Cotton

Pests of Field Crops in Southern Africa

COTTON

(Gossypium hirsutum, Malvaceae)

Text Box: See general pest section. As a major pest of cotton, often appearing in at least two successive generations, this bollworm has been extensively researched and recommended control measures are based upon scouting for eggs on a designated number of plants within a field. Damage in cotton often appears as clean round holes in bolls of all sizes, but with plenty of frass in evidence. Damaged flowers tend to be shed, leaving “flared” squares. TOP

Text Box: On occasion, weevils of the genus Alcidodes (probably A. erythropterus) (Coleoptera: Curculionidae) cause damage to cotton crops. As the damage is very sporadic, it is more of interest than of economic importance, and is very characteristic. The beetles girdle the main stems of the plants about 50 cm from the top of the plant, leaving a ring of frayed fibres around the stem. The upper part of the stem dies, and often breaks off. The beetles are thought to lay their eggs in the swollen tissue just below the girdled area and the larvae feed downwards in the stem. Attack is sporadic, and rarely concentrated, although many plants within a field may be affected. This was seen in an area where soyabean plants and a species of Malvaceous weed were also attacked and the field of cotton in question may have had as many as 1% of the plants affected. Although the damage to the plants is more likely to be seen than the weevils themselves, they too have a rather distinctive appearance. They are typical chunky weevils of up to 10 cm in length, with elongated snout, hump-backed body and robust legs, and a tendency to “play dead” when disturbed. The thorax and head are black, while the elytra are rusty-red, pitted with several longitudinal rows of parallel punctures and dusted with golden scales in patches. TOP

Text Box: See general section. Red spider mite is an important pest of cotton, since both the crop and the pest enjoy warm, dry climates. It has traditionally been controlled in Zimbabwe by the operation of an acaricide rotation scheme which controls the chemicals used in different parts of the country. Sprays are, as is normal for most cotton pests, applied only when scouting results show that threshold levels have been reached. TOP

Text Box: Thrips are usually abundant in cotton fields, particularly in the flowers, and may actually have more significance by acting as a source of infestation for neighbouring crops than being serious in cotton itself. This is certainly the case with the flower thrips, Frankliniella spp. (see general section). However, since cotton is generally planted early in the season while it is still hot and before the rains set in (ideal conditions for thrips), Thrips tabaci and Caliothrips sp. have on occasion been recorded as causing fairly serious damage to emerging seedlings. This early in the season, few natural enemies would be around to provide any sort of natural control, and if populations are heavy, the cotyledonous leaves may wither or show distortions which can appear similar to herbicide damage. Since the rows of cotton are wide and the plants small, spraying is wasteful and difficult, but systemic seed dressings can give excellent control. The main problem with this latter method of control is that cotton has traditionally been planted by sowing three or four seeds per planting station, and then thinning them later when emergence is assured. This would be very wasteful of expensive seed dressing materials. More recently there has been a trend to reduce the number of seeds planted per hectare drastically, while protecting them with seed dressings to control soil pests and early-season sucking pests. The arrival of reasonable rains or the use of flood irrigation will certainly go a long way toward controlling early-season thrips as they have a part of their life cycle in the soil (see general section). TOP

African bollworm

Alcidodes beetle

Alcidodes beetle

Red Spider Mite

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Þ Cotton Leafworm

Þ Cotton Lygus

Red Spider Mite

Þ Cotton Stainers

Þ False wireworms (see general section)

Þ Red Bollworm

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Þ Spiny Bollworm

Þ Termites (see general section)

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African bollworm

Text Box: See general section. Cotton may be infested by Bemisia tabaci in low altitude areas, and mixed populations of this and Trialeurodes vaporariarum at higher altitudes. TOP

Bollworm on cotton.

Whitefly adults (above) and nymphs (below) on cotton.

Alcidodes weevil.

Red spider mite on the underside of a leaf.