Enumerating Populations of Adults of the Red Locust, <i>Nomadacris septemfasciata</i> (Serville), in its Outbreak Areas in East and Central Africa

Scheepers, Cathrine; Gunn, D. L.

doi:10.1017/s0007485300053608

Cited by 16 publications

(9 citation statements)

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“…In order to do this, something must be known of the relation between population size and the likelihood that gregarious groups will form. Little is known at present (Symmons, 1963;Scheepers & Gunn, 1958), and the nature of the relation would become better understood if regular assessments of population-size were made. Alternatively, one could search in a manner designed to discover ' control-worthy ' targets only.…”

Section: The Value Of Assessments Of the Size Of Outbreak-area Populamentioning

confidence: 99%

Assessing the size of populations of adults of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas by means of a helicopter

Symmons

1966

Bull. Entomol. Res.

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Populations of the red locust, Nomadacris septemfasciata (Serv.), can build up to large numbers in certain of the grass plains of south-central Africa. Adult populations are always under-dispersed, and estimates of their size can be made economically only by counting the locusts flushed on line traverses of considerable length. It is necessary, in such traverses, to determine the width of the strip swept, the proportion of the locusts in it that are flushed, and the accuracy with which these are counted. Investigations into the use of a three-seat Hiller helicopter for this purpose were carried out in south-west Tanzania in 1964–65. It proved possible to cause virtually all the locusts in a strip the width of the rotor blades to rise ahead of the machine, provided it was flown at a height of about 10 ft. and at less than 10 m.p.h. and neither in the early morning nor in the late evening. A comparison of estimates of the size of the population in a 1-sq.-mile plot made by Land-Rover traverses with those made by helicopter, both when the traverse strip was marked on the ground and when it was assessed from within the helicopter, showed no evidence of systematic errors in estimating from the helicopter either the number of locusts flushed or the width of strip. Further, two observers simultaneously counting locusts rising from one strip achieved very similar results. Thus, estimation of the size of a complete outbreak-area population by means of a helicopter is clearly possible.

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Section: The Value Of Assessments Of the Size Of Outbreak-area Populamentioning

confidence: 99%

Assessing the size of populations of adults of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas by means of a helicopter

Symmons

1966

Bull. Entomol. Res.

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“…The estimates for Hyparrhenia are especially significant as this is a tall straight open grass giving little ground cover, so that movement, either along the ground or between the stalks, would have been comparatively easy. Some locusts have been seen to move along the ground (Scheepers & Gunn, 1958) but it is clear that, at least under the conditions under which population assessments are carried out, the proportion involved is negligible. When the vehicles are travelling with the wind, estimation by this means is beset with serious difficulties; the locusts are flushed further ahead than when the vehicles are moving against the wind, and it is possible that they do not distinguish between the first two Land-Rovers; moreover, the distance makes counting from Land-Rover 3 difficult.…”

Section: Errors In Estimating the Number Of Locusts Present In A Strimentioning

confidence: 99%

“…In order to plan a control campaign against adults of this species, especially with a large proportion of inexperienced officers, an objective method of assessing the size of the population was needed. Counts of adult locusts flushed by a moving motor vehicle have provided an answer to this problem, and this technique has been described by Scheepers & Gunn (1958). In addition, an attempt has been made to study the dynamics of outbreak-area populations.…”

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The assessment of the size of populations of adults of the red locust, Nomadacris septemfasciata (Serville), in an Outbreak Area

Symmons¹,

Dean²,

Stortenbeker³

1963

Bull. Entomol. Res.

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Estimates of the size of populations of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas have been made in Tanganyika from counts of adults caused to fly up by the approach of a slowly moving vehicle. For research purposes it was desirable both to improve and estimate the accuracy of such assessments. For control purposes it was desirable to devise an objective aerial assessment technique which could be used in areas where ground methods were either impracticable or expensive. The errors involved in both ground and air estimates can be divided into those concerned with the ratio of locusts counted in the sample area to those actually present, and those concerned with the distribution of the locusts.It was found that a Land-Rover moving at five miles an hour, with the wind blowing from a rear quarter, flushed a high and virtually constant proportion of locusts during most of the day and in the most important grasses of the Rukwa outbreak area, throughout the dry season. Variation in locust density and in the state of the grass had little effect, if any, on the value of the equivalent strip (e), i.e., a strip whose width is such that the number of locusts in it is equal to the number flushed by the vehicle in a strip equal in width to its own. The change with time, and thus presumably with locust behaviour, in the value of e, estimated as recommended, was small but significant.Counts by various observers in one vehicle gave similar results and it seems likely that, even with high numbers, there was no systematic mis-estimation. This is almost certainly true for counts of under a hundred locusts flushed per 0·1 mile. With upwards of one thousand 0·1-mile counts in each assessment, it is believed that the random errors due to mis-counting of the locusts flushed will be negligible.The locusts are always distributed in an over-dispersed manner, consequently normal techniques for ascertaining the standard error of estimate cannot be used. Three assessments were made by the methods recommended, using a grid of two-mile squares; in one case, where the distribution was very gregarious, the most densely populated sector was more intensively sampled. The standard error of assessments laid out in this way was estimated. The method of analysis overestimates the standard error to some degree. Alternatively, the standard error resulting from the nature of the locust distribution can be computed for assessments where the 0·1-mile counts can be shown to be distributed log-normally.Provided the population assessment is carried out under the conditions stipulated above, the additional errors will be minor. The effect of such errors will be likely to be so small that the estimated standard error, derived from the lognormal or assessment-grid analysis, may be regarded as a valid estimate of the standard error of the population estimate.An attempt was made to estimate populations of the red locust in part of an outbreak area from locusts flushed by a ‘ Prospector ’ aircraft. The trials suggested that, given a clear rear view, red locusts which were flushed could be identified as such. Further, the number of locusts flushed could be counted at low densities, and it is thought likely that a reasonably objective technique could be developed for estimation at higher densities.Flushing appeared to be most effective at an aircraft height of 20 ft. or less and when the direction of flight was with the wind. However, the majority of locusts present failed to take off and the proportion which did rise varied very greatly. The spraying of concentrated ammonia under pressure increased the efficiency of flushing.A large-scale comparison of air and ground assessments suggested that, under the prevailing conditions, a considerable proportion of the locusts could be flushed only from the dense concentrations. At all densities the proportion of locusts which the aircraft caused to rise was a small fraction of those present. With flushing of such a low level of efficiency, little idea could be obtained of the size of the locust population at a moderate or low density.

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“…Both these areas are bounded by a tree-line and the Lake Rukwa shore and it is almost certain that the number of locusts which leave, except in swarms, is negligible (SCHEEPERS & GUNN, 1958). On the other hand within the grassy plains locusts can and often do move, thus though it is possible to treat each plain as a self-contained unit it is not possible to study the changes in the population distribution pattern, from the data on which this paper is based, in any subdivision of such an area.…”

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THE PATTERNS OF DISTRIBUTIONS OF ADULTS OF THE RED LOCUST (NOMADACRIS SEPTEMFASCIATA SERVILLE) IN AN OUTBREAK AREA

Symmons¹

1963

Entomologia Exp Applicata

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The coefficient of dispersion, an index of the relative proportion of the population at high and low densities, was calculated for a number of outbreak area populations of the red locust. The comparison of these indices showed that the proportion of the population at high densities was greatest near the middle of the adult locust's life. The data suggested that the proportion of locusts at high densities was greater with higher mean locust densities and after the formation of swarms. On the other hand this value was probably lower in a habitat made up of patches of tall and short grass or of burnt‐off ground and unburnt stands, than in uniform grassland, and was probably also lower after insecticide attack than before. Résumé LES MODES DE DISTRIBUTION DES ADULTES DU CRIQUET NOMADE (NOMADACRIS SEPTEMFASCIATA SERVILLE) DANS UN FOYER D'ORIGINE Les populations adultes du criquet nomade dans certains foyers d'origine ont été estimées pendant un certain nombre d'années. La technique employée consiste à conduire un véhicule suivant un tracé préétabli de façon à couvrir systématiquement la superficie et de compter le nombre de sauterelles chassées d'une bande de la largeur d'un Land Rover; les nombres sont notés tous les 160 meGtres. Le coefficient de dispersion (la variance divisée par la moyenne) a été calculé par le comptage des criquets s'envolant dans les sections de 160 m. Celui‐ci est un index de la proportion relative de criquets à des densités hautes et basses. Il est indépendant de la grandeur totale de la population de sorte qu'il est possible de faire des comparaisons tant entre les distributions de la population dans des régions différentes et les distributions existant à des périodes différentes dans la même région. La comparaison de ces indices démontra qu'en général le coëfficient de dispersion était le plus élevé vers le milieu du stade adulte ***Guin juillet, aôut) c.à.d. qu'à ce moment la plus grande proportion de la population était concentrée sur la plus petite surface. A cause du nombre d'exemples relativement petit it est difficile de déterminer l'effet indépendant d'autres facteurs sur le coefficient de dispersion. Il est cependant probable qu'à une densité moyenne de criquets plus élevée la proportion de criquets était plus grande à des densités élevées. En outre la densité des criquets formant des essaims est en effet treGs élevée; la formation d'essaims tendait à accroître le coefficient de dispersion. Un traitement insecticide paraissait réduire le coëfficient de dispersion. Ceci n'est pas surprenant puisque seuls les essaims en repos on été attaqués. Enfin it est possible que la proportion de criquets à des densités élevées était plus grande dans un habitat relativement uniforme que dans un habitat consistant en des morceaux de terre avec des herbes longues el courtes ou en des terres incinérées et des plages non incinérées.

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Enumerating Populations of Adults of the Red Locust, Nomadacris septemfasciata (Serville), in its Outbreak Areas in East and Central Africa

Cited by 16 publications

References 5 publications

Assessing the size of populations of adults of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas by means of a helicopter

Assessing the size of populations of adults of the red locust, Nomadacris septemfasciata (Serv.), in their outbreak areas by means of a helicopter

The assessment of the size of populations of adults of the red locust, Nomadacris septemfasciata (Serville), in an Outbreak Area

THE PATTERNS OF DISTRIBUTIONS OF ADULTS OF THE RED LOCUST (NOMADACRIS SEPTEMFASCIATA SERVILLE) IN AN OUTBREAK AREA

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