Phytocoenologia
32 (4)
645 – 664
Berlin – Stuttgart, December 10, 2002
The habitat functions of vegetation in relation to the
behaviour of the desert locust Schistocerca gregaria
(ForskaK l) (Acrididae: Orthoptera) – a study in
Mauritania (West Africa)
by Heike Culm see, Freiburg
with 6 figures and 4 tables
Abstract. The functions of vegetation for the behaviour of gregarious desert locust hoppers (Schistocerca gregaria [ForskaK l]) are considered in respect to different scales: single
plant (taxon, life form), plant community (species composition, dominance, density), and
patterns in vegetation and landscape. In the semi-desert of Mauritania, two typical desert
locust habitats are phytosociologically characterized. The perennial communities of the
tree and shrub layer belong to the alliance A c aci o -P an i cio n with an oceanic Euphorbia
balsamifera-Salvadora persica community in a dune-wadi complex characterized by Zygophyllum waterlotii, and a more continental Acacia ehrenbergiana-Panicum turgidum
community in the inselberg landscape of the Mauritanides. The spatial-temporal movement of a hopper band was observed. Hopper band movement depends mainly on wind
direction, but is also influenced by patterns in vegetation and landscape morphology.
Places giving protection and for moulting are provided by perennials and dense vegetation
patches. Length of stay is determined by predation, moulting and food availability. Feeding behaviour in the natural habitat depends on food plant preference and the availability
of palatable plant species within the plant community. In this study, feeding damage is
evaluated both qualitatively and quantitatively using the method Density Analysis with
Estimation of Feeding Damage.
Keywords: behaviour, A ca ci o -P a n ic io n , habitat preference, semi-desert.
Introduction
Searching behaviour is an active movement by which insects seek resources.
It is guided by sensory information, perceived from the external environment, and internal information (Bell 1990). Bell (1990) further states that
the perception of the external environment is likely to be dependent on a
hierarchical system (i.e. resource item, patch, clusters of patches) and related to environmental structure and resource distribution. Vegetation provides the main resource and structural element in the habitat of phytophagous insects. Krato chwil & Schwabe (2001) distinguish between resource
(food) and requisite (living, protection, reproduction) functions of vegetation as a habitat for animal species. To understand the relationship between
resources and consumers, requisites and users, differences in the objectives
lead the investigations to focus on different scales (Wiens 1989). AccordDOI: 10.1127/ 0340-269X/ 2002/0032-0645
0340-269X/02/0032-0645 $ 5.00
© 2002 Gebrüder Borntraeger, D-14129 Berlin · D-70176 Stuttgart
646
H. Culmsee
ingly, the observer has to use the appropriate scale to perceive the different
functions of vegetation as a habitat.
In the present study, the behaviour of gregarious hoppers of the desert
locust Schistocerca gregaria (ForskaKl) (Acrididae: Orthoptera) in their natural habitats is investigated. Field observations were carried out in the semidesert of Mauritania (West Africa), which is one of the major breeding and
recession regions of the desert locust (FAO 1997). The temporal-spatial
movements of hopper bands are studied in relation to abiotic and biotic
factors. The habitat functions of vegetation are considered on different
scales, which extend from the single plant (taxon, life form) to the plant
community (species composition, dominance, density), up to vegetation
patterns according to landscape features.
The paper contributes to the understanding of the following items in
habitats and behaviour of desert locust hoppers:
– An overview of the plant communities occurring in two typical desert
locust habitats in Mauritania is provided with emphasis on landscape
structure.
– A detailed description of the temporal-spatial movement of a hopper
band by means of an example including an evaluation of factors influencing the movement is compared to literature, which mainly exist for East
Africa (Kennedy 1939, Ellis & Ashall 1957, Uvaro v 1977, Ro ffey &
Sto wer 1983).
– It is well-known that Schistocerca gregaria is highly polyphagous. Nevertheless, some plants are never accepted (Chapman & Swo rd 1997),
and others are preferred (Chapman 1990), but the range of plants eaten
in laboratory experiments may not coincide with food plant use in the
field (Uvaro v 1977). Furthermore, food choice under natural conditions
is highly dependent on what is available (Chap man & Swo rd 1997).
Therefore, food intake is analysed in respect to preferences and abundance of plant species in natural habitats. This study is the first in Mauritania which has been carried out on the plant community level.
– Concluding remarks are drawn about the migration and feeding behaviour arising from a functional and multi-scalar analysis of vegetation as a
habitat.
Study area
The study was carried out in typical desert locust habitats (FAO 1997) in
the western provinces of Mauritania (Inchiri and Trarza), which are ecoclimatically part of the sahelo-saharan zone (Po po v et al. 1991), and have
an annual summer rainfall of 100 to 250 mm (Barry 1989).
The main study area (study area 1, N 18°13’, W 15°48’) is located in
proximity to the Atlantic Ocean, close to the capital Nouakchott (Fig. 1).
The landscape in this region of the Senegalo-Mauritanian Basin, is characterized by the mainly consolidated Quaternary sand dunes, and the Quaternary ocean, lake and river deposits, which form interdune plains
(Caruba & Dars 1991).
The desert locust Schistocerca gregaria
647
Fig. 1. Location of study areas 1 and 2 in Mauritania (West Africa) including climatic
diagrams by Walter & Lieth (1960 – 67).
Study area 2 (N 19°40’, W 14°11’) is situated close to the mining town
of Akjoujt within the Mauritanides, a range of Palaeozoic mountains extending NW-SE (Caru ba & Dars 1991). This heavily eroded landscape is
distinguished by a pattern of inselbergs (isolated mountains that arise
abruptly from the surrounding area), regs (bedrock plains), sandy wadis
(valleys) and temporary lakes (mares d’hivernage, Quézel 1965), which are
filled up with clay deposits. Climatically, this region is more continental
than the first study area.
648
H. Culmsee
Material and methods
The fieldwork was mainly carried out in the autumn of 1996 and the autumn/winter of 1997/1998, just after the annual rainy season (Fig. 1).
Vegetation and landscape analysis
The vegetation was analysed using the phytosociological approach according to Brau n-Blanq uet (1964), which was modified for studies in semiarid regions as proposed by Deil (1984). Additionally, landscape features
(morphology, slope, exposition), soil type (using the classification of
Mitchell 1984), and type and distribution of vegetation were analysed in
large scale landscape transects of 1800 m up to 4200 m in length.
Observation of hopper band displacements
The geographical position of the hopper band was determined every two
hours, starting each day at 8 a.m., using the Global Positioning System
(Garmin 45). Detailed information on landscape and vegetation structure
(according to the transect method described above) was recorded corresponding to the migration route. Furthermore, the activities of predatory
birds were continuously observed and monitored. The daily observations
finished at 6 p.m. Estimation of hopper band sizes and number of individuals followed the methods described by Wilps & Dio p (1997).
Vegetation dependent specification of food intake
Density analysis with estimation of feeding damage
For this study, a method was developed to investigate the feeding damage
on vegetation without disturbing the hopper band’s migration. After the
hoppers had passed through a homogenous vegetation stand, as defined by
Dierschke (1994), adjoining squares of 50 cm × 50 cm in size were laid out
in rows. For each square, plant species, number of individuals per species,
feeding damage per individual plant according to the Feeding Damage Index (FDI) shown in Tab. 1, and vegetation cover in percentage were defined.
This method was also applied to field trials using hoppers originating
from captive breeding, which were released into cages temporarily set up
in the natural environment. Trial sites were sandy wadis with well developed annual vegetation. Cages open at the top with translucent plastic walls
of 1 m in height were used, which the hoppers could not climb up. 16
squares were laid out over an area of 2 m × 2 m (size of cages). The released
hoppers were of the same instar (3rd, 4th, or 5th instar) and the population
density was kept constant at 50 hoppers per m2, which is equivalent to the
average hopper density during migration (Wilps 1997). Each square was
649
The desert locust Schistocerca gregaria
Tab. 1. Feeding Damage Index (FDI) demonstrating damage symptoms caused by the
desert locust Schistocerca gregaria in relation to the estimated loss of leaves and flowers/
fruit.
FDI
Damage symptoms
Leaf loss
Flower/Fruit loss
0
1
2
3
4
free of damage symptoms
lightly damaged
moderately damaged
heavily damaged
dead
0 – 10 %
11 – 25 %
26 – 60 %
61 – 99 %
100 %
0%
chewed on
several consumed (up to 50 %)
many consumed (more than 50 %)
none left
examined before and after a feeding time of six hours, using the method
described above.
Data analysis
Data were analysed qualitatively and quantitatively. Regarding the qualitative analysis, the median FDI was evaluated for each plant species. For the
quantitative analysis, the results obtained from all squares observed during
one feeding period within each homogenous and phytosociologically defined vegetation stand were firstly summarized into one relevé. Then, all
relevés undertaken in similar plant communities were examined in relation
to dominance and constancy of the plant species. The total number of individuals was set against 100 % to make the relevés comparable. Dominance
is defined as the percentage of the total number of individuals. Constancy
is a factor evaluated from the number of relevés in which the species occurs.
Results
Patterns in vegetation and landscape
A synopsis of the plant communities occurring in study areas 1 and 2 is
given in Tab. 2.
Tab. 2. Synopsis of plant communities (constancy table).
CS: character species, DS: diagnostic species.
Quaternary dune-wadi ecosystem in proximity to the Atlantic Ocean (study area 1):
consolidated dunes (9), consolidated dunes, sand mixed with gypsum (10), special site
nebka (11); inter-dune compact gypsum soil covered with a thin sand layer (12), interdune compact gypsum soil (13), dune crests, sand mobile on the surface (14), consolidated
dunes south of study area 1 (8).
Inselberg landscape of the Mauritanides (study area 2): inselberg rocks (1), upper, middle
and lower regs (2 – 4), sandy wadis with gravel (5), sandy wadis (6– 7), mares (15– 17).
Further rare species: in 2 & 3: Cymbopogon schoenanthus, in 4: Combretum aculeatum,
in 5, 12 & 15: Convolvulus fatmensis, in 8: Acacia senegal, in 11, 12, 14 & 15: Portulaca
oleracea, in 12 & 13: Anastatica hierochuntica, in 15: Cucumis ficifolius, in 15 & 16: Caylusea hexagyna.
650
H. Culmsee
The desert locust Schistocerca gregaria
651
652
H. Culmsee
Fig. 2. Plant communities of the Zygophyllum waterlotii vegetation complex in proximity
to the Atlantic Ocean in Mauritania, West Africa (N 18°13, W 15°48), corresponding to
geomorphological and edaphic conditions. Landscape units I to VII corresponding to
figures in table showing distribution of plant communities below.
The desert locust Schistocerca gregaria
653
Located in proximity to the Atlantic Ocean, the dune-wadi ecosystem
of the main study area 1 is characterized by the Zygophyllum waterlotii
vegetation complex, which is rich in succulent and evergreen trees and
shrubs. An overview of plant communities corresponding to landscape
units is given in Fig. 2. Perennial plant communities are classified as oceanic
components of the A ca c i o - Pa n i c i on (Barry et al. 1981) with Maerua
crassifolia and the tussock-grass Panicum turgidum as characteristic species
of the alliance. Physiognomically, Euphorbia balsamifera dominates the
consolidated dunes. Together with Salvadora persica, it forms the Euphorbia balsamifera-Salvadora persica dune community with diffusely distributed plant individuals (Tab. 2, col. 9– 11). Variants of the community can
be differentiated depending on edaphic conditions and distinctive land
forms, such as dune crests, slightly undulating parts, valleys and nebkas.
Nebkas are hills built up by humified aeolian material which accumulates
around trees like Salvadora persica (Killian 1945, Walter & Breckle
1990). Further in the south, one can find the Leptadenia pyrotechnica variant of this vegetation type, enriched in Soudanian elements (Tab. 2, col.
8). Therophyte communities, rich in psammophile annuals of tropical and
Saharan origin, build up the herb layer of the dunes. On compact gypsum
soil in the plains, the dwarf shrub Zygophyllum waterlotii forms vegetation
stands in a contracted mode with scattered trees of Capparis decidua
(Tab. 2, col. 12– 13). Superimposed is a facies of the Corchorus depressus
community dominated by the annual Zygophyllum simplex. This community belongs to the alliance A e r vo -F a go n io n (class B oe rh a vi o Te ph r os i e te a , Barry et al. 1986). Most of its species mainly grow in the
plains, but some of them extend to the dunes.
In the inselberg landscape of study area 2, perennial vegetation appears
mainly in the lower parts of the regs, in sandy wadis and temporary lakes.
The Acacia ehrenbergiana-Panicum turgidum community (A ca ci o - Pa n i c io n ) is found on fixed sand and sandy regs (Tab. 2, col. 3– 7). On deep
sand deposits, this community is enriched with Soudanian species, such as
Euphorbia balsamifera, Boscia senegalensis and Chrozophora brocchiana. In
the mares, which are partly cultivated areas, the Psoralea plicata community
is found, characterized by Psoralea plicata and scattered Capparis decidua
(Tab. 2, col. 15– 17). Depending on top soil conditions, varying types of
herb communities extend from the wadis over the regs and up to the rocks
of the inselberg mountains. Some of them are psammophile therophyte
communities with elements of Saharan (e.g. Farsetia sp., Tribulus sp., Heliotropium bacciferum) and tropical (e.g. Indigofera sp., Tephrosia vicioides,
Limeum obovatum) origin. Other species growing on rocky ground can be
included in the A ste r i sc o -F or sk a l e te a (Quézel 1965). A third group,
which is widely distributed, is classified as part of the A er vo -F ag on i o n.
Vegetation as a habitat to live in: migration and protection
Fig. 3 shows the migration route of a small desert locust hopper band in
study area 1, which was followed for a period of three weeks. The tempo-
Fig. 3. Migration route of a small Schistocerca gregaria hopper band in the Quaternary dune-wadi-landscape (study area 1). Migration from east to west, thus fig. to be read from right to left. Dunes in landscape transect are presented twice as high as actually
observed.
654
H. Culmsee
The desert locust Schistocerca gregaria
655
ral-spatial movement is set in relation to wind direction, bird attacks,
moulting, landscape and vegetation patterns.
At the beginning of the observation, the hopper band migrated within
the interdunes. On 8th November, it entered the dune system and had
passed through it by 21st November. In the evening of 21st November, it
moved into the interdune plain and subsequently progressed towards the
following dune chain.
The hopper band migrated mostly downwind and covered a total distance of 4 km, measured as the crow flies between the over-night resting
places. Due to numerous daily changes in the direction of migration, the
total distance migrated amounted to 7.4 km.
Deviations from the main direction of migration occurred frequently
and were caused by various biotic and abiotic factors. In the morning of
8th November, the hoppers were heavily attacked by a flock of 20 birds
(Cursorius cursor) and migrated towards a dense Zygophyllum waterlotii
cluster within the interdunes. In parallel observations made on other hopper bands which migrated through the interdunes in the same area, the
hoppers stayed for hours or even days at the same place, permanently
watched by birds standing in front of or circling the Zygophyllum
waterlotii patches. The hopper band described in Fig. 3 only stayed in the
Zygophyllum waterlotii patch for about 2 hours, leaving at noon on 8th
November to continue westwards, entering slightly undulating dunes with
diffuse perennial vegetation. On 11th November, the hoppers entered a
highly structured dune area and passed through it by marching through a
deep dune valley, at the end of which they climbed up to the dune crest.
On 13th to 15th November, instead of following a clear direction, the movements of the hoppers became aimless due to the onset of moulting into the
5th instar stage. As far as possible, moulting takes place in dense vegetation,
perennials being preferred like Panicum turgidum and Zygophyllum
waterlotii. The following straightforward migration was repeatedly interrupted by bird attacks, occurring most frequently on 17th/18th November. The hoppers tried to escape into dense perennial plants and migration
looked more like a chase from one shrub to the next. Nonetheless, the
groups of predatory birds (up to 20 Cursorius cursor in the interdunes and
up to 15 Alaemon alaudipes, Oenanthe deserti and Lanius excubitor in the
dunes at a time) reduced the initial 20000 hoppers to less than 5000.
During the following days, the hoppers moved more or less directly
west, until they underwent moulting into the adult on 24th/25th November.
Vegetation as a food resource
Tab. 3 presents an overview of the plant species occurring at the two study
sites which are consumed or refused by desert locust hoppers. As indicated
by the qualitative analysis, the hoppers feed on most of the plants. However, a taxon dependent ingestion is also elucidated and can be expressed as
groups of plants which are preferentially consumed, consumed or strictly
rejected. In some cases, the strictly rejected species produce sticky milky
656
H. Culmsee
Tab. 3. Food preferences of desert locust hoppers (Schistocerca gregaria) evaluated with
the FDI = Feeding Damage Index (Tab. 1). Asterix (*) = perennial species. Left columns
(FDI): P = preferred (FDI 3 & 4), PF = consumed, flowers/fruit preferred, PL = leaves
preferred, but flowers/fruit rejected, C = consumed (FDI 1 & 2), R = rejected (FDI = 0),
RF = leaves rejected, but flowers/fruit consumed. Right columns (A = abundance of the
plant species): 3 = 25 – 50 % cover, 2 = 5 – 25 % cover, 1 = less than 5 % cover, but number
of individuals more than 6 per relevé, + = less than 5 % cover, number of individuals less
than 6 per relevé, r = rare.
The desert locust Schistocerca gregaria
657
sap like in Euphorbiaceae and Asclepiadaceae. Attractiveness of some plant
species might change with their age. Fig. 4, for instance, demonstrates that
Euphorbia scordifolia is rejected when young and containing milky sap, but
eaten when senescent.
The selective food choice is reflected in the proportion of plant individuals damaged within a vegetation type. Fig. 5 depicts an example, representing relevés of feeding damage caused by 5th instar hoppers within the herb
layer of the Euphorbia balsamifera dune community. Since the annual herb
Euphorbia scordifolia, generally rejected by the hoppers, occurs in huge
quantities (more than 50 % of the total number of individuals), the total
feeding damage in these stands is negligible. The other 18 species are consumed, some of which become heavily damaged. Each of the 18 species
contain only a small number of individuals (< 2 %) when regarded on their
own. However, together they represent about half the individuals within
this community on which the hoppers feed.
Fig. 6 demonstrates contrary results for the cage trials within the Acacia
ehrenbergiana-Panicum turgidum community in sandy wadis in study area
2. There are several species which occur in high quantities. Limeum obovatum is rejected by the 4th instar hoppers (21 % of all individuals), but Farsetia agg., Indigofera sessiliflora, Fagonia olivieri and Heliotropium bacciferum, which also occur in high quantities, are consumed or preferentially
consumed.
Fig. 4. Feeding damage on leaves of the annual Euphorbia scordifolia (Euphorbiaceae)
caused by 5th instar Schistocerca gregaria hoppers. Left columns: proportion of fresh
individuals compared to the total number of individuals of E. scordifolia in relevés 1 to
8; no feeding damage on fresh, milky sap-containing plants. Right columns: proportion
of senescent (wilting) plant individuals compared to the total number of individuals of
E. scordifolia in relevés 1 to 8. FDI = Feeding Damage Index (Tab. 1).
658
H. Culmsee
Fig. 5. Quantitative analysis of feeding damage in the herb layer of the dunes of study
area 1, caused by 5th instar Schistocerca gregaria hoppers. L = damage on leaves. F =
damage on flowers/fruit. FDI = Feeding Damage Index (Tab. 1).
Fig. 6. Quantitative analysis of feeding damage in the herb layer of sandy wadis of study
area 2, caused by 4th instar Schistocerca gregaria hoppers from a breeding station. Field
trials in cages in the natural habitat. L = damage on leaves. F = damage on flowers/fruit.
FDI = Feeding Damage Index (Tab. 1).
Discussion
Patterns in vegetation and landscape
The A ca c i o -P a n i c i on (Barry et al. 1981), is widespread in the whole
southern Saharan belt (Mo no d 1986, Schulz 1988) and coincides with
large parts of the major desert locust breeding habitats (FAO 1997). Two
forms of the Acacia-Panicum vegetation can be distinguished: a contracted
The desert locust Schistocerca gregaria
659
and a diffuse, savannah-like form. The patterns within this dominant perennial vegetation type and its distribution in the landscape determines the
structure of desert locust habitats.
In both study areas, the underlying herbal communities belong, on one
hand, to the A e r vo -F a go n io n (Barry et al. 1986), widely distributed all
over the Saharan region (FAO 1997). On the other hand, there are annual
communities, rich in both Saharan and tropical species. The feeding damage
list of mainly annual species eaten by Schistocerca gregaria hoppers (Tab. 3)
is only partly comparable to other regions (e.g. Ellis & Ashall 1957,
Bro wn 1947, Guichard 1955). It might be possible to compare food plant
preferences at a higher taxonomic level, like genus or family, as done by
Deil (1994) for plant communities over wide regions.
Vegetation as a habitat to live in: migration and protection
Uvaro v (1977) explains the synchronization of migration behaviour of
hoppers as a result of their similar individual responses to external conditions. Movement of insects can be directed by the abiotic environment,
such as orientation relative to a wind current, but can also be highly affected by biotic factors like resource distribution or the influence of other
individuals (Bell 1990). During the migration of the hopper band described in Fig. 3, vegetation patterns according to landscape features and
distinctive landscape morphology play an important role for deviations
from the main down-wind direction. Dense perennial vegetation patches,
like Zygophyllum waterlotii and the tussock grass Panicum turgidum, provide a more or less guaranteed protection against predatory birds. On this
scale, the results are highly comparable to existing studies which mainly
exist for East Africa (Kennedy 1939, Ellis & Ashall 1957, Uvaro v 1977,
Ro ffey & Sto wer 1983).
Ellis & Ashall (1957) conducted observations on hopper bands of different sizes in the Commiphora-Acacia bush of East Africa. Here, small
hopper bands (less than 20000 hoppers) moved fairly consistently with the
wind, but movement was canalised by clear spaces such as roads. Hoppers
went around big trees or other obstacles which, in contrast, does not always
hold true for bigger hopper bands. Kennedy (1939) noticed that visual
attraction is an efficient way of making hoppers gather in higher and more
conspicuous vegetation, while a lack of vegetation and wind stimulates
wandering. Displacement of hopper bands is also known to be much more
rapid than usual within the entire migration period when approaching
dense Suaeda patches of an East African salt plain (Ro ffey &Sto wer 1983).
Observations of aimless movement during the two days after hatching from
the 4th to 5th instar is supported by similar observations made by Ellis &
Ashall (1957).
Predatory birds can have an enormous impact on desert locust populations. Greathead (1966) estimated that some hundred to a thousand hoppers are being consumed per day per bird in East Africa. Wilps & Dio p
(1997) observed that hopper bands, initially consisting of some 100000 in-
660
H. Culmsee
dividuals, were reduced to only a few thousand individuals after being attacked by birds. Ashall & Ellis (1962) observed the consumption of eight
million young hoppers, mainly by birds within 14 days. In comparison,
trials on the recovery of hoppers in different types of vegetation demonstrate hoppers being protected most in large Panicum turgidum (Ro ffey &
Sto wer 1983).
Vegetation as a food resource
Schistocerca gregaria is highly polyphagous as defined by Chapman
(1990) – that means feeding on a range of plants from a number of different
families, but does not imply that all plants are equally accepted, and some
can be totally unacceptable. Selective feeding of desert locust nymphs is
reported from field studies (Bro wn 1947, Ghao ut et al. 1991, Uvaro v
1977), cage experiments in the field (e.g. Husain et al. 1946, Wilps & Dio p
1997) and laboratory experiments (e.g. Blaney et al. 1985). Diet breadth
of Acridomorpha is presumably largely genetically determined, but is also
dependent on what is available (Chapman & Swo rd 1997). The sensory
system mainly responds to plant secondary compounds whose occurrence
and composition are characteristic for each plant taxon. Food intake is governed by a balance of these phagostimulatory and deterrent chemicals in
the plants, which determines meal size (Chapman 1990). Therefore, even
for polyphagous species, most plants are not eaten in maximal amount
(Chapman & Swo rd 1997).
As shown in Tab. 3, there are clear preferences within the consumed
plants, some are consequently rejected. For instance, fruit of Fabaceae and
leaves of Heliotropium bacciferum are clearly preferred to other consumed
taxa. Bro wn (1947) noted that egg deposition takes place in vegetation
which is preferably eaten by young hoppers, and Bashir et al. (2000) found
that egg deposition is positively influenced by Helitropium sp. This supports the result that Heliotropium bacciferum is one of the species preferentially consumed. Some plant taxa are consequently rejected by Schistocerca
gregaria nymphs, such as Euphorbiaceae, Asclepiadaceae, Tiliaceae, Cucurbitaceae on the family level, and others on the genus or species level. Within
the Euphorbiaceae, Chrozophora brocchiana might be an exception due to
its lack of milky sap. In the case of Euphorbia scordifolia, it is also noted
that fresh, milky sap-containing plants are avoided by 5th instar hoppers,
while wilted plants are consumed (Fig. 4). This observation can be explained by a reduction of unpalatable plant compounds in senescent plants
(Bernays & Lewis 1986). Laboratory trials also demonstrate that older
hoppers preferably consume dry plant material (Lewis & Bernays 1985)
and the nutritional requirements vary with growth and development (Bernays & Simpso n 1990). In the case of most Poaceae, flowers and fruit are
preferably eaten, probably due to their high protein contents, while leaves
are often rejected. Causes for rejection of leaves are controversially discussed in the literature: The hypothesis that C4 grasses are rejected due to
the thick-walled vascular bundle sheath cells (Caswell & Reed 1976),
The desert locust Schistocerca gregaria
661
could not be proved to be significant by Bo utto n et al. (1978). Chapman
(1990) also states that leaf hardness is unlikely to be of major importance
in the choice of food. For the C4 grass Cenchrus biflorus it is seen that the
leaves are preferentially eaten, but the spiny fruits are rejected.
Polyphagy requires an individual grasshopper to move away from the
plant on which it is currently feeding to search for other plant individuals
(Chapman & Swo rd 1997). The movement depends on the abundance and
palatability of plants in the natural habitat. Therefore, among other aspects
explained above, for the movement of a desert locust hopper band, the
availability of palatable plants in a plant community is an important factor.
Desert locust hoppers mainly feed on annual plants and low growing
perennials during their march. The results (Fig. 5 and 6) show that the
feeding damage highly depends on the composition of the plant communities in respect to the herb layer and the dominance of certain species. In
the dune area of study area 1, the overall dominant annual Euphorbia scordifolia is generally rejected. As a consequence, the hoppers have to search
longer for food than, for instance, in the sandy wadis of study area 2, where
several annuals, each of them with percentage densities of 10 to 20, are
consumed. Therefore, in study area 1, the attractiveness of species which
occur only in low individual numbers, is higher in terms of the quantity of
food intake by the desert locust, while their consumption is negligible in
study area 2.
Concluding remarks
Parallel observations on different scales lead to the perception of the different functions of vegetation as a habitat for desert locust hopper bands
(Tab. 4).
Migration of the small hopper band observed in this study mainly follows wind direction, but it is also influenced by spatial patterns in landscape, mode and density of the dominant, widely distributed perennial vegTab. 4. The scale of observation in vegetation and landscape as a habitat for Schistocerca
gregaria.
Habitat function
Scale of observation in vegetation
Migration
patterns in vegetation (contracted-diffuse); vegetation density; landscape morphology (canalisation effect)
life form: perennials, particularly dense tussock grass and bushy
shrubs; patterns in vegetation: patches
life form: perennials
life form: tall perennials, only when used as resting places at the same
time, mainly annuals and low growing perennials during migration;
plant taxa: food choice (preferred, consumed, avoided species/
genera/families);
plant community: quantitative food intake = feeding damage
Protection
Moulting place
Food resource
662
H. Culmsee
etation type A c a c i o- P a n i ci on . Due to the high scale of observation,
these spatial relationships are highly comparable over large distances from
West to East Africa.
The time during which hoppers stay in vegetation is determined by predator protection (especially during moults) and food availability. Protection
is given by dense vegetation clusters, and also by single shrubs or tussock
grasses in the diffusely distributed vegetation.
Desert locust feeding behaviour in a special habitat is firstly determined
by food plant preferences of Schistocerca gregaria, which feeds polyphagously, but selectively. Secondly, it depends on the availability of palatable
species within the plant community in which the hoppers feed, i.e. species
composition and abundance.
Acknowledgements. This study was executed under the framework of a desert locust
research project, sponsored by the Federal Ministry of Economic Co-operation and Development (Germany) and co-ordinated by the Gesellschaft für Technische Zusammenarbeit (GTZ). I would like to thank two reviewers – Dr. C. Hemp (Bayreuth) and one
anonymous – for very helpful advice on the manuscript. I also would like to express my
gratitude to all the colleagues from CLAA and GTZ in Mauritania, especially Dr. H.
Wilps (now Cairo), for discussion, advice and help to carry out this study.
References
Ashall, C. & Ellis, P. E. (1962): Studies on numbers and mortality in field populations of
the desert locust (Schistocerca gregaria ForskaK l). – Anti-Locust Bull. 38: 1– 63.
Barry, J.-P. (1989): Le problème des divisions bioclimatiques et floristiques au Sahara.
Note VII: Les confins saharo-sahéliens en Mauritanie. – Bull. Soc. Bot. Fr., Actual.
Bot. 136 (3/4): 67– 84.
Barry, J.-P., Celles, J. C. & Maniere, R. (1981): Le problème des divisions bioclimatiques
et floristiques au Sahara Algérien. Note III: L’analyse de la végétation de la region
d’In-Salah et de Tamanrasset (Sahara Central et Méridional). – Naturalia monopeliensia, Ser. Bot. 44: 1– 48.
Barry, J.-P., Celles, J. C. & Musso, J. (1986): Le problème des divisions bioclimatiques et
floristiques au Sahara. Note V: Du Sahara au Sahel. Un essai de définiton de cette
marche africaine aux alentours de l’Adrar des Iforas. – Ecologia Mediterranea 12:
187– 235.
Bashir, M. O., Hassanali, A., Rai, M. M. & Saini, R. K. (2000): Changing oviposition preferences of the desert locust, Schistocerca gregaria (ForskaK l), suggest a strong species
predisposition for gregarisation. – J. Chem. Ecol. 26: 1721 – 1733.
Bell, W. J. (1990): Searching Behavior Patterns in Insects. – Annu. Rev. Entomol. 35:
447– 467.
Bernays, E. A. & Lewis, A. (1986): The effect of wilting on palatability of plants to Schistocerca gregaria, the desert locust. – Oecologia 70: 132 – 135.
Bernays, E. A. & Simpson, S. J. (1990): 4. Nutrition. – In: Chapman, R. F. & Joern, A.
(eds.): Biology of Grasshoppers, pp. 105 – 127. – J. Wiley & Sons, New York.
Blaney, W. M., Winstanly, C. & Simmonds, M. S.J (1985): Food selection by locusts: An
analysis of rejection behaviour. – Entomol. exp. appl. 38: 35– 40.
Boutton, T. W., Cameron, G. N. & Smith, B. N. (1978): Insect Herbivory on C3 and C4
Grasses. – Oecologia 36: 21 – 32.
The desert locust Schistocerca gregaria
663
Braun-Blanquet, J. (1964): Pflanzensoziologie. – Springer, Berlin. 330 pp.
Brown, E. S. (1947): The distribution and vegetation of egg-laying sites of the Desert
Locust (Schistocerca gregaria Forsk.) in Tripolitania in 1946. – Bull. Soc. Fouad 1.
Entom. 31: 287 – 306.
Caruba, R. & Dars, R. (1991): Géologie de la Mauritanie. – CRDP, Nice. 321 pp.
Caswell, H. & Reed, F. C. (1976): Plant-Herbivore Interactions; the indigestibility of C4
Bundle Sneath Cells by Grasshoppers. – Oecologia 26: 151 – 156.
Chapman, R. F. (1990): 2. Food Selection. – In: Chapman, R. F. & Joern, A. (eds.): Biology of Grasshoppers, pp. 39 – 72. – J. Wiley & Sons, New York.
Chapman, R. F. & Sword, G. A. (1997): 10. Polyphagy in Acridomorpha. – In: Gangwere,
S. K., Muralirangan, M. C. & Muralirangan, Meera (eds.): The Bionomics of Grasshoppers, Katydids and Their Kin, pp. 183 – 195. – CAB International, Wallingford, New
York.
Deil, U. (1984): Zur Vegetation der Gebirgstihamah am Beispiel des Beckens von At
Tur. – In: Kopp, H. & Schweizer, H. (eds.): Jemen-Studien 1: 225 – 235. – Reichelt,
Wiesbaden.
– (1994): Klassifizierung mit supraspezifischen Taxa und symphylogenetische Ansätze
in der Vegetationskunde. – Phytocoenologia 24: 677 – 694.
Dierschke, H. (1994): Pflanzensoziologie. – Ulmer, Stuttgart. 683 pp.
Ellis, P. E. & Ashall, C. (1957): Field Studies on diurnal behaviour, movement and aggregation in the Desert locust (Schistocerca gregaria ForskaK l). – Anti-Locust Bull. 25:
105 pp.
FAO (1997): Atlas of desert locust breeding habitats. – FAO, Rome. 122 pp. & 56 maps.
Ghaout, S., Louveaux, A., Mainguet, A. M., Deschamps, M. & Rahal, Y. (1991): What
defenses does Schouwia purpurea (Cruciferae) have against the desert locust? Secondary compounds and nutritive value. – J. Chem. Ecol. 17 (8): 1499 – 1515.
Greathead, D. J. (1966): A brief survey of the effects of biotic factors on populations of
the Desert Locust. – J. Appl. Ecol. 3: 239 – 250.
Guichard, K. M. (1955): Habitats of the Desert Locust (Schistocerca gregaria ForskaK l) in
Western Libya and Tibesti. – Anti-Locust Bull. 21: 1– 33.
Husain, M. A., Mathur, C. B. & Roonwal, M. L. (1946): Studies on Schistocerca gregaria.
XIII. Food and feeding habits of the desert locust. – Indian J. Ent. 8: 141 – 163.
Kennedy, J. S. (1939): The behaviour of the desert locust (Schistocerca gregaria (Forsk.))
(Orthopt.) in an outbreak Centre. – Trans. R. ent. Soc. London 89 (10): 385 – 542.
Killian, C. (1945): Un cas très particulier d’humification au désert, due à l’activité des
microorganismes dans le sol des Nebkas. – Revue Canadienne de Biologie 4 (1): 3–
36.
Kratochwil, A. & Schwabe, A. (2001): Ökologie der Lebensgemeinschaften. – Ulmer,
Stuttgart. 756 pp.
Lewis, A. C. & Bernays, E. A. (1985): Feeding behavior: selection of both wet and dry
food for increased growth in Schistocerca gregaria nymphs. – Entomol. exp. appl. 37:
105– 112.
Mitchell, C. W. (1984): Soils. – In: Cloudsley-Thompson, J. L. (ed.): Sahara Desert:
pp. 41 – 55. – Pergamon, Oxford.
Monod, T. (1986): The Sahel Zone North of the Equator. – In: Evenari, M., Noy-Meir,
I., & Goodall, D. W. (eds.): Ecosystems of the World 12B, Hot Deserts and Arid
Shrublands: 203 – 243. – Amsterdam.
Popov, G. B., Duranton, J.-F. & Gigault, J. (1991): Etude écologique des biotopes du
criquet pèlerin S. gregaria (ForskaK l, 1775) en Afrique nord-occidentale. – CIRAD/
PRIFAS, Montpellier. 744 pp.
664
H. Culmsee
Quézel, P. (1965): La végétation du Sahara: du Tchad à la Mauritanie. – Geobotanica
Selecta 2: 333 pp.
Roffey, J. & Stower, W. J. (1983): Numerical changes in the desert locust, Schistocerca
gregaria (ForskaKl), in a seasonal breeding area with special reference to ecology and
behaviour. – FAO, Rome. 75 pp.
Schulz, E. (1988): Der Südrand der Sahara. – Würzb. Geogr. Arb. 69: 167 – 210.
Uvarov, B. P. (1977): Grasshoppers and Locusts. A Handbook of General Acridology
2. – Centre for Overseas Pest Research, Cambridge, 613 pp.
Walter, H. & Breckle, S.-W. (1990): Spezielle Ökologie der Tropischen und Subtropischen
Zonen. – Ökologie der Erde 2. – Fischer, Stuttgart. 461 pp.
Walter, H. & Lieth, H. (1960 – 67): Klimadiagramm Weltatlas. – Fischer, Jena.
Wiens, J. A. (1989): Spatial scaling in Ecology. – Functional Ecology 3: 385 – 397.
Wilps, H. (1997): Ecology of Schistocerca gregaria (ForskaK l): observations in West Africa
from 1990 to 1994. – In: Krall, S., Peveling, R. & Ba Diallo, D. (eds.): New Strategies
in Locust Control, pp. 9 – 17. – Birkhäuser, Basel.
Wilps, H. & Diop, B. (1997): Field investigations on Schistocerca gregaria (ForskaK l)
adults, hoppers and hopper bands. – In: Krall, S., Peveling, R. & Ba Diallo, D. (eds.):
New Strategies in Locust Control, pp. 117 – 128. – Birkhäuser, Basel.
Address of the author:
Heike Cu lmsee, Institute of Biology II/Geobotany, University of Freiburg, Schänzlestr. 1,
D-79104 Freiburg, Germany. E-mail: [email protected]