Cotton stainers (Dysdercus spp.)
Several types of bugs attack cotton. They are late season pests and they usually attack during the flowering and fruiting stages of the crop. The most important are the cotton stainers.
Stainer bugs are between 14 and 24 mm long. They are bright red, yellow or light grey with an orange tinge depending on the species, and with black bands. Stainer bugs are late season pests. They appear when the bolls are ripening. Female lays whitish yellow eggs in moist soil or in crevices in the ground. They hatch to produce reddish-orange nymphs. Initially the nymphs are wingless, but wings develop gradually as the nymphs grow.
The nymphs are found together in the area where the eggs have been laid and later disperse to look for food. Both nymphs and adults feed on the bolls, but adults cause the most serious damage. They pierce through the boll and suck the seeds reducing germination capability and the quality of the seed oil and the cake.
Furthermore, they cause severe indirect damage by transmission of a fungus (Nematospora sp.), which leads to internal boll rot and stain of the lint with typical yellow colour, hence the name 'cotton stainers'. The nymphs feed mainly on seeds in open bolls reducing the seeds's oil content and their germination capacity. Severe bug attack affects yield, oil content and the marketability of the crop.
- Cotton stainers are attacked by a range of natural enemies. The most important are assassin bugs, ants, spiders, birds and parasitic flies.
- Caging chickens in cotton plots using chicken wire may control cotton stainers; about 15 birds will keep about 0.1 ha free of stainer bugs. This is a good option for small plots grown next to the homestead.
- Preventive control measures are sanitation; remove cotton plants and all its debris as well as ratoon cotton as soon as harvesting is over. Keep stores clean. Cotton should be grown strictly as an annual with a close (dead) season. Hand pick and destroy the bugs, this is feasible in small plots and at the beginning of infestations, and will help to reduce population density.
- Custard apple leaf extract is recommended for control of these bugs (PAN).
- The baobab tree is one of the main host plants of stainer bugs. If cotton is grown where baobab occurs, the soil and trunk of the baobab tree should be sprayed to kill the nymphs hatching from eggs laid around the stem.
- Tanzania: pyrethrum formulation with black wattle extract as UV light stabiliser is used.