Halting the march of African armyworm

For crop and livestock farmers in Tanzania, the month of December brings both blessing and curse, for as rains arrive, so does a devastating pest. African Armyworm are actually caterpillars of a night flying moth, Spodoptera exempta, which lays its eggs on grasses and cereal crops. Within a few days the larvae hatch, and subsequently grow to around 30mm, dark striped and voracious, with up to 1000 caterpillars occupying each square metre. In such numbers, they are able to devastate an area of grassland or crop in a few hours, before characteristically 'marching' to the next source of food. In Tanzania, serious outbreaks of armyworm occur in nine years out of ten, causing up to 90 per cent losses of crops and pasture in bad years. As the number of caterpillars and moths increases, so the plague spreads, aided by inter-tropical convergence winds, which carry the moths to the north through Kenya to Ethiopia, and even Yemen, or south to Malawi, Zambia and Mozambique.

Biological warfare

credit:D.Grzywacz, NRI

As a migratory pest, armyworm control is a responsibility of the government in Tanzania. Previously chemical sprays have been used, and have been funded in part by donors, but in recent years, donor funding for chemical pesticides has been withdrawn and, with the government unable to fund adequate provision, farmers have been left to either pay the $10 per hectare needed for pesticide application, or face the threat of total crop loss. The withdrawal of donor support has, however, led the government to look for a lower cost solution in the form of nuclear polyhedrosis virus, or NPV. The virus occurs naturally in armyworm caterpillars, but generally spreads too late and too slowly to prevent crop loss. Mass-production of NPV, for use in place of a chemical spray, has, however, proved to be an effective tool against caterpillars in other regions. In Brazil, for example, the method has been used to control the velvetbean caterpillar (Anticarsia gemmatalis) in soya.

Work to develop an NPV solution to armyworm began in 1999, with a joint project between the Tanzanian government and the UK's Natural Resources Institute (NRI). The virus is extracted from the bodies of infected caterpillars and is then processed to a powder, which can be mixed with water and sprayed on plants where moths have laid their eggs. Tests following ground and aerial spraying have shown the method to be highly effective, killing up to 98 per cent of caterpillars before they are large enough to inflict serious crop damage. Since the virus, once sprayed, spreads naturally in the caterpillar population, the method is ideal for tackling a mobile pest found over large areas. Specific to the armyworm, the virus is of no danger to humans, livestock or other insects. This makes it particularly suitable for use on pastures, since livestock can safely graze on the same day that spraying occurs, which is not an option when chemical sprays are used.

Pre-emptive strike

Unlike chemical pesticides, NPV does not have an immediate 'knock down' effect, as it takes about four days to infect and kill the larvae. Early application - during the first few days after the larvae hatch - is therefore essential if adequate crop protection is to be achieved. This requires early warning of outbreaks, through regular monitoring of moth numbers. Formerly, monitoring in Tanzania was done by government employees, who reported back to a central control office, from where information was distributed to farmers, though often not very effectively. In the last three years, however, a community-based monitoring system has been implemented successfully in several high-risk districts, where armyworm forecasters have been elected and trained to monitor male moth numbers through the use of pheromone traps. By recording the number of moths caught during a week, as well as the rainfall, the forecasters can predict when outbreaks will occur. During training they are also asked to identify the best means of spreading that information within their community, for example, through local leaders, schools, churches or mosques, or through posters.

Once farmers receive warning of an imminent outbreak, they are urged to inspect their crops for the tiny, newly hatched larvae, and take necessary action. At present, that action is likely to be either use of a chemical, for those who can afford the cost, or a non-chemical alternative such as neem. While Tanzania's Plant Protection Advisory Committee has recommended to the government that NPV be developed as a cost-effective means of armyworm control, consistent mass production of a high quality product is still two or three years away. In particular, further research is needed to develop efficient and cost-effective methods to 'harvest' infected caterpillars and find the right formulation for the virus-bearing product. Wilfred Mushobozi, head of the armyworm project, is confident however, that the NPV-based pesticide will be extremely cost-effective, at between one and two US dollars per hectare, and will be far simpler and safer to use than chemical sprays. A national programme is planned to formulate the NPV powder and distribute it, either at the beginning of each season, or on the basis of a national early warning system.