Attraction

Chemical, visual, and acoustic signaling are used to attract potential mates. Attraction of mates can be accomplished by either sex in Coleoptera, but only females of Lepidoptera release sex pheromones and only males of Orthoptera stridulate.

Sex pheromones greatly improve the efficiency with which insects find potential mates over long distances in heterogeneous environments (Cardé 1996, Law and Regnier 1971, Mustaparta 1984). The particular blend of compounds and their enantiomers, as well as the time of calling, varies considerably among species. These mechanisms represent the first step in maintaining reproductive isolation. For example, among tortricids in eastern North America, Archips mor-tuanus uses a 90:10 blend of (Z)-11- and (E)-11-tetradecenyl acetate, A. argy-rospilus uses a 60:40 blend, and A. cervasivoranus uses a 30:70 blend. A related species, Argyrotaenia velutinana also uses a 90:10 blend but is repelled by (Z)-9-tetradecenyl acetate that is incorporated by A. mortuanus (Cardé and Baker 1984). Among three species of saturniids in South Carolina, Callosamia promethea is active from about 10:00-16:00, C. securifera from about 16:00-19:00, and C. angulifera from 19:00-24:00 (Cardé and Baker 1984). Bark beetle pheromones also have been studied extensively (e.g., Raffa et al. 1993). Representative bark beetle pheromones are shown in Fig. 4.2.

Sex pheromones may be released passively, as in the feces of bark beetles (Raffa et al. 1993), or actively through extrusion of scent glands and active "calling" (Cardé and Baker 1984). The attracted sex locates the signaler by following the concentration gradient (Fig. 4.3). Early studies suggested that the odor from a point source diffuses in a cone-shaped plume that expands downwind, the shape of the plume depending on airspeed and vegetation structure (e.g., Matthews and Matthews 1978). However, more recent work (Cardé 1996, Mafra-Neto and Cardé 1995, Murlis et al. 1992, Roelofs 1995) indicates that this plume is neither straight nor homogeneous over long distances but is influenced by turbulence in the airstream that forms pockets of higher concentration or absence of the vapor (Fig. 4.4). An insect downwind would detect the plume as odor bursts rather than as a constant stream. Heterogeneity in vapor concentration is augmented by pulsed emission by many insects.

CHALCOGRAN (£,ZM2,4)-METHYL DECADIENOATE

Representative pheromones produced by bark beetles. Pheromones directly converted from plant compounds include ipsdienol (from myrcene), trans-verbenol, and verbenone (from a-pinene).The other pheromones shown are presumed to be synthesized by the beetles. From Raffa et al. (1993).

| Typical responses of male noctuid moths to the sex pheromone released by female moths. From Tumlinson and Teal (1987).

| Models of pheromone diffusion from a point source. The time-averaged Gaussian plume model (a) depicts symmetrical expansion of a plume from the point of emission. The meandering plume model (b) depicts concentration in each disc distributed normally around a meandering center line. The most recent work has demonstrated that pheromone plumes have a highly filamentous structure (c). From Murlis et al. (1992) with permission from the Annual Review of Entomology, Vol. 37, © 1992 by Annual Reviews.

| Models of pheromone diffusion from a point source. The time-averaged Gaussian plume model (a) depicts symmetrical expansion of a plume from the point of emission. The meandering plume model (b) depicts concentration in each disc distributed normally around a meandering center line. The most recent work has demonstrated that pheromone plumes have a highly filamentous structure (c). From Murlis et al. (1992) with permission from the Annual Review of Entomology, Vol. 37, © 1992 by Annual Reviews.

Pulses in emission and reception may facilitate orientation because the anten-nal receptors require intermittent stimulation to avoid saturation and sustain upwind flight (Roelofs 1995). However, Cardé (1996) noted that the heterogeneous nature of the pheromone plume may make direct upwind orientation difficult over long distances. Pockets of little or no odor may cause the attracted insect to lose the odor trail. Detection can be inhibited further by openings in the vegetation canopy that create warmer convection zones or "chimneys" that carry the pheromone through the canopy (Fares et al. 1980). Attracted insects may increase their chances of finding the plume again by casting (i.e., sweeping back and forth in an arcing pattern until the plume is contacted again) (Cardé 1996). Given the small size of most insects and limited quantities of pheromones for release, mates must be able to respond to very low concentrations. Release of less than 1 ug sec-1 by female gypsy moth, Lymantria dispar, or silkworm, Bombyx mori, can attract males, which respond at molecular concentrations as low as 100 molecules ml-1 of air (Harborne 1994). Nevertheless, the likelihood of attracted insects reaching a mate is small. Elkinton et al. (1987) reported that the proportion of male gypsy moths responding to a caged female declined from 89% at 20 m distance to 65% at 120 m. Of those males that responded, the proportion arriving at the female's cage declined from 45% at 20 m to 8% at 120 m, and the average minimum time to reach the female increased from 1.7 min at 20 m to 8.9 min at 120 m (Fig. 4.5). Therefore, the probability of successful attraction of mates is low, and exposure to predators or other mortality factors is relatively high, over modest distances.

Visual signaling is exemplified by the fireflies (Coleoptera: Lampyridae) (e.g., Lloyd 1983). In this group of insects, different species distinguish each other by variation in the rhythm of flashing and by the perceived "shape" of flashes produced by distinctive movements while flashing. Other insects, including glowworms (Coleoptera: Phengodidae) and several midges, also attract mates by producing luminescent signals.

Acoustic signaling is produced by stridulation, particularly in the Orthoptera, Heteroptera, and Coleoptera, or by muscular vibration of a membrane, common in the Homoptera. Resulting sounds can be quite loud and detectable over con-

ab bc bcd

6 time

60 80 Distance from source (m)

Effect of distance on insect perception of and arrival at a pheromone source. Proportion (mean ± SD) of male gypsy moths responding at 20,40,80, and 120 m from a pheromone source (black bar), mean proportion of those responding that reached the source within a 40-min period (gray bar), and the average minimum time to reach the source (white bar); n = 23. Values followed by the same letter do not differ significantly at P <0.05. Data from Elkinton et al. (1987).

siderable distances. For example, the acoustic signals of mole crickets, Gryllotalpa vinae, amplified by the double horn configuration of the cricket's burrow, are detectable by humans up to 600 m away (Matthews and Matthews 1978).

During stridulation, one body part, the file (consisting of a series of teeth or pegs), is rubbed over an opposing body part, the scraper. Generally, these structures occur on the wings and legs (R. Chapman 1982), but in some Hymenoptera sound also is produced by the friction between abdominal segments as the abdomen is extended and retracted. The frictional sound produced can be modulated by various types of resonating systems. Frequency and pattern of sound pulses are species specific.

Sound produced by vibrating membranes (tymbals) is accomplished by contracting the tymbal muscle to produce one sound pulse and relaxing the muscle to produce another sound pulse. Muscle contraction is so rapid (170-480 contractions per second) that the sound appears to be continuous (Matthews and Matthews 1978). The intensity of the sound is modified by air sacs operated like a bellows and by opening and closing opercula that cover the sound organs (R. Chapman 1982).

Such mechanisms greatly increase the probability of attracting mates. However, many predators also are attracted to, or imitate, signaling prey. For example, some firefly species imitate the flash pattern of prey species (Lloyd 1983).

Oplan Termites

Oplan Termites

You Might Start Missing Your Termites After Kickin'em Out. After All, They Have Been Your Roommates For Quite A While. Enraged With How The Termites Have Eaten Up Your Antique Furniture? Can't Wait To Have Them Exterminated Completely From The Face Of The Earth? Fret Not. We Will Tell You How To Get Rid Of Them From Your House At Least. If Not From The Face The Earth.

Get My Free Ebook


Post a comment