Pests of Field Crops in Southern Africa

TERMITES

(Isoptera: Termitidae)

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In the drier regions of southern Africa, termites, often popularly called "white ants", can cause a good deal of damage to a variety of crops.  A number of species have been implicated:
Macrotermes spp., which may be recognised by their large size, white bodies and golden-red heads, and the large mounds ("ant hills") they build.  
Odontotermes spp., which are smaller (about 4 mm long), white-bodied, with golden heads, and which build smaller mounds with large open ventilation shafts, particularly in black “vlei” areas. 
Ancistrotermes latinotus, a small species that looks very similar to the above but which does not form mounds at all, merely living in series of underground hives or occupying portions of hives belonging to other termites.
Microtermes spp., which is a minute termite (about 2 mm long), similar in appearance to the above two, and which also does not build mounds.

The harvester termite, Hodotermes mossambicus, belongs to a different family and is not included here, as it is a more specific pest of grasses and pastures.  
Macrotermes sp. are often erroneously called harvester termites, but they differ in appearance (being much paler in body colour than the pigmented harvesters, although of similar size) and in their food-collecting habits (they do not wander freely, but build mud tunnels over their food source).

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Text Box: Among the crops which have been subject to termite damage are cotton, deciduous fruit trees, groundnuts, maize, pasture grasses, plantation trees, sorghum, soyabeans, sugar cane, tea, tobacco and wheat.  Also attacked are lawns and various ornamentals.

Plants are readily attacked when they have been damaged or have been under some form of stress, such as drought or sometimes even water logging.

Not surprisingly, timber is sometimes damaged by the species mentioned, as dry wood is their natural food, but other species are usually implicated.

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Text Box: Damage takes on various forms, depending on the crop.  In cotton, most of the damage takes place in the first nine weeks after planting, and the termites (Ancistrotermes, Odontotermes and Microtermes spp. particularly) tend to penetrate and hollow out the taproot and stem from beneath the soil.   Affected plants then simply wilt and become desiccated.

In maize, the smaller termite species may hollow out the roots and lower stem, but this seems to occur after physiological maturity of the crop. The result is lodging and then rapid covering of the fallen plants with earth workings so that they can be consumed, cobs and all.  Sometimes Macrotermes spp. from a nearby mound may attack the crop, and they approach from above the ground, simply covering the lower stem with earth and climbing up the plant.

In groundnuts, damage appears from about twelve weeks after planting, and taproots and subterranean pods may be hollowed out.  This allows the penetration of fungi into the pods, one of which in particular, Aspergillus flavus, produces a substance (aflatoxin) which is highly toxic to cattle and poultry.

In other plants, damage may be more obvious, with earth-workings around the stem bases.  Often, weakened or stressed plants (from drought or a spell of water logging) are attacked.

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Text Box: A number of different forms of individuals exist in termite communities.  Every colony has a king and queen, whose sole concern is reproduction, and these are often enclosed in an earthen cell where the hugely distended and helpless queen produces thousands of eggs.  If destroyed, the royal couple can be replaced by complementary royalties which are reared by the workers. The soldier caste is easily recognised by the enlarged, pigmented head and usually well-developed jaws. “Soldiers” surround the openings of mounds or earth workings and protect the workers while they are on food forays.  The greater part of the colony consists of the workers, which are concerned with food gathering, nest building and care of the eggs and young.  They also feed all the other members of the colony, which are unable to feed themselves. Many termites of the family Termitidae cultivate a fungus within the nest, and this is a source of food for the royal pair and the young.

When the rains arrive, activity in the colony increases, the hives are enlarged, and a winged form appears.  These termites are fully developed sexually, and have the task of mating and founding new colonies.  Vast flights of them appear when weather conditions are ideal, and after mating, the wings are dropped and each pair finds a new home.  They form an important source of food for many other animals, and relatively few succeed to start new colonies. 

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Text Box: Control of termites has proven difficult.  There is little doubt that in themselves termites would be very susceptible to a number of chemicals, but the difficulty lies in their inaccessibility and in their time of attack.  Most of the currently available chemicals are short-lived and the attack invariably comes at the end of the crop's life.  A successful chemical would need to have some persistence or to be able to move down the soil profile to provide any control, so insoluble compounds or those that are readily adsorbed onto clay particles would be of no use unless they were incorporated.  A couple of new insecticides are currently being researched for termite control, and have received registration for certain crops.  One of these, fipronil, has given some success either as a seed dressing, or as a furrow treatment at planting, and another, imidachloprid, has given some success when applied as a high-volume spray to the plant bases at the appropriate time (when attack is likely).

As far as termite mound control is concerned, it may not be the nearest mound that is responsible for termite damage in a field, since frequently no mound is made and the subterranean colony is apparent only by earth workings on the soil surface.  If, however, a nearby mound has been found to be responsible, then attempts can be made to eliminate this by direct application of an insecticide with some persistence, such as gamma-BHC which is usually available in ant remedies.  Contact insecticides such as synthetic pyrethroids may serve only to eliminate those individuals which come into direct contact with them, but are unlikely to reach the termite queen .

Sometimes an attempt is made to destroy a colony by removing the queen.  This may be an extremely laborious task, since the queen may be concealed at some depth.  Some termites, however, have the ability to replace the lost queen by putting in her place reproducing females. 

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Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

Macrotermes mound.

Soldier and worker termites.

Termite damage to groundnuts.

Macrotermes soldier.

Harvester termites (below) are pigmented, unlike crop damaging termites above.