Abstract
Following the successful introduction ofEpidinocarsis lopezi (De Santis) for biological control of the cassava mealybug (CM)Phenacoccus manihoti Mat.-Ferr. in southwestern Nigeria in 1981 and 1982, 11 groups of cassava fields were sampled every 2 weeks up to 1988 for impact assessment. After 1984, CM populations remained mostly below 10 per tip despite the presence of native hyperparasitoids, demonstrating the long-term success of biological control byE. lopezi in the region. Indigenous polyphagous coccinellids were found only during peak host densities, whereas the specificE. lopezi was common throughout the year. During some periods, percentage parasitism indicated delayed density dependence. Since 89% of all sampled cassava tips had no CM at all and the parasitisme is very mobile, parasitization rates were also calculated for individual infested tips (N=4,878). Parasitism increased slightly with host density on tips having between 1 and 10 CM of the 3rd and 4th instars, indicating positive density dependence. Such tips comprised 64% of all infested tips. At higher host densities, parasitism rates fell rapidly. The results are discussed in view of different theories on population regulation by biological control agents.
Résumé
Suite à l'introduction d'Epidinocarsis lopezi (De Santis) en vue d'une lutte biologique contre la cochenille du manioc (CM)Phenacoccus manihoti Mat.-Ferr. dans le sud-ouest du Nigéria en 1981 et 1982, 11 groupes de champs situés dans 2 régions ont été échantillonnés toutes les 2 semaines jusqu'en 1988, afin d'estimer l'impact de cet auxiliaire. Les populations du ravageur sont restées au-dessous de 10 CM par pousse la plupart du temps, malgré la présence d'hyperparasitoïdes indigènes, prouvant ainsi le succès continu de la lutte biologique parE. lopezi dans la région. Les coccinelles indigènes et polyphages n'ont été présentes que pendant les sommets de population de la CM, tandis que le parasitoïde spécifique était commun toute l'année. Pendant certaines périodes la courbe du parasitisme suggère une dépendance de la densité retardée. Comme 89% de toutes les pousses sont exemptes de la cochenille et comme le parasitoïde est très mobile, les taux de parasitisme ont été calculés sur la base des pousses individuelles. Malgré la grande variance des données couvrant 4 878 pousses infestées, une légére augmentation du parasitisme et, par conséquent, une dépendance positive de la densité sont constatées sur toutes les pousses ayant entre 1 et 10 CM du 3e et 4e stade. De telles pousses représentent 64% de toutes les pousses infestées. Aux densités d'hôte plus élevées le degré de parasitisme diminue rapidement. Les résultats sont discutés à la lumière des différentes théories sur la régulation des populations par les ennemies naturels.
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Hammond, W.N.O., Neuenschwander, P. Sustained biological control of the cassava mealybugPhenacoccus manihoti [Hom.: Pseudococcidae] byEpidinocarsis lopezi [Hym.: Encyrtidae] in Nigeria. Entomophaga 35, 515–526 (1990). https://doi.org/10.1007/BF02375086
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DOI: https://doi.org/10.1007/BF02375086