the study took place in mauritania , one of the key countries for the implementation of preventive control and hosting desert locust permanent habitats ( popov et al . the country is also historically an important locust migration route between the sahel and the maghreb in africa ( ceccato et al . 2007 ) . the study was conducted during and shortly after two successive annual rainy seasons ( between july and december ) , in 2012 and in 2013 . the studied areas are annually monitored by the survey teams of the national anti - locust center of mauritania and correspond to the traditional desert locust breeding and gregarization areas ( popov et al . field sampling was oriented by locust presence in these areas ( see fig . 1 and cisse et al . 1.sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . 
  sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . the sampling methodology used to estimate the desert locust adult densities was consistent with the fao guidelines ( cressman 2001 ) . at least ten samples were taken at an individual site ( survey stop ) where green vegetation was present . each sample consisted of counting the number of individuals in a foot transects of 100 m in length and 1 m in width . parallel transects were separated by at least 50  m. the average number of locusts per 100  m was determined from the ten samples and converted to density per hectare . the collected data were focused on locust behavior ( isolated or scattered solitarious adults ; grouped transiens or gregarious adults ) and body - coloration ( usually brown for solitarious adults ; bright pink for transiens , pink , or yellow for gregarious adults ) ( symmons and cressman 2001 ) . phase polyphenism confers to locust species , and desert locust in particular , considerable variation in behavioral , morphological , anatomical , and physiological characteristics , as population densities and environmental conditions of the habitat area change ( pener and simpson 2009 ) . the extreme gregarious adult forms are noticeably different from the extreme solitarious adult forms in various traits such as coloration , wing length , hind femur length , a bulge in the cheeks , a depression in the crest of the pronotum , a shortening of the prozona , and a constriction in the thorax ( duranton and lecoq 1990 ) . in the transiens adult forms ( congregans or dissocians , depending on phase change direction ) , phase characteristics are independently and continuously induced or shifted to either direction in response to changes in the density of the population ( rao 1942 , pener and simpson 2009 ) . also , the swarm migration and group oviposition are well known as typical gregarious - phase characteristics . in the field , prominence was given to behavior rather than appearance in characterizing the phase status of individuals because body color and morphological changes could occur later ( rogers et al . also , during these 2  yr of field work , no major outbreak was in progress . ( 2013 ) , with a solitarious status or gregarious status ( hence including most transiens forms ) . samples were conducted in 148 sites and adult locust found in 130 sites ( fig . 1 , also described in cisse et al . the vegetation variables of status and cover were collected for each site following the fao guidelines ( cressman 2001 , cisse et al . if mixed dry and green vegetation was found with no signs of new growth , it was noted green or  shooting. the cover vegetation variable was based on the density of the vegetation compared with bare soil : it was denoted as we assessed the prediction of desert locust adult phase with the statistical model using adult density , vegetation status and cover as established in cisse et al . this prediction model is a multivariate logistic regression model of the form : logit[p(gregarious ) ]  adult density  +  vegetation status  +  adult density : vegetation cover . table 1 presents the results of its adjustment from historical data realized in cisse et al . ( 2013 ) . by introducing the new field observations as explanatory variable in this adjusted statistical model , we could compute the probability that each sampled population of desert locust adult was gregarious according to its density and the status and cover of the vegetation . we then compared these predictions with the phase status observed in the field according to behavior . the probability of being gregarious from the model was compared with the characterized phase status through a simple confusion matrix . data were analyzed with the statistical software r version 3.2.2 ( r development core team 2015 ) . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover.coefficientestimatesez valuepr(>|z|)intercept1.89971680.039710347.839<2e16adult density0.00255450.000141318.082<2e16greening vegetation0.08145730.17160300.4750.635shooting vegetation1.99471650.30273576.5894.43e11dry vegetation0.68886790.22303443.0890.002green vegetation0.08929480.05119911.7440.081adult density : low cover0.00326990.00038438.509<2e16adult density : medium cover0.00036970.00015202.4310.015vegetation cover from cisse et al ( 2013 ) . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover . the study took place in mauritania , one of the key countries for the implementation of preventive control and hosting desert locust permanent habitats ( popov et al . the country is also historically an important locust migration route between the sahel and the maghreb in africa ( ceccato et al . 2007 ) . the study was conducted during and shortly after two successive annual rainy seasons ( between july and december ) , in 2012 and in 2013 . the studied areas are annually monitored by the survey teams of the national anti - locust center of mauritania and correspond to the traditional desert locust breeding and gregarization areas ( popov et al . field sampling was oriented by locust presence in these areas ( see fig . 1 and cisse et al . 1.sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . 
  sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . the sampling methodology used to estimate the desert locust adult densities was consistent with the fao guidelines ( cressman 2001 ) . at least ten samples were taken at an individual site ( survey stop ) where green vegetation was present . each sample consisted of counting the number of individuals in a foot transects of 100 m in length and 1 m in width . parallel transects were separated by at least 50  m. the average number of locusts per 100  m was determined from the ten samples and converted to density per hectare . the collected data were focused on locust behavior ( isolated or scattered solitarious adults ; grouped transiens or gregarious adults ) and body - coloration ( usually brown for solitarious adults ; bright pink for transiens , pink , or yellow for gregarious adults ) ( symmons and cressman 2001 ) . phase polyphenism confers to locust species , and desert locust in particular , considerable variation in behavioral , morphological , anatomical , and physiological characteristics , as population densities and environmental conditions of the habitat area change ( pener and simpson 2009 ) . the extreme gregarious adult forms are noticeably different from the extreme solitarious adult forms in various traits such as coloration , wing length , hind femur length , a bulge in the cheeks , a depression in the crest of the pronotum , a shortening of the prozona , and a constriction in the thorax ( duranton and lecoq 1990 ) . in the transiens adult forms ( congregans or dissocians , depending on phase change direction ) , phase characteristics are independently and continuously induced or shifted to either direction in response to changes in the density of the population ( rao 1942 , pener and simpson 2009 ) . also , the swarm migration and group oviposition are well known as typical gregarious - phase characteristics . in the field , prominence was given to behavior rather than appearance in characterizing the phase status of individuals because body color and morphological changes could occur later ( rogers et al . also , during these 2  yr of field work , no major outbreak was in progress . ( 2013 ) , with a solitarious status or gregarious status ( hence including most transiens forms ) . samples were conducted in 148 sites and adult locust found in 130 sites ( fig . 1 , also described in cisse et al . the vegetation variables of status and cover were collected for each site following the fao guidelines ( cressman 2001 , cisse et al . if mixed dry and green vegetation was found with no signs of new growth , it was noted green or  shooting. the cover vegetation variable was based on the density of the vegetation compared with bare soil : it was denoted as we assessed the prediction of desert locust adult phase with the statistical model using adult density , vegetation status and cover as established in cisse et al . this prediction model is a multivariate logistic regression model of the form : logit[p(gregarious ) ]  adult density  +  vegetation status  +  adult density : vegetation cover . table 1 presents the results of its adjustment from historical data realized in cisse et al . ( 2013 ) . by introducing the new field observations as explanatory variable in this adjusted statistical model , we could compute the probability that each sampled population of desert locust adult was gregarious according to its density and the status and cover of the vegetation . we then compared these predictions with the phase status observed in the field according to behavior . the probability of being gregarious from the model was compared with the characterized phase status through a simple confusion matrix . data were analyzed with the statistical software r version 3.2.2 ( r development core team 2015 ) . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover.coefficientestimatesez valuepr(>|z|)intercept1.89971680.039710347.839<2e16adult density0.00255450.000141318.082<2e16greening vegetation0.08145730.17160300.4750.635shooting vegetation1.99471650.30273576.5894.43e11dry vegetation0.68886790.22303443.0890.002green vegetation0.08929480.05119911.7440.081adult density : low cover0.00326990.00038438.509<2e16adult density : medium cover0.00036970.00015202.4310.015vegetation cover from cisse et al ( 2013 ) . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover . the study took place in mauritania , one of the key countries for the implementation of preventive control and hosting desert locust permanent habitats ( popov et al . the country is also historically an important locust migration route between the sahel and the maghreb in africa ( ceccato et al . 2007 ) . the study was conducted during and shortly after two successive annual rainy seasons ( between july and december ) , in 2012 and in 2013 . the studied areas are annually monitored by the survey teams of the national anti - locust center of mauritania and correspond to the traditional desert locust breeding and gregarization areas ( popov et al . field sampling was oriented by locust presence in these areas ( see fig . 1 and cisse et al . 1.sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . 
  sampling sites in mauritania where desert locust adults were found ( n  =  130 ) . the gray lines show the isohyetes of mean annual rainfall in mm ( data from agrymet / rim ) . the sampling methodology used to estimate the desert locust adult densities was consistent with the fao guidelines ( cressman 2001 ) . at least ten samples were taken at an individual site ( survey stop ) where green vegetation was present . each sample consisted of counting the number of individuals in a foot transects of 100 m in length and 1 m in width . parallel transects were separated by at least 50  m. the average number of locusts per 100  m was determined from the ten samples and converted to density per hectare . the collected data were focused on locust behavior ( isolated or scattered solitarious adults ; grouped transiens or gregarious adults ) and body - coloration ( usually brown for solitarious adults ; bright pink for transiens , pink , or yellow for gregarious adults ) ( symmons and cressman 2001 ) . phase polyphenism confers to locust species , and desert locust in particular , considerable variation in behavioral , morphological , anatomical , and physiological characteristics , as population densities and environmental conditions of the habitat area change ( pener and simpson 2009 ) . the extreme gregarious adult forms are noticeably different from the extreme solitarious adult forms in various traits such as coloration , wing length , hind femur length , a bulge in the cheeks , a depression in the crest of the pronotum , a shortening of the prozona , and a constriction in the thorax ( duranton and lecoq 1990 ) . in the transiens adult forms ( congregans or dissocians , depending on phase change direction ) , phase characteristics are independently and continuously induced or shifted to either direction in response to changes in the density of the population ( rao 1942 , pener and simpson 2009 ) . also , the swarm migration and group oviposition are well known as typical gregarious - phase characteristics . in the field , prominence was given to behavior rather than appearance in characterizing the phase status of individuals because body color and morphological changes could occur later ( rogers et al . also , during these 2  yr of field work , no major outbreak was in progress . ( 2013 ) , with a solitarious status or gregarious status ( hence including most transiens forms ) . samples were conducted in 148 sites and adult locust found in 130 sites ( fig . 1 , the vegetation variables of status and cover were collected for each site following the fao guidelines ( cressman 2001 , cisse et al . if mixed dry and green vegetation was found with no signs of new growth , it was noted drying. if the vegetation present was yellow only , it was noted  dry. vegetation could also be completely  green or shooting. the cover vegetation variable was based on the density of the vegetation compared with bare soil : it was denoted as we assessed the prediction of desert locust adult phase with the statistical model using adult density , vegetation status and cover as established in cisse et al . this prediction model is a multivariate logistic regression model of the form : logit[p(gregarious ) ]  adult density  +  vegetation status  +  adult density : vegetation cover . table 1 presents the results of its adjustment from historical data realized in cisse et al . ( 2013 ) . by introducing the new field observations as explanatory variable in this adjusted statistical model , we could compute the probability that each sampled population of desert locust adult was gregarious according to its density and the status and cover of the vegetation . we then compared these predictions with the phase status observed in the field according to behavior . the probability of being gregarious from the model was compared with the characterized phase status through a simple confusion matrix . data were analyzed with the statistical software r version 3.2.2 ( r development core team 2015 ) . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover.coefficientestimatesez valuepr(>|z|)intercept1.89971680.039710347.839<2e16adult density0.00255450.000141318.082<2e16greening vegetation0.08145730.17160300.4750.635shooting vegetation1.99471650.30273576.5894.43e11dry vegetation0.68886790.22303443.0890.002green vegetation0.08929480.05119911.7440.081adult density : low cover0.00326990.00038438.509<2e16adult density : medium cover0.00036970.00015202.4310.015vegetation cover from cisse et al ( 2013 ) . logistic regression model from cisse et al . ( 2013 ) of the form : logit[p(gregarious ) ]    adult density  +  vegetation status  +   adult density : vegetation cover . vegetation cover from cisse et al ( 2013 ) . from the 130 sites where adult locusts were found , we observed 98 sites with solitarious adults and 32 sites with gregarious adults . regarding vegetation status , 104 sites were green , 5 shooting , and 21 drying . vegetation cover was low in 50 sites , medium in 51 , and dense in 29 sites . comparison with the model showed eight errors of prediction for the phase status ( table 2 , fig . the model predicted solitarious for one of the gregarious observations leading to a false negative rate of 3.8% ( table 2 ) . the false positive rate was a bit higher ( 6.7% ) with seven solitarious observations predicted as gregarious . altogether , these tests led to a relatively small overall error of prediction of 6.1% ( 1overall accuracy , table 2 ) . 2.comparison of observed phase status in the field with the model prediction of the phase status from density , vegetation cover , and status ( expressed as a probability of being gregarious ) . table 2.confusion matrix of the model prediction from density , vegetation cover , and status ( prediction threshold taken at 0.5 of probability to observe gregarious locusts ) versus the observed phase status in the fieldpredicted solitariouspredicted gregariouserrorsobserved solitarious977false positive rate = 7/104 = 6.7%observed gregarious125false negative rate = 1/26 = 3.8%errorsfalse omission rate = 1/98 = 1.0%false discovery rate = 7/32 = 21.9%overall accuracy = 122/130 = 93.8% comparison of observed phase status in the field with the model prediction of the phase status from density , vegetation cover , and status ( expressed as a probability of being gregarious ) . confusion matrix of the model prediction from density , vegetation cover , and status ( prediction threshold taken at 0.5 of probability to observe gregarious locusts ) versus the observed phase status in the field this study allowed checking the robustness of our previous findings on desert locust adult gregarization thresholds focusing on historical data analysis ( cisse et al . 2013 ) . despite the diversity of field workers who contributed to the collection of historical data in mauritania the false negative predictions would be the most problematic in a management context as it would indicate that gregarious individuals are solitarious . fortunately , the model presented a low rate of false negatives with these new data . this indicates that an operational use of this model could help in determining if a given locust density and vegetation situation is at risk or not . treatments could be decided on this base , though the locust density measure might be another source of errors . indeed , a certain amount of caution should be exercised when the results of foot transects are extrapolated to an entire area of green vegetation . this method may be appropriate for homogenous distribution but is not likely to account entirely for clumpiness in locust distribution within the green vegetation . several studies have recognized that preventive control strategy is the only effective strategy ( duranton and lecoq 1990 , lecoq 2004 , van huis et al . locust control should never be spontaneous ; it must be rational , based on a risk assessment . this requires a cross - analysis process of locust densities and environmental factors , mainly vegetation , rainfall and time factor ( beginning , middle , or end of seasonal rainy period ) . the staff involved in implementation of the preventive control strategy needs specific indicators for when or where chemical treatment should be done . in this respect , the statistical model of cisse et al . a simple routine that transforms locust density and vegetation status and cover into a probability to find gregarious locusts could be incorporated into the custom geographic information systems , ramsesv4 ( reconnaissance and management system of the environment of schistocerca ) , that is used to analyze field data for decision - making in each desert locust affected country ( cressman and hodson 2009 ) . lastly , the statistical model could be improved further by analyzing data of all locust - affected countries according to our methodology . from this , this study was supported by funds from the fao commission for controlling the desert locust in the western region ( clcpro ) .
previous studies investigated the effect of vegetation on density thresholds of adult desert locust gregarization from historical data in mauritania . 
 we examine here the prediction of locust phase based on adult density and vegetation conditions using the statistical model from cisse et al . compared with actual behavior of desert 
 locust adults observed in the field in mauritania . from the 130 sites where adult locusts were found , the model predicted the phase of desert locust adults with a relatively small error of prediction of 6.1% . 
 preventive locust control should be rational , based on a risk assessment . 
 the staff involved in implementation of the preventive control strategy needs specific indicators for when or where chemical treatment should be done . in this respect , we show here that the statistical model of cisse et al 
 . may be appropriate .