CONTROLLING COCOA PESTS
IN SOUTHEAST ASIA WITH
THE BLACK COCOA ANT

A contributed integrated pest management paper

Early this century cocoa planters in Indonesia observed that the presence of a species of ant, Dolichoderus thoracicus, was associated with greatly reduced damage caused by mirids, an important group of pests attacking cocoa. During the last decade a great deal of research has gone into establishing the effectiveness of the ant in reducing damage caused by mirids and other cocoa pests. The possible problems that may result from the presence of the ant were also investigated, and techniques for its manipulation that are simple, cheap, and reliable were developed.

In the major cocoa-growing areas of the world, the crop is afflicted by a number of insect pests, the most serious of which is the group of bugs commonly known as capsids or mirids. Damage is caused by the mirid when it sucks the sap from various parts of the tree. Pod damage is common and, depending on the mirid species involved, other parts such as pod peduncle, shoot, and even the hardened stem may be attacked.

Mirid damage to cocoa pods. Click on the thumbnail to see the full size picture.

Recently, in the cocoa-growing countries of Southeast Asia, notably Indonesia and Malaysia, a new insect pest has emerged and has overtaken mirids in importance. The cocoa pod borer, Conopomorpha cramerella, can cause heavy losses and is a difficult pest to control because the larva bores through the floor of the egg directly into the cocoa pod where it lives and feeds, and only emerges when it is ready to pupate. The insect is therefore safe from insecticides most of its life.

Cocoa pod borer damage to mucilaginous coat surrounding cocoa beans.

Various control strategies have been used against the cocoa pod borer, but there is a great deal of dependence on insecticides. Currently, a farmer practising insecticidal control of the borer is likely to spray a synthetic pyrethroid every other week throughout the year. This practice increases cost of production substantially besides being a threat to the ecosystem. Furthermore, development of insecticidal resistance is a worry.

The important pests of cocoa, including C. cramerella, can be controlled using a species of ant indigenous to some countries in Southeast Asia. Research has established that the black cocoa ant, can bring about a significant reduction in losses due to mirids, the cocoa pod borer, black pod disease, and rats.

Healthy cocoa pods with black cocoa ants and mealybugs.

Concerning black pod disease, it was feared that large numbers of the ant crawling between the ground and the cocoa tree would increase transmission and spread of disease organisms, in particular the fungus (Phytophthora palmivora) responsible for black pod disease. It was therefore a pleasant surprise when an experiment showed that the incidence of black pod damage was much lower in plots where D. thoracicus was abundant, compared with plots where there were few or no ants. It is known that certain flies which feed on black pods are also attracted to the sugary exudates occurring around fresh mirid lesions. So a reduction in mirid damage also black pod incidence too.

The ant is absent in most cocoa farms and a technique for its manipulation has been developed so that it can be introduced and established where it is needed. There are four primary components to bear in mind when manipulating D. thoracicus: the crop environment, the ant, the mealybug, and the natural enemies. These components will be discussed in the context of two requirements vital to all organisms, a place to live and food.

Cocoa is commonly grown under partial shade provided by leguminous trees such as Gliricidia, Leucaena, Albizia and/or coconut palms. Of these shade trees, coconut palms clearly favour the black cocoa ant. Where the ant is well established, active trails can be seen running between the cocoa trees and the crowns of coconut palms, even those more than 60 ft (18 m) tall. A variety of homopterans, including C. hispidus, reside on the leaves and fruit of the coconut palm and D.thoracicus tends them for their honeydew, a source of sustenance.

More importantly, the coconut palm is an excellent source of shelter for the ant. The coconut palm has huge leaves or fronds, and each leaf consists of many leaflets. Aggregations of D.thoracicus can be found under some of these leaflets. When a leaf becomes old and drops from the palm, the drying leaflets form long tubes; D.thoracicus loves to nest in these tubes.

In the absence of coconut palms, other sources of shelter are available but not abundant in the average cocoa farm. Strong colonies of D.thoracicus can be found nesting in the cocoa leaf litter if the layer is thick. Most cocoa farms do not have a thick layer of leaf litter. When leaves on the cocoa tree happen to overlap, small aggregations of ants, without eggs and brood, can be found between the leaves. However, neither the cocoa leaves nor the leaves of leguminous shade trees can provide nesting sites comparable in stability to that of a tube of dried coconut leaflet.

Black cocoa ants, eggs and brood within artificial nest of cocoa leaf litter.

The knowledge that D.thoracicus likes to nest in the cocoa leaf litter and coconut leaflet tubes has been used to construct artificial nests for manipulating the ant. Artificial nests are made either by stuffing cocoa leaf litter into polythene bags or by tying coconut leaflets into bundles. Placed in an area where D.thoracicus activity is high, the artificial nests are quickly colonized. A colonization rate of more than 90% within a month is normal. The colonized artificial nests can then be collected and introduced into the area desired. Uncolonized artificial nests are also used to aid ant establishment and to supplement natural nesting sites when they are lacking.

The species of ant for use in biological control must be amenable to manipulation if it is to be used successfully in pest management. Several behavioral and biological characteristics of D.thoracicus favour the use of the species for this purpose:
The black cocoa ant does not sting and is not particularly pugnacious. It bites when strong colonies are disturbed, but the bite is well tolerated by farm workers.
Many queens are present in a nest (polygyny). In some species of ants a colony consists of several nests, one of which contains the queen; in others, a single queen is present in a nest. Since the queen is the only individual in a colony which is capable of reproduction, when “harvesting” nests, the polygynous habit makes it very unlikely that a nest would be without at least a queen.
The black cocoa ant readily colonizes suitable nesting sites.
There are no behavioral boundaries between colonies (unicolonial) and it is possible to get a large contiguous population of D.thoracicus in a farm. In contrast, many ants maintain “no-man’s land” between colonies even though they are of the same species. This results in patchy distribution of the population.
The ant has a propensity to spread rapidly within the crop.

The ant has a close mutualistic relationship with the mealybug Cataenococcus hispidus. The primary source of food for D.thoracicus seems to be the honeydew produced by the mealybug. In an experiment, isolated colonies of D.thoracicus with only C. hispidus available for their source of food remained in good health, and even increased in number, for eight weeks when the experiment was terminated. In return, the mealybug receives protection from its natural enemies. It was shown experimentally that C. hispidus, in the absence of D.thoracicus, and otherwise unprotected from its natural enemies, was quickly decimated.

Black cocoa ant tending mealybugs. Droplets of honeydew are food for the ant.

In a cocoa farm densely populated by D.thoracicus, large aggregations of C. hispidus are common on the peduncle of the cocoa pod and the pod itself, as well as on the stems. The mealybug obtains its nutrients by sucking sap from the tissues where they sit and this has caused concern that C. hispidus itself is damaging. However, experiments have shown no such adverse effect. This is just one example of an insect feeding on a crop that does not cause any loss and can even be beneficial.

From field observations, the black cocoa ant was seen to also obtain its nutrients from other sources including dead or dying insects, dead snails, bird droppings, and honeydew from a variety of homopterans such as aphids, scale insects, and mealybugs. It is fortuitous that D.thoracicus tends C. hispidus in preference to the other honeydew-secreting insects because some of these species are damaging to the crop.

The importance of C. hispidus to D.thoracicus means that the mealybug has to be introduced and established together with the ant. Mealybugs are generally regarded as pests, hardly as beneficial insects. Strange as it may seem, techniques have actually been developed for manipulating and increasing C. hispidus. Basically, these techniques involve placing mature female mealybugs (males are not necessary because the female C. hispidus does not need to mate to reproduce) near the pod peduncle and then stapling a cocoa leaf over them to provide shelter from rain which is very detrimental to the young mealybugs known as “crawlers”. In one technique the mature female mealybugs are scraped off cocoa pods, put into little pockets made of muslin cloth, and stapled near the pod. The mature mealybugs are retained in the pocket but the tiny crawlers can escape and are likely to settle on the pod peduncle. Until the mealybugs are established, supplementary food for D.thoracicus may need to be given in the form of chicken dung or fish.

Cost is always an important consideration when deciding on pest control options. In a study in Malaysia it was determined that the cost of establishing and maintaining the black cocoa ant for 30 months over an area of 7 ha was US$22 per ha per year. The cost of insecticidal control of the cocoa pod borer alone is estimated to be US$192 per ha per year. Looking at monetary cost (and disregarding the social and environmental costs of using insecticides), pest control employing the black cocoa ant therefore has a clear advantage over insecticidal control.

One difficulty in manipulating D.thoracicus is that both it and its mutualist, C. hispidus, have their natural enemies. The most important natural enemies of D.thoracicus are other species of ants and the control of these antagonists is crucial during the initial phase of establishment. One strategy is to introduce a large number of D.thoracicus at once to overwhelm the antagonistic ants. Another is to use a toxic bait to reduce the population of antagonistic ants before introducing D.thoracicus. The toxicant and its concentration are selected so that the worker ants do not die immediately after consuming the bait but live long enough to share the poison with other members of the colony, particularly the queen. Once a strong population is established, D.thoracicus can not only hold its own against the antagonistic ants, but even invade their territory.

Farm worker carrying paper bags filled with colonized artificial nests.

A strong population of D.thoracicus also affords good protection to C. hispidus from its natural enemies. However, slugs are voracious predators of C. hispidus, and D.thoracicus does not seem to be able to provide effective protection against them. During wet spells when slugs are active, it may be necessary to apply a molluscicide on the tree collar.

To achieve good control of C. cramerella and other pests of cocoa it is necessary to be able to not only introduce and establish D.thoracicus, but also ensure that the ant population is high and well distributed. In addition, the population should be self-sustaining and should spread aggressively: this has the benefit of lowering cost and reducing the grower’s inputs, thus making it an attractive pest management option.

All these requirements for successful ant manipulation may seem like a tall order. Nature provides some inspiration: There are natural populations of D.thoracicus which densely colonize large cocoa farms for many years. Ant manipulation efforts in Malaysia have been encouraging too: After some early failures, the success rate is now very high.

Enough is known about D.thoracicus for its use to be an effective and attractive option for managing major pests of cocoa in Southeast Asia that is clean and inexpensive. Nevertheless, much more needs to be done to understand the complex biological system involved so that we can simulate nature more elegantly.

References

Ho, C.T. & Khoo, K.C. (1997) Partners in biological control of cocoa pests: Mutualism between Dolichoderus thoracicus (Hymenoptera: Formicidae) and Cataenococcus hispidus (Hemiptera: Pseudococcidae). Bulletin of Entomological Research 87: 461-470.
Khoo, K.C. & Chung, G.F. (1989). Use of the black cocoa ant to control mirid damage in cocoa. The Planter, Kuala Lumpur 65: 370-383.
Khoo, K.C. & Ho, C.T. (1992). The influence of Dolichoderus thoracicus (Hymenoptera: Formicidae) on losses due to Helopeltis theivora (Heteroptera: Miridae), black pod disease and mammalian pests in cocoa in Malaysia. Bulletin of Entomological Research 82: 485-491.
See, Y.A. & Khoo, K.C. (1996). Influence of Dolichoderus thoracicus (Hymenoptera: Formicidae) on cocoa pod damage by Conopomorpha cramerella (Lepidoptera: Gracillariidae) in Malaysia. Bulletin of Entomological Research 86:467-474
Way, M.J. & Khoo, K.C. (1992). Role of ants in pest management. Annual Review of Entomology 37: 479-503.

  More ant references.

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