Tidskrift/serie: Rapport - Sveriges lantbruksuniversitet, Institutionen för växtskyddsvetenskap
Utgivare: SLU, Institutionen för växtskyddsvetenskap
Redaktör: Berger A.
Utgivningsår: 1995
Nr/avsnitt: 4
Författare: Bahana J.W.
Adress: International Red Locust Control Organisation for Central and Southern Africa - Ndola, Zambia
Ingår i...: Natural Plants Products as Pesticides. Proceedings from the first National Symposium i Zambia held in Lutsaka, 2nd-5th August of 1994
Titel: The use of pheromone traps to monitor the African armyworm Spodoptera exempta Walker in Zambia and the possibility of timely application of natural plant products to control outbreaks
Huvudspråk: Engelska
Målgrupp: Rådgivare
Nummer (ISBN, ISSN): ISSN 1104-6422, ISRN SLU-VÄXT-R-4-SE

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Infestations of the African armyworm, Spodoptera exempta, are characterised by their sudden appearance followed by high crop devastation in a short time. Quite often, agricultural extension staff is caught unaware and farmers are equally unprepared to control the pest and thus unable to prevent crop losses. A national pheromone trap network has been initiated in Zambia and can be expanded as far as possible for agricultural extension staff and farmers to manage. Monitoring of armyworm moth movements and forecasting of outbreaks will facilitate timely application of insecticides. The scouting for egg masses as well as young larvae is done where and when high moth catches are recorded in the traps. Because S. exempta is specialised in its host range, it is argued that there must be chemicals in non-host plants that can be exploited to control armyworm infestations. Such substances would be more easily accessible and cheaper to farmers than the conventional insecticides. Moreover, optimal application, in terms of timing and area would reduce the amount of chemicals applied.

1. Introduction

The African armyworm, Spodoptera exempta, Walker, is a serious and often devastating pest of cereal crops such as maize, sorghum, millet, wheat, barley, rice and forage grasses. Its impact is particularly remarkable in maize, the staple food in most of Africa, including Zambia.

The main distribution areas of the pest is eastern Africa, (including Kenya, Somalia, Sudan, Tanzania, Uganda) and central and southern Africa (Botswana, Malawi, Mozambique, Swaziland, South Africa, Zambia and Zimbabwe). Populations are also found in West Africa and the Indian Ocean Islands.

The armyworm is a migratory pest, which, in eastern and southern Africa, displays a distinct migratory pattern (Rose et al., 1968). The insect apparently starts breeding along the coast of East Africa and Mozambique further south. A progression of outbreaks then follows two directions: one northerly, from Tanzania to Ethiopia and across into Yemen, and the other to the south towards South Africa. However, there are indications that initial breeding might also take place in Angola leading to outbreaks spreading into Zambia and further south into Botswana and Zimbabwe (Bahana, 1993, unpubl.).

In general, the migration dynamics can be divided into three phases. The first one is characterised by very low populations of mainly 'solitary' nature. These populations are very difficult to detect and often pass undetected. The second phase consists of 'gregarious' populations which appear suddenly and are widespread. They are more conspicuous because of the large number of larvae and the devastation damage they cause to crops and grasses. The development of the second phase happens in a very short time and the infestations are referred to as primary outbreaks. The third phase refers to secondary outbreaks deriving from moths which have migrated from the primary outbreaks areas. Secondary outbreaks are much more widespread and cause even heavier crop losses. More generations will follow thereafter before the pest disappears until the next season.

2.Monitoring moths arrival with pheromone traps

Monitoring of armyworm populations originally consisted of light traps, particularly in East Africa (Brown et al., 1969). However, this method was found inadequate because of the large numbers of different insect species such traps attract. Moreover, light traps require experienced operators to identify the many species that are caught, and are limited to sites with electricity supply. With the advent of pheromone lures a funnel trap bearing a pheromone bait, was developed and has been in use since late 1970's in Kenya, Tanzania and Malawi.

In Zambia armyworm monitoring is very recent. In 1990/1991 a network of pheromone traps was established with about 15 trap stations. The network has now expanded to about 45. By 1995/1996 the number will increase to about 100. The present traps presently in operation in Zambia as well as in other member countries of IRLCO-CSA (Botswana, Kenya, Malawi, Mozambique, Swaziland, Tanzania, Uganda and Zimbabwe) utilise a bait impregnated with an artificial sex pheromone from female S. exempta. It remains active in field conditions for about 10 weeks.

Pheromone traps have been found to catch high numbers of moths when there is an influx in an area. The capture will consist of sexually mature males and this may be taken to mean egg laying by females in the next few days, following mating. This information is critical to a forecaster.

3. Forecasting of armyworm outbreaks

During the armyworm season, i.e. November to May, traps are observed on a daily basis and records are reported to a central forecasting office (Mt Makulu Research Station) every Monday. For the trap data to be sent on time, the reports are sent by radio, telephone, telex or any other fast way of delivery. When increasing numbers of moths are caught on consecutive nights, this may indicate a forthcoming outbreak. The forecaster then alerts the extension services, equally as fast as possible, about the possibility of an outbreak in areas where high catches were recorded by traps.

When the extension service receives a forecast, insecticides and spray equipment must be procured with minimum delay. Farmers in the prone areas are also warned to start scouting for egg masses or young caterpillars before damage to crops is done. If the pest is found, reports are then made to the nearest agriculture office, where help can then be given.

In the season of 1992/1993 Zambia experienced widespread outbreaks of the armyworm. Many farmers were caught unprepared and as a result heavy crop losses occurred. Although actual figures may not be available, reports from districts that were affected indicate that thousands of tons of crops may have been lost due to the pest. Farmers often reported the presence of the pest when the larvae were in their fourth to sixth instar stage. However, through a number of traps operating at that time it was possible to detect the arrival of the moths and therefore foresee the subsequent outbreaks well in time. It appears that the feed-back of the trap information to the farmers has been lacking.

4. Accuracy of forecasts

The question that arises is "How accurate is a forecast based on pheromone traps?". Broza et al. (1991) found that trap data can be used to idenfif\r potential outbreak areas within a given region. He was thus able to locate egg masses 5-6 days after peak catches in nearby traps. On the other hand, no egg masses or armyworm larvae were found during the same period in areas where low number of moths were recorded. The accuracy and usefulness of a forecast largely depends on the speed of reporting of trap records to the forecasting office. Pheromone traps alone can not contribute to an accurate forecast. A number of studies are being undertaken on how to improve forecasting of armyworms. Results so far show that rainfall is most likely to occur over an area which has also experienced convergence of air masses. This is determined by examining Cold Cloud Duration (C CD) using satellite imagery. If such an area also records high numbers of moth catches for a number of successive days, the probability of an outbreak will be very high. This is because moths are concentrated by wind convergence, and an outbreak is more likely to occur where there is rainfall.

5. Role of natural plant products

There are numerous methods that have been tried by farmers to control armyworm outbreaks, many in desperation. Such desperate methods, which may give temporary respite, include, digging trenches around field, beating to death the matching caterpillars, sweeping the larvae out of the fields etc. Where outbreaks repeatedly occur in the same area, massive numbers of caterpillars may die of fungal or viral infection. These have been reported to be crushed and mixed with water. The mixture is then sprayed on fresh infestation and interesting results have been obtained. Research conducted by Desert Locust Control Organisation in this area show that naturally occurring pathogens can be exploited for the control of armyworm outbreaks.

Timely application of insecticide for the control of armyworm outbreaks is critical for saving the crops. Unavailability in addition to high prices make it difficult for farmers to control armyworms. Moreover, by the time a farmer obtains insecticide, the pest has already spread into the field and therefore insecticides have to be sprayed over a wide area. This may result in an increased pollution of the environment as well as higher costs for the pesticides.

However, work on natural plant products is still to be undertaken. S. exempta is selective and can be referred to as oligophagous since it only feeds on graminaceous plants. It is likely that there are chemical substances which deter the armyworms from feeding on other plant groups. If such chemicals are present in all non-host plants, then they would not be difficult to find.

The correct use of a network of pheromone traps can serve as an important means for locating high moth populations and therefore potential outbreak areas. Such a monitoring system can be managed by field extension staff who will supply the pheromone traps at the beginning of the season in prone areas with vulnerable crops. In case of high numbers of moths being caught, scouting for eggs and young larvae has to be carried out.

Once egg masses are located, small extract of plant products probably within easy reach of the farmers, can then be applied at an optimum time when larvae are at their most susceptible stage over a much smaller area. The time has come for a serious search for chemical products within the wide array of plant species that are non-hosts to S. exempta.

6. Literature references

Brown, E.S., Betts, E. and Rainey, R.C. 1969. Seasonal changes in distribution of the African armyworm, Spodoptera exempta (Wlk.)(Lep., Noctuidae), with special reference to Eastern Africa. Bull. Ent. Research 58 (IV): 661-728.

Broza, M., Brownbridge, M. and Sneh, B. 1991. Monitoring secondary outbreaks of the African armyworm in Kenya using pheromone traps for timing of Bacillus thuringiensis application. Crop protection 10 (3): 229-233.

Rose, D.J.W, Page, W.W., Dewhurst, C.F., Riley, J.R., Reynolds, D.E., Pedgley, D.E. and Turner, M.R. 1968. Downwind migration of the African armyworm moth Spodoptera exempta, studied by mark-and-capture and by radar. Ecological Entomology 10 (3): 299-313.