Control is more effective against the young larvae because they are localized on smaller areas, they are flightless, their movement is restricted and they are more susceptible to insecticides (Grigorov, 1976). Once developed, a locust plague is almost impossible to stop or control. Control measures include destroying egg masses laid by invading swarms; digging trenches to trap nymphs; using hopper dozers (wheeled screens) that cause locusts to fall into troughs containing water and kerosene; using poison baits; and dusting and spraying swarms and breeding grounds using aeroplanes (Anon., 2004b).
In poor countries, control of the Moroccan locust is very difficult. Armed with picks and shovels, and a small amount of pesticide, the few remaining farmers in the Khajaalwan Valley (northern Afghanistan) were the last line of defence against what experts called "the worst infestation of Moroccan locusts (D. maroccanus) the region has seen in 30 years". The UN and local authorities tried to mobilize the remaining men in a "search and destroy" mission against the locust plague. Local farmers and shepherds monitored the mountainside pastures where the eggs were laid approximately 2 cm below the surface, until they hatch. Long trenches were dug by hand to capture the newly hatched locusts as they started their mass migration towards the fields. Entire families carrying branches and brooms corralled the hoppers into the trenches and buried them. The locust mulch could be excavated later and used as fertiliser. The second line of defence was 1300 men across the north of Afghanistan, equipped and trained by the UN to use the agency's small store of pesticides, which they delivered up to the fringes of the cultivated land. In addition, there was one mobile sprayer in the district that could cover 250 ha per day. Once the locusts mature and start to fly they are virtually impossible to control effectively (Dillon, 2002).
These insects are driven by heat to look for grains of corn left in the fields after the harvest. According to experts, the invasions could be avoided by taking special preventive measures in fields between September and April. First, it is necessary to identify the sites where the locusts lay their eggs. Then the fields have to be ploughed to bring the nests to the surface, because they are hidden 3 or 4 cm underground. This ensures their destruction by the elements (Macri, 2004). In Afghanistan, the Food and Agriculture Organization of the United Nations (FAO) helped to set up community control mechanisms, whereby farmers were trained to monitor where the eggs were being laid and to kill the vulnerable young hoppers as they emerged from the ground, by driving them into trenches and burying them. This method, known as mechanical control, only works if it is carried out every year and if communities can be mobilized on a large scale (FAO, 2002a).
The entomopathogenic control agent Metarhizium anisopliae var. acridum, has received considerable attention over the last 15 years as a viable biopesticide-alternative to chemicals. The fungus is highly specific to the Acrididae, the family of short-horned grasshoppers to which the majority of economically important grasshoppers and locusts belong. It can be mass-produced relatively easily on artificial solid substrates, and when formulated in oil, can be applied under a range of environmental conditions using current chemical application technology. The biopesticide resulting from this research and development has been registered for use in parts of Africa (Green Muscle®) (Thomas et al., 2000).
Previously it had only been possible to use mycoinsecticides in humid environments such as glasshouses against aphid pests. The Green Muscle® formulation technology has transformed the development and use of mycoinsecticides, demonstrating their practical application in the harshest of environmental conditions (deserts) against an extremely mobile and difficult target pest. The efficacy of the product has been demonstrated against all the major acridid species in Africa, through numerous field trials undertaken in collaboration with African national programmes. It has been repeatedly demonstrated that Green Muscle® can reduce grasshopper populations by more than 95%. Due to its persistence, Green Muscle® offers better long-term control than any other grasshopper control product, requiring fewer applications and with much lower associated risks to farmers, their families and the environment. Green Muscle® is registered for use in South Africa and is recommended by the FAO, for use in environmentally sensitive areas. The technology has been licensed to commercial companies, and the royalties and license fees generated from the sale of the product by the licensees will be accumulated in a Trust Fund for disbursement within Africa. This project used the CABI Bioscience MycoHarvester with Dyson technology (CABI, unda).
Even though the details mentioned above imply that M. anisopliae can be a successful biological control agent for locusts and grasshoppers, the effectiveness of the pathogen, as with many biocontrol agents, can be highly variable (Bateman and Thomas, 1996). Without understanding and being able to predict this variability, confidence and widespread uptake of this green alternative may be affected (Klass et al., 2004). Recent research has identified that the key constraint that affects efficacy is temperature. M. anisopliae develops and kills its host most rapidly at approximately 30°C (Arthurs and Thomas, 2001). It shows no development below approximately 10°C or above 40°C (Ouedraogo et al., 1997; Thomas and Jenkins, 1997). In Spain, Klass et al. (2004) developed a pathogen-performance model that can accurately predict the speed of kill in a field environment.
Quesada-Moraga and Santiago-Alvarez (2000a) studied the susceptibility of D. maroccanus fourth-instar nymphs to an isolate of an undescribed species of Steinernema spp. (glaseri group). The mortality data, 24 hours after treatment, was directly related to the infective juvenile (IJ) dosage and gave an LC50 of 126 IJ per nymph. The nymphs that died 24 hours after the treatment were used to produce new infective juveniles that maintained the infectivity against fourth-instar D. maroccanus nymphs. The authors considered that the use of this nematode for biological control of Moroccan locust outbreaks is possible.
In Spain, experiments were carried out using the entomopathogenic bacteria Bacillus thuringiensis. For the first time, adults of the Moroccan locust were fed with wheat seedlings containing Bacillus thuringiensis subsp. aizawai spore/crystal suspension. The bacteria caused 54% mortality in newly moulted adults, 8 days after treatment. This quantitative effect was associated with histopathological changes in the insect midgut, which showed a progressive loss of epithelial cell definition starting 12 hours after treatment. The epithelial cells sloughed off into the midgut lumen were vesicle-like structures. More extensive cellular disintegration was observed after longer incubation times (Quesada-Moraga and Santiago-Alvarez, 2001c). The second set of forty B. thuringiensis isolates, representing 20 serotypes, were assayed using the one dose method, against third-, fourth- and/or fifth-instar nymphs of the Moroccan locust. Three successive experiments were carried out. In the first, four quite active strains were selected (belonging to serovars aizawai, mexicanensis, soonchoen and kim), which were further assayed with randomly selected strains among the remaining ones. Serovar aizawai appeared to maintain a high activity towards nymphs in the three experiments, and could be regarded as an appropriate biological control agent against the Mediterranean locust (Quesada-Moraga and Santiago-Alvarez, 2003).
Chemical control is most effective against the young larvae (nymphs). Insecticides used include organophosphates and pyrethroids. Suggested doses vary depending on the origin of the chemicals. For example, malathion is effective as a 40% solution or 50% e.c. at 2 litres/ha against early- or middle-instar larvae and at 3 litres/ha against later-instar larvae and adults.
Khodzhaev et al. (1987) tested pyrethroids for the control of D. maroccanus. They found that a ground application of fenvalerate and deltamethrin (0.4 to 0.5 litres in 300 litres liquid/ha), and an aerial application of fenvalerate or an emulsifiable concentrate containing 10% alpha-cypermethrin (6 or 50 litres liquid), gave levels of control reaching 95 to 100%, which were higher than comparison compounds, such as malathion.
Chitin inhibitors were later included in control methods. In Morocco, insecticides were applied in June 1992 to sparse, ephemeral grasses that supported a sedentary population of grasshoppers and locusts (mainly D. maroccanus). Areas were sprayed with either diflubenzuron at 60 g a.i./ha in 2 litres of dilutent, or with malathion at 600 g a.i./ha in 0.5 litres of dilutent, with three replicates per treatment. The malathion treatment showed a rapid and significant reduction in numbers of both nymphs and adults, followed by a slow recovery that started 10 days after spraying. The diflubenzuron treatment produced a slower decline in nymphal and adult numbers, but the populations remained low 10 days after spraying until the end of monitoring. Malathion- and diflubenzuron-treated nymphal populations differed significantly for the first 10 days after treatment. They did not differ from days 13 to 18, but diverged significantly from days 20 to 30. Adult populations were also reduced in both treatments (Bouaichi et al., 1994a).
Malathion has been shown to have a great initial effect, but three treatments were needed to cover the entire hatching period. The chitin inhibitors diflubenzuron and flufenoxuron, were observed to act more slowly, but persisted for more than 3 weeks, so that one treatment covered most of the hatching period. They had less effect on non-target arthropods compared to malathion (Arias Giralda and Jiménez Vinuelas, 1995).
Bouaichi et al. (1994b) studied barrier treatments with diflubenzuron. The insecticide was applied in advance of mobile hopper bands of D. maroccanus in grassland and cereal crops. Bands entering wheat and barley crops (69 to 120 tillers/m²; ear emergence stage), treated with diflubenzuron barriers (50 m deep) at a rate of 60 g a.i./ha within the treated area, experienced high mortality at subsequent moults. There was a significant positive correlation between the proportion of the treated groups moulting and the rate of population decline.
In Spain, Giralda and Jiménez Vinuelas (1996) tested diflubenzuron and flufenoxuron at ultra-low volumes against newly-hatched hoppers of the Moroccan locust. Flufenoxuron was applied at 100 g/litre and 25 g a.i./ha, and diflubenzuron was applied at 9 g/litre and 45 g a.i./ha. In pastures with rich grazing, both chitin inhibitors gave total efficiency after a few days of feeding on treated pasture. The authors recommended that the optimal period for the application of these chitin inhibitors is from the presence of newly-hatched hoppers to the appearance of second-instar hoppers.
Quesada-Moraga et al. (2000) applied the LD50 of diflubenzuron (1.5 g a.i./nymph) to fifth-instar nymphs of D. maroccanus. The authors studied the reproductive capacity of nymphs that reached the adult stage. The number of egg-pods was significantly reduced after treating the females, but was not affected by treating the males. The adult longevity of treated nymphs was significantly reduced.
The combination of 100 ml diflubenzuron (Dimilin 480 SC) and 500 ml mineral oil per ha was successfully used in Bulgaria (NSPP, 2003).
Field Monitoring/Economic Threshold Levels
The economic injury level for the Moroccan locust is two to five nymphs per m² (NSPP, 2000).
Quesada-Moraga and Santiago-Alvarez (2000c) analysed the relationship between temperature and developmental times for completing anatrepsis in the egg stage using a non-linear Logan type III model. The optimal temperature estimated for the development of eggs during anatrepsis was 24.7°C; the lower and upper thermal thresholds were 9°C and 31°C, respectively. Once the embryos completed anatrepsis, only those incubated at 15°C continued morphogenesis beyond stage XIV (diapause stage) without a low-temperature exposure period. For catatrepsis, temperature and developmental time were linearly and inversely related. Linear regression was used to estimate the lower developmental threshold and the number of degree-days required for catatrepsis. Both decreased with longer exposure to the low temperature; the former from 13.8°C to 10.5°C and the latter from 212.8 to 171.5 degree-days, following 30 and 90 days at 10°C, respectively. Their results improve the ability of decision support systems for Moroccan locust pest management by providing better forecasts to land managers and pest advisors.