Alternative methods of controlling the brown locust, Locustana pardalina (Walker)
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Date
2003-07
Authors
Price, Roger Edward
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
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.
Description
Keywords
Brown locust, Locustana pardalina, Alternative control methods, Natural enemies, Mechanical control, Baiting, Fipronil barriers, Insect growth regulators, Green Muscle myco-insecticide, Microbial agents, lPM strategy, Locusts -- Control -- South Africa -- Karoo, Thesis (Ph.D. (Zoology and Entomology))--University of the Free State, 2003