ALTERNATIVE MANAGEMENT SYSTEMS FOR TERMITES IN AGROECOSYSTEMS
Termite attacks on annual and perennial crops, especially in the semi-arid and sub-humid tropics, cause significant yield losses. In general, damage by termites is greater in rain-fed than irrigated crops, during dry periods or droughts than periods of regular rainfall, in lowland rather than highland areas, and in plants under stress (lack of moisture, disease or physical damage), rather than in healthy and vigorous plants. In particular, exotic crops are more susceptible to termite attacks than indigenous crops.
Termites causing greater losses in agriculture belong to the following families and genera: Hodotermitidae (Anacanthotermes and Hodotermes), Kalotermitidae (Neotermes), Rhinotermitidae (Copotermes, Heterotermes, and Psammotermes), and Termitidae (Amitermes, Ancistrotermes, Cornitermes, Macrotermes, Microcerotermes, Microtermes, Odontotermes, Procornitermes, and Syntermes).
The extent to which termites are a problem to agricultural crops, the nature of loss they cause and the plant species they infest are very much related to the geographic region concerned.
Lack of taxonomic understanding has been a major impediment to the study and management of tropical termites. Species numbers are high and many of those are undescribed or poorly described. Farmers and extension personnel often lack proper training. Very few specialists are able to identify tropical termite species and many of the economically important groups lack proper taxonomic revision, resulting in a large number of incorrect, doubtful or incomplete identifications. Under these conditions it is very difficult to accurately collect information on the biology and economic importance of each species. Therefore, it is strongly recommended to establish and develop regional reference collections, identification guides and training programs for extension staff.
Termites can attack plants at any stage of development from the seed to the mature plant.
Various species of termites build large mounds often containing many thousand of individuals. Termites construct shallow subterranean foraging galleries radiating from the nest for distance of up to 50m. The main galleries give rise to a network of small galleries from which foraging parties can exploit potential food resources over extensive areas of land. Termites forage directly on underground plant material. Seedlings are either cut just below or above the soil surface. In the latter case termites gain access from soil-covered galleries impinging on the base of the plant. Usually, the seedlings are completely severed, resulting in lowered plant populations.
In Africa, the reputation of termites as pests is coupled with the presence of large mounds in crops or close to trees. Termites are widespread in Africa, restricted mainly by desert areas and the lower temperatures found at higher altitudes. There are approximately 20-50 damaging termite species in savanna and forest ecosystems in the family Termitidae. The majority of species feed on plant material, living or dead, dung or soil rich in organic material. The greatest pest potential exists within the subfamily Macrotermitinae, which has a symbiotic association with the fungus Termitomyces. The most economically important genera throughout Africa are Macrotermes, Odontotermes, Pseudacanthotermes, Ancistrotermes and Microtermes. These genera differ characteristically in their biology and mode of attack. Macrotermes spp. build large epigeal nests (mounds) from which they forage outwards for distances up to 50m in galleries/runways. They attack plants at the base of the stem, ring-barking or cutting them through completely. Odontotermes spp. build both subterranean and epigeal nests. Damage is due to feeding either under soil sheeting on the outer surface of the plants or on the roots. Microtermes spp. and Ancistrotermes spp. have diffuse subterranean nests and attack plants from below ground by entering the root system and tunneling up into the stem, hollowing it out and frequently filling it with soil.
Microtermes and Odontotermes species have always been associated with groundnuts in semi-arid tropical countries of Africa, causing yield losses between 10 and 30%. Management measures include the use of resistant groundnut varieties, cultural practices, botanical insecticides, and minimal application of synthetic insecticides either to the soil or as a seed dressing. These treatments form a barrier, which repels or kills foraging termites.
Among cereal crops, maize is the most often damaged by termites. Microtermes and Ancistrotermes attack maturing and mature maize plants, while Macrotermes spp. cause damage to seedlings. Species of Odontotermes, Allodontermes, and Pseudacanthotermes can defoliate maize seedlings or consume the entire plant. Yield losses of from 30 to 60% have been reported in some parts of Africa. Maize plants attacked early in the season can compensate damage with new growth. One of the options for farmers to manage termites is to sow at a higher rate. The other option is to dress the seeds with insecticides.
The most damage to sugarcane is done by genera Amitermes, Pseudacanthotermes, Macrotermes, Odontotermes, Microtermes and Ancistrotermes. Yield losses of 18% were reported in Sudan, and 5-10% in central Africa. In Nigeria plant germination failure of up to 28% has been reported. The most common damage to sugarcane is the destruction of the setts (planting material). The usual method of prevention is to dip the setts in various formulations of chlorinated hydrocarbons before planting, or to spray them in the furrows before filling in.
Yams and cassava are grown from tuber and stem cuttings, respectively, and are consistently attacked as seed pieces by Amitermes, a predominantly root-feeding species. Ancistrotermes, Macrotermes, Odontotermes, Microtermes and Pseudacanthotermes are also involved in damaging the maturing crops, as well by hollowing out stems at ground level. The current management strategy consists of treating setts with aldrin dust.
Termite species in the genera Allondotermes, Ancistrotermes, Hodotermes, Microtermes and Odontotermes have been reported to damage cotton especially in the drier parts of Africa. Management measures include broadscale application of chlorinated hydrocarbons or seed dressings, and baiting with chopped grass treated with insecticides.
Other Crops (Table 10)
Termite problems in agriculture in Southeast Asia largely affect perennial tree crops. The most economically important genera throughout Southeast Asia are Microtermes, Coptotermes, Odontotermes, Macrotermes, Trinevitermes and Heterotermes.
Microtermes obesi is a small, fungus-growing, subterranean termite that has a diffuse network of brood-rearing and nesting chambers in the ground. It can be very widespread where it occurs. The termite attacks freshly planted sugarcane setts in Thailand and the north of Peninsular Malaysia, resulting in poor establishment of plantations. Yield losses of 12% have been recorded in Pakistan. Species such as Heterotermes, Coptotermes, Odontotermes, Macrotermes and Trinervitermes also attack sugarcane. Current chemical control involves the use of granular insecticides, such as fipronil, applied into the soil when the setts are planted.
Coptotermes curvignathus in Southeast Asia causes damage to a number of commodity crops, as well as fruit trees (see Termites in Forestry). Crops that appear to be rather susceptible include rubber (Hevea brasiliensis), oil palm (Elaeis guineensis), coconut (Cocos nucifera), kapok (Ceiba pentandra), mango (Mangifera spp.) and papaya (Carica papaya). In oil palm and coconut, termites usually attack the spear region of the palm. Once the spear is destroyed, the tree dies, as it is the only growing vegetative part of the palm. Some crops are rarely attacked and, therefore, appear to be more resistant. These include coffee (Coffea arabica), tea (Camellia sinensis) and a number of fruit trees such as cocoa (Theobroma cacao), durian (Durio zibethenus), mangosteen (Garcinia mangostana), rambutan (Nephelium lappaceum), nutmeg (Myristica fragrans) and cloves (Syzygium aromaticum).
Other crops (Table 1)
In Australia, several species in the genera Mastotermes, Heterotermes, Coptotermes, Amitermes and Microcerotermes have been reported to attack a range of field crops (Table 10). However, most incidences are reported in the tropical areas, chiefly during the dry season or in low-rainfall areas, with Mastotermes darwiniensis as the key species. Mastotermes may cause significant damage to sugar cane and other produce, but only in areas where the crops are cultivated on other than rainforest soils (Mastotermes is absent from tropical rainforest regions.). Elsewhere in the country termite damage to field crops is sporadic, and in general associated with longer periods of dry weather or other special circumstances. For example, in southeastern Australia Coptotermes and Heterotermes can attack certain varieties of potatoes in dry summers, and in semi-arid regions cereal crops in the first year of cultivation on fallow land may be attacked by Amitermes neogermanus.
Mastotermes darwiniensis poses major problems to horticulture in the tropical Australia (see Table 10 and Termites in Forestry). For example, losses of cashew, mango and avocado trees to this species can be as high as 30% in the Northern Territory. In tropical Queensland, the importance of Mastotermes as a pest species in horticulture is likely to increase, as plantings of exotic fruit trees extend more and more into areas outside former rain forest. Elsewhere in Australia, localized damage, most notably by species of Coptotermes, to fruit trees, tea trees (Melaleuca) and grapevines has been recorded. But in most cases, termite attack is selective and focuses on plants that are past their prime or otherwise stressed (for example root-bound seedlings). Sources of moisture, including drip irrigation, may attract termites to some of these sites in the first place.
The main approach to dealing with the problems caused by Mastotermes to the horticultural industry in tropical Australia is a combination of plantation management and use of aggregation and baiting techniques. The POPs chemical mirex is the currently registered active ingredient in baits for Mastotermes control. (Northern Territory and Western Australia only). Alternative bait toxicants and other management strategies are under investigation.
On some islands of the South Pacific (northern Cook Islands, Tuvalu) Neotermes rainbowi hollows out the trunk of coconut palms. If attack starts in mature palms, termite activity has limited impact on yield until the palms snap off in storms at the point where the greatest damage has been done. Species of Neotermes in the South Pacific can also damage other tree crops such as Citrus and cocoa, and occasionally some annual crops. In experiments with the biological control agents Metharizium anisopliae (fungus) and Heterorhabditis sp. (nematode), colonies of Neotermes could be eliminated from palms and other trees. However, plantation management and tree care alone can be successful in preventing a build-up of Neotermes populations.
Termites are abundant and diverse in most parts of South America, particularly in tropical lowland forests, savannas and grasslands. Several native termite species have been reported as agricultural pests. As natural habitats are gradually replaced by urban and agricultural space, new pests are discovered, while the increasing trade with other continents facilitates the introduction of new pests. However, information on the pest status of termites in South America is still very limited. There are no estimates of the economic losses caused by termites or the relative importance of each species. Agricultural termite pests are more numerous and much less studied than urban pests. Furthermore, termite research and expertise in South America are highly concentrated in Brazil, while the fauna and termite problems of large countries, such as Colombia, are virtually unknown. A total of 53 termite species are reported as agricultural pests, and 15 species as pests of both agriculture and structure.
The main pest species belong to Rhinotermitinae (Heterotermes and Coptotermes) and Nasutitermitinae (Cornitermes, Procornitermes, Syntermes and Nasutitermes).
The most affected crops in South America are sugarcane, upland rice, maize, cotton, groundnuts, soybean, coffee, cassava, and some vegetables. Termite damage includes injury to roots, injury to leaves and stems, and injury to woody tissue by kalotermitids.
In the Amazon, the largest biome of South America (6 million km2), Nasutitermes, Coptotermes, and Heterotermes may cause variable damage to crops such as cassava, maize and fruit trees. In the Cerrado, which is savanna, termites are abundant and diverse. Because large areas of the Cerrado are being replaced by agroecosystems, termites may become a menace to some crops.
Perennial tree crops that are affected by termites in South America include several fruit trees, ornamental trees, palm trees and coffee. Damage to fruit trees is often restricted to seedlings and young trees, which may be killed by termites. Termites feed on several parts of the plant, especially the roots. Heterotermes tenuis seems to be the most important species causing these problems. Mature trees, including palms and coffee, may be attacked by drywood termites, particularly Neotermes, and by arboreal-nesting termitids Nasutitermes and Microcerotermes. However, in general, termites seem to be minor pests of tree crops in South America and, in many cases, damage is presumed based only on the presence of termites.
Introductory remark is provided in (Termites as Structural Pests)
Alternatives to POPs for control of termites in agriculture is attracting renewed interest following increasing restrictions on the use of POPís. Some progress has been made in this respect in agricultural settings involving termites.
Management of termites generally varies according to the type of termites involved and can be conceived in three ways: (i) preventing termites gaining access to the plants, (ii) reducing termite densities in the vicinity of the plants and (iii) rendering plants less susceptible to attack by termites. The three approaches may overlap in some cases.
Conventionally, placing persistent insecticidal barriers in the soil around the roots has prevented damage to plants by subterranean termites. Organochlorines (lindane, aldrin, dieldrin, chlordane and heptachlor) were, in the past, applied almost exclusively as seed dressing or to the soil in planting holes or as furrows treatments. Attack on crops, especially by Macrotermitinae in Africa and India, have been prevented by poisoning the mounds with organochlorines. Following increasing restrictions on the use of POPs less persistent insecticides, such as organophosphates (chlorpyrifos, iodofenphos, isofenphos), carbamates (carbosulfan, carbofuran), and pyrethroids (permethrin, decamethrin, deltamethrin), have been used as alternatives, however their low persistence often necessitates repeated applications. Recently, controlled-release formulations of some non-persistent insecticides were tried and found to be effective and long-lasting. However, these formulations are not cost-effective for the majority of low-income farmers in developing countries.
A number of techniques are used to reduce termite densities in the vicinity of the plants. They include cultural methods, plant insecticides and biological control.
Deep plowing or hand tillage exposes termites to desiccation and to predators, thus reducing their number in the crops. Pre-planting tillage also destroys the tunnels built by termites and restricts their foraging activities and associated damage to crops. Removal of the queen and/or destruction of the nest have frequently been used by farmers as a traditional method for control of mound-building termites. Mounds are physically destroyed, flooded or burnt with straw to suffocate and kill the colony.
Crop rotation may be useful in reducing the buildup of termites since intensive monoculture for long periods makes plants more susceptible to termite attack. However, winged adults of some termite species are capable of moving in from other sites if preferred hosts are planted in the field used for rotation.
Intercropping is the most effective cultural practice used by small-scale farmers in Sub-Saharan Africa to manage insects that have specific host ranges. However, controversial results have been reported for termites. For example, intercropping maize and beans resulted in significant reduction of ground tunneling by termites but did not reduce termite damage on the plants. Intercropping groundnut with sunhemp did not affect termite abundance or damage to the groundnuts. On the other hand, intercropping in forestry has been suggested as a means of retaining termite diversity in the crop in order to prevent them from achieving pest status. Certain grasses are intercropped with different crops in western Africa to repel termites.
The removal of residues and other debris from the field may reduce potential termite food supplies and hence lead to a reduction in termite numbers and subsequent attack. On the other hand, leaving residues in the field or adding further organic matter could provide alternative food to which termites will be attracted, thereby reducing levels of attack on the main crop. However, this is not true in all cases. For example, in most parts of Sub-Saharan Africa, monocropped groundnut is cultivated after cereal crops. Post harvest debris from cereal plantings is often left behind. It serves as food for Microtermes and Macrotermes, which at the same time attack ground crops. Termite infestation of 100% was observed in groundnut crops where the plant residues were rated very high.
Mulches may either increase or decrease the incidence of termites depending on whether they have any repellent properties. Non-decomposed organic matters such as plowed weeds and green manure have been associated with termite attack on cotton in Malawi and Sudan.
Various parts of plants and plant extracts are known to be either toxic or repellent to pests of agriculture, and are widely used in rural settings. However, no specific recommendations have been made for their large-scale utilization. Some of these extracts have been investigated in the laboratory and proven effective against termites. Plant extracts, such as those of neem, wild tobacco and dried chili, have been used to control termites in the field and storage warehouses.
Wood ash heaped around the base of the trunk of coffee bushes has been recorded as preventing termite infestations. Wood ash has also been reported to repel termites from date palms. Wood ash has also been used to protect stored yams, maize straw, tree seedlings, if it is mixed into plant nursery beds or applied as a layer. However, the potential benefits of using wood ash for termite management require verification.
Many natural enemies (predators, parasites and pathogens) attack termites in nature. Biological control is the use of these natural enemies in termite management. It constitutes a more environmentally acceptable alternative to traditional chemical control measures. The US Environmental Protection Agency encourages the development and the use of biological control agents (http://www.epa/pesticides/citizens/advantages). Biological control can be accomplished in three ways:
Termites have a wide variety of predators, both opportunist and specialist, but ants are the greatest enemies of termites in all regions of the world. Although ants limit termite numbers under natural conditions, their suitability for use as biological control agents for target termite management has yet to be ascertained. The practice of traditional farmers of using dead animals, meat bones and sugarcane husks to poison Macrotermes mounds was recently used to develop baits for predatory ants and tested in maize fields for termite control in Uganda. The protein-based baits attracted significant number of ants and more ants established nests near maize plants, this in turn reduced termite damage and increased grain yield.
Among the pathogens (bacteria, viruses, protozoa and fungi), entomopathogenic fungi so far offer the best prospect as termite control agents. Fungi infect their host through the cuticle and do not need to be ingested. In fact, spores of candidate fungi, such as Metarhizium, pass through the termite gut without causing any harm. Conidia of entomopathogenic fungi could be spread through the colony and the nest by contact and grooming between contaminated and uncontaminated termites. However, factors such as repellency of fungal conidia, the removal and burial of fungus-killed termites, and behavioural changes in infested termites (in the case of Metarhizium they congregate at the base of a nest where they are easier to quarantine by nest mates) may limit the spread of the disease in the colony. Application methods to overcome some of the limitations include inundating of termite nests or sites of high termite activity with formulated products, and use of low doses of the insecticide imidacloprid in combination with the fungus. The insecticide has a synergistic effect. More termites die from the combined treatment than from the combined mortality caused by each agent alone.
The entomopathogenic fungus, Metarhizium anisopliae, has recently been developed into a product for control of termites in buildings in the USA, Brazil and Australia. Colonies of some species could be killed when nests were inundated with conidia. With a similar approach it was possible to destroy colonies of Macrotermes and Ondotermes in Kenya. Successful control of Cornitermes colonies has also been achieved in pasture in South America. The effectiveness of M. anisopliae to control termites in maize cropping system was demonstrated for the first time in Kenya and Uganda. The fungus, formulated as granules and applied as seed treatment, resulted in reduction of plant lodging and a subsequent increase in maize yield comparable to that obtained with the chemical insecticide lindane.
The use of the fungus M. anisopliae in bait stations is under investigation in Australia, and may represent a new potential alternative for the control of termites.
Although termites may damage healthy plants, unhealthy and stressed plants are generally more susceptible to termite attack. Therefore, cultural practices should aim at maintaining or enhancing plant vigor.
The use of good quality seed, healthy seedlings, and appropriate transplanting procedure is more likely to produce healthy plants.
Intensive cropping for long periods may reduce soil fertility and structure. This in turn may lead to reduced crop vigor and an increase in susceptibility to termite attack. Crop rotation, including fallow periods, is recommended as a measure to improve soil fertility and structure. However, in a number of cases, crop rotation can lead to greater levels of termite attack.
In general, crops showing resistance or tolerance to termites are indigenous while the susceptible crops are exotic. In Africa, sorghum and millet are more resistant to termites than maize and groundnut. Presumably indigenous crops have evolved defense mechanisms against the local termite fauna. Cultivars of a particular crop may also differ in susceptibility to termites. This has prompted research on the selection of cultivars for resistance to termites. Some varieties of groundnut and maize have been found to be resistant.
Weeds competing with crops for nutrients, light and water may lead to stress and hence increased susceptibility to termite attack. For example, higher incidence of termite attack on cotton was noted in Sudan in areas where the sedge Cyperus rotundus was abundant.
Deficiency or excess of water may stress plants and encourage termite attack. In general, attack on crops and trees is greater in drier areas and during dry periods. Overall, annual rainfall is important but the temporal distribution of rainfall through the growing season may be more significant. Various measures may be taken to reduce water stress and, therefore, maintain plant vigor. It is recommended to water nursery stock prior to planting out and to plant at the correct time to avoid drought periods.
The use of inorganic fertilizers enhances plant vigor and hence the ability to withstand pest damage. Application of nitrogen, phosphorus and potassium in wheat, barley and yam has been observed to reduce termite incidence.