A dynamic tubular olfactometer to evaluate the repellent effect of plants on aphids

Abstract

Aphids are important crop pests that are difficult to control, even more that the use of synthetic insecticides is increasingly restricted by legislation (Ecophyto II+ plan). The use of repellent companion plants (CPs) could, in the long term, be a lever for controlling these pests. Some CPs have the capacity to emit volatile organic compounds (VOCs) that can disrupt the establishment and reproduction of these insects. In association with cash crops, repellent CPs could maintain aphid populations at a level of crop damage economically acceptable to farmers. The study of CPs and in particular their repellent capacity is therefore an essential prerequisite for their use in agronomic models. The olfactometer is a tool of choice for studying the effect of odours on insect movement. However, the olfactometers currently available are essentially used to test the choice between several odour sources and are mainly used to measure attraction phenomena (e.g. to pheromones). They are sometimes used to measure a repellent effect, but only in an indirect way: the insect goes towards the odour that is the least unpleasant. They also allow testing only a few individuals at the same time and are thus very time-consuming.
At the « Plants and cropping Systems in Horticulture » research unit (INRAE, Avignon, France), we have therefore developed a new type of dynamic airflow tubular olfactometer that allows us to directly study the repellent effect of CPs on a cohort of aphids (more
than 40 individuals simultaneously). We can also capture the odours (VOCs) emitted by the plants on devices such as Tenax® cartridge and SPME fiber at any time during the repellent test, and then characterise the VOCs emitted by gaz chromatography-mass spectrometry (GC-MS). In addition, the device is equipped with cameras to monitor the insect behaviour in real time and to establish a repulsion index (RI) for each aphid-plant modality.
The device was validated by tests on CPs but also on negative controls (host plants or absence of odours) and positive controls (ß-farnesene). Finally, this device was designed to be scalable: it can be dismantled and adapted to other types of experiments and different odour sources.