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Migrating locusts, especially the desert locust (Schistocerca gregaria), have been feared in Africa for thousands of years as famine-inducing pests. Instead of simply waiting for outbreaks to occur, attempts are being made to take preventive action against these pests. Since the breeding areas of the desert locust are distributed across the entire Sahel region, the Arabian peninsula, Pakistan and India, a gigantic logistical and organizational effort is required. Every year, millions of dollars are spent on these preventive control measures, which are still unable to prevent locust plagues completely. The outbreaks in 1987/88 and 1993/94 are the most recent examples. Exactly how large potential disasters caused by gigantic locust swarms may be and whether the effort and expense involved in preventing them pays off economically has never been systematically investigated. The Deutsche Gesellschaft für Technische Zusammenarbeit (GTZ) project, 'Integrated Biological Control of Grasshoppers and Locusts', has attempted to assess, on the basis of the available data, what the costs and potential benefits are and to identify the difficulties involved in developing an effective strategy.

Melia volkensii extracts were tested for the first time under field conditions in the southern Tamesna Desert (Republic of Niger). S. gregaria larvae caught in the field and larvae hatched from egg pods brought from Germany to Agadez were both treated with these extracts. Although these experiments had pilot character in every respect, a number of important findings were made, some of them unexpected. Application of an oil‐based formulation containing 1000 ppm of biologically active compounds (10 liters per hectare) caused retarded growth and a considerable delay in the onset of sexual maturity in the imagoes, i.e. it was possible to reduce by 30–40 days the potential reproductive period of adult S. gregaria individuals that had been treated with Melia extract as larvae. Moreover, initial studies indicated a high likelihood that active compounds contained in Melia extracts are capable not only of halting the feared gregarization process that S. gregaria undergoes, but indeed of reversing it. Zusammenfassung Biologisch aktive Inhaltsstoffe aus Melia volkensii. Ihre Wirkung als Wachstumshemmer und Phasenmodulatoren der Wüstenheuschrecke Schistocerca gregaria (Forskal) (Orth., Cyrtacanthacrinae) Melia volkensii‐Extrakte wurden zum ersten Mal unter Freilandbedingungen in der südlichen Tamesna‐Wüste (Republik Niger) getestet. Appliziert wurden diese Extrakte sowohl gegen im Freiland gefangene S. gregaria‐Larven als auch in Agadez gezüchtete Locusts. Obwohl diese Experimente in jeder Hinsicht Pilotcharakter besaßen, wurden wichtige und z. T. überraschende Ergebnisse erzielt. Die Applikation einer 1000 ppM‐haltigen öligen Formulierung (10 l/ha) bewirkte ein retardiertes Larvenwachstum sowie eine erst sehr spät einsetzende Geschlechtsreife der Imagines, d. h. die mögliche Reproduktionsdauer adulter S. gregaria, als Larven mit Melia‐Extrakt appliziert, konnte um 30–40 Tage verringert werden. Weiterhin machten es erste Untersuchungen äußerst wahrscheinlich, daß in Melia‐Extrakten Wirkstoffe enthalten sind, die den gefürchteten Gregarisierungsprozeß von S. gregaria nicht nur anhalten, sondern auch umkehren können.