Bioactivity of essential oil-based nano-biopesticides toward Rhyzopertha dominica (Coleoptera: Bostrichidae)

Essential oils (EOs) are promising active ingredients to produce biopesticides, although their physicochemical characteristics are a critical issue to develop commercial formulates. Repellent and toxic activity of EOs against crop and stored product pests has been widely investigated in the last decades; however, no information is available on the occurrence of adverse undesirable behavioral responses (i.e. habituation) toward these repellents in target pest species. In this study, stable EO-based nano-emulsions from commercial fennel (Foeniculum vulgare), mint (Mentha x piperita) and sweet orange (Citrus sinensis) EOs were developed, and their repellence was tested against a major stored product pest, the lesser grain borer Rhyzopetha dominica. Besides, the occurrence of habituation in R. dominica adults following successive exposure to the repellent formulations was evaluated, considering the main characteristics of this kind of non-associative learning. Nanometric droplet size was achieved for all developed nano-emulsions (<200 nm). All the tested EO-based nano-formulations were repellent to R. dominica, whereas F. vulgare-based nano-emulsions triggered lower repellence both in area choice and arena bioassays. The occurrence of habituation was validated for the strongest repellents, M. piperita and C. sinensis formulations and the decline of R. dominica responses was frequency-dependent. Furthermore, insects completely recovered their responsiveness toward the biopesticides just after 24 h from the end of exposure. The decline of the responsiveness in R. dominica adults was attributable to real learning process, since motor and sensory fatigue were excluded by testing stimulus specificity and dishabituation. The results demonstrated that habituation could occur for repellent EO-based formulations, thus this behavioral process can reduce the effectiveness of these kind of treatments against R. dominica and should be considered to articulate adequate IPM programs against stored product pests.