Beneficial thresholds for Coccinella 7-punctata L. (Col., Coccinellidae) as a predator of cereal aphids in winter wheat: results of population investigations and computer simulations

: 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.