Biological control of Otiorhynchus sulcatus by insect parasitic nematodes, Heterorhabditis spp., at low temperatures : a systems analytical approach

The black vine weevil, Otiorhynchus sulcatus, is an important pest in ornamentals and nursery stock in The Netherlands. The larvae, which feed on the root system of the plant, can be controlled by insect parasitic nematodes, Heterorhabditis. However, the presently available isolates of the nematode are ineffective at temperatures below 12-13°C, causing problems in black vine weevil control in open cultures. In this study, options to improve control by Heterorhabditis are explored, using a systems analytical approach. First, the nematode behavioural processes involved in host finding and control were studied and characterized. These processes are nematode movement, immobilization and remobilization near the soil surface, accumulation near an attractive insect (arrestment), penetration and aggregation of nematodes among insect hosts. The influence of temperature (9 and 20°C) and host species ( O . sulcatus or the more attractive and susceptible Galleria mellonella) on nematode behaviour was assessed to determine the contribution of these factors to control success. Knowledge of behavioural processes was integrated into a systems simulation model that relates the control success to the underlying behavioural processes. The model simulates movement of nematodes in space and time from the moment of application on a sand column until penetration into a host. The model for O . sulcatus at 9°C was most sensitive to changes in the parameters characterizing aggregation and arrestment. Parameters characterizing penetration, the proportion infectious nematodes and the relative penetration rate, had a moderate effect on model outcome. Options for improvement were evaluated by relating the sensitivity of the model to genetic and phenotypic variation found in the nematodes. The amount of variation was assessed by comparing behavioural traits between and within Heterorhabditis isolates. Aggregation and arrestment are host related and there is little variation in Heterorhabditis for these traits. There is phenotypic variation in the proportion infectious nematodes. The most promising option to enhance control of O . sulcatus by Heterorhabditis at low temperatures is, therefore, to improve production and storage conditions to increase and stabilize the proportion infectious nematodes.