Protecting crops from bollworms

are large caterpillars often seen living and feeding in tomato fruit, but they also attack many other vegetable and field crops such as cotton and sorghum.
Adults are fat brown moths, which fly at night and lay single round yellowish-white eggs measuring about 0,5mm in diameter, usually on the upper part of the plant.
A single female can lay many hundreds of eggs.
The young larvae are green to reddish brown caterpillars, which grow to about 4cm long.
They have dark markings and a light stripe along the length of each side of their bodies.
The fully-grown caterpillars drop to the ground to pupate a few centimetres beneath the soil surface.
Pupae are smooth, dark brown with two spines at one end.
The larvae caterpillars feed on leaves, flowers and fruit.
The leaf damage can reduce leaf area, which slows plant growth, and the flower feeding can prevent fruit formation.
When they burrow in the fruit they are difficult to reach and control with insecticide.
The damage may cause the fruit to drop or make it more susceptible to secondary fungal and bacterial diseases.
Experts encourage farmers to practise scouting as it enables them to detect infestations early, preferably for the presence of eggs, since the larvae are well protected once they move into the flowers and fruits.
By this stage, the damage caused is severe.
Crop rotation can only help to prevent build up of populations if it is done over large areas since the adult moths can move quite long distances.
This is unlikely to be practical for smallholders.
Hand picking of eggs and larvae can be an effective method if infestations are not too severe.
Hens can help by eating larvae and pupae at certain times of crop development, although they should not be allowed near seedlings or plants with fruit since their scratching and pecking will cause damage.
Farmers are encouraged to destroy infested fruit compose or burn infested plants after harvest.
Since this pest can infest several other crops, including maize, sorghum and cotton, it is important to ensure that infested crop residues are carefully destroyed to prevent the pest switching backwards and forwards between these different hosts.
If the infestation is severe, pesticide sprays may be required.
The aim should be to target the young caterpillars before they have entered the fruit.
Farmers should apply the pesticide using a knapsack sprayer fitted with fine medium hollow cone nozzle and follow procedures on safe and selective application.
Care should be taken not to use the same pesticide time after time since this pest can develop resistance very quickly.