EFFECT OF MIXED CROPPING ON SOME INSECT PESTS OF BRASSICAS; REDUCED BREVICORYNE BRASSICAE INFESTATIONS AND INFLUENCES ON EPIGEAL PREDATORS AND THE DISTURBANCE OF OVIPOSITION BEHAVIOUR IN DELIA BRASSICAE

Abstract

Intercropping brassicas with taxonomically unrelated plant species reduced infestations of the aphid Brevicoryne brassicae L. and the rootfly Delia brassicae (Bohé) by over 60% compared with those on brassicas grown in pure stand. Twice as many carabid and staphylinid predators of the immature stages of the rootflies were trapped on intercropped than on brassicas areas, but when they were excluded similar reductions in rootfly eggs occurred suggesting that predation was not an important factor suppressing rootflies in intercrops. Similar numbers of female D. brassicae entered mixed stands indicating that the response of the flies to host‐plant stimuli was not disturbed by non‐host plants. Flies were more active in the laboratory in mixed plantings than in stands of host plants or of non‐ host plants with host‐plant odour blown over them, and their rate of departure from these treatments was correlated with their activity. Oviposition by D. brassicae was reduced in mixed stands. The proximity of host and non‐host plants influenced the diversionary effects of the non‐host plants on oviposition behaviour, so that when 50 cm apart or less, maximum reduction in oviposition occurred. Single row‐intercropping, therefore, appears to be the best arrangement of plants for reducing rootfly attack and was most effective when the intercrop provided at least 50% ground cover between the rows at the time of pest invasion. RÉSUMÉ Effet de l'association culturale sur quelques insectes nuisibles aux choux: réduction des attaques de Brevicoryne brassicae, conséquences pour les entomophages prédateurs épigés, perturbation du comportement de ponte de Delia brassicae L'association des choux avec des plantes sans relations taxonomiques réduit d'environ 60% les attaques de Brevicoryne brassicae et Delia brassicae par rapport aux attaques observées sur cultures pures de choux. Deux fois plus de coléoptères prédateurs des stades larvaires de mouches sont capturés en cultures associées qu'en cultures pures de choux; mais même après élimination des carabidées et staphylinidées, en observe une réduction similaire des effectifs d'ceufs de la mouche de choux, ce qui suggère que la prédation n'est pas un facteur important de régression des populations de D. brassicae en cultures associées. L'entrée d'effectifs comparables de femelles de D. brassicae dans les cultures associées montre que le vol en réponse aux stimuli de la plante‐hôte n'est pas perturbé par les plantes non‐hôtes. Au laboratoire les mouches sont plus actives sur cultures associées ou sur plante non‐hôte mais recevant une odeur de plante‐hôte; leur taux d'envol est dans ces cas proportionnel à leur activité. La ponte de D. bassicae est réduite en cultures associées. La distance entre plantes‐hôtes et non‐hôtes influence l'effet perturbateur des plantes non‐hôtes sur le comportement de ponte, si bien que la réduction la plus efficace est observée pour des distances égales ou inférieures à 50 cm. L'association par rangs alternés paraît être la meilleure disposition pour réduire les...

Response by Specialist and Generalist Natural Enemies to Agroecosystem Diversification: A Selective Review

“…The third study mentioned (ICRISAT: see Bhatnagar and Davies [1982]) found both pest movement and natural enemies to be important in determining pest abundance patterns. Other recent studies that have examined both herbivore movement and enemycaused mortality have suffered from insufficient sampling to determine enemy activity (Andow 1983), or inadequate replication and experimental control (Tukahirwa & Coaker 1982). In one study (Salick 1983), numbers of enemies were insignificant.…”

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Host‐plant selection by insects – a theory based on ‘appropriate/inappropriate landings’ by pest insects of cruciferous plants

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