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  • G. K. Bracken (a1)


The order of feeding preference on plant parts of rape (Brassica napus L.) by sixth instar (L6) larvae of bertha armyworm, Mamestra configurata Walker, in laboratory tests was leaves, bracts, immature pods, and mature pods. Field sampling showed that upper, immature pods are attacked with greater frequency than lower, more mature pods. In a greenhouse test, a slightly larger proportion of L6 larvae was recorded on inflorescences of rape plants during darkness than during light; the proportion of larvae found on the inflorescences was about 20%. Larvae pupated more quickly, formed heavier pupae, and gained more dry weight when fed exclusively from mid L4 on leaves than when fed mature or immature pods; mortality for larvae fed mature pods was 30% compared with none for larvae fed leaves or immature pods.The finding that pods are neither as preferred nor as adequate a dietary source as leaves is discussed in relation to the phenology of leaf retention of rape on the economic impact of bertha armyworm infestations.

Dans des expériences de laboratoire, l'ordre de préférence dans la déprédation des parties de la plante de colza, Brassica napus L., par les larves de sixième stade (L6) de la légionnaire bertha, Mamestra configurata Walker, était : les feuilles, les bractées, les jeunes gousses et les gousses pleinement développées. L'échantillonnage sur le terrain montrait que les jeunes gousses, plus hautes, étaient attaquées plus souvent que les gousses inférieures, plus développées. Dans un essai en serre, on constatait une proportion légèrement supérieure de larves L6 sur les inflorescences de plantes de colza en période de noirceur qu'en période de clarté; la proportion de larves trouvées sur les inflorescence était d'environ 20%. La pupaison était plus rapide, les pupes plus lourdes et le gain de poids sec supérieur chez les larves nourries exclusivement de feuilles à partir du stage intermédiaire L4 que chez celles nourries de gousses entièrement développées ou non. La mortalité des larves nourries de gousses entièrement développées était de 30% comparativement à zéro pour celles nourries de jeunes gousses.La découverte que les gousses ne sont pas une source alimentaire aussi recherchée ni aussi appropriée que les feuilles est examinée en relation avec la phénologie de la rétention des feuilles de colza sur l'incidence économique des infestations de légionnaires bertha.



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  • G. K. Bracken (a1)


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