Einfluss der Pflanzedichte und -architektur auf Abundanz und innnerpflanzliche Verteilung stängelminierender Schadinsekten in Winterraps

Abstract

The effect of the plant density and plant architecture on the abundance and within-plant distribution of the stem borers Psylliodes chrysocephala L., Ceutorhynchus napi Gyll. and C. pallidactylus (Mrsh.) was investigated by using various sowing rates (30, 60 and 90 seeds/m2) of winter oilseed rape in randomised field plot experiments with 6 repetitions each in 1999/00 to 2001/02 (2001/02: sowing rates 30 and 60 seeds/m2). An open-pollinated cultivar was used in 1999/00 and a hybrid cultivar in 2000/01 and 2001/02. The immigration of adult beetles was recorded by yellow water traps. Emergence traps and water trays were used to estimate the abundance and activity density of P. chrysocephala adults, respectively. The density of P. chrysocephala eggs/m2 was determined by flooding of soil samples. To assess the number and within-plant distribution of the egg batches of C. napi and C. pallidactylus as well as the larvae of all three species, plant samples were collected randomly along a transect. The petioles and stems were dissected under a microscope.The sowing rate of 30 seeds/m2 led to significantly lower plant density and larger oilseed rape plants which produced significantly higher numbers of leaves and lateral buds. The adults of P. chrysocephala showed a significant preference for higher sowing rates in one of two trials. At the low plant density, their activity density was clearly lower than at high plant densities. In 2001/02 the number of P. chrysocephala eggs/m2 increased with increasing number of plants/m2. When samples were collected more distant from the plant, the abundance of egg batches dropped at low plant density more strongly than at higher densities. With all three species the number of larvae/plant at the lowest sowing rate (30 seeds/m2) was significantly higher than at the high sowing rates (60 and 90 seeds/m2); however with C. napi this difference was statistically significant only in one of three years. In contrast, the number of weevil larvae/m2 was not influenced by plant density; however with P. chrysocephala larvae it increased in all trials with increasing number of plants/m2.In 2000/01 the infestation of terminal buds by P. chrysocephala larvae at sowing rates of 60 and 90 seeds/m2 was significantly higher than at 30 seeds/m2. In this year vigorous growth of the hybrid oilseed rape cultivar in the autumn enabled an early development of lateral buds and leaves. The differences in plant architecture, as caused by various plant densities, resulted in different within-plant distribution of egg batches and larvae of C. napi and C. pallidactylus in stems and leaves; this can be related to the oviposition preference of these species for specific plant growth stages and organs.The seed yield per hectar and the 1000-kernel-weight were not affected significantly by different plant densities. In spite of a higher pest infestation, the seed yield per single plant significantly increased with decreasing plant density.