Interactions between the stem-mining weevils Ceutorhynchus napi Gyll. and Ceutorhynchus pallidactylus (Marsh.) (Coleoptera:Curcullonidae) in oilseed rape

1 The rape stem weevil Ceutorhynchus napi Gyll. and the cabbage stem weevil Ceutorhynchus pallidactylus (Marsh.) share the same habitat and food resource within the stems of oilseed rape, Brassica napus L. var. oleifera. Interactions occurring between these two sympatric species on this host were studied under both field and laboratory conditions. 2 The oviposition preference of C. pallidactylus and the within-plant distribution of eggs and larvae were examined in field plots of oilseed rape. Female C. pallidactylus tended to lay their eggs in plants already infested by eggs and larvae of C. napi rather than in uninfested plants. The within-plant distribution of the egg batches of C. pallidactylus did not differ significantly between uninfested plants and those preinfested by C. napi. Ovipositing females of C. napi and C. pallidactylus generally showed a significant preference for plants with larger stem diameter. 3 Laboratory choice tests provided further evidence for the oviposition preference of C. pallidactylus. Females laid significantly more eggs in leaves of plants that had been previously infested by C. napi than in leaves of previously uninfested plants. 4 Larvae of C. pallidactylus showed a significant shift of their feeding niche towards the stem base when feeding in individual plants attacked by both species. This possibly indicates ressource partitioning between C. pallidactylus and C. napi. The within-plant distribution of C. napi larvae was not affected by the simultaneous attack of C. pallidactylus. 5 The size of the head capsule of full-grown larvae of C. napi and C. pallidactylus was not significantly correlated with the diameter of the stem of their host plant or with the number of conspecific larvae within individual plants.