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University of the Free State (1999)

Environmental impact of acridicides in the Karoo

Roux RWJ

Titre : Environmental impact of acridicides in the Karoo

Auteur : Roux RWJ

Université de soutenance : University of the Free State

Grade : Master of Science (MS) 1999

Outbreaks of the brown locust, Locvstana pardalina (Walker), occur almost every year in the semi-arid Karoo region of South Africa and southern Namibia. Locust outbreaks are combated with the extensive application of broad-spectrum synthetic pyrethroid acridicides to hopper bands and fledgling swarms within the Karoo before migrating swarms can escape and threaten food security within the southern African region. Controlling locusts early in their life cycle also prevents millions of rands of losses to veld pastures within the Karoo. However, the repeated application of acridicides, especially if used irresponsibly, may have a negative effect on the unique biodiversity in the Karoo biome and there is increasing public concern regarding the possible environmental impact of locust control. This thesis describes the environmental impact of three different acridicides on the non-target fauna and flora in the Karoo. The acridicides studied were the synthetic pyrethroids, deltamethrin (Decis®) and esfenvalerate (Sumi Alpha®) and the insect growth regulator (IGR) diflubenzuron (Dimilin®). The acridicides were applied at registered or recommended dose rates using a vehicle-mounted motorised Solo applicator. The trials were undertaken in the Hanover district in the Northern Cape Province, which lies within the recognised high frequency outbreak area of the brown locust. Each acridicide was applied to individual 9 ha plots in three habitat types, consisting of mainly grass, mainly shrub and a mixed grass-shrub habitat. A control plot, with no acridicide treatment, was also selected in each vegetation type. The environmental impact of the acridicides was monitored at different seasons during 1996-1997 by comparing the impact of an early summer application (October) with an early winter application (April). The abundance and diversity of the fauna in each of the plots was monitored with pitfall traps, Moericke traps and sweep-netting at one day prior to acridicide treatment and then at intervals of two days, two weeks, two months, six months and one year after treatment. The initial knockdown action of the acridicides was evaluated on one occasion, two days after application. Fauna were stored in alcohol and later identified to the level of Order or Family. The floral composition of each of the plots was monitored by using the wheel-point method before acridicide treatment and then at one year post-application. Two days following treatment, significantly more dead insects were collected in the deltamethrin treated plots compared to the other acridicide-vegetation plots, indicating that deltamethrin had the greatest knock-down activity of the acridicides evaluated. The pyrethriods had an immediate negative impact on the insect fauna and at two weeks post-application the abundance and diversity of the fauna seemed to be lower than in the untreated control plots. However, the fauna soon recovered and at two months after treatment no statistically significant differences could be detected between virtually all the treated and untreated plots. None of the acridicides therefore had any long-term impact on either the abundance or diversity of the fauna. None of the acridicides had any detectable impact on the vegetation composition within the plots. Variations in the abundance of some plant species were attributed to seasonal differences with a lush growth of pioneer species during the relatively wet season of 1997, while grazing pressure by sheep on some palatable species was considered to be important during the drier 1996 season. The results of this study therefore indicate that locust control operations in the Karoo, using recognised acridicides at registered dosage rates, are unlikely to have any long-term detrimental effects on the environment.

Présentation (NRF)

Page publiée le 26 octobre 2017