Source:TheHerald
Date:10 December 2005
Author: Elita Chikwati
THE Department of Agricultural Research and Extension Services (Arex) has warned farmers of migratory pests which are a problem during the rainy season. Pests such as armyworm and locusts usually migrate to Zimbabwe between November and early March during which they could damage young cereal crops. Arex research officer Mr Kwadzanayi Mushore said the tracking of the armyworm infestations was difficult since the moths were nocturnal and could move long distances with the Inter Tropical Convergence Zone winds. The pests usually affect pastures, maize and sorghum and if not controlled on time, cause extensive damage to crops. Detection and possible control should therefore begin as early as possible. "Farmers can expect outbreaks of armyworm when they see sudden swarms of moths around cooking fires or candles," Mr Mushore added. Armyworm moths are identified by their dark brown wings which have black spots while the hind wings are white with dark edges. Farmers are urged to increase scouting for any suspicious caterpillars in fields and in pastures and make a report to the nearest Arex offices or the Plant Protection Unit. The moths do not form swarms like locusts but shifting winds could result in high concentrations of moths and ultimately, high concentrations of army worm.
The most effective way of predicting armyworm infestations, Mr Mushore said, was through pheromone moth traps, which make use of a synthetic female hormone to attract males. Light traps could also be used for this purpose. The armyworm larvae exist in solitary and gregarious phases. The solitary form of armyworm was pale green and was rarely recognisable. Small populations of the solitary phase do little or no crop damage. Armyworm caterpillars are most dangerous in the gregarious form when they gather together and are found in large numbers on plants. In this form, the caterpillars are up to 30mm long and have dark stripes that make them appear black. The caterpillar's head is black with a white V-shaped mark. The appetite of this gregarious form is such that a heavy infestation can consume rangeland grasses, wheat, barley, pastures sorghum and finger millet faster than 400 head of cattle per hectare and vast areas can be destroyed overnight.
When there is an outbreak, chemical control should be employed since it
is most effective. Cabaryl 85 percent WP, which is commonly used in Zimbabwe,
is sprayed at the rate of 625g per 200 litres of water for a hectare. The
addition of molasses to the mixture enhances its effectiveness.
Other chemicals which are registered for the control of armyworm are cypermethrin,
trichloforn and malathion. Chemical control, though effective, has a disadvantage
of poisoning pastures and water sources.