Alternative methods of controlling the brown locust, Locustana pardalina (Walker)
Price, Roger Edward
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Outbreaks of the brown locust, Locustana pardalina (Walker), occur almost annually in the semi-arid Karoo region of South Africa and southern Namibia. Current suppressive control strategy relies on the application of fast-acting, synthetic pyrethroid insecticides, applied as ultra low volume drift sprays, to control gregarious brown locust targets at source within the Karoo outbreak region. However, the negative impact that the repeated application of insecticides may have on the rich diversity of endemic invertebrates and reptiles found in the Nama-Karoo biome is of great concern to landholders and conservationists. How to reduce the insecticide load and minimise the environmental impact in the Karoo and yet at the same time control this serious agricultural pest has become a controversial issue. There is thus an urgent need for more environmentally benign methods of locust control, as an alternative to the current spraying of insecticide. As part of a locust research project initiated by the Plant Protection Research Institute, Pretoria, the potential of various alternative methods of controlling the brown locust were evaluated against gregarious hopper populations in the laboratory and in the field. It was first important to update the available information on the background level of control provided by natural enemies and diseases of the brown locust. Although a range of natural enemies were found to prey upon the various life stages, their impact on brown locust populations in the present study was negligible. Of particular interest was a study of the impact of the sarcophagid fly, Wohlfarhtia pachytyli, which is a well-known facultative parasite of late instar brown locust hoppers and fledglings. However, field data suggested that the potential of the fly as a biological control agent may have been over estimated in the past, as the fly failed to cause more than 6% mortality of fledgling swarms in the present study. Before the first insecticides became available at the turn of the zo" century, farmers had to resort to mechanical methods to protect their crops and pastures from the ravages of locusts. Turning back the clock, the destruction of locust egg beds and the harvesting of locusts were re-examined as control methods. Excavation of locust eggs gave effective control, but the disturbance of the friable soils in the Karoo would damage the vegetation cover and cause severe erosion problems and is therefore not advocated. Harvesting of live locusts using nets or vacuum machines was not practical due to the avoidance behaviour of locusts. However, the harvesting of locust cadavers lying on the soil surface following insecticide spraying, once they had dried out and insecticide residues had broken down, was possible. With their high protein and fat content, the processing of locust cadavers into animal feed may become economically viable in future. Before organo-chlorine insecticides became available in the 1940s, bran bait containing sodium arsenite was extensively used for brown locust control. The baiting technique was re-evaluated in the present study using minute dose rates of the phenyl-pyrazol insecticide, fipronil, dissolved in water and mixed into wheat bran as the edible carrier. Bran bait containing 0.02% fipronil 200Se (Regent®) was prepared on site and was broadcast by hand onto the soil surface around bushes occupied by hopper bands as overnight roosting sites. Excellent control (>95%) of small and medium sized hopper bands was achieved, as long as baiting was undertaken shortly after sunrise, before hoppers scattered from the baited area. Baiting large band targets, or baiting later in the day once hoppers became active, was not effective. Baiting with 0.02% Regent® proved very effective if applied to compact, roosting hopper bands. It was also inexpensive and was easy to prepare and apply, requiring basic equipment and limited training. However, the logistics of the bulk transport, preparation and application of locust baits under operational conditions appear daunting. Insecticide barrier treatments using fipronil (Adonis® 5UL), applied to 21m-wide strips of Karoo vegetation at a dose rate of 12.5g a.i./ha, were used to intercept gregarious brown locust hopper bands marching through the veld. Barriers of Adonis® proved very effective against mobile L2-L3 bands and against small L4-L5 bands, giving >90% control within 48 hours. However, barriers sometimes failed to adequately control large and mobile L5 bands that had sufficient momentum to march through barriers before the majority of hoppers acquired a lethal dose of Adonis®. Barriers also proved less effective where the vegetation density was sparse or where the vegetation was unacceptable to locusts. The size and density of the hopper bands and the time of day when bands made contact with the barriers also appeared to influence efficacy. Despite these factors, Adonis® barriers were still considered to have potential for the control of brown locust hopper bands in the more remote areas of the Karoo, especially during the early stages of an outbreak when hopper bands are still young. However, barriers would have to be judiciously applied to restrict the environmental impact of Adonis® against non-target organisms. Large-scale operational trials are recommended. Insect Growth Regulators (IGRs) have shown promise when applied as barrier treatments against various locust and grasshopper species. However, laboratory experiments with the IGRs, flufenoxuron and teflubenzuron, applied to leaf discs and fed to L5 brown locust hoppers at dose rates of 3-l5Ilglg, gave variable mortality of 30-70%, with most mortality occurring as the hoppers attempted to moult. In another experiment, diflubenzuron (Dimilin OF6®), was sprayed onto maize plants at volume rates of l-3.f;ha and subsequently fed to L2 brown locust hoppers in the laboratory. Dimilin OF6® produced 100% mortality of L2 hoppers within Il days at all application rates, as long as hoppers were continuously exposed to treated vegetation. However, irregular exposure to Dimilin® during the inter-moult period produced unsatisfactory mortality, as the product is evidently non-accumulative and is readily excreted. The fact that brown locust hoppers have to be regularly exposed to IGR-treated vegetation, combined with the sporadic feeding behaviour and high mobility of brown locust hopper bands in the Karoo, would probably make IGR barriers unsuitable for brown locust control operations. In collaboration with nBC and the LUBILOSA programme (CABI Bioscience, Ascot, UK), the locust-killing fungus, Metarhizium anisopliae var. acridum, was imported and evaluated by PPRI locust researchers as a myco-insecticide agent in laboratory and field trials against the brown locust. Under suitable application conditions the myco-insecticide, applied at a standard dose rate of lOOgconidia/ha, regularly produced >90% mortality of hoppers maintained in cages, although speed of kill was slow, with median lethal times of 10.3 and 13.4 days for the ground and aerial application trials respectively. In most cases, acceptable >90% mortality was not achieved for at least three weeks after application. Despite the slow speed of kill, the myco-insecticide agent was considered a significant advance in locust control and the product was subsequently registered as Green Muscle® in South Africa in 1998. However, the lack of a knock-down action and the slow kill currently makes Green Muscle® unsuitable for operational use in the Karoo. The thousands of individual hopper bands treated during control campaigns, and the high mobility of bands, would make the recognition of treated and untreated targets by locust officers impossible. The hot and dry Karoo climate is also usually detrimental for the survival and transmission of fungal conidia, while the thermoregulation behaviour of brown locust hoppers enables them to effectively delay the onset of Metarhizium mycosis. An alternative application strategy needs to be developed and tested before Green Muscle® can be recommended for brown locust control. Other pathogenic micro-organisms evaluated in the laboratory for brown locust control were certain acid-tolerant strains of Bacillus thuringiensis and an entomopoxvirus isolated from a West African grasshopper, Odaleus senegalensis (De Geer). Unfortunately, none of these microorganisms proved virulent to the brown locust. The alternative locust control methods evaluated against the brown locust were all ranked according to various performance criteria and compared with the conventional spraying of ULV insecticides. Of the alternative control methods, only Adonis® barrier treatments and Regent® bait showed sufficient promise for brown locust control. However, none of the alternatives were considered suitable under all locust control situations to entirely replace the spot spraying of conventional ULV insecticides, which will thus remain the backbone of brown locust control strategy. Recommendations on the development of an lPM strategy for brown locust control, to incorporate barrier treatments and baiting in certain areas of the Karoo in order to complement conventional insecticide spraying, are given.