Pests of Field Crops in Southern Africa

African Bollworm

(Lepidoptera: Noctuidae)

 

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The African, or as it used to be, Heliothis bollworm (Helicoverpa armigera) can clearly be classified as a general pest since it has so many different host plants, but its method of attack tends to be peculiar to the crop in question.  It has earned for itself many common names, amongst which are "American bollworm" (a misnomer as it is an African pest), tobacco budworm, maize earworm and tomato fruitworm.  It has an extremely wide distribution, and can be regarded as one of the major pests in southern Africa.

 

The adult is a typical nocturnal moth of insignificant appearance, having brown to greyish-brown forewings marked with wavy transverse lines, and beige hindwings with darker brown edges.  The wingspan is about 35 mm.

 

The small, yellowish-white eggs are ribbed and rather dome-shaped.  The mature larvae, depending on their food source, vary widely in colour from shades of green to pink, brown or black, but may usually be recognised by three darker lines running down the middle and sides of the dorsal surface, separated by two broad, pale bands.  Laterally, they usually have white bands which are punctuated by the dark breathing pores, whatever their basic colour.  The larvae have the full complement of legs: three pairs of jointed legs at the anterior end, and five pairs of fleshy “prolegs” along the body.  When fully grown, they may reach a length of up to 40 mm.  Pupae are mahogany-brown and are usually found in the soil.

Text Box: H. armigera has a very wide host range, usually feeding on flower buds or developing seeds and fruits, and only occasionally on leaves.  It has been recorded on beans, castor beans, citrus, coffee, cotton, cape gooseberry, groundnuts, maize, peas, sorghum, soyabeans, sunflower, tobacco, tomato, drying wheat ears, lettuce heads, paprika (peppers) and even rosebuds.   It also attacks a number of ornamentals, particularly of the legume family.

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Text Box: Newly-hatched larvae feed mostly on surface tissues of the plant, but most damage is caused by feeding on the buds and fruiting parts.  On beans, tomatoes, cotton bolls, etc., the larvae bore right into the fruit, with the rear end of the body often protruding.  On cotton, the damage is often to very young bolls, with the result that the plant aborts many of these.  On maize, after eating the silks on developing cobs, they sometimes feed on the soft seeds at the tip of the cob, and on tobacco they damage developing leaves right in the bud.

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Text Box: The female moth is capable of laying several hundred eggs during its life, these being deposited singly at dusk on leaves, stalks, buds or tassels of the host plant.  Moths appear to be attracted into a crop once flowering has begun.  The incubation period on a cotton crop is about three days in warm weather, and eight or more days at the cooler times of year.  The hatching larvae first consume their eggshells, then proceed to the flower buds and fruit, often hollowing them out completely.  When fully grown, some 18 days later in warm weather, or up to 50 days later in cooler weather, they leave the plant and burrow into the soil where pupation takes place.  They have occasionally been known to pupate in the tip of maize cobs.  Some pupae over-winter during the cold months, so that the pupal period may last for several weeks, but during the warmer months, it can be as short as 14 days, increasing progressively as the temperature drops.

The moths emerge from the soil at the end of this time, and after about four days begin to mate and subsequently lay eggs.  Moths live for only one to two weeks but deposit hundreds of eggs in this time.

Rate of development therefore is affected by time of year (temperature), but also by the food plant.  The increasing production of winter vegetable crops, particularly peas, which are a favoured host of H. armigera, and the very wide host range of crops which offer an uninterrupted sequence of availability, seem to have enabled this pest to reach a status unparalleled by many other insects.

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Text Box: Control of H. armigera can be achieved by a variety of chemicals depending on the crop concerned.  Very often a short-lived chemical is necessary as the fruit is attacked shortly before harvest.  Such is the case particularly in tomatoes, but also in other crops such as beans, peas, gooseberries and sweetcorn.  For these cases, chemicals such as mevinphos and endosulfan have been recommended.  Other chemicals registered for control of this pest are carbaryl, tetrachlorvinphos, methomyl, acephate, monocrotophos and the synthetic pyrethroids.  Many of these have in latter years, however, been judged unsuitable for vegetable crops in particular, and safer formulations such as formulated Bacillus thuringiensis (a bacterial disease of many insects) have become available. 

The habit this pest has, of entering the fruit, boll or pod, affords it a good deal of protection against chemical sprays, often making control almost impossible.  As with many caterpillar pests, best results are achieved if control measures are taken against the young stages, and therefore scouting procedures are recommended for early detection. These have been highly developed for cotton crops, and are based on the number of eggs found within the crop.  In some seasons, maize can come under fairly heavy attack by bollworms, which eat the silks protruding at the end of the cobs, and then penetrate the cob tips themselves.  Very often, this pest does not require control measures as only the very tip grains are consumed, and these are often lost in the harvesting process anyway. If left unsprayed they also come under attack by a large range of natural enemies.  However, there have been a few cases where the damage was excessive, and as usual, control is awkward because the pest is well sheltered within the cob. If a history of this problem has been experienced, the only chance of control will be by sending through scouts to look for eggs or very young larvae on the silks before they penetrate further.  This will be the only chance of applying insecticides effectively.

Another problem experienced with bollworm is that it has a tendency world-wide for the development of resistance to chemicals, particularly synthetic pyrethroids.  For this reason, there are quite severe restrictions with regard to the use of synthetic pyrethroids in Zimbabwe, in an attempt to prolong their usefulness in controlling H. armigera. 

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Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

Bollworm on ripening wheat.

Bollworm damage to cotton flower bud.

Bollworm on cotton flower.

Bollworm on young maize cob.

Bollworm on cotton

Bollworm entry point shows an infested cob.

Bollworm eggs on cotton flower.