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RECENT LOCUST/GRASSHOPPER OUTBREAKS IN BRAZIL
State of Mato Grosso (1984-88 ; 1991-92)
The state of Mato Grosso (MT) is located in the western central region of Brazil. Several species of grasshopper occur normally in the region ; among the most common are Tropidacris cristata (L. 1758) and Rhammatocerus schistocercoides. Until 1983 none of these species were economically important. However, since 1984, Mato Grosso has suffered the most serious locust outbreaks which have occurred in the country in the last 40 years (Cosenza et al. 1990).
From 1984-1988 swarms of R. schistocercoides originating in the Indian reserves of Nambiquara and Parecis migrated, infesting an enormous area between 12° and 15°W, from Vilhena in the state of Rondonia to the border with the state of Goias (Fig. 2) (Cosenza 1987).
Rhammatocerus schistocercoides has only one generation per year. Females lay eggs in October-November, which hatch in November-December. The hopper stage passes through five nymphal instars, reaching the adult stage in April-May. Migration occurs from August to September, mating in September-October (Cosenza 1987). The species is predominantly graminivorous, feeding primarily on native grasses and rice. They also feed on sugar cane, maize, sorghum, pasture grasses and soybean.
From 1984 to 1988 a control campaign against R. schistocercoides was undertaken by the Government of Brazil with FAO assistance in 1986 (TCP/BRA/4508). Approximately one million hectares were treated during this period. However, because the populations declined all control activities were suspended from 1989-1991. A resurgence was reported in January, 1992, however, when numerous hopperbands were located in northeastern Mato Grosso, in the localities of Paranatinga, Pacovao, Nova Brasilandia, Campo Novo, Agua Boa, Comodoro and Cocalinho (Fig. 2) (Barrientos 1992a, unpublished).
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Fig. 2. Map of the western central region of Brazil, showing the locust infestation area in the state of Mato Grosso. |
In response to a recent request from the Government : FAO approved a technical Cooperation Project (TCP/BRA/2256) to assist in a control campaign in 1992-1993. The project envisaged emergency assistance for chemical control in Mato Grosso and a strong biological control research program in several other states, including Mato Grosso.
State of Rio Grande do Sul (1991-92)
The grasshopper problem in Rio Grande do Sul (RS) began in 1986 (Technicians of the Plant Protection Service and cattlemen / farmers, pers. comm.). Since then, population densities have increased gradually every year. In February-March, 1991, it was estimated that 80,000-100,000 ha of pensacola pasture (Paspalum notatum) were infested in northwestern and southwestern RS (Barrientos 1992a) (Fig. 1).
Most species involved (Rhammatocerus conspersus, R. pictus, Staurorhectus longicornis) are univoltine, with eggs hatching annually by October/November. Although population densities are sometimes very high (15-20 grasshoppers/m2 and in extreme cases up to 30-40/m2), none of the species mentioned has been observed to form bands or swarms. The problem is, therefore, considered as a grasshopper type.
Damage was caused primarily to pensacola pasture (either native or improved) and secondarily to maize, soybean, rice and natural grass. Since cattle raising is the predominant activity in southwestern RS, the problem involved more cattlemen than farmers. A control campaign with FAO assistance (TCP/BRA/ 0152) was undertaken by the Government from November, 1991, to January, 1992. Although there were many difficulties in implementation of the campaign by the Brazilian Institute for the Protection of the Environment and Renewable Natural Resources (IBAMA), 40,000 ha were treated in the southwestern region of RS ; the outbreak was suppressed by February, 1992.
State of Paraíba (1990-92)
Grasshopper problems in Paraíba (PB) (northeastern region) are caused mainly by Schistocerca pallens and Stiphra robusta (Melo-Leitao, 1838). An increase in populations of these species has been observed since 1987. The problem has been associated with a severe drought that began in the region in 1985 and lasted almost six years. In Paraiba (Fig. 1) S. robusta causes major damages to algaroba trees (Prosopis juliflora) and cashew plantations, while S. pallens attacks mainly pastures and subsistence crops (maize and beans).
Main infestations occur in Eastern Paraíba, localities of Barra de Santa Rosa, Curjao, Soledade, Inga, Itabaiana and Mogeiro. Population densities in 1988 averaged 30 grasshoppers/m2 in pastures and subsistence crops and more than 50 grasshoppers/m2 in algaroba trees. The Federal and State Governments carried out control operations in approximately 28,000 ha in 1990 and 8,000 ha in 1992. As the infested area expands, there is a major concern about the sugar cane crop, which is the major crop in the region, occupying 1,830,000 ha in northeastern Brazil.
Subject to adequate rainfall, S. pallens is known to have at least two generations per year (Duranton et al. 1987). During the dry season (September-January/February) adults remain sexually immature. Nymphs are present from February to June-July and in some areas until August.
In a recent review (24-28.08.92) of the grasshopper situation in the states of Paraíba and Rio Grande do Norte, it was found that all populations of S. pallens located in northern Rio Grande do Norte (localities of Jandaira, Pedro Avelino and Açu) were sexually immature adults, while those located in Paraiba (locality of Inga) were in the nymphal stage (2nd-5th instar) or fledglings. These differences in life cycle are due to climatic variations: very dry in the localities of Rio Grande do Norte and very humid and warm in the locality of Inga. Control operations should be undertaken when S. pallens is in nymphal or sexually immature adult stage.
States of Rio Grande do Norte and Pernambuco, northeastern region (1992)
Schistocerca pallens and Stiphra robusta also cause agricultural problems in other Northeastern States. In 1986 the Federal and State Governments of Pernambuco (PE) implemented a control campaign to suppress an outbreak of S. pallens. The operation was successful but monitoring grasshopper populations did not continue after the campaign, leading to an upsurge of the problem in 1992. The infestation this time extended to Rio Grande do Norte (RN), where S. pallens and S. robusta caused damage of importance to cashew trees, citrus and maize crops. Approximately 28,000 ha of the above crops were treated in May-June, 1992.
FACTORS RESPONSIBLE FOR LOCUST/GRASSHOPPER OUTBREAKS IN BRAZIL
There are many factors that have a direct effect in increasing locust grasshopper/populations, e.g., climatic and biotic (predators, parasites, diseases) factors. From the above, climatic factors are the most important and for some species (e.g., Schistocerca gregaria and Locusta migratoria migratorioides) they can determine the occurrence of an outbreak (Lecoq 1991).
In the case of Brazil, however, other factors are perhaps more important in originating locust/grasshopper outbreaks. Barrientos (1992b) pointed out the following:
a. Changes in land management and ecological disturbances. Prior to 1985, farmers grew winter wheat and summer soybean in northwestern and southwestern regions of RS for 15 years. During this period chemicals were used continuously to control major pests. Indirectly they controlled native local populations of grasshoppers, which were not a problem at that time and also eliminated natural enemies and predators of grasshoppers.
In 1986 pensacola-improved pasture grass was introduced and the land was dedicated mainly to cattle raising. This terminated application of insecticides, resulting in the multiplication of grasshoppers.
b. Failure to continue control operations for several consecutive years. For example, spraying against grasshopper populations in RS was not carried out for the first time until 1991-92 (Barrientos 1992b).
c. Deforestation and introduction of new crops. Deforestation may well explain the locust outbreak in the state of Mato Grosso, where large areas of land (typically savannah) have been opened to agriculture (rice, maize, sugar cane and soybean crops) in the last 10-15 years. These crops provide food and shelter for local grasshopper species which are not controlled.
d. Abandonment of traditional crops and cultivated fields which creates suitable habitats for locust/grasshopper breeding. An example of this is the case of cotton in northeastern Brazil. Until 1976 there used to be 3,500,000 ha of cotton in the northeastern region but at present it is reduced to 800,000 ha. Infestation by the cotton weevil (Anthonomus grandis), prolonged droughts, lack of investment money and low commercial prices were the main reason for this decrease in cotton production (Robson de Macedo Vieira, Director National Centre for Cotton Research, Paraíba, pers. comm.).
e. The abandonment of monitoring and vigilance once the outbreaks are suppressed.
