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The symptom of S. exempta attack is gross feeding damage to foliage, growing points and young stems. Severe infestation results in total defoliation or destruction of the plant to ground level.
Forecasting and Control
Effective forecasting and management of pests like S. exempta, whose long-range migrations result in the rapid spread of outbreaks from country to country, requires international cooperation. This is most easily achieved if regional organizations have responsibility for coordinating information and, if possible, the infrastructure required for both forecasting and control. Several national crop protection services, especially in eastern, central and southern Africa and the Yemen, have departments with special responsibility for control of migrant pests, including armyworm. These may assist farmers in countries where armyworm is a notifiable pest, by providing and/or applying insecticide. The Desert Locust Control Organization for Eastern Africa (DLCO-EA) and the International Red Locust Control Organization for Central and Southern Africa (IRLCO-CSA) have regional responsibilities for armyworm. Where infestations are widespread, governments have the option of requesting (or hiring) spray aircraft from them. International cooperation also allows the development of a strategic approach to limiting the development and spread of outbreaks. By focusing attack on early season infestations providing sources of migrants, it should be possible to reduce or eliminate the occurrence of subsequent outbreaks, or sequences of outbreaks, downwind. Such an approach has been developed and implemented by DLCO-EA and its member countries. For further discussion of strategic control of S. exempta, see Cheke and Tucker (1995), Rose et al. (1996), and references therein. Forecasting An effective forecasting system is essential for successful management of Spodoptera exempta, to allow preparations to be made in time to control infestations and reduce crop losses. There may be little time to respond as infestations frequently go unnoticed until larvae become conspicuous at the fourth instar and the amount of damage then increases exponentially with every day that passes. A forecasting system for S. exempta has been in operation in East Africa since 1969. All forecasts and warnings are currently based on: the distribution of armyworm populations currently reported through the monitoring system; the distribution of previously reported populations, with allowances made for their development with time (timing of oviposition following large trap catches of moths and subsequent hatching and development of larvae, timing of emergence of, and emigration by, moths from earlier outbreaks); predominant winds during the periods of moth migration, and synoptic and meso-scale zones of wind convergence in which moths could become concentrated; the distribution of rainstorms with associated wind convergence capable of concentrating moths, particularly early in armyworm seasons; and historical precedents for the development anticipated. The necessary information is derived from monitoring outbreaks and sampling larvae. Accurate monitoring and prompt reporting of armyworm outbreaks is essential for forecasting and control. The procedures developed in member countries of DLCO-EA involve: searches for newly hatched larvae one week after first high catches in pheromone traps, especially catches associated with the first heavy rains after a drought period; larval sampling to determine the age of outbreaks; reporting outbreaks to local district agricultural offices and national plant protection services immediately they are found, preferably with samples of larvae; assessment of larval age by national plant protection service laboratories. Details of these procedures are given by Rose et al. (1996). Trap data from moth trap networks Networks of moth traps in each country are the most effective way of monitoring armyworm populations. Pheromone traps are recommended for widespread use in national networks. However, they catch only sexually receptive males so some light traps, which catch immature, migrating moths (sometimes in large numbers), should be included at selected sites where electricity and expertise for sorting and identification of catches is available. Detailed information on trap design, pheromone lures (chemistry, formulation and availability), and siting and operating traps is provided by Rose et al. (1996). Meteorological data Forecasting services require rainfall, windfield and rainstorm distribution data. These are generally available through national meteorological services with which close cooperation is essential. In recent years, satellite remote-sensing data have become available at an affordable cost. In East Africa, Cold Cloud Duration data from Meteosat are used to identify locations of rainstorms with the potential to concentrate moths, as well as to determine whether there has been little or no, intermittent and scattered, or prolonged and widespread, rainfall on a regional scale. This information is used to guide monitoring to locate outbreaks and to assess the level of infestation that can be expected, given the known influence of weather, especially rainfall, on the population dynamics of S. exempta (see section on Biology and Ecology). Historical data archives Archives of S. exempta trap and outbreak data have been accumulated nationally and regionally in eastern Africa (these archives are stored also at DLCO-EA, Nairobi and, up to 1988, at the Natural Resources Institute, Chatham, UK). The archive data are now routinely accessed by forecasters and provide analogues of outbreak distribution and trap characteristics for the evaluation of current armyworm situations. Computerised databases, using the specific data management system 'WormBase', are established in eastern Africa (Crop Protection Branch in Kenya, Pest Control Services, Tanzania, DLCO-EA), and are being introduced in IRLCO-CSA countries of central and southern Africa. Historical meteorological data are available from national meteorological services. Forecasts and warnings have different levels of urgency: forecasts are prepared weekly, or every 2 weeks, based on information received at the national or regional offices from the monitoring systems and describe expected future armyworm developments. They may be regional or national in scale. An expert system to produce computer-assisted forecasts has been developed at DLCO-EA. A warning is issued as an alert to the immediate potential occurrence or redistribution of infestations, ideally while they are still in the moth or early larval stages. Warnings should be sent by the quickest possible means (telephone, radio, or broadcast media and newspapers) to agricultural offices in the affected areas, to be acted on immediately. Verification of the reliability of forecasts and warnings is essential if their accuracy and value are to be improved. This is now undertaken routinely by the Regional Forecasting Office at DLCO-EA, Nairobi, by plotting locations of reported outbreaks in relation to predictions, using a computer. Reasons for errors are analysed to avoid repetition. Further information on forecasting is given by Rose et al. (1996).
Weed-free maize crops greater than 50 cm high are unlikely to become infested by newly hatched larvae of S. exempta because the leaves are too tough to allow them to establish. However, if larvae are able to develop on grass weeds, subsequent infestation of the crop may occur. Farmers are advised to keep crops free of grass weeds but, if fields do become infested, to leave the weeds until the larvae have pupated or been controlled. Some maize varieties are more susceptible to attack than others, e.g. Katumani, a dryland variety grown widely in Kenya. These varieties are most at risk where probabilities of armyworm infestation are high.
Predators and parasitoids of S. exempta are never numerous enough to achieve natural control of outbreaks. The nuclear polyhedrosis virus (SpexNPV) has been used to control infestations by spraying with a water suspension of diseased larvae. The feasibility of laboratory production and formulation of the virus has been demonstrated. Recent field trials in Tanzania indicate that SpexNPV could have a potential role as a substitute for chemical insecticides in strategic armyworm management programmes (Grzywacz et al., 2008). A formulation of Bacillus thuringiensis has also been identified as promising. Full laboratory and field evaluation of these products may result in their adoption in the future.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Losses to agricultural production due to S. exempta are frequently significant and intermittently devastating on a local, national and regional scale. Significant losses are most consistently reported from eastern and southern Africa. However, in recent decades, the frequency of reports from West Africa has increased, possibly due to the extension of suitable grassland habitats following forest and bush clearance for agriculture. Outbreaks are occasionally reported from elsewhere in the species' range, notably from Papua New Guinea, northern Australia and Hawaii, USA.
Damage to cereal crops results principally from direct attack on young plants by larvae hatching or dispersing into the crop as first instars, and by invasion of the crop by older larvae from adjacent wild grasses. Where these invasions are caused by late-instar larvae moving from heavily infested grasslands, even maturing crops can be totally destroyed. If drought conditions follow an outbreak, plants may not recover from defoliation and replanting may fail to produce a crop.
Smallholders are particularly vulnerable to the effects of infestation as they rarely have the resources for effective control or spare seed for replanting. Infestations frequently affect large areas, eliminating the possibility of relief by mutual support and assistance between farmers. Government crop protection and extension services may be able to provide only limited assistance as a result of financial and logistical constraints.
Yield reduction caused by defoliation in maize is almost directly proportional the percentage of leaf area available to the larvae at the time of attack. Reported losses range from 9% in plants attacked at the early whorl (four leaves) stage to 100% in those damaged at the pre-tassel stage. The ability of young maize plants to recover from armyworm damage depends on the position of the apical meristem at the time of attack and the amount of root development when the larvae cease feeding. Damage is always serious if the apical meristem is affected but, as it remains at the base of the plant until near to the pre-tassel stage, it may be below ground during the outbreak and remain undamaged.
Tentative nominal action thresholds for control measures have been determined for maize. To avoid yield losses of >15%, action thresholds for early whorl maize should be taken as 200 second (II), 80 third (III), or 20 fourth (IV) instar larvae per 100 plants. Serious damage develops rapidly once larvae reach the IV instar.
Replanting maize after armyworm have eaten the first-sown plants to the ground is frequently unsatisfactory as the optimum planting dates will have been missed. Yield losses of 6% have been estimated for each day's delay after the optimum planting date in high-rainfall areas in Kenya. Late planting may result in much higher losses in areas with less rainfall; yield losses of up to 92% have been recorded in such areas in Malawi and Kenya.
In sorghum, millet, rice and Eragrostis tef, armyworm damage may stimulate tillering which can, in favourable conditions, increase yield. If subsequent rainfall is adequate for crop growth and development, yield losses may be limited, providing the damage occurs before the critical grain-initiation stage has been reached.
Damage to pasture and rangeland can be extensive and severe. Armyworm damage to grasses and the consequent advantage to dicotyledonous weeds results in changes in the composition of the sward which may be re-inforced by drought and overgrazing. However, good rainfall after infestation is an important factor in pasture recovery. Surveys in Tanzania indicate that effects of infestation may last for more than eight weeks but, in areas with good rainfall, they seldom last more than 5 weeks. In Kenya, vegetation changes in infested pastures have been reported to persist for many years before good grass cover has been restored by management of dicotyledonous weeds. As a general rule, control measures for the protection of pasture are not recommended unless larval densities exceed 10/m².
Deaths among cattle grazing recently infested pasture have been reported by herdsmen in southern Ethiopia, Somalia (where 100 cattle were reported to have died on one occasion), and Kenya, as well as in southern Africa. Speculations as to the causes of death include high cyanide levels induced in Cynodon spp. grasses by armyworm damage, and ingestion of larvae or fungal mycotoxins on armyworm faeces.
See Rose et al. (1995) and references therein.