An investigation into the combination of the parasitic nematode Phasmarhabditis hermaphrodita and cedarwood oil to control pestiferous slugs

Several slug species are voracious pests of agricultural crops in northern Europe and are difficult to control. The parasitic nematode Phasmarhabditis hermaphrodita has been developed as a slug control product, but there is little information about whether it could be combined with other control methods (such as essential oils) to enhance its efficacy. Here, we carried out experiments in propagators with lettuce at three different time periods (July, September and October), and tested the following treatments: water (untreated control), cedarwood oil, P. hermaphrodita, cedarwood oil and P. hermaphrodita, and Tween 80 (used as an emulsifier for the cedarwood oil solution). Lettuce was grown in propagators with either 10 Deroceras reticulatum or 5 Arion vulgaris and the percentage of lettuce eaten over 14 days (as well as weight, the number of live slugs and eggs produced) was recorded. Cedarwood oil reduced slug damage, slug numbers and slug eggs in the experiments with D. reticulatum, and P. hermaphrodita performed well in two out of three experiments. The mixture of P. hermaphrodita and cedarwood oil was superior in reducing the proportion of lettuce eaten compared to single doses of each treatment in one out of three trials. In propagators with A. vulgaris all treatments performed poorly. In summary, P. hermaphrodita and/or cedarwood can be used to reduce damage by D. reticulatum, but are ineffective at controlling A. vulgaris. Slugs from the genus Arion continue to be a difficult group to control.