Information Processing

Several types of information are processed by searching insects. Some cues are nondirectional but alert insects to the presence of resources or initiate search behavior. A nondirectional cue may alter the threshold for response to other cues (cross-channel potentiation) or initiate behaviors that provide more precise information (Bell 1990). For example, flying bark beetles usually initiate search for their host trees only after exhausting their fat reserves. Emerging adults of parasitic wasps gather information about their host from odors emanating from host frass or food plant material associated with the emergence site (Godfray 1994). Wasps emerging in the absence of these cues may be unable to identify potential hosts.

Directional information provides the stimulus to orient in the direction of the perceived resource. For example, detection of attractive chemicals without airflow initiates nondirectional local search, whereas addition of airflow stimulates orientation upwind (Bell 1990). Accuracy of orientation increases with signal intensity. Signal intensity decreases with distance and increases with density of the source (Elkinton et al. 1987, M. Stanton 1983). Concentration of attractive odors remains higher at greater distances from patches of high host density compared to patches of low host density (Fig. 3.12). Insects move upwind in circuitous fashion at low vapor concentration, but movement becomes increas-

Fashion Diffusion Curve

Distance downwind

| Odor concentration downwind from patches of two host densities: the low-density odor curve represents patches a and c, whereas the high-density curve represents patch b. The curves reflect an ideal situation in which diffusion is overshadowed by convection resulting from wind. In still air, odor concentration cannot be changed by altering host-plant density. Attractive areas shown as rectangles are actually irregular in shape. Attractive zones for low-sensitivity herbivores (threshold T1) are stippled; those for high-sensitivity herbivores (T2) are shaded. From M. Stanton (1983).

ingly directed as vapor concentration increases upwind (Cardé 1996). Insects integrate visual, chemical, and acoustic signals to find their resources, switching from less precise to more precise signals as these become available (Bell 1990).

2. Responses to Cues Visual cues include host silhouettes and radiant energy. Some species find arboreal resources by orienting toward light. Aphids are attracted to young, succulent foliage and to older senescent foliage by longer-wavelength yellow, but this cue is not a good indicator of host species (Dixon 1985). Aphids, Pemphigus betae, migrating in autumn may discriminate among susceptible and resistant poplar trees on the basis of prolonged leaf retention by more susceptible hosts (N. Moran and Whitham 1990). Many bark beetles are attracted to dark-colored silhouettes of tree boles and can be attracted to other cylindrical objects or prevented from landing on tree boles painted white (Goyer et al. 2004, Strom et al. 1999). Some parasitic wasps detect their wood-boring hosts by means of infrared receptors on their antennae (Matthews and Matthews 1978).

The importance of flower color and color patterns to attraction of pollinators has been a topic of considerable research (Chittka and Menzel 1992, Heinrich 1979, Wickler 1968). Reds and blues are more easily detected in open or well-lit ecosystems; hence they are more common in tropical and grassland ecosystems, white is more readily detected under low-light conditions, such as in the under-stories of forests. Ultraviolet designs, detectable by insects, provide important cues to insect pollinators. Insects can detect ultraviolet "runways" or "nectar guides" that guide the insect to the nectaries (Eisner et al. 1969, Heinrich 1979, Matthews and Matthews 1978). Some floral designs in the orchid genus Ophrys resemble female bees or wasps and produce odors similar to the mating pheromones of these insects. Male bees or wasps are attracted and unwittingly pollinate these flowers while attempting to copulate (Wickler 1968).

Nonhosts that are visually similar to hosts can interfere with host discovery by insects that rely on visual cues. For example, Hamback et al. (2003) reported that leaf-feeding beetles, Galerucella spp., were significantly less abundant on purple loosestrife, Lythrum salicaria, that were surrounded by nonhost or artificial shrubs than on hosts that were not surrounded by nonhost or artificial shrubs.

Many plant chemicals are highly volatile and are the basis for floral and other plant odors. Prominent among these are the monoterpenes, such as verbenone in verbena flowers and alpha-pinene in conifers, and aromatic compounds, such as vanillin (Harborne 1994). These represent attractive signals to pollinators and to herbivores adapted to feed on a particular plant. Some odors repel some insects. For example, verbenone and 4-allylanisole, in the resin of various trees, repel some bark beetle species (Hayes et al. 1994). Verbenone is present in the bark of certain conifers (including western redcedar, Thuja plicata, and Pacific silver fir, Abies amabilis) and likely influences orientation by bark beetles among tree species in diverse forests (Schowalter et al. 1992). Nonattractive or repellent odors from nonresources can mix with attractive odors in the airstream of more diverse ecosystems and disrupt orientation (Fig. 3.13). For example, volatile

Dendroctonus Rufipennis

Elements of host odor perception in insects. From Visser (1986) with permission from the Annual Review of Entomology, Vol. 31, © 1986 by Annual Reviews. Please see extended permission list pg 569.

Elements of host odor perception in insects. From Visser (1986) with permission from the Annual Review of Entomology, Vol. 31, © 1986 by Annual Reviews. Please see extended permission list pg 569.

chemicals from nonhost angiosperms disrupted attraction of spruce beetles, Dendroctonus rufipennis, and western pine beetle, D. brevicomis, to host conifer odors (Poland et al. 1998). However, not all insects attracted to host odors suffer disruption by nonhost odors (Hamback et al. 2003).

Predators often are attracted to prey pheromones or odors from damaged plants that indicate the presence of prey (Kessler and Baldwin 2001, Stephen et al. 1993,Turlings et al. 1990,1993,1995). Pheromones are known from more than 1000 insect species (Mustaparta 2002).

Attractiveness of volatile biochemicals to insects is species specific (Mustaparta 1984). A chemical that is attractive to one species may be unattractive or even repellent to other, even related, species. For example, among sym-patric Ips species in California, I. pini and I. paraconfusus both are attracted to ipsdienol, but I. paraconfusus also incorporates ipsenol and cis-verbenol in its pheromone blend, whereas I. latidens is attracted to ipsenol and cis-verbenol but not in the presence of ipsdienol (Raffa et al. 1993). Plants that depend on dipteran pollinators often produce odors that resemble those of carrion or feces to attract these insects. Peakall and Beattie (1996) and Peakall et al. (1987) described pollination of Australian orchids by male ants and wasps during pseudocopulation, suggesting that the chemical stimulus is similar to the mating pheromone produced by potential mates. Sex pheromones (see Chapter 4) often are more attractive when mixed with host volatiles (e.g., Raffa et al. 1993), indicating prior discovery and evaluation of suitable hosts.

Studies have shown that detection of relevant odors is genetically encoded but response can be modified through learning (see the following section). Insects have a relatively simple nervous system, composed of receptor neurons that detect chemical signals, interneurons that integrate and convey information, and motor neurons that elicit the behavioral response. Olfactory receptor neurons are located in various sensilla, primarily on the antennae. Volatile chemicals apparently diffuse through the cuticle and bind to receptor proteins that are highly selective for biologically relevant molecules (Mustaparta 2002).These proteins transport the odor molecule to a neuronal membrane that contains receptor proteins genetically coded for specific molecules; each receptor neuron expresses proteins specific to certain odor molecules. Therefore, the discrimination power of an organism depends on the number of different neuron types (Mustaparta 2002).

Once a chemical attractant is detected in the air or water current, the insect begins a circuitous search pattern that involves continually turning in the direction of increasing odor concentration (Cardé 1996). However, insects following the odor trail, or plume, are far from assured of reaching the source. Odor plumes often are disrupted by turbulence, resulting from habitat heterogeneity (e.g., substrate or canopy irregularities) (Mafra-Neto and Cardé 1995, Murlis et al. 1992). For example, Fares et al. (1980) found that openings in forest canopies created sites of soil warming and convective eddies that dissipated chemical plumes. Elkinton et al. (1987) found that male gypsy moth response to a caged female declined from 89% at 20 m distance to 65% at 120 m. Of those that responded, arrival at the female's cage declined from 45% at 20 m to 8% at 120 m (see Chapter 4).

If an insect successfully arrives at the source of attractive cues, it engages in close-range gustatory, olfactory, or sound reception (Dixon 1985, Raffa et al. 1993, Städler 1984). Contact chemoreceptive sensilla generally have a single pore at the tip, with unbranched dendrites from 2-10 receptor cells (R. Chapman 2003, E. Städler 1984) and are located on antennae, mouthparts, or feet (Dixon 1985, E. Städler 1984). These sensors provide information about nutritive value and defensive chemistry of the resource (R. Chapman 2003, Raffa et al. 1993). Certain plant chemicals act as phagostimulants or as deterrents (R. Chapman 2003). For example, cucurbitacins (the bitter triterpenes common to Cucurbitaceae) deter feeding and oviposition by nonadapted mandibulate insects but are phagostimulants for diabroticine chrysomelid beetles (Tallamy and Halaweish 1993). Predators also may avoid prey containing toxic or deterrent chemicals (Stamp et al. 1997, Stephens and Krebs 1986). Many parasitic wasps avoid hosts marked by wasps that oviposited previously in that host (Godfray 1994). Because hosts support only a limited number of parasitoid offspring, often no more than one, avoidance of previously parasitized hosts reduces competition among larvae within a host.

Acoustic signals include the sounds produced by cavitating plant cells and by potential mates.Water-stressed plants often produce audible signals from the collapse of cell walls as turgor pressure falls (Mattson and Haack 1987). Cavitation thus is a valuable cue to stressed, and potentially more suitable, plants. Attraction to this signal may partly explain the association of bark beetles with water-stressed trees (Mattson and Haack 1987, Raffa et al. 1993).

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