Insecticidal and repellent properties of garlic essential oil on the edaphic aphid Pemphigus bursarius

The repercussions of insecticide intensive use, such as the decline of non-target species and the emergence of resistant insect populations, have led to the need to develop nature-based solutions for crop protection both above- and belowground. Among these alternatives, essential oils have received particular attention given their ability to alter insect pest physiology and behaviour. In this context, we aimed to assess the insecticidal and repellent properties of garlic essential oil (gEO) on the edaphic aphid Pemphigus bursarius, a root-feeding species that colonizes the belowground systems of market gardening vegetables such as lettuce and chicory. We investigated lethal and sublethal toxicological effects, with a particular focus on sublethal behavioural responses and their neurotoxic basis, and independently assessed repellent effects. Topical application of low concentrations of gEO (0.1%) was sufficient to induce a significant decrease in aphid survival 24 hours after treatment, although the LD50 was around 2.5%. We subsequently tested aphids exposed to 0.1% of gEO for changes in locomotion, measuring their velocity, dispersion and exploratory behaviour. All the aforementioned metrics were significantly reduced compared with the control group and converged on the conclusion that gEO impaired locomotor performance. Moreover, the observed locomotor deficits were associated, at least in part, with a neurotoxic effect of gEO, as aphids also exhibited significantly lower acetylcholinesterase activity. In a dual-choice device, aphids displayed positive directed responses to sections of chicory roots but avoided conditions where the LD50 of gEO was applied, independently of the presence or absence of root sections. These results suggest a repellent effect of gEO that is stronger than the attraction induced by host plant chemical cues. In conclusion, the present work paves the way to an in-depth understanding of belowground insect chemical ecology and points out the potential of gEO as a bioactive agent in sustainable pest control strategies.