Male moths compete to arrive first at a female releasing pheromone. A new study reveals that additional pheromone cues released only by younger females may prompt males to avoid them in favor of older but more fecund females.

Chemical signals mediate many of life's processes. For organisms that use these signals to orient and navigate in their environment, where and when these cues are encountered is crucial in determining behavioral responses. In air and water, fluid mechanics impinge directly upon the distribution of odorous molecules in time and space. Animals frequently employ behavioral mechanisms that allow them to take advantage of both chemical and fluid dynamic information in order to move toward the source. In turbulent plumes, where odor is patchily distributed, animals are exposed to a highly intermittent signal. The most detailed studies that have attempted to measure fluid dynamic conditions, odor plume structure, and resultant orientation behavior have involved moths, crabs, and lobsters. The behavioral mechanisms employed by these organisms are different but generally integrate some form of chemically modulated orientation (chemotaxis) with a visual or mechanical assessment of flow conditions in order to steer up-current or upwind (rheo- or anemo-taxis, respectively). Across-stream turns are another conspicuous feature of odor-modulated tracks of a variety of organisms in different fluid conditions. In some cases, turning is initiated by detection of the lateral edges of a well-defined plume (crabs), whereas in other animals turning appears to be steered according to an internally generated program modulated by odor contacts (moth counterturning). Other organisms such as birds and fish may use similar mechanisms, but the experimental data for these organisms is not yet as convincing. The behavioral strategies employed by a variety of animals result in orientation responses that are appropriate for the dispersed, intermittent plumes dictated by the fluid-mechanical conditions in the environments that these different macroscopic organisms inhabit.

We characterized single upwind surges of flying male Helothis virescens moths in response to individual strands of pheromone generated experimentally in a wind tunnel. We then showed how this surge functions in this species as a basic 13.4-cm, 0.38-sec-long building block that is strung together repeatedly during typical male upwind flight in a normal pheromone plume. The template for a single iteration, complete with crosswind casting both before and after the straighter upwind surging portion, was exhibited by males flying upwind to pheromone and experiencing ifiament contactsjust frequently enough to produce successful upwind flight to the source, as hypothesized by an earlier model. Also as predicted, with more frequent filament contact by males, only the straightest upwind portions of the surges were reiterated, producing direct upwind flight with little crosswind casting. Electroantennogram recordings made from males in free Tfight upwind in a normal point source pheromone plume further support the idea that a high frequency offilaments encountered under the usual pheromone plume conditions promotes only these repeated straight surges. In-flight electroantennogram recordings also showed that when filament contacts cease, the casting, counterturning program begins to be expressed after a latency period of 0.30 sec. Together these results provide a plausible explanation for how male and female moths, and maybe other insects, fly successlly upwind in an odor plume and locate the source of odor, using a surging-casting, phasictonic response to the onset and disappearance of each odor strand.In the quest for understanding how male moths fly upwind and locate females (1-5) there have been suggestions (6, 7) that all odor-mediated flight in moths, including host plant location by females, may be explained by two programs, optomotor anemotaxis (2) and self-steered counterturning (8), that are turned on and modulated by odor fluctuations. It has also been pointed out (9, 10) that many other kinds of insects flying upwind to odor exhibit behavior somewhat similar to moths', and these other insects may also use these same mechanisms in odor-source location. Knowledge about the mechanisms used by moths should therefore be important for understanding the neuroethology of olfaction, the evolution of pheromone and host-plant-insect systems, and the potential application of pheromones in insect control.The physical structure of a pheromone plume created by a point source of odor is not a time-averaged homogeneous cloud. Turbulence causes the plume to break up into strands of odor-laden air (filaments) interspersed with pockets of clean air where little or no odor is present (11,12). The physical intermittency of these filaments is a requirement for male moths to sustain their upwind progress; they will not continue to fly upwind upon entering a homogeneous cloud of pheromone (4, 13) but will if this cloud is alternated with swaths of clean air (14).Recently, results from experiments investigating the highspeed beh...

The neural computations used to represent olfactory information in the brain have long been investigated. Recent studies in the insect antennal lobe suggest that precise temporal and/or spatial patterns of activity underlie the recognition and discrimination of different odours, and that these patterns may be strengthened by associative learning. It remains unknown, however, whether these activity patterns persist when odour intensity varies rapidly and unpredictably, as often occurs in nature. Here we show that with naturally intermittent odour stimulation, spike patterns recorded from moth antennal-lobe output neurons varied predictably with the fine-scale temporal dynamics and intensity of the odour. These data support the hypothesis that olfactory circuits compensate for contextual variations in the stimulus pattern with high temporal precision. The timing of output neuron activity is constantly modulated to reflect ongoing changes in stimulus intensity and dynamics that occur on a millisecond timescale.