Soyabeans

Pests of Field Crops in Southern Africa

SOYABEANS

(Glycine max, Family Fabaceae)

Text Box: See general section. Presently bollworm is of little importance in soyabeans, and has seldom been seen in significant numbers. It is, however, potentially serious since low numbers can inflict damage on the crop by feeding directly on the developing pods. Neat round holes in the pods are diagnostic. It should not be confused with semi-looper, which is the more likely pest to occur in soyabeans, but is easy enough to distinguish by counting the number of pairs of prolegs on the caterpillar stage: bollworm have the full complement of five pairs of prolegs, while semi-loopers have only three pairs. It should be noted that the freely-occurring viral disease which kills semi-loopers, will not have an effect on this bollworm, so that distinguishing between the two species may be important. TOP

Text Box: See Bean section. As with green stinkbugs, these slender, spiny, brown bugs tend to appear during the pod development stage, and they pierce and suck the developing seeds within the pods. They too can result in malformed and discoloured seeds. The bugs are very elusive, and may only be noticed on close examination. They tend to be in heaviest populations on the last green crops of the season. TOP

African Bollworm

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General Leafeaters

Þ Leaf Rollers

Bean Stem Maggot

African Bollworm

Text Box: See Bean section. Bean stem maggots occasionally attack soyabeans but the damage caused has never been considered to be important. The first sign of their presence is the appearance of drooping and wilting leaves. Close examination will reveal that the petiole bases are swollen and minute maggots or pupae can be discovered within the swellings, which appear to choke up the vascular system feeding the leaves. TOP

Bean Stem Maggot

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Þ Soyabean Semi Loopers

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Text Box: A number of other leaf-eating species can be found on soybeans. These include caterpillars such as Spodoptera littoralis, common green grasshoppers as well as the brightly coloured elegant grasshoppers and the spectacularly coloured gold tortoise beetles. They are usually fairly solitary and of little importance. In addition, chafer beetles can occur at the start of the season, causing an abundance of the typical “zig-zag” holes in the leaves, but the damage is usually fleeting and the plants soon grow out of it. TOP

Text Box: Snout beetles are often found in soyabean crops, where the damage is confined to the leaf edges as described earlier. Occasionally, an unusual species has been found in abundance on soyabeans and foliage damage has been more serious than usual. TOP

General Leafeaters

Text Box: Green stinkbugs (Nezara viridula) tend to become concentrated in the crop towards the end of the season during bean development. In some seasons, the numbers seem to be much higher than in other seasons, and damage to developing seeds can cause fairly serious losses in the form of down-grading of beans. The damaged seeds are often discoloured or malformed. TOP

Text Box: See general section. These tiny insects are seldom noticed unless one is specifically collecting insects, but thrips of the genus Sericothrips can reach very high numbers within a soyabean crop, particularly in hot, dry seasons. Fortunately, they are of no importance, although the streaks and flecks that their feeding causes will be noticed on the young leaves. TOP

Bollworm damage to pod.

Bollworm.

Juvenile green stinkbugs on Soya.

Bean bug on soya pod.

Common green grasshopper on Soya. (above)

Elegant grasshopper on Soya. (right)

Spodoptera littoralis on Soya. (left)