Entomopathogenic nematodes (EPN) and entomopathogenic fungi (EPF) are pathogens which require an insect host to fulfil their life cycle and are commonly used as biocontrol agents. Studies have shown that a combination of EPN and EPF can result in greater insect mortality when compared to the agents applied alone. This interaction could possibly reduce the cost and quantities of single dose applications, inferring a synergistic effect. However, combinations of these pathogens may also result in additive or antagonistic interactions which would be of no benefit to growers. Hylobius abietis (large pine weevil) and Otiorhynchus sulcatus (black vine weevil) cause economic damage to forestry and horticultural crops, respectively. Large pine weevil larvae develop in the tree stumps, emerge as adults and feed on the bark of newly planted saplings, causing extensive mortality. Black vine weevil larvae kill plants by feeding on the roots and adults cause cosmetic damage. In this investigation, laboratory and field trials were conducted against large pine weevil larvae to determine whether interactions occur if EPN and EPF were applied simultaneously. EPN and EPF have already been shown to interact synergistically against the vine weevil in laboratory conditions. These experiments were repeated to investigate if synergistic interactions could be reproduced. Synergistic interactions were found with EPN and EPF against the large pine weevil in some laboratory experiments. However, antagonism and additivity were also recorded and synergism could not be predictably achieved. The field trial against the large pine weevil resulted in additivity. This indicates that the combined application of EPN and EPF would not be a better option than single applications. Initial interaction experiments against the black vine weevil did not show similar results to previous publications. However, larvae used in previous publications were younger than in this investigation which may indicate that interactions are dependent on larval age.