Simulation studies of Senegalese Grasshopper ecosystem interactions I: the ecosystem model

The Senegalese grasshopper is a major pest of crops in Sahelian Africa. It has three generations per year and undergoes a 7-8-month long embryonic diapause during the long dry season. Egg pods collected at the end of the rainy season from mid-August to the end of September (the known period of induction of the diapause) were placed in incubation in semi-natural conditions, and were humidified on various dates between November and August. The results reveal a rather complex diapause system, which is undoubtedly part of a bet-hedging strategy that is well adapted to maximizing overall survival and reproduction in the variable rains of the Sahel. In this strategy, some eggs hatch on early wet season rains and survive well if there is follow-up rain, while others remain in diapause and hatch when the wet season is assured. (Résumé d'auteur

Section: Discussionmentioning

confidence: 98%

Egg survival strategies of the Senegalese grasshopper during the dry season in the African Sahel

Maïga

Lecoq

Morand

2010

“…It was made possible for the grasshoppers to delay death by basking in the sun (behavioural fever -a pathogen suppressing strategy described by Blanford et al (1998). It was assumed in the model that grasshoppers, infected but still alive, eat and oviposit normally and that Green Muscle did not harm the alternative prey or other insects in the model (Axelsen 2009). Availability of alternative prey for the birds in the model was simulated to be proportional to the grasshopper density ).…”

Section: The Modelmentioning

confidence: 99%

“…The ecosystem model (Axelsen 2009) simulates the dynamics and migration of O. senegalensis along a five-location, south-north transect running from the southern Sahel to the pastoral zone south of the Sahara desert. The model simulation includes: a natural Sahelian grass in the grassland areas (representing the natural vegetation); millet; natural Sahelian grass as weeds in millet fields prior to manual weeding after 1 month; O. senegalensis; an dipteran egg pod predator Bombyliidae sp., representing all bombyliid species; a coleopteran egg pod predator Tenebrionidae sp., representing all egg pod predatory beetles; the grasshopper Acrotylus blondelii Saussure 1884 as an alternative prey to the egg pod predators; 24 bird species; and an alternative prey for the birds ).…”

Section: The Modelmentioning

confidence: 99%

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Simulation studies of Senegalese Grasshopper ecosystem interactions III: optimization of grasshopper control

Maïga¹,

Axelsen²,

Badji³

et al. 2009

Self Cite

Large tracts of millet fields in the Sahel are sprayed with chemical insecticides to control the grasshopper Oedaleus senegalensis, but it is difficult to assess the impact of these pesticide applications on the grasshopper's population dynamics due to its migratory behaviour. The grasshoppers migrate along a route which runs from south to north. Therefore, a simulation model that can simulate population development and migration on a simple south-north transect was used to assess the impact of treatment with pesticides, including the biopesticide Green Muscle 1 , in terms of grasshopper control on two south-north transects in the Sahel. The results indicate that the various actual pesticide treatments had a considerable overall impact on local grasshopper pressure but had little impact on sites further along the transect, because they were only carried out curatively in the millet fields themselves. The results however suggest that preventive treatment of both grassland and millet fields in the south would be more efficient, and thereby represents an alternative strategy. It would, moreover, appear that this could represent an environmentally sensitive alternative if carried out using Green Muscle, but even preventive use of a traditional chemical pesticide would be more environmentally sensitive than spraying pesticides solely on millet fields curatively and over a larger area.

“…Furthermore, each location was split into two compartments, millet fields and grassland, occurring in varying respective proportions. The model has been described in detail by Axelsen (2009) and has also been used to assess the effects of actual treatments from 2003 and 2004 in eastern Niger and to establish how to optimize pesticide treatments (Maiga et al 2009 (Appendix 1), but a small number of parameters had to be found by calibration to available population density data. The data comprised two datasets from Niger and one from Senegal.…”

Section: Introductionmentioning

confidence: 99%

Simulation studies of Senegalese Grasshopper ecosystem interactions II: the role of egg pod predators and birds

Axelsen¹,

Petersen²,

Maïga

et al. 2009

Self Cite

We present a simulation study of the role of birds and egg pod predators in the population dynamics of the Senegalese grasshopper. The model was an energetics-driven ecosystem model that included a natural Sahelian grass, millet, grasshoppers, the egg pod predators bombyliidae and Tenebrionidae, and 24 species of birds that prey on grasshopper nymphs and adults. The model simulated the rain-dependent dynamics and migration along a five-location south-north transect in Niger and Senegal, and the main outputs were the grasshopper pressure on the transect, measured as accumulated transect grasshopper days, and the density of eggs ready to emerge the following rainy season. The model was calibrated using observations from transects in Niger (2003Niger ( , 2004 and Senegal (2004).When input data from Niger 2003 and 2004 were applied, simulated reductions in grasshopper pressure during the rainy season from egg pod predators and birds, respectively, were within a range of 18-25%, but taken together the overall reduction in grasshopper pressure ranged between 35 and 37%. Using input from Senegal (2004) the birds accounted for reductions of around 25%.When the impact of the natural enemies was measured in terms of reduction in the density of eggs hatching the following year, the simulated impact of the egg pod predators ranged between 45 and 83% and that of the birds between 33 and 39%, and the two groups combined caused reductions between 60 and 75%.Analyses of the sensitivity to the bird species, added one by one, showed 'predator saturation' along the Niger transects (with low grasshopper density) but not along the Senegal transect.