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Wageningen University (2021)

Don’t kill your allies : the impact of chemical and biological locust and grasshopper control on birds

Mullié, Wim C.

Titre : Don’t kill your allies : the impact of chemical and biological locust and grasshopper control on birds

Auteur : Mullié, Wim C.

Université de soutenance : Wageningen University

Grade : Doctor 2021

Résumé partiel
Before the large-scale introduction of chemical insecticides after the Second World War (WW2), birds were considered worldwide as welcome natural allies to control acridids. For their role in acridid control they were worshipped in ancient times and in the first half of the 20th century protected in many countries worldwide. Until WW2, mechanical control e.g. by harvesting grasshoppers with ingenious dozers or digging trenches to interrupt marching hopper bands were chemically supplemented by massive use of arsenicals. The large scale introduction of Dieldrin in locust control around 1955 marked a turning point, as for the first time a persistent chemical became available to treat hopper bands which provided toxic residues killing hoppers emerging afterwards. The high toxicity for man and wildlife and the persistence of Dieldrin lead to a worldwide ban twenty years later, but in locust control it took until 1989 before its use eventually was discontinued. Meanwhile, organophosphorous insecticides had dominated the acridid arena since WW2 with varying contributions of carbamates, pyrethroids and some other chemical families. The interest in birds as natural allies had faded because of a widely held believe that the insecticides did a better job. In addition, the impact on birds of their widespread and large-scale use in Africa had neither been studied, nor assessed for its potential consequences for acridid control.

It was against this background that the studies described in this thesis were undertaken to compare the efficacy on acridids and the impact on birds of non-selective chemical and selective biological insecticides, to contribute to efficient locust and grasshopper management with the lowest environmental footprint. Two oganophosphorous insecticides, fenitrothion and chlorpyrifos, both commonly used in acridid control until the present day and the entomopathogenic fungus Metarhizium acridum were studied. Both organophoshates caused direct avian mortality (2-7%) and morbidity through anti-cholinesterase poisoning. They also reduced the availability of arthropod prey leading to reductions in bird densities, reduced body mass, nest desertion and debilitation of fledglings. Acridid numbers returned to pre-spray densities by 21 days post-spray but bird numbers remained 25-50% lower than controls. By using a novel approach, mimicking bird feeding behaviour by using fine tweezers to pick up affected locust nymphs, it was shown that nymphs accumulated residues by secondary uptake generally peaking from 3 h to 24 h post-spray. This would have been missed by the current approach of using pitfall traps, sweep nets or tethered insects which are unlikely to represent realistic residue levels. Therefore current regulatory frameworks likely underestimate risks of acridid control, and do neither take into account gorge feeding and depletion of insect prey by the use of non-selective chemical insecticides.


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