Alternative methods of controlling the brown locust, Locustana pardalina (Walker)

[摘要] Outbreaks of the brown locust, Locustana pardalina (Walker), occur almost annually in thesemi-arid Karoo region of South Africa and southern Namibia. Current suppressive controlstrategy relies on the application of fast-acting, synthetic pyrethroid insecticides, applied as ultralow volume drift sprays, to control gregarious brown locust targets at source within the Karoooutbreak region. However, the negative impact that the repeated application of insecticides mayhave on the rich diversity of endemic invertebrates and reptiles found in the Nama-Karoo biomeis of great concern to landholders and conservationists. How to reduce the insecticide load andminimise the environmental impact in the Karoo and yet at the same time control this seriousagricultural pest has become a controversial issue. There is thus an urgent need for moreenvironmentally benign methods of locust control, as an alternative to the current spraying ofinsecticide. As part of a locust research project initiated by the Plant Protection ResearchInstitute, Pretoria, the potential of various alternative methods of controlling the brown locustwere 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 controlprovided by natural enemies and diseases of the brown locust. Although a range of naturalenemies were found to prey upon the various life stages, their impact on brown locustpopulations in the present study was negligible. Of particular interest was a study of the impactof the sarcophagid fly, Wohlfarhtia pachytyli, which is a well-known facultative parasite of lateinstar brown locust hoppers and fledglings. However, field data suggested that the potential ofthe fly as a biological control agent may have been over estimated in the past, as the fly failed tocause 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 resortto mechanical methods to protect their crops and pastures from the ravages of locusts. Turningback the clock, the destruction of locust egg beds and the harvesting of locusts were re-examinedas control methods. Excavation of locust eggs gave effective control, but the disturbance of thefriable soils in the Karoo would damage the vegetation cover and cause severe erosion problemsand is therefore not advocated. Harvesting of live locusts using nets or vacuum machines was notpractical due to the avoidance behaviour of locusts. However, the harvesting of locust cadaverslying on the soil surface following insecticide spraying, once they had dried out and insecticideresidues had broken down, was possible. With their high protein and fat content, the processingof locust cadavers into animal feed may become economically viable in future.Before organo-chlorine insecticides became available in the 1940s, bran bait containing sodiumarsenite was extensively used for brown locust control. The baiting technique was re-evaluated inthe present study using minute dose rates of the phenyl-pyrazol insecticide, fipronil, dissolved inwater 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 bushesoccupied by hopper bands as overnight roosting sites.Excellent control (>95%) of small and medium sized hopper bands was achieved, as long asbaiting 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 noteffective. Baiting with 0.02% Regent® proved very effective if applied to compact, roostinghopper bands. It was also inexpensive and was easy to prepare and apply, requiring basicequipment and limited training. However, the logistics of the bulk transport, preparation andapplication of locust baits under operational conditions appear daunting.Insecticide barrier treatments using fipronil (Adonis® 5UL), applied to 21m-wide strips ofKaroo vegetation at a dose rate of 12.5g a.i./ha, were used to intercept gregarious brown locusthopper bands marching through the veld. Barriers of Adonis® proved very effective againstmobile 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 hadsufficient momentum to march through barriers before the majority of hoppers acquired a lethaldose of Adonis®. Barriers also proved less effective where the vegetation density was sparse orwhere the vegetation was unacceptable to locusts. The size and density of the hopper bands andthe 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 ofbrown locust hopper bands in the more remote areas of the Karoo, especially during the earlystages of an outbreak when hopper bands are still young. However, barriers would have to bejudiciously applied to restrict the environmental impact of Adonis® against non-targetorganisms. Large-scale operational trials are recommended.Insect Growth Regulators (IGRs) have shown promise when applied as barrier treatments againstvarious 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 doserates of 3-l5Ilglg, gave variable mortality of 30-70%, with most mortality occurring as thehoppers attempted to moult. In another experiment, diflubenzuron (Dimilin OF6®), was sprayedonto maize plants at volume rates of l-3.f;ha and subsequently fed to L2 brown locust hoppers inthe laboratory. Dimilin OF6® produced 100% mortality of L2 hoppers within Il days at allapplication 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 bandsin 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), thelocust-killing fungus, Metarhizium anisopliae var. acridum, was imported and evaluated by PPRIlocust researchers as a myco-insecticide agent in laboratory and field trials against the brownlocust. Under suitable application conditions the myco-insecticide, applied at a standard dose rateof lOOgconidia/ha, regularly produced >90% mortality of hoppers maintained in cages, althoughspeed of kill was slow, with median lethal times of 10.3 and 13.4 days for the ground and aerialapplication trials respectively. In most cases, acceptable >90% mortality was not achieved for atleast three weeks after application.Despite the slow speed of kill, the myco-insecticide agent was considered a significant advancein locust control and the product was subsequently registered as Green Muscle® in South Africain 1998. However, the lack of a knock-down action and the slow kill currently makes GreenMuscle® unsuitable for operational use in the Karoo. The thousands of individual hopper bandstreated during control campaigns, and the high mobility of bands, would make the recognition oftreated and untreated targets by locust officers impossible. The hot and dry Karoo climate is alsousually detrimental for the survival and transmission of fungal conidia, while thethermoregulation behaviour of brown locust hoppers enables them to effectively delay the onsetof Metarhizium mycosis. An alternative application strategy needs to be developed and testedbefore Green Muscle® can be recommended for brown locust control.Other pathogenic micro-organisms evaluated in the laboratory for brown locust control werecertain acid-tolerant strains of Bacillus thuringiensis and an entomopoxvirus isolated from aWest African grasshopper, Odaleus senegalensis (De Geer). Unfortunately, none of these microorganismsproved virulent to the brown locust.The alternative locust control methods evaluated against the brown locust were all rankedaccording to various performance criteria and compared with the conventional spraying of ULVinsecticides. Of the alternative control methods, only Adonis® barrier treatments and Regent®bait showed sufficient promise for brown locust control. However, none of the alternatives wereconsidered suitable under all locust control situations to entirely replace the spot spraying ofconventional ULV insecticides, which will thus remain the backbone of brown locust controlstrategy. Recommendations on the development of an lPM strategy for brown locust control, toincorporate barrier treatments and baiting in certain areas of the Karoo in order to complementconventional insecticide spraying, are given.

[发布机构] University of the Free State