das (1990); 34. Bhalla et al. (1988); 35. Shah et al. (1989); 36. Sen-Sarma (1987)

The use of azadirachtin, an isolate of neem seed kernels, is recommended against defoliators in places where it is economical because it is said to be safe to humans (Sen-Sarma 1987).

Behavioural and genetical methods of control

Many insect species (particularly those that mate predominantly at night) communicate sexually with volatile chemicals called pheromones. For many pests, pheromones have been analysed and synthesised. As a bait in appropriately designed traps, pheromones are effective in luring conspecific insects (of the sex that responds to the pheromones), even when densities are so low that the insects are otherwise undetectable. Pheromone traps are thus useful for monitoring insect population levels so that other control measures can be instituted at the most appropriate time. Pheromones are also used in orchards to 'confuse' mating efforts and thus reduce oviposition. This latter application may be impractical for agroforestry conditions except perhaps under unforeseen circumstances that may arise in the future.

Sterile insect methods have been used successfully against insects that are pests at low density (e.g. screwworm flies in America) or pests that need to be exterminated on accidental introduction into a new area (e.g. medflies in California). These methods would generally be too costly for agroforestry.

Plant varietal resistance

Breeding programmes can produce tree varieties that are resistant to a particular pest species that causes severe damage. This has been done with leucaena and some bred cultivars are being developed which are quite effective against the leucaena psyllid. Unfortunately, resistance in this species is associated with high tannin content, which decreases the forage value of the plant. However, the resistance mechanism in leucaena is not understood, and this retards progress in selecting for resistance as breeders have no way of assaying their progress on the basis of chemical analysis rather than on observed resistance to the pest (A. Castillo and H.M. Shelton, unpublished data).

Transgenic potato and cotton plants, with the gene for producing Bacillus thuringiensis toxin introduced into their genome, are now being investigated for possible cultivation. This may be a possibility for defoliating pests (usually caterpillars) of tree legumes that are killed by the toxin.

Cultural methods or ecological management

Cultural control methods are diverse and rely on a good understanding of the pest species' ecology in relation to the production system. Not all would be appropriate for pest management in forage tree systems because the environment is not manipulated to the extent it is in cropping systems. However, combinations of plants in mixed systems can be selected to ensure that pest populations are not inadvertently enhanced by the provision of a good alternative host for a particular pest species. Also, stored seed may be protected by manipulating the storage environment and practising good sanitation nearby.

Biological control

Effective biological control has many advantages, especially if it is of the self-sustaining type. Unfortunately the success rate of biocontrol is, in general, quite low. A substantial effort in locating species-specific parasitoids or predators is necessary, as shown by the exploration for leucaena psyllid natural enemies (Waage 1990). Furthermore, an appropriate research base on the species status of the natural enemies and host would substantially reduce the possibility of costly mistakes, which is an ever-present possibility (Paterson 1991). It is noteworthy that the leucaena psyllid is thought to have 'host races' (Waage 1990), which is often a good hint that host-specific sibling species may be present.

Conclusions

Alternatives to chemical control are available for dealing with insect pests of tree legumes. Biological and varietal control are probably best suited to the requirements of forage production and to the means and available resources of the grower. Chemicals can be quite effective even if little is known about the pest species, and the same chemical can be used to kill insects of several species. In contrast, alternative methods of control require a substantial understanding of each insect pest species as the design of the treatment centres around the unique adaptations and ecology of each species. To date, such understanding of forage tree legume pests is lacking.

Insect biocontrol has been successful in the past with relatively little research effort and the programmes that have worked well are the most widely quoted, for example, the Australian bug (Icerya purchasi) in California (Doutt 1958). Most introductions in biocontrol projects have, however, not succeeded and to improve the success rate we must follow basic principles in species theory (Paterson 1991). The appropriate research on the host relationships of parasitoids in nature and the specificity of predator-prey relationships in nature must also be conducted for each species involved. Climate matching between the proposed biocontrol site and the area that is explored for natural enemies may enhance the chances of success (Kennett et al. 1966).

Finally, the breeding of resistant varieties may be aided considerably by an understanding of the chemical or other basis of resistance and of the way in which the target insects are affected.

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