Figure 4.

Temporal structure of the odor plume. Plume snapshot (upper part): the wind transports odorants away from the source along its main direction. The mean odorant concentration in the odor plume decreases with the square of the distance but local vertices mix together odorized and clean air, making the local odor concentration vary considerably around the average. As a result, odor plumes are highly intermittent signals, consisting of series of whiffs, clumps and blanks (odor/no-odor events). Foraging insects cannot rely on a chemical gradient but use the fast temporal dynamic of the plume to locate distant mates and host plants (Budick and Dickinson, 2006; Cardé and Willis, 2008). EAG recording (lower part): the complex structure of odor plumes can be visualized with the EAG technique (Vickers et al., 2001; Riffell et al., 2008; Nagel and Wilson, 2011) since the insect antenna responds gradually and dynamically to odor stimuli. Here, the EAG was recorded from an Agrotis ipsilon antenna attached to a walking red palm weevil (Rhynchophorus ferrugineus). The weevil was attracted to an odor source containing 10 µg of its aggregation pheromone (4-methyl-5-nonanol, ferrugineol) mixed with 10 µg of the main component of the A. ipsilon pheromone, (Z)7-dodecenyl acetate (Z7-12:Ac). Z7-12:Ac is thus used as a tracer of ferrugineol. The distance between the starting point of the insect and the odor source it reached was 1.75 m. Portions of the EAG recording corresponding to a whiff, a blank, and a clump of whiffs are enlarged and show single detection events (red dots).