Entomopathogenic nematodes (EPN) of the families steinernematidae and Heterorhabditidae are a promising prospective control agent of the large pine weevil, Hylobius abietis L. (Coleoptera: Curculionidae), a major pest in Irish forestry. Applications of EPN to tree stumps in Ireland and the UK have been shown to significantly reduce adult H. abietis populations emerging from the stumps. To date however, little work had been done on the host-finding abilities of applied EPN in the field and how those EPN will interact with adult H. abietis either already on site or those that emerge from the stumps. The main objectives of this study were to investigate the behavioural interactions between two EPN species and adult H. abietis and to assess one of the methods used to investigate the impact of applied EPN strains on the indigenous EPN population. To investigate if adult H. abietis react to the presence of EPN, one thousand nematodes were applied to the head region of adult weevils and grooming responses were recorded (Chapter 3). Insects that had Steinernema carpocapsae applied on average groomed for significantly longer (44.16 seconds), than those that had Heterorhabditis downesi applied (11.33 seconds). In order to gain an insight into the possible cause of these different insect reactions to these two EPN species observations of nematode behaviour on adult H. abietis were carried out (Chapter 3). Infective juveniles (IJs) of S. carpocapsae were more likely to be observed standing or moving across the body of the insect while H. downesi IJs were more likely to be observed not moving. It is thought that this difference in behaviour has caused the differences observed in H. abietis grooming reactions. Adult H. abietis were tested to see if the presence of EPN in the substrate would alter the amount of time the insects would take to leave the area (Chapter 3). There was no difference found in the time taken to leave S. carpocapsae or H. downesi treated substrates when compared to controls. The impact of EPN presence in the substrate on H. abietis feeding was also investigated (Chapter 3). Steinernema carpocapsae was found to reduce the level of feeding on bark discs after two days when compared to controls in a choice test. This difference was not found with H. downesi or after two days post-application with either nematode species. Adult H. abietis that had been exposed to EPN did not significantly change the amount of feeding on bark discs compared to non-exposed controls (Chapter 3). In all nematode treatments, infected weevils fed more than non-infected weevils in that treatment. Weevils that died during the assay fed more than those that did not die when exposed to a low dose of H. downesi but not in the other EPN treatments. As weevils that died that were not exposed to EPN also fed more it is thought that this is not due to nematode exposure. Weevils were able to mount an immune response to both S. carpocapsae and H. downesi as encapsulated IJs of each species were found upon dissection of the exposed insects. Foraging strategies of entomopathogenic nematodes have been the subject of several studies but most have been based on simple substrates such as agar and have not reflected the ability of certain EPN species to infect hosts in cryptic field habitats. The presence of twigs in the substrate increased H. abietis larval mortality by S.carpocapsae. Insects that were able to feed differed in their mortality from those that could not in a sand/peat mixed substrate but this difference was not found in a sand only substrate (Chapter 4). The presence of twigs or insect feeding did not alter the mortality of H. abietis larvae to H. downesi in a sand substrate. The various life stages of H. abietis were tested for susceptibility to EPN infection (Chapter 4). The adult proved to be relatively insusceptible to EPN infection at low doses, with a two day exposure of 200 IJs killing none of the adult weevils exposed. Continuous exposure on filter paper at doses up to 4000 IJs resulted in a higher mortality rate. In a peat medium with an eight hour exposure mortality was different for each EPN species tested with S. carpocapsae killing more adult insects than both S. feltiae and H. downesi at all doses. When larvae and pupae were tested for EPN susceptibility it was found that larvae were highly susceptible to even low concentrations for both continuous and limited exposure. Pupae were found to be less susceptible than callow adults as insects that fully pupated during assays became readily infected by EPN (Chapter 4). The amplified fragment length polymorphism (AFLP) method for assessing levels of hybridisation between EPN strains was assessed to examine the level of accuracy possible with this method (Chapter 5). A certain level of inheritance patterns of strains of known parentage was possible but this level proved too low for statistically valid examination. The AFLP method proved to be difficult to reproduce when assessing EPN gene flow.