A closer look at the dreaded bollworm

Home Focus A closer look at the dreaded bollworm

WINDHOEK – The African bollworm is a pest of major importance in most areas where it occurs. It damages a wide variety of food, fibre, oilseed, fodder and horticultural crops. It is a major pest due to its high mobility, its ability to feed on many species of plants, its high fecundity and reproductive rate, and its capacity to develop resistance to pesticides.

The habit of feeding inside parts of the plant during most of its development makes bollworms less vulnerable to insecticides. Pesticides should be applied before the caterpillars bore into the fruits/pods. The African bollworm has a strong ability to develop resistance to insecticides. Currently there is a widespread occurrence of resistance in bollworms to popular synthetic pyrethroids in Africa and elsewhere.

The African bollworm has been reported on 35 crops and 25 wild host plants in eastern and southern Africa. The severity of the damage varies between crops, regions and locations, and between seasons. The adult moth is fleshy, yellowish-brown with a dark speck, greyish irregular lines and a black kidney-shaped mark on the forewings. The hind wings are whitish with a black patch along the outer margin. The moth is about 14 to 18 mm long with a wingspan of 35 to 40 mm. They are relatively strong fliers, dispersing widely within areas where the host plants are found. They can also be carried by strong winds.

Moths lay a large number of eggs, and the life cycle may be completed in a short time under warm conditions. Eggs hatch in three to five days. Larval and pupal periods last 17 to 35 and 17 to 20 days, respectively. The life cycle is completed in 25 to 60 days depending on temperature.

Sanitation (Clean cultivation)

•Remove and destroy plant residues immediately after harvesting.

•Plough the soil after harvesting. This exposes pupae, which may then be killed by natural enemies or through desiccation by the sun.

Mechanical control

•Handpick and destroy eggs and small caterpillars. This is feasible in small plots or when infestations are low. For instance, it has been reported that in Ethiopia when pest damage in sorghum is minor, farmers shake the plant to induce the caterpillar to drop from the sorghum head and pick them by hand. It is very important to detect small caterpillars before they enter the fruits.

•If African bollworms are detected in the field, sort out the harvested crop very thoroughly and remove the caterpillars manually. This is particularly important for export crops to minimise/avoid rejection in the importing country.

Maize has been commonly used as a trap crop. Preference for maize has been observed to be so strong that cotton plots remained almost clear of eggs when bordered with a few rows of maize. However, this attractiveness appears to be inconsistent.

Regularly observe the populations of bollworms on the trap crop and, if necessary, spray them with a suitable pesticide to control them.

Crop rotation

Avoiding planting crops after each other that are susceptible to bollworm like cotton, maize, sorghum, tobacco, soybean, and tomato may help to reduce/prevent build up of bollworm populations. In countries with two distinct seasons (wet and dry), it has been recommended to plant rice followed by beans during the rainy season, and cotton or small grains in the second cropping. Spraying practices can harm natural enemies. This is particularly detrimental early in the season since natural enemies that may otherwise have built up and suppressed the pest are killed. Substitution of broad spectrum pesticides with selective biopesticides .

Ants are important predators of the African bollworm. Ant activity on the crops can be encouraged by changing crop composition, by weed management and by provision of alternative food sources. Plants that offer alternative food sources on the canopy, such as plant exudates or honeydew producing insects, are more attractive to ants. Ants could also be attracted to crops by providing crushed sugar cane, or sprayed sugar solution. Ants would then complete their diet with protein by preying on insects, including caterpillars of the African bollworm. A good example is the BioRe Project in Tanzania, where sunflower is used as a trap crop in and around organic cotton fields. Cannibalism and predation by ants (in particular the bigheaded ants Pheidole spp.) on sunflowers causes high mortality among bollworm caterpillars (Cherry et al, 2003).

Chickens can help by eating caterpillars and pupae at certain time of crop development in small fields. They can be allowed to roam freely on lands not yet planted or where hardy crops are growing. However, they should not be allowed near seedlings or plants with fruit since they may cause damage by scratching and pecking (Dobson et al, 2002; Elwell and Maas, 1995).

Birds that eat pests can be encouraged to visit crop fields. Some changes will encourage them to nest and stay in the area, and this can lead to a permanent increase in local predatory bird populations. For example, groundnut plants are close to the ground and birds cannot use them as vantage points for spotting insect prey. From a perch, however, birds can easily identify prey and swoop down to devour them. Bird perches are resting places for predatory birds to rest and to look for preys such as insect pests of cotton, peanuts, and cowpeas. Predatory birds prefer to look for prey in field crops where they have places to rest. To make bird perches, use bamboo or wooden poles or tree branches. Erect either of these at regular intervals in the field.

Physical methods

•Handpick damaged fruits and collect those that fall down. Destroy the damaged fruits by cutting into small pieces, or place them in sealed sacks and dry under the sun. Do not put them immediately in a compost pit or burying them will enable the matured caterpillars to pupate into the soil.

•Handpick and destroy eggs and caterpillars. It is very important to detect small caterpillars before they enter the fruits. This is feasible at low infestations and in small plots.

•Weed if necessary. Destruction of weeds that may harbour caterpillars is important to prevent African bollworm infestation.

•Plough the field to expose the pupae to predators and the sun.