Establishment and persistence of the entomopathogenic nematodes, Steinernema feltiae and Heterorhabditis bacteriophora
Objective of this study was to investigate establishment and persistence of the entomopathogenic nematodes (EPNs), Steinernema feltiae and Heterorhabditis bacteriophora in different field crops and rotation regimes. A total number of 10,980 soil samples was collected and investigated for the presence of EPN over the period 2001 until 2004. In potatoes and pasture establishment of H. bacteriophora often failed, whereas this species established well in beans, oil seed rape, wheat and barley. It dropped considerably after tillage and during the winter. The longest persistence of H. bacteriophora was detected 23 months after release in beans followed by wheat as cover crop over red clover and pasture. In this field larvae of the pea weevil Sitona lineatus (Coleoptera: Curculionidae) were detected in soil samples and found infected with the released nematode population. Persistence of H. bacteriophora was high in pots containing larvae of the black vine weevil Otiorhynchus sulcatus (Coleoptera: Curculionidae) on strawberries. Mortality of the insect surpassed 90% although nematodes had been applied by dipping strawberry roots a months before eggs were laid into the pots. Released in November on a field infested with cabbage root fly larvae Delia radicum (Diptera: Anthonomyiidae), S. feltiae was recorded in 100% of the soil samples in February and in 80 % of the soil samples after 9 months when the monitoring stopped. The comparison on persistence in the absence of host insects with the results obtained in the field strongly indicates that persistence is influenced by the presence of suitable host insects and indirectly by the crop. Thus the antagonistic potential of EPN populations can be preserved if host insects are provided by managing a suitable crop rotation regime.