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.

No abstract

No abstract

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.

No abstract

Syrphid larvae as well as adults and larvae of Propylaea 14-punctata (L.) and Coccinella 7-punctata L. are the most dominant aphid predators in winter wheat of Middle and Eastern Germany. The aphid-coccinellid interaction was investigated in a 4-year project aimed at the definition of beneficial thresholds (density of a predator guild necessary to keep a pest under control) for these species. Eight field studies and four cage experiments were performed to calculate the infestation reducing effect of coccinellids, particularly C. 7-punctata. The obtained data were used to estimate beneficial thresholds for the whole predator complex and especially for coccinellids whereby all predators have been converted into predator units (PU), e.g. fertile female of C. 7-punctata = 1.00 PU, larva = 0.33 PU, Episyrphus balteatus larva = 0.46 PU. In the field studies the calculation of the coccinellid related effect within the antagonist potential has not yet succeeded. The cage experiments have to be seen as case studies under special conditions. Therefore activities concentrated on the improved and validated simulation model GTLAUS including the submodels COCCISEP and WHEAT. Simulation runs with this tritrophic interaction model including the field count and cage trail data have shown that the beneficial threshold of C. 7-punctata to control cereal aphids under average conditions in Middle and Eastern Germany varies likely between 8 and 20 PU/m2. However, an isolated evaluation of C. 7-punctata or another single predator species within the beneficial potential is undoubtedly not realistic. It seems to be better to calculate summarized effects of the whole predator community weighted as PU.

No abstract

No abstract