Moth, plume

Our neurophysiological studies have focused on three important properties of the sex-pheromonal signal its quality (chemical composition of the blend), quantity (concentrations of components), and intermit-tency [owing to the fact that the pheromone in the plume downwind from the source exists in filaments and blobs of odor-bearing air interspersed with clean air (47, 48)]. Each of these properties of the pheromonal message is important, as the male moth gives his characteristic behavioral responses only when the necessary and sufficient pheromone components A and B are present in the blend (44), when the concentrations and blend proportions of the components fall within acceptable ranges (49), and when the pheromone blend stimulates his antennae intermittently (39, 50). In our studies, we examine how each of these important aspects of the odor stimulus affects the activity of neurons at various levels in the olfactory pathway. 

A third important characteristic of a female moth s sex-pheromone plume is its nonuniformity. Simulation of odor plumes using ionized air has shown clearly that a plume is not a simple concentration gradient but instead is distinctly filamentous and discontinuous (47,48). Furthermore,... 

The behavior of the animal in response to flow is important, not just the flows themselves. An animal searching for the source of an odor moves in the direction of increasing stimulus intensity and stays within the boundaries of the plume. If an animal needs to sample odor in turbulence frequently, it may have to reduce speed, to untenable levels in the case of moths or birds, or to an energetically... 

Mafra-Neto, A. and R. T. Cardd. Fine-scale structure of pheromone plumes modulates upwind orientation of flying moths. Nature 369, 142-144 (1994). 

This review considers evidence for selective forces that mold the chemical signal and behavioral response, including the characteristics of pheromone dispersal and the plume-tracking maneuvers that influence the success of mate finding. Recent reviews that consider mechanisms of evolutionary change in moth pheromones include Phelan (1992,1996,1997), Lofstedt (1993), Linn and Roelofs (1995), and Lofstedt and Kozlov (1996). 

Baker T. C. and Haynes K. F. (1987) Maneuvers used by flying male oriental fruit moths to relocate a sex pheromone plume in an experimentally shifted wind-field. Physiol. Entomol. 12, 263-279. 

Pheromones are powerful modulators of insect behavior. Since the isolation and identification of the first pheromone, (10E, 12Z)-hexadec-10,12-dien-l-ol, the sex attractant of the silk moth Bombyx mori, thousands of other insect pheromones have been identified. Our understanding of the sensory apparatus required for pheromone detection has also increased significantly. Coincidentally, B. mori was instrumental in many of these advances (see below). Volatile pheromones are detected by a specialized olfactory system localized on the antennae. The precise recognition of species-specific nuances in the structure and composition of pheromone components is essential for effective pheromone-based communication. The pheromone olfactory system of species studied so far exhibits remarkable selectivity towards the species-specific pheromone blend. Pheromones are emitted in low (fg-pg) quantities and are dispersed and greatly diluted in air plumes. Thus, pheromone olfaction systems are among the most sensitive chemosensory systems known. (Schneider et al., 1968). This chapter summarizes efforts (particularly over the past 10 years) to understand the molecular basis for the remarkable selectivity and sensitivity of the pheromone olfactory system in insects. The chapter will also outline efforts to design compounds that interfere with one or more of the early events in olfaction. 

If odor-evoked slow temporal patterns actually provide higher brain centers with information about the odor quality, identification and discrimination cannot be instantaneous as many of the temporal features in the response profiles appear late or even after offset of odor exposure. Honeybees need 500 ms for a response to (non-sexual pheromone) odors but at least 1 second of stimulation is required for a correct discrimination (J. Klein, unpublished, cited in Galizia el al., 2000a). Thus, it appears that time is an important factor in discrimination tasks involving non-pheromonal odors and the slow temporal patterns could theoretically contribute to an olfactory code. In contrast, these temporal patterns would be too slow to encode information about sexual pheromones. Male moths, for example, must be able to respond to rapid changes in stimulus intermittency when moving upwind in pheromone plumes in search of a calling female. 

In a third study, single unit recordings from the moth Heliothis virescens placed in a wind tunnel corroborated the previously described results by showing how well single neurons follow the fine-scale temporal characteristics of a natural odor plume (Vickers et al., 2001). Also in this study it was clear that the occurrence of a stimulus over time heavily influences the temporal structure of the response to a given stimulus. Both stimulus intensity and dynamics of the odor plume had an effect on the time course of the PN spike pattern. Furthermore, no wave-like periodicity in PN spiking could be observed and PN spike frequency did only rarely match the frequency range of local field potential oscillations that has been reported from moths (M. sexta Heinbockel et al., 1998). 

Artichoke Plume Moth Gypsy Moth Peachtree Borer Pink Bollworm Spruce Budworm Tobacco Budworm Western Pineshoot Borer... 

Similar experiments have been conducted in which insect damage or mating suppression have been demonstrated with other insect species. Table IV shows that 7.5 gram a.i./acre was highly effective in suppressing mating of the artichoke plume moth. 

Table IV. Percent artichoke plume moths mated in control and pheromone-treated plots.

Like many other insects, moths attract mates by long-distance pheromones. Females produce these pheromones in specialized abdominal glands. Chemically, they are acetates, often active in precise mixtures of geometric isomers. Males fly upwind, following the females pheromone plume to the somce, and mating ensues. In a typical experiment, a female moth, or just the pheromone, serves as odor source. An air current from that source helps to attract males who fly upwind to the pheromone source and attempt to mate. With this technique, we can compare the effects of known pheromones from different, related species on one species (species specificity). We can also test the attractiveness of different compounds that are stracturally similar to a known pheromone. In the laboratory, a wind tunnel, where available, is ideal, for this experience. 

Key words Pheromone detection, chemotaxis, computational neuroscience, moth, olfaction, odour plumes. 

Olfactory receptor neurons not only have to be selective to a spectrum of molecules, they must also report the quantity or concentration of molecules in the environment and also temporal properties. Receptor neurons respond to fluctuations in stimulus concentration with changes in action potential firing frequency. We also see very clear diversity in the temporal responses of ORNs responsible for pheromone detection in S. litoralis (J. Mackenzie et al, unpublished) which may suggest that the Ifont end of the pheromone detection system in the moth is tuned to different frequency information which is presumably rich in turbulent odour plumes (see Section 4). In solving the problem of chemieal source localisation the capture of temporal information has already been demonstrated to be important [15]. 

Therefore, what is relevant to the animal behaviourally and how it interacts with the environment determines its biological setup (Figure 2). The odour stimuli in which moths are primarily interested are plumes with fast temporal features and complex blends. This has implications for the form and function of the AL and its inputs. For instance, it has been suggested that there are ORNs specifically designed for detection of flux in pheromone components. 

Figure 7. a) Typical flight behaviour of a male moth tracking a pheromone plume released by a female moth, b) Trace of a typical pheromone search compared to the structure of the pheromone plume. (Image on top by Ishida Morizumi 2002 [40] bottom image by J.Hildebrand). 

It has been suggested that the key properties of chemical plumes that influence moth chemical search behaviour are [41] ... 

Hence, given the complex structure and dynamics of chemical plumes male moths cannot rely on gradient-based chemotaxis alone to find a mate but need to resort to a more advanced search behaviour. Indeed, the search behaviour of male moths is described to consist of at least two stereotypic behaviours (Figure 7, top) ... 

Surprisingly, when the male moth looses the plume, it usually finds it again closer to the source than before [44]. In addition, it always maintains an altitude close to that of the plume and usually its body is not totally aligned with the wind direction (Figure 8) [45]. When the male moth is getting closer to the source the casting frequency increases while its speed decreases. This culminates in a landing close to the pheromone source and a putative reward. 

Using both sensory systems, the male moth flies slowly upwind in the direction of the source when it finds a pheromone plume, performing a number of turns in a quite regular structure when the plume filaments are lost. 

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