It is generally accepted that climate change is likely to disrupt host-parasite relationships, but little consideration has been given to how this may affect agricultural pests. Here, we study the potential influence of climate change on the pest slug Deroceras reticulatum and its nematode parasite Phasmarhabditis hermaphrodita. The European slug D. reticulatum is a major pest of agriculture and has become invasive in many countries throughout the world, including the Americas, Asia and Australasia. P. hermaphrodita strongly inhibits feeding and causes mortality in host slugs. The parasite is widely distributed in the slugs’ native range but is very rare in some countries where the slug is invasive, for example the USA. The influence of temperature on D. reticulatum has been studied previously but the temperature response of P. hermaphrodita is unknown. The current study showed that while host and parasite have similar optimum growth temperatures, the parasite is much less tolerant of temperatures above optimum than the host. As a result, when parasites were present, slug feeding increased significantly above the slug’s optimum of 14 up to 24 °C as the inhibitory effect of the parasites lessened. Conversely, in the absence of parasites, slug feeding significantly decreased over the same temperature range. This finding suggests that given adequate moisture, rising temperatures may increase slug problems in D. reticulatum’s European home range, but possibly reduce problems in invasive territories such as the USA. The relationship between high temperatures and poor parasite performance is supported by studying prevalence of Phasmarhabditis spp. in the UK where a significant negative relationship exists between the mean summer high temperature and parasite prevalence.