In the present study, we investigated the natural control of aphids by predators in wheat fields in a low (L) and high-input cropping region (H) of Germany during a 10-year period. Data for the statistical analyses were obtained from weekly after the start of aphid emergence. The mean annual aphid indices, calculated as the sum of Sitobion avenae (Fabr.), Rhopalosiphum padi (L.), Metopolophium dirhodum (Walk.)(Homoptera: Aphididae), were 30.4 and 81.5 · 10 3 aphid days per m 2 , for L and H, respectively. Nine predator fractions were analysed: Coccinella septempunctata L., adults (1) and larvae (2), Propylea quatuordecimpunctata (L.) (Coleoptera: Coccinellidae) adults (3) and larvae (4), syrphid larvae (mostly Episyrphus balteatus [De Geer] (Diptera: Syrphidae)) (5), Chrysoperla carnea Steph. (Neuroptera: Chrysopidae) larvae (6), and adult carabids (7), staphylinids (8) and spiders (9). The two sites were comparable in terms of the mean size of the overall predator community, expressed in predator units (PU): 4.9 PU/m 2 (L) vs. 5.4 PU/m 2 (H). Most predator fractions responded numerically to increasing aphid densities. The numerical response was strongest in syrphid larvae, scarcely detectable in adult coccinellids, and virtually non-existent in epigeic arthropods. Multiple regression models revealed indirect relationships between the weekly overall predator community densities (PU/m 2 ) and individual predator fractions (individuals/m 2 ) and absolute rates of aphid density increase (individuals/m 2 ) one or two weeks after baseline. A site-independent reduction of the aphid density increase to nil (y = 0) was observed at 3.9 to 4.2 PU/m 2 . Consequently, the 2.7 times higher aphid density at H cannot be attributed to the presence of fewer predators or lower effects of the overall predator community or of any individual predator fraction.
The present study deals with entomoparasites of Coccinella septempunctata L. in cereals, mainly in winter wheat. Sections of monthly sampled adult ladybirds during one year in different habitates at Staaken (Berlin) gave two peaks of parasitization with Perilitus coccinellae (Schrank) (Hym., Braconidae): (1) 33% in March (hibernation site); and (2) 12.5% in June (winter wheat). Populations of the ladybird investigated during the seasons in 1993 and 1994 in winter wheat at three locations (Berlin‐Staaken, Fläming, Magdeburger Börde) showed a similar sharp increase in degree of parasitization with Perilitus coccinellae in the second half of June. Maximum of parasitization reached from 12.3% (Fläming, 1993) to 25% (Börde, 1993) and was a result of the act of the first newly emerged adult wasps. Main parasite of the coccinellid pupae was Phalacrotophora fasciata (Fallen) (Dip., Phoridae) with parasitization rates between 4.9% (Börde) and 20.2% (Fläming). Oomyzus scaposus (Thompson) (Hym., Eulophidae, Tetrastichinae) was found only at two locations with low rates: 0.9% (Fläming) and 1.4% (Berlin). Experiments in climatic chambers with alternating temperatures gave new results for the effect of Perilitus on the fertility of matured female ladybirds. At relatively high temperatures (25°C middle of day) daily numbers of coccinellid eggs increased for a period of a week immediately after parasitization. Though mortality was high (70.6%) at this condition, half of the surviving females began egg deposition 12 days after emergence of the Perilitus larva.
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