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Plantwise Technical Factsheet

migratory locust (Locusta migratoria)

Host plants / species affected
Eleusine coracana (finger millet)
Ipomoea batatas (sweet potato)
Manihot esculenta (cassava)
Musa (banana)
Musa x paradisiaca (plantain)
Oryza sativa (rice)
Panicum miliaceum (millet)
Pennisetum glaucum (pearl millet)
Phaseolus (beans)
Poaceae (grasses)
Saccharum officinarum (sugarcane)
Sorghum bicolor (sorghum)
Triticum (wheat)
Triticum aestivum (wheat)
Washingtonia flabelliformis (California fan palm)
Zea mays (maize)
List of symptoms/signs
Fruit  -  external feeding
Fruit  -  frass visible
Growing point  -  external feeding
Inflorescence  -  external feeding
Inflorescence  -  frass visible
Leaves  -  external feeding
Leaves  -  frass visible
Seeds  -  external feeding
Seeds  -  frass visible
Stems  -  external feeding
Stems  -  visible frass
Whole plant  -  external feeding
Whole plant  -  frass visible
Symptoms
Symptoms are not very specific and they depend on the type of plant attacked and the degree of hunger of the pest. The leaves are usually the first plant parts to be attacked and these can be chewed almost completely or if they are rather hard, the major veins, especially the midribs, are left. In cereals, varying proportions of the ripening grains are chewed back. Seed pods and fruits may also be attacked. When hungry, the locusts may chew stems and bark.
Prevention and control

Introduction

Following major plagues in Africa from well-defined outbreak areas, one of the first initiatives for the control of migratory locusts was the establishment of dedicated bases for locust survey and control in the outbreak areas with the aim of preventing swarms from forming and leaving these areas. In West Africa, an internationally supported control organization was formed. It was formally established in 1952 as the Organisation Internationale contre le Criquet Migrateur Africain (OICMA) (Taylor, 1972). The infrequency of plagues of L. migratoria in relation to the cost of these infrastructures has not allowed this organization to persist. Even on Madagascar, where outbreaks and plagues are more frequent, the Bureau central de Lutte contre le Criquet Migrateur stopped functioning in the early 1990s and the Centre d'Observation antiacridienne de Betioky no longer exists. However, plans are underway to revive the locust control structure on Madagascar. A similar organization existed in the former Soviet Union, but the current situation is not clear, now that many outbreak areas are situated in new independent republics (Uvarov, 1977).

With or without national or regional control organizations, outbreaks became less frequent in many regions or disappeared altogether, because agricultural development destroyed the locusts' breeding habitats. In China, this breeding habitat was eliminated after a massive engineering scheme to reclaim the flood plain between the Yellow and Yangtze rivers for irrigated cropping (Chen et al., 1981; Farrow, 1984). However, routine monitoring of this region for migratory locust is still carried out. In southern Mindanao, transmigration and the development of multi-cropping systems also led to the demise of the grassland habitats of migratory locust in Cotobato (Roffey, 1972; Farrow, 1974a). Similar developments occurred in other areas, like the Central Niger flood plain and the deltas of the Volga and Danube rivers. Nevertheless, outbreaks of L. migratoria have continued to arise unpredictably at widely separated locations as a result of other human activities that encourage the formation of grasslands.

Mechanical Control

Various mechanical control techniques have been used in many countries, especially in the Philippines, China and Madagascar. The most common one is driving the hoppers into trenches and then killing them by various methods. Digging up of egg pods has been practised and also beating the hoppers with branches or killing them with flame-throwers. Most of these techniques are very labour-intensive and can only be used in or close to inhabited areas. They are still practised in some places by the local population, but they have mostly been replaced by the application of chemical insecticides.

Chemical Control

The swarms and hopper bands of L. migratoria are relatively dense and slow moving and make better targets than most locusts for insecticidal control from the air or ground. Control initially involved the application of several (now banned) persistent organochlorine insecticides. However, these products are no longer permitted because of their undesirable environmental side effects. They have been substituted with a range of organophosphates such as fenitrothion, and, to a lesser extent, with pyrethroids such as deltamethrin (Steedman, 1990). These chemicals have to be applied repeatedly as blanket sprays as they are not very persistent.

A range of insect growth regulators (IGRs), which have minimal environmental effects, has been tested against locusts but they are not yet widely used. The latest chemical product is Fipronil, which belongs to the family of phenylpyrazoles. It is more persistent than most products and can be applied in barriers. Its main drawback is its very wide spectrum of action, raising concerns about its environmental impact when used in large-scale applications. Although most locust control products are nowadays applied as ultra-low volume (ULV) sprays and dusts, the practice of baiting is still in use. The advantage of this is that only small amounts of insecticide are applied with high precision, but it is very labour intensive and expensive in terms of transporting the bait material.

Biological Control

There has been considerable interest in developing biological control against locusts in order to reduce chemical usage. A number of agents found in the wild have been assayed against L. migratoria both in the laboratory and in the field. Classical biological control programmes, using an introduced agent against a native locust, or programmes to augment locust natural enemies, are unlikely to prevent outbreaks because of the rapidity with which locusts multiply from a very low population base. The rate of multiplication can vastly exceed the numerical response of most predators and parasites. Moreover, most of the traditional breeding areas that produced regular outbreaks have declined and outbreaks now often occur in unexpected places. However, areas that still produce outbreaks from time to time, could be targeted for the release of biological agents, especially pathogens, which could at least reduce the number of outbreaks. The best candidates for such an approach appear to be protozoans of the Phylum Microspora, like Nosema spp. or the recently identified Johenrea locustae. An interesting approach reported in the press was the use of thousands of 'specially trained' chickens to gobble up locusts invading grasslands and gardens in Xinjiang, China. The effectiveness of this method was not reported. Even though it is not always possible for natural enemies to prevent outbreaks, they may be able to prevent some, and almost certainly contribute to their demise. It is, therefore, advisable to conserve these organisms in the environment. There are indications that large-scale applications of chemical insecticides have a severe impact on these beneficial organisms and may even have led to outbreaks of grasshoppers.

Field trials of natural enemies against L. migratoria appear to have been conducted only with strains of entomopathogenic fungi and the bacterium Pseudomonas pseudoalcaligenes (Lu et al., 1996; Yang et al., 1996). A new biological product has recently been developed by the LUBILOSA programme of CAB International, IITA, GTZ and CILSS (Lomer et al., 1997a, b). Based on the fungus Metarhizium anisopliae var. acridum (previously identified as M. flavoviride), it was initially targeted for use against the Desert Locust, Schistocerca gregaria. During its development, it became clear that the fungus could infect most members of the Superfamily Acridoidea (short-horned grasshoppers). In fact, it turned out to be almost totally specific to this group. It appears to be safe for humans and other vertebrates, and though it can infect other species of insects when they are under stress, no infections have been noted at recommended doses under field and semi-field conditions. This product is now produced commercially in South Africa under the name Green Muscle. Though the product has not yet been tested in the field against L. migratoria, it proved sufficiently virulent in the laboratory. Different strains of the same fungus have been field tested in China and Madagascar with promising results (Lu et al., 1996; unpublished report of Montana State University).

Impact
Damage to crops occurs only in the gregarious phase. Main crops affected are cereals. When attacked in the vegetative stage, they may still recover and produce grain, though the yield may be less. However, cereals are often attacked when grains are being formed, and these are preferentially consumed. Pastures can also be seriously damaged.

The extent of damage has not often been estimated. For example, no reliable figures are available for the African continent. Some estimates from southern and eastern Africa made during the plagues of the 1930s were unfortunately based on both migratory and red locusts that swarmed at the same time during those years. These estimates are in the order of tens of thousands to about a hundred thousand US dollars a year. Better estimates are available from Madagascar and these are typically several hundred thousand US dollars in plague years. In Queensland, Australia in 1973-74, total crop losses were estimated at 1.5 million Australian dollars. Crops worth 10 million pesos were lost in the Philippines in 1911-12 and between 1925 and 1934 annual losses ranged from 67,000 to more than 5 million pesos.
Related treatment support
Pest Management Decision Guides
Wang, F.; CABI, 2017, English language
Wang, F.; CABI, 2017, Chinese language
Liu, Y. M.; CABI, 2016, Chinese language
Liu, Y. M.; CABI, 2016, English language
Simwinga, V.; CABI, 2014, English language
 
External factsheets
IRRI Factsheets, International Rice Research Institute (IRRI), English language
BSES Factsheets, BSES Limited, English language
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