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close this book Locust handbook
close this folder 5. Southeast Asian locusts
View the document Oriental migratory locust-Locusta migratoria manilensis
View the document Javanese grasshopper-Valanga nigricornis
View the document Bombay locust-Nomadacris succincta (formerly Patanga succincta)

Oriental migratory locust-Locusta migratoria manilensis

 


Fig. 129. The distribution of the Oriental Migratory Locust.

The Oriental Migratory Locust is a subspecies of Locusta migratoria and its distribution in southeast Asia is shown in Fig. 129. It has long been an important agricultural pest in the region; the earliest records of plagues come from China in 707 BC and the Philippines in the sixteenth century. Locusta is widely distributed in low-lying grasslands, and gregarious populations have been produced in several outbreak areas.

Life cycle

There can be as many as five generations a year depending on the temperature; there are more generations in warmer areas. For example, 3-5 generations in the Philippines; 4 in Thailand; 3-4 in China at latitude 25ºN and 1-2 at latitude 40ºN.

This species is very similar to the African Migratory Locust but is smaller. Solitary adults can be green or brown but as the population becomes more gregarious there are more brown varieties. Maturation takes 3-4 weeks.

Eggs are laid in a variety of soils including volcanic ash, alluvium and sand. There is a wide range in the number of egg pods laid and the number of eggs that they contain. Females can lay 2-7 pods (a maximum of 12 has been recorded) at intervals of 4-15 days. The number of eggs per pod can be as low as 15 and as high as 100. Solitary locusts lay more eggs than the gregarious phase.

Hoppers hatch from 10 to 24 days after the eggs were laid. At this time they are grey-brown in colour but this changes with age according to population density and habitat. Solitarious hoppers are green or brown, brown being more likely in dry conditions. At high densities the colour becomes a reddish-brown or brownish-orange with distinctive black markings. There are usually five instars but some females go through an extra moult. The hopper development periods recorded in four countries are given in Table 16.

TABLE 16

Malaysia

26-32 days

China (Taiwan)

37-52 days

Philippines

43-61 days (Males)

 

42-58 days (Females) Mean 48

Thailand

27-39 days (Males five instars)

 

30-40 days (Females six instars)

 

Behaviour

There is little information on solitary hoppers but gregarious hoppers form basking groups and bands which can march up to 4 km/day. Solitary adults fly at night but gregarious adults fly during the day.

Outbreaks, plagues and seasonal movements

Outbreaks of the Oriental Migratory Locust have occurred in several parts of the distribution area, in western Malaysia, Sabah, eastern China and the Philippines (Fig. 129). There seem to be two types of habitat likely to provide ideal conditions for population increase. The first is a region of low-lying intermittently flooded ground such as that in the delta areas of the Hwang-Ho in China. The second is an area of tropical grassland where forests have been cleared. Local people practice shifting cultivation for a few years then abandon their plots for a new area. The abandoned plots are invaded by grasses such as Imperata species which provide food and shelter for locusts. Nearby open spaces used for cultivation are ideal laying sites.

China

Recent studies suggest that outbreaks occurred every 9-11 years between 1906 and 1956 but 75% only lasted for one year. They were more likely to occur after dry summers and autumns followed by warm winters. It is thought that the dryness provided more laying sites; warm winters put less stress on the eggs so that a higher proportion survived. Limited flooding also lead to population increase but continued flooding had the opposite effect. Unlike Locusta in Africa swarms produced in China do not migrate very far. They expand outwards from the breeding areas but are restricted to the river valleys by the surrounding uplands. Movement northwards takes them to areas that are too cold and to the south it is too wet for successful breeding. By 1962 the outbreak areas had been changed by flood control, drainage, cultivation and afforestation. Since then there does not appear to have been another outbreak.

Philippines

There have been four plagues this century: 1919-1929, 1932-1939, 1941-1949 and 19581960. Analysis of the plagues has suggested that they originated from an area in southern Mindanao around Sarangani Bay. This was an area of natural grassland. The soil is porous containing a high proportion of volcanic ash so that in any month when the rainfall is less than 50 mm there can be local droughts.

The last outbreak occurred in 1958 and is one of the few where the early stages were recorded. In Mindanao the rainy season lasts from April to November when the island is under the influence of the summer monsoon. The wettest months are July and August with average falls of 106 mm. Although rain falls every month there is a drier season between December and March when the winds are mainly from the northeast. March is the driest month with an average rainfall of 45 mm.

Analysis of the previous plagues suggested that they occurred after a period of drought or below average rainfall. In 1957 concentrations of adults were controlled in June around Sarangani Bay. In November and December increasing densities of locusts were observed. By early February 1958 hoppers and clouds of locusts were reported some 100 km north of Sarangani Bay in the central plain of Cotabato. The rainfall from October 1957 to March 1958 was 133 mm, about 22 mm per month. This was the driest period on record since January 1949.

In mid-March there were reports of mature locusts in groups and at the end of the month small swarms were seen in the outbreak area. Laying occurred in April and hoppers and adults appeared in May and June. By July and August there were many reports of concentrations of solitariform locusts in central Cotabato as well as the outbreak area. In September there was a northward shift in the infested area and several reports of day-flying swarms which covered a reported area of 5478 ha.

There were fewer reports during the following dry season but locust activity increased again in 1959 and terminated in late 1960. There were no reports of swarms outside southern Mindanao.

The movement of swarms within the Philippines is not clear. Analysis of earlier plagues suggested that there was a gradual northward extension across the archipelago over seven years (some 1500 km). There have also been outbreaks on other islands, for example, Palawan, Bohol, Panay in 1952, Luzon 1959, Masbate 1962-1964 and Negros 1965.

Outside the Philippines it is thought that locusts invaded Taiwan from Luzon and invaded Sulawesi, probably from Mindanao, but movements from Sabah and Sulawesi to the Philippines have also been considered.

The origin of swarms in Peninsula Malaysia and Sabah is also unclear but both have areas of cleared forest with extensive grasslands providing ideal local breeding conditions which could lead to outbreaks.

Recent outbreak in Japan

There was an outbreak of Locusta migratoria manilensis in southern Japan in September 1986. Swarms were reported on Mageshima Island which is located to the south of Kyushu and west of Tanegashima. The island has been uninhabited since 1980 and the vegetation is a mixture of wild grasses and abandoned rice fields.

There are two generations of Locusta a year and population increase is thought to have begun in 1984 following dry conditions. On 20 November 1985 a grass fire burnt a quarter of the island. The effect of this appears to have been to concentrate the locusts in the remaining areas of vegetation for feeding, this was followed by further concentration as the females preferred to lay in the open sites of burnt land. This laying took place in spring 1986.

The grass fire also allowed good growth of a favoured food plant, Eulalia (Miscanthus sinensis) so that there was plenty of young green vegetation for the emerging hoppers.

By September, when the island was visited after fishermen had reported seeing swarms, locusts were found widely over the whole island. There were marching bands of hoppers reported at densities up to 1000/m² and flying swarms were seen between 26 September and 21 October. Spraying was carried out in December and the locusts were said to have been controlled.

Decline in Locusta plagues

Since 1960 Locusta has become a minor local pest in the Philippines. This would appear to be the result of increasing land use in those areas where grassland had predominated. Not only rice and maize are cultivated but cassava, pineapple and citrus; other areas have been planted with imported grasses and used for ranching. There are thus few sites where locusts can live and breed undetected. In both the Philippines and China Locusta has declined as an agricultural problem, however, its wide distribution and ability to live successfully in newly created grassy habitats make it a potential danger elsewhere in the region where land use change is in progress. It is also important to remember that if a pest has not been a problem for several years it does not mean that it will not be again.