As locusts are called species from the family of grasshoppers (Acrididae), the (morphological) in two in appearance and physique and different behavioral forms ( morphs ) occur that lives like most grasshoppers individually a "solitary" shape, and a " Gregarian " form (or wandering form), whose individuals join together to form large swarms that leave their original habitat together. These can devastate entire areas in the event of mass occurrences. Several African countries are regularly hit by swarms of locusts. A single swarm of locusts can consist of more than a billion animals, which corresponds to a weight of 1,500 tons. Since these insects eat about their own body weight of plant material per day, the economic damage to the countries concerned is considerable.
Locusts (English locusts ) in the strict sense only species in which a solitary and a traveling form can be clearly distinguished, although it can often be transitional forms with intermediate morphology (see the definition.). In the case of species that occur in morphologically clearly distinguishable forms, one speaks generally of polymorphism . In migratory locusts, the two forms are genetically identical, which form is formed is determined exclusively by environmental stimuli. In contrast to the mostly genetically determined polymorphism, the newly coined term polyphenism is preferred today . Swarm development it is not only the adult locusts ( Adulti or imagines ). Often the larval stages ( called nymphs in grasshoppers ) already combine to form large swarms that migrate over land. Migratory locusts are not a clearly definable group. For example, there are some species that group together to form swarms, but whose individuals cannot be distinguished from those living individually. Some authors class these as migratory locusts, others not.
The following species are counted among the migratory locusts:
- Schistocerca gregaria
- Schistocerca picifrons . Central America and Northern South America
- Schistocerca cancellata . southern south america
- Schistocerca interrita . Peru
- Nomadacris septemfaciata . Sub-Saharan Africa and Madagascar
- Patanga succincta . South asia
- Austracris guttulosa . Australia. only adult swarms, phases morphologically indistinguishable
- Anacridium melanorhodon . Sahel region of Africa. Phases distinguishable by color, preferably acacia leaves
- Locusta migratoria . Europe, Africa, Asia, Australia, New Zealand
- Locustana pardalina . southern africa
- Oedaleus senegalensis . Old world tropics
- Gastrimargus musicus . Australia
- Pyrgodera armata . Mediterranean area to Central Asia. so far only swarms of nymphs observed.
- Chortoicetes terminifera . Australia. swarming individuals equally colored, but different behavior.
- Austroicetes cruciata . Western Australia.
- Aiolopus simulatrix (synonym: A. savignyi ). Africa
- Ceracris kiangsu . China. monophag of bamboo species
- Calliptamus italicus . Europe, North Africa, North Asia
- Dociostaurus maroccanus . North Africa, Mediterranean region to Central Asia
- Rhammatocerus schistocercoides . South America: Brazil
- Gomphocerus sibricus (synonym: Aeropus sibiricus ). Europe, North Asia
- Melanoplus spretus . North America. extinct
- Melanoplus sanguinipes . North America. only raves at extreme density
- Melanoplus differentialis . North America. only raves at extreme density
Other species, including the Egyptian migratory locust ( Anacridium aegyptium , native to the Mediterranean region) are sometimes included due to their relationship and appearance, although they never form swarms of locusts.
Migratory locusts occur on all continents with the exception of Antarctica. The European migratory locust ( Locusta migratoria ) has appeared repeatedly throughout Central Europe for centuries. The last major plague of locusts occurred in Central Europe in 1749. Nevertheless, the animals were still found in the lower reaches of the Danube and in the Volga steppes into the 19th century . It has now become rare in Europe. In Africa, on the other hand, there are four types of locust: the desert locust ( Schistocerca gregaria ), the locust, the red locust ( Nomadacris septemfasciata ) and the brown locust ( Locustana pardalina ). The most common and damaging species in Africa is the desert locust. Their occurrence ranges from North Africa and Southern Europe to the steppes of Kazakhstan and India. In Central Asia, besides the European migratory locust, the Italian beautiful hedge and the Moroccan migratory locust are the most feared. In the eastern parts of Australia, the Australian plague locust ( Chortoicetes terminifera ) is widespread and causes the greatest economic damage there.
Phases / forms
Migratory locusts come in two forms, namely as largely localized, solitary animals ( solitary phase ) and as migrating swarm animals ( gregarious phase ). The transition from the solitary to the gregarious phase is triggered by the hormone serotonin , which is produced when enough solitary animals meet, especially when they touch. The number of conspecifics that the animals see, smell or feel when their hind legs touch is decisive for the change to swarming animals. The swarm behavior goes hand in hand with an increase in the serotonin concentration in parts of the nervous system.
The two phases differ in behavior and coloration as well as morphologically (e.g. ratio of wing length to length of the talus). The morphological differences between the solitary and swarming grasshoppers are so great that they were assigned to different species until the 1920s. In contrast to grasshoppers, solitary grasshoppers have a greater ability to reproduce, live inconspicuously in mostly remote areas and are of no economic importance; Gregarians, on the other hand, stay in groups, show a characteristic imitation behavior and a synchronous development and finally migrate together from their areas of retreat.
Way of life using the example of the African desert locust
In contrast to other species of locust, the females of the African desert locust do not lay their eggs once, but several times a year. Embryonic development takes about 20 days at a temperature of 36 ° C. The animals only hatch when the air humidity is very high, i.e. generally during or after rain. After hatching, the locusts, which are hemimetabolic insects, go through five larval and nymph stages, each of which is terminated by a molt . While the first stage (worm-shaped larva) takes five days to complete, all subsequent stages take about six days. After the last molt, the locusts need around 16 to 18 days to reach sexual maturity.
The animals living individually in the solitary phase are adapted to the dry climate of semi-deserts . If the ecological conditions such as high temperature, loose soil conditions and rain favor egg development and if the population density - i.e. the number of individuals per area - exceeds a certain level, offspring are produced that differ from the initial population both in appearance and in behavior. After a few generations, the typical wandering form has formed ( gregarious phase), the individuals of which are larger and darker and have larger wings. In the case of the African desert locust, the preferred temperature for the transition from one phase to the other is between 20 and 30 ° C. Mass migrations as an expression of highest activity only take place between 27 and 40 ° C. According to the latest research, swarming behavior is triggered when the animals frequently receive contact stimuli from conspecifics on their hind feet, i.e. when they walk around in large numbers. The transition itself from one phase to another is likely to be controlled by one or more gregarization pheromones .
The following types of pheromones are assumed in locusts based on behavioral experiments:
- Gregarization pheromones that bring about the transition from the solitary to the gregarous phase
- Solitarization pheromones that cause the transition from the gregarous to the solitary phase
- Maturation pheromones that cause the animals to mature rapidly
- pheromones produced by males that stimulate oviposition
- Sex pheromones
- Aggregation pheromones that help grasshoppers to “gather together”.
To prevent the locust populations from growing, insecticides such as organophosphates (e.g. malathion ), carbamates (e.g. bendiocarb ) and synthetic pyrethroids (e.g. deltamethrin ) are used, so that the number of larvae is reduced. Intensive research is also being carried out into biological locust control agents (such as pheromones). The advantages are obvious: pheromones allow a species-specific control of the grasshoppers, do not damage the natural enemies of the grasshoppers and at best lead to low environmental pollution.
Similar effects are achieved with the ingredients of neem tree oil . The most important active ingredients are azadirachtin , salannin , meliantriol , nimbin and nimbidin . Azadirachtin is the main component of neem oil and is obtained from the pressed seeds of the neem tree. The substance inhibits larval development, while meliantriol directly protects the crops and scares off locusts. For humans, mammals and many other insects, however, the neem ingredients are relatively harmless.
If the animals have already formed a swarm, commercially available insecticides are used. This is most effective at dawn, when the animals are still inactive. At this point, a large amount of insecticide can be airborne over the area, ideally killing the entire swarm.
In China, around the turn of the millennium, ducks were successfully used against a locust plague.
- Martin Battran: Migratory locusts - a constant threat to Africa. In: Naturwissenschaftliche Rundschau. Volume 58 (7), 2005, pp. 357-362.
- RF Chapman: A biology of locusts. (= The Institute of Biology's Studies in Biology, 71). E. Arnold, London 1976, ISBN 0-7131-2618-3 .
- VM Dirsh: Genus Schistocerca. (= Series Entomologica, 10). W. Junk, The Hague 1974, ISBN 90-6193-120-7 .
- H. Weidner: The migratory locusts. (= Die Neue Brehm Bücherei. Volume 96). Akad. Verlagsgesellschaft Geest and Portig K.-G., Leipzig 1953 (Westarp-Wiss.-Verl.-Ges., Hohenwarsleben 2003, ISBN 3-89432-571-2 ).
- DH Whitman: Grasshopper Chemical Communication. In: The biology of grasshoppers. 1990, chap. 12, p. 357.
- Hannelore Kluge: Neem tree - the power of the Indian wonder plant. Ludwig, Munich 1996, ISBN 3-7787-3580-2 .
- Bernhard Lübbers : The locust plague of 1749 in Bavaria and Franconia. Perceptions and coping strategies of an early modern natural disaster. In: Bayerisches Jahrbuch für Volkskunde 2018, pp. 97–110, ISSN 0067-4729 .
- With pheromones against the locust plague - biologists want to fight the pests with fragrances
- Extensive information on swarms of locusts, numerous press releases
- Locusts: With the wind across the Atlantic
- ↑ a b Meir Paul Pener & Stephen J. Simpson (2009): Locust phase polyphenism: update on. Advances in Insect Physiology Vol. 39. Academic Press. ISBN 0123814286 Preview on Google Books
- ↑ Hojun Song (2011): Density-Dependent Phase Polyphenism in Nonmodel Locusts: A Minireview. Psyche Volume 2011, Article ID 741769, 16 pages. doi : 10.1155 / 2011/741769 (open access).
- ^ Bernhard Lübbers: The locust plague of the year 1749 in Bavaria and Franconia. Perceptions and coping strategies of an early modern natural disaster . In: Bavarian Yearbook for Folklore . 2018, p. 97-110 .
- ^ FAO Locust watch: Locusts in Caucasus and Central Asia
- ^ Australian plague locust. Australian Government Department of Agriculture and Water Resources, October 24, 2016, accessed February 6, 2017 .
- ↑ Michael L. Anstey et al .: Serotonin Mediates Behavioral Gregarization Underlying Swarm Formation in Desert Locusts. In: Science . Vol. 323, 2009, p. 627. (English)
- ↑ spiegel.de