African bollworm moths, which have a wingspan of 40mm, are strong fliers. The forewings are reddish-brown and often tinged with irregular green lines and blotches, while the hindwings are whitish brown with a light brown margin. Fully grown caterpillars are initially creamy-white, with rows of black spots and short hair along the back. With maturity, they become dark green or almost black with grey-yellow longitudinal stripes. This pest targets more than 30 different crops, including maize, cotton, sunflower, wheat and potatoes.
The caterpillars are prolific feeders, attacking leaves, flowers, growing points, fruits and buds during the growing, flowering and fruiting stage. Damage to the leaves affects their ability to photosynthesise and so stunts growth or kills the plant. Damage to flowers reduces fruit yields. The first sign that a caterpillar is present is a circular hole bored into the fruit – you may even see it halfway out of the fruit as it feeds. Other than damaging the fruit, the hole serves as an entrance point for secondary diseases.
Because the species is highly mobile, can migrate through a large range of plant species, has a high breeding rate and can build up resistance to certain chemicals, early detection is crucial if it is to be controlled. Combating it once it has disappeared into the fruit is difficult. Check regularly for the presence of eggs and small caterpillars and implement control measures immediately.
African bollworm caterpillars are prolific feeders, attacking the leaves, flowers, growing points, fruits and buds during the growing, flowering and fruiting stage.
The female lays a number of tiny yellowish-white eggs on the host plant. In warm conditions, the eggs can take three to five days to hatch. Just before hatching, they turn brown. When ready to pupate, the caterpillar drops to the ground and burrows into the soil. The pupa (chrysalis) is brown with two spines at the rear end. Pupation takes 15 to 20 days.
Methods of control
- Chemical: Synthetic pyrethroid-based insecticides are widely used for control, but in a number of countries the pest has resistance to them. Another option is the nuclear polyhedrosis virus (Bacillus thuringiensis) or one of its sub-species. Several commercial preparations are available under the trade names Dipel, Florbac 70 DG, Thuricide HP and XenTari. These control the bollworm caterpillar only and leave other beneficial insects unharmed. If these preparations are unavailable, a botanical preparation such as neem (Azadirachta) can be used.
- Natural predators: The bollworm fly (Dejeania bombylans) is a specialist predator of bollworms and parasitises their eggs. Affected eggs appear black. They should be left alone, as the emerging female fly will parasitise surrounding eggs. Other natural predators include parasitic wasps, ants, lacewings, ladybirds, assassin bugs, spiders and birds.
- Other measures: On a small-scale operation, remove the eggs and caterpillars manually. Burn stubble or residue after harvesting or plough the soil to expose pupae to predators. As female bollworm moths may prefer certain plants for laying their eggs, try inter-planting valuable crops with less valuable ones.
For cotton, plant chickpea, cowpea, onion, millet, marigold, sunflower, sorghum and maize, and for tomatoes, plant marigolds.
This series is your guide to identifying pests. The control methods discussed are merely suggestions. For help with area- or crop-specific control, consult an agricultural extension officer.
Paul Donovan is a biologist who has worked with reptiles and insects for more than 30 years in zoological collections in the UK. He is currently based in Botswana, where he advises farmers on the use of biological control agents. Email him at [email protected]