Michel Lecoq

Do outbreaks affect genetic population structure? A worldwide survey in Locusta migratoria, a pest plagued by microsatellite null alleles

An understanding of the role of factors intrinsic to a species’ life history in structuring contemporary genetic variation is a fundamental, but understudied, aspect of evolutionary biology. Here, we assessed the influence of the propensity to outbreak in shaping worldwide genetic variation in Locusta migratoria, a cosmopolitan pest well known for its expression of density‐dependent phase polyphenism. We scored 14 microsatellites in nine subspecies from 25 populations distributed over most of the species’ range in regions that vary in the historical frequency and extent of their outbreaks. We rejected the hypothesis that L. migratoria consists of two genetically distinct clusters adapted to habitats either rarely (nonoutbreaking) or cyclically (outbreaking) favourable to increases in population density. We also invalidated the current subspecific taxonomic classification based on morphometrics. Bayesian inferences indicated evidence of a homogenizing effect of outbreaks on L. migratoria population structure. Geographical and ecological barriers to gene flow in conjunction with historical events can also explain the observed patterns. By systematically assessing the effects of null alleles using computer simulations, we also provide a template for the analysis of microsatellite data sets characterized by a high prevalence of null alleles.

Recent progress in desert and migratory locust management in Africa. Are preventative actions possible

Abstract Locust control in Africa has been the focus of considerable controversy over the last 15 y. Many control aspects were called into question following the last large plagues of 1987–88 (Desert Locust) and 1996–2000 (Malagasy Migratory Locust): recommended preventative strategy, environmental impact of insecticides used, socioeconomic importance of the locusts. Many projects were launched to improve survey and control methods and find alternative solutions to chemical pesticide treatments. Recent progress is reviewed. Survey and application techniques showed dramatic improvements with GIS, GPS technology and precision spraying. Barrier applications are again available, thanks to new persistent pesticides. Biological control with mycopesticides now appears very promising and undoubtedly the technique will gradually be adopted. The environmental aspects are taken into better account and an IPM approach is now possible. The EMPRES program of FAO is committed to reinforcing national early warning and early reaction capacities for preventative control of the Desert Locust. In Madagascar a new National Locust Center has recently been set up. In many places local research capabilities are improving. However, certain very fundamental aspects remain unsolved. Among others, the sustainability of preventative control systems is an important concern. Many affected countries are amongst the poorest in the world. They currently do not have sufficient financial resources to fund intensive control campaigns, nor even the basic minimum to ensure prevention. A realistic solution to prevent locust plagues requires stronger commitment of governments and donors, sustainable small but flexible national locust control units for prevention, and especially the creation of emergency action plans, ready to quickly mobilize and organize complementary resources in the event of an upsurge. Emergency funds are thus essential and should be created. Locust control seems now to depend more on political and institutional choices than on scientific and technological innovations.

Desert locust management: from ecology to anthropology

Abstract Plague locusts are a major problem in many developing countries. For the desert locust, control by a preventive strategy proved effective, and led to a dramatic decrease in outbreak frequency and duration over the last 40 y. Principal problems are now of an organizational nature. Natural-risk management plans for locust outbreaks, associated with new financial mechanisms, must be implemented in order to ensure early reaction, efficiency of control and sustainability of the preventive approach. Studies of these outbreaks should be based on the ecological mechanisms considered to date, but also be broadened to include economic, social, organizational and cultural mechanisms that have been largely overlooked in the past. New concepts, such as risk-management systems, stakeholder strategies, and governance, should be given serious consideration. A new approach to locust issues is suggested, using techniques derived from sociological and anthropological sciences.

Outbreaks, gene flow and effective population size in the migratory locust, Locusta migratoria: a regional-scale comparative survey.

The potential effect of population outbreaks on within and between genetic variation of populations in pest species has rarely been assessed. In this study, we compare patterns of genetic variation in different sets of historically frequently outbreaking and rarely outbreaking populations of an agricultural pest of major importance, the migratory locust, Locusta migratoria. We analyse genetic variation within and between 24 populations at 14 microsatellites in Western Europe, where only ancient and low‐intensity outbreaks have been reported (non‐outbreaking populations), and in Madagascar and Northern China, where frequent and intense outbreak events have been recorded over the last century (outbreaking populations). Our comparative survey shows that (i) the long‐term effective population size is similar in outbreaking and non‐outbreaking populations, as evidenced by similar estimates of genetic diversity, and (ii) gene flow is substantially larger among outbreaking populations than among non‐outbreaking populations, as evidenced by a fourfold to 30‐fold difference in FST values. We discuss the implications for population dynamics and the consequences for management strategies of the observed patterns of genetic variation in L. migratoria populations with contrasting historical outbreak frequency and extent.

Field Trial with the Entomopathogenic Fungus Metarhizium anisopliae var. acridum against Bands of the Grasshopper Rhammatocerus schistocercoides in Brazil

The efficacy of a mycoinsecticide formulated in vegetable oil was tested in Brazil against the grasshopper Rhammatocerus schistocercoides . A set of experiments was conducted in the Chapada dos Parecis region (Mato Grosso state), a permanent zone of outbreaks for this pest. Experiments were performed in zones of natural vegetation, against grasshopper bands in the third nymphal instar. Three nymphal bands were treated with a mycoinsecticide formulation based on conidia of the entomopathogenic fungus Metarhizium anisopliae var. acridum ( =M. flavoviride ), strain CG 423. Three non-treated bands were used as control. The application was made with the aid of a hand-held ULV sprayer adjusted to deliver 2 l of the formulation ha -1 , each containing 1 2 10 13 conidia. Treatments were limited to the surface of the grasshopper bands and their immediate borders (5-10 m). The efficacy of the mycoinsecticide was evaluated through band survival after treatment (grasshopper numbers, surface, density, behaviour and daily movement of the band), allowing the insects to move freely in their natural environment. Insects were regularly surveyed and maintained in the laboratory, allowing estimates of the infection rate. Field and laboratory studies showed a clear effect of the product 10 days after treatment. At 14 days post-spraying, mortality caused by the mycoinsecticide in the field was approximately 88%.

Genetic variation for parental effects on the propensity to gregarise in Locusta migratoria.

BackgroundEnvironmental parental effects can have important ecological and evolutionary consequences, yet little is known about genetic variation among populations in the plastic responses of offspring phenotypes to parental environmental conditions. This type of variation may lead to rapid phenotypic divergence among populations and facilitate speciation. With respect to density-dependent phenotypic plasticity, locust species (Orthoptera: family Acrididae), exhibit spectacular developmental and behavioural shifts in response to population density, called phase change. Given the significance of phase change in locust outbreaks and control, its triggering processes have been widely investigated. Whereas crowding within the lifetime of both offspring and parents has emerged as a primary causal factor of phase change, less is known about intraspecific genetic variation in the expression of phase change, and in particular in response to the parental environment. We conducted a laboratory experiment that explicitly controlled for the environmental effects of parental rearing density. This design enabled us to compare the parental effects on offspring expression of phase-related traits between two naturally-occurring, genetically distinct populations of Locusta migratoria that differed in their historical patterns of high population density outbreak events.ResultsWe found that locusts from a historically outbreaking population of L. migratoria expressed parentally-inherited density-dependent phase changes to a greater degree than those from a historically non-outbreaking population.ConclusionBecause locusts from both populations were raised in a common environment during our experiment, a genetically-based process must be responsible for the observed variation in the propensity to express phase change. This result emphasizes the importance of genetic factors in the expression of phase traits and calls for further investigations on density-dependent parental effects in locust phase change. More population samples with different outbreak histories need to be analyzed to demonstrate that differences in propensity to gregarise evolve because of different outbreak histories.

Field trials of fipronil for control of Rhammatocerus schistocercoides (Rehn, 1906) hopper bands in Brazil

Abstract Field trials in the state of Mato Grosso (Brazil) assessed the efficacy of fipronil against Rhammatocents schistocercoides hopper hands (8th instar) in natural vegetation within the context of a preventive control strategy. Thirteen experimental plots, with 14 hopper bands, were sprayed at doses ranging from 2.15 to 11.8 g a.i./ha. Mortality increased with higher doses in a shorter time, but all tested doses resulted in 100% mortality within one to four days. The dose to control hopper bands depends on the speed of control required, according to the potential threat to crops: preventive control in natural vegetation (4 g a.i./ha) or curative control in crops (8 g a.i./ha dose could he used).

The use of Metarhizium anisopliae var. acridum against the grasshopper Rhammatocerus schistocercoides in Brazil

Abstract The control of grasshoppers in Brazil has been based exclusively on chemical insecticides (fenitrothion and malathion). However, as these products are known to be harmful to the environment, their massive use has caused concerns. In the face of pressure against their use, the development of alternative methods became imperative. Some species of entomopathogenic fungi can supplement or even replace chemical insecticides in the control of grasshoppers. An integrated research project began in Brazil in 1993 with the specific objective of developing bioinsecticides based on entomopathogenic microorganisms, especially fungi, to control grasshoppers. Activities centered in surveys, characterization, production, formulation, and field evaluation. Emphasis was given to developing the fungus Metarhizium anisopliae var. acridum as the most promising biocontrol candidate. It is now known that this pathogen can be used efficiently in the control of Rhammatocerus schistocercoides in Brazil and we are verifying its effects on non-target organisms, including other Orthoptera, Diptera and Hymenoptera.

Drought and an exceptional outbreak of the oriental migratory locust, #Locusta migratoria manilensis# (Meyen 1835) in Indonesia (Orthoptera : Acrididae)

Serious outbreaks of oriental migratory locusts, #Locusta migratoria manilensis# (Meyen 1835), were reported on different islands of the Indonesian archipelago during the rainy season of 1997-98, especially in southern Sumatra. In the long term, the intensive deforestation that has been under way over the last 20 years is certainly partially responsible for these outbreaks. The drought that prevailed in the region in 1997 also seems to be a major recent factor that prompted these outbreaks. (Authors abstract)

Demographic processes shaping genetic variation of the solitarious phase of the desert locust

Between plagues, the solitarious desert locust (Schistocerca gregaria) is generally thought to exist as small populations, which are particularly prone to extinction events in arid regions of Africa and Asia. Given the high genetic structuring observed in one geographical area (the Eritrean coast) by former authors, a metapopulation dynamics model involving repeated extinction and colonization events was favoured. In this study, we assessed the validity of a demographic scenario involving temporary populations of the solitarious phase of the desert locust by analysing large‐scale population genetic data. We scored 24 microsatellites in 23 solitarious population samples collected over most of the species range during remission. We found very little genetic structuring and little evidence of declining genetic diversity. A Bayesian clustering method distinguished four genetically differentiated units. Three groups were largely consistent with three population samples which had undergone recent bottleneck events. Nevertheless, the last genetically homogeneous unit included all individuals from the remaining 18 population samples and did not show evidence of demographic disequilibrium. An approximate Bayesian computation treatment indicated a large population size for this main genetic group, moderately reduced between plague and remission but still containing tens of thousands of individuals. Our results diverge from the hypothesis of a classical metapopulation dynamics model. They instead support the scenario in which large populations persist in the solitarious phase of the desert locust.