Chris Kiwawulo
11 November 2009
Kampala — SAM Ilakut planted one acre of sweet potatoes in the aftermath of the famine brought about by floods that destroyed crops in Teso sub-region. As he inspected his garden recently,about two weeks ago, he saw several grayish heavy-bodied moths with wing spans of 105-110 millimetres hovering over his potato garden in Amuria district.
Ilakut thought these were normal butterflies and paid little attention to them. Little did he know that they were laying eggs on the potato leaves. The eggs are spherical and translucent with a slight greenish tint.
Seven days later, Ilakut found hornworms (larvae) that had embarked on destroying his potato plantation. At first, he handpicked a few, but was overwhelmed on seeing that they were multiplying at a faster rate each other day.
Henry Bagiire, the agriculture state minister, says the worms consume much foliage leaving only bare stems and petioles of plants.
"Sometimes the hornworms display army worm-like habits of moving together in large numbers. Larvae always hide under large leaves at the base of plants."
Bagiire adds that the host plants (primary food) for hornworms are sweet potatoes and the morning glory.
According to scientific research, the hornworms can feed on the host plants for up to a month, which damages their growth. Their presence can also be established when large, black droppings are found on the plant leaves or on the ground beneath the plant.
Some farmers in Teso have already started slashing the vines to get rid of the pests. Herbert Akol Okurut, the Nabuin Zonal Agricultural Research and Development Institute research officer, says slashing the vines affects the growth of the root tubers.
He says collecting and destroying worms (handpicking), was the cheapest way to eliminate them. Okurut says older larvae may require a higher dosage of the insecticide.
Teso is a leading producer of sweet potatoes and supplies markets in Kampala, Jinja and Mbale areas. The crop matures quickly and was meant to address the food insecurity in the sub-region that was last year hit by floods.
Life cycle
Hornworms can survive dump temperatures even during winter in the soil as hard-shelled, brown pupae. Large adult moths, known as sphinx emerge during warm periods and deposit spherical green eggs on the undersides of leaves of host plants.
The larvae hatch a week later and feed on foliage and fruit for three to four weeks until they have fully developed. Pupation occurs in the soil and adults emerge two to four weeks later to lay second generation eggs.
Description of each stage
Bagiire says each of the stages of a hornworm has different characteristics and behaviour.
Eggs - Nearly spherical and about 1mm, in diameter. The translucent egg has a greenish tint.
Larvae - The first instar of the sweet potato hornworm has a white body and a black anal horn. Later instars are green or brown with prominent, slanted black markings on each side of the body and a black anal horn. The head is also green or brown with three dark stripes on each side. A fifth instar hornworm may be 90mm or more in length.
Pupae - The reddish-brown pupae are about 15mm wide and 64mm long. The large tongue case has a pincher - handle - like appearance.
Adult - greyish heavy bodied moth with a wing span of 105 to 110 mm. The hind wings and abdomen bear bright pink bands.
Control
Bagiire says hornworms can be controlled both mechanically and chemically. In small gardens, he says, hornworms can be controlled by picking them from plants and destroying them, but in big fields, chemicals may be used.
"Contact insecticides like cypermethrin and deltamethrin can be used for a quick knock down effect. Systemic insecticides like dimethoate and imidacloprid can also be sprayed," he advises.
Scientists at the Kemper Centre for Home Gardening also advise the use of biological control means where bacterial insecticides containing bacillus thuringiensis kurstaki are effective when larvae are smaller. It may take several days for the caterpillars to die, but feeding generally stops shortly after treatment.
Farmers may also use cultivation. Here, they apply a method called disking or rot tilling after harvest thereby destroying pupae in soil.
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