Bollworms (Helicoverpa armigera, Earias spp and Pectinophora gossypiella)
Bollworms are among the most serious cotton pests. They feed in the bolls, damaging lint and seed and causing considerable reduction in yield and quality. The main bollworms are the African bollworm (Helicoverpa armigera), spiny bollworm (Earias spp) and the pink bollworm (Pectinophora gossypiella). Caterpillars of the false codling moth (Cryptophlebia leucotreta) are sometimes found boring in the bolls.
1) African bollworms (Helicoverpa armigera)
It bores into the boll often with the hind part of the body exposed outside the boll. If younger bolls are attacked they normally show a yellowish colour and the bracteoles open out (flared square). One caterpillar can damage a number of bolls and buds by moving from one to the other. Infested bolls and buds drop prematurely.
2) Spiny bollworms (Earias biplaga / Earias insulana)
The adults are moths, about 12 mm long with a wingspan of about 20-22 mm. The forewings are white, peach, metallic green to straw yellow in colour according to the species. Fully-grown caterpillars are up to 18 mm long. Their body bears numerous fleshy spines. Caterpillars vary in colour from brown and deep orange to grey and yellow. Caterpillars bore into flower buds, young shoots and maturing bolls. Hollowed tender shoots whither and die.
3) Pink bollworm (Pectinophora gossypiella)
The moths are small (about 8-9 mm long) with grey brown or brown forewings covered with dark spots. The hind wings are whitish, broader than the fore wings, and have a long fringe on the posterior margin. The pink caterpillars are up to 15 mm long and have 2 broken transverse red bands on each segment of the body. This is usually a late season pest.
The caterpillars bore into the flower buds and young bolls causing shedding. Caterpillars feeding on flowers spin the petals together, causing the formation of what is called "rosette flowers" which do not open up. The bolls open prematurely and may also rot or drop to the ground. The most important damage is caused by caterpillars penetrating bolls, where they feed on the seeds and soil the lint with frass and excrements. The pest is carried over to the next season and crop as a diapause-caterpillar mainly within the cottonseeds.
- Practise field hygiene. Remove and destroy old crops and plant debris after harvesting or let cattle graze in the field after the picking is over.
- Crop rotation with plants not related to cotton (avoid kenaf, okra, abutilon and other malvaceas) may help to reduce attack by the pink bollworm and the spiny bollworm, but it is unlikely to be effective against the African bollworm, since this pest feed on many different plants.
- Mixed cropping helps to reduce attack by bollworms; plant composition and combinations are important to optimise the benefits. Some plants may act as trap crops and/or may attract natural enemies that will then predate on bollworms.
- In Tanzania, early sowing of "Ukiriguru" varieties is strongly recommended. These varieties have the ability to compensate for early crop loss of fruiting points caused by either physiological stress or by the African bollworm, provided soil moisture and nutrients are not limiting. Thus, the early sown cotton (sown between the end of November and end of December) may lose its bottom crop, but can compensate later by producing a crop during the main rains in March-April. If bollworms attack later, then the early sown crop would have set its main crop and will therefore escape damage (Nyambo et al., 2003).
- Encourage natural enemies like ladybird beetles, lacewings, spiders etc.
- Direct control measures are spraying with neem spray or a garlic-chilli-onion-repellent and Bt. For more information on Neem click here. For information on Bt click here.
- In Malawi and Zimbabwe, thresholds based on egg numbers have been used successfully in cotton since 1961. Spraying was recommended at an average of one egg per two plants in twice-weekly counts. In the Sudan Gezira, over two eggs or caterpillar per 18 plants, and in Australia two eggs per metre of row were used as thresholds (CABI, 2000). It has been argued that control thresholds based on damage are easier to use and more economical than those based on pest density. In the case of cotton, damaged buds are easier to detect and sample than either eggs or small caterpillars. Studies in Tanzania indicate that spraying at damage threshold of 10 to 20% would give adequate protection to the crop. Further fine-tuning of damage thresholds should be concentrated during the first four weeks of flowering when most of the damage by this pest occurs. (Kabissa, 1989).
- Bollworms can also be removed by hand picking. This helps when their numbers are low and in small fields.