Phytophagous insects show specialized feeding habits. In general, each species feeds on a restricted range of taxonomically related plant species and in addition limits its feeding to particular plant parts. Most studies on insect-plant rela­ tionships concern pest species in agricultural systems, which do not show the stability of the insect's original ecosystem. In monocultures host plants of high quality are present over large areas. Despite the biased nature of these in­ teractions, some generations of insects are forced to disperse and must find new host plants on which feeding results in growth and reproduction. (182). Host selection in phytophagous insects consists of a sequence of behavioral re­ sponses to an array of stimuli associated with host and nonhost plants. The insects are equipped with sensory receptors enabling them to perceive these stimuli. Plant stimuli involved include in varying proportions visual, mechani­ cal, gustatory, and olfactory characteristics (158). Plant odors have been considered of minor importance in host selection for many insect species, especially those that perform as r-strategists (93). This view needs to be reconsidered because such insects as aphids (21, 121, 122), whiteflies (173), and rice brown planthoppers (117), possess the full equipment of olfactory receptors and are attracted by host plant odors. In the present review selection was made mainly from the literature of the last 20 years. Floral relationships and social insects were excluded. Readers are invited to consult related reviews (49, 145, 158) as well as Dethier (27) for older Iiterature .

/pdf/host-odor-perception-in-phytophagous-insects-51veq4864u.pdf

Host odor perception in phytophagous insects

http://web.as.uky.edu/Biology/faculty/cooper/SENSORY%20Biology%20class/Evolution%20of%20Insect%20Olfaction.pdf

Evolution of insect olfaction.

Tree killing bark beetles and their vectored fungal pathogens are the most destructive agents of conifer forests worldwide. Conifers defend against attack by the constitutive and inducible production of oleoresin, a complex mixture of mono-, sesqui-, and diterpenoids that accumulates at the wound site to kill invaders and both flush and seal the injury. Although toxic to the bark beetle and fungal pathogen, oleoresin also plays a central role in the chemical ecology of these boring insects, from host selection to pheromone signaling and tritrophic level interactions. The biochemistry of oleoresin terpenoids is reviewed, and the regulation of production of this unusual plant secretion is described in the context of bark beetle infestation dynamics with respect to the function of the turpentine and rosin components. Recent advances in the molecular genetics of terpenoid biosynthesis provide evidence for the evolutionary origins of oleoresin and permit consideration of genetic engineering strategies to improve conifer defenses as a component of modern forest biotechnology.

Defensive resin biosynthesis in conifers

Publisher Summary This chapter reviews the existing data on numbers of chemoreceptors on the legs, mouthparts, and antennae of insects and describes the physiological characteristics of the sensilla. It discusses the possible significance of numbers of sensilla in relation to insect behavior and evolution. The picture that emerges is fragmentary, but the fragments should serve to indicate fruitful lines of research, which will lead to a better understanding of the physiological significance of chemoreceptor populations and the numerical differences that occur between them. It is apparent that observed behavior will often be the outcome of interactions between several, even many, chemosensory inputs. It is not, therefore, to be expected that the input from a single receptor will be directly related to behavior except in very simple situations. This may be true even where a receptor is tuned to a key stimulus. For example, bark beetles have very few olfactory sensilla compared with many moths and, as a consequence, they are probably less sensitive to their attractant pheromone. However, bark beetle pheromones are produced at host trees that have a characteristic odor. This is also attractant to the beetles and occurs in much greater concentrations than the pheromone so that attraction from a distance can occur without a very sensitive sensory system.

Chemoreception: The Significance of Receptor Numbers

When searching for suitable hosts in flight, especially in mixed forests, conifer- inhabiting bark beetles will encounter not only suitable host trees and their odours, but also unsuitable hosts and nonhost trees. Rejection of these trees could be based on an imbalance of certain host characteristics and/or a negative response to some nonhost stimuli, such as nonhost volatiles (NHV). 2 Recent electrophysiological and behavioural studies clearly indicate that conifer- inhabiting bark beetles are not only able to recognize, but also to avoid, nonhost habitats or trees by olfactory means. Green leaf volatiles (GLV), especially C6-alcohols, from the leaves (and partly from bark) of nonhost angiosperm trees, may represent nonhost odour signals at the habitat level. Specific bark volatiles such as trans-conophthorin, C8-alcohols, and some aromatic compounds, may indicate nonhosts at the tree species level. Flying bark beetles are also capable of determining whether a possible host is unsuitable by reacting to signals from conspecifics or sympatric heterospecifics that indicate old or colonized host tree individuals. 3 Combined NHV signals in blends showed both redundancy and synergism in their inhibitory effects. The coexistence of redundancy and synergism in nega- tive NHV signals may indicate different functional levels (nonhost habitats, species, and unsuitable hosts) in the host selection process. Combinations of NHV and verbenone significantly reduced the number of mass attacked host trees or logs on several economically important species (e.g. Dendroctonus ponderosae, Ips typographus, and I. sexdentatus). 4 We suggest a semiochemical-diversity hypothesis, based on the inhibition by NHV of bark beetle host-location, which might partly explain the lower outbreak rates of forest insects in mixed forests. This 'semiochemical-diversity hypothesis' would provide new support to the general 'stability-diversity hypothesis'. 5 Natural selection appears to have caused conifer-inhabiting bark beetles to evolve several olfactory mechanisms for finding their hosts and avoiding unsuit- able hosts and nonhost species. NHV and unsuitable host signals have potential for use in protecting trees from attack. The use of these signals may be facilitated by the fact that their combination has an active inhibition radius of several metres in trap test, and by the observation of area effects for several trees near inhibitor soruces in tree protection experiments. Furthermore, incorpora- tion of negative signals (such as NHV and verbenone) and pheromone-based

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1461-9555.2004.00202.x

Olfactory recognition and behavioural avoidance of angiosperm nonhost volatiles by conifer-inhabiting bark beetles

The stimulation effect of the two enantiomers of the male-produced aggregation pheromone, ipsdienol, was tested by electrophysiological recordings from single olfactory cells in females of two species,Ips pini andI. paraconfusus. The results demonstrated two types of receptor cells, each specialized to one of the optical configurations. This suggests that separate acceptors (membrane receptors) for (+)- and (−)-ipsdienol are produced by the beetles' receptor cells. The dose-response curves obtained for the 92% “pure” enantiomers and racemic mixtures indicated no synergistic or inhibitory interaction of the enantiomers on the receptor cells. The results could be explained by activation of one acceptor type in each cell group.

Receptor discrimination of enantiomers of the aggregation pheromone ipsdienol, in two species ofIps

Electrophysiological recordings inIps pini were made from single olfactory cells stimulated by six concentrations of eleven compounds—either pheromones or host constituents. The receptor cells were grouped according to their differential responses to these biologically significant compounds. Cells specialized to either ipsdienol or ipsenol (pheromones for severalIps species) appeared to be relatively uniform in their sensitivity to all compounds tested. Another group of cells specialized tocis- andtrans-verbenol were more variable in their sensitivities to the substances tested. The activation of separate groups of cells by the different pheromones and host compounds shows (by exclusion) that behaviorally expressed syner-gistic as well as inhibitory action mainly is due to an interaction in the central nervous system rather than an interaction of the compounds on the receptors. All “ipsdienol cells” responded to both enantiomers of ipsdienol. Although some differences between responses to the enantiomers by individual cells were observed, these did not form a consistent pattern, and no net differences could be found between the eastern and western populations ofI. pint.

Specialization of olfactory cells to insect-and host-produced volatiles in the bark beetleIps pini (say)

Responses of single olfactory cells on the antennal club ofIps pini have been recorded electrophysiologically. The majority of cells were strongly activated by either/or of the two behavior-modifying chemicals, the aggregation pheromone ipsdienol and the aggregation inhibitor ipsenol. Simultaneous stimulation with these terpence alcohols showed that ipsenol had no inhibitory effect on the receptor responses to ipsdienol. It appears that the behavioral inhibition by ipsenol on the attraction of ipsdienol results from central integration of the information from separate receptor cells rather than blockage at the peripheral receptors.

Responses of single receptor cells in the pine engraver beetle, Ips pini (SAY) (Coleoptera: Scolytidae) to its aggregation pheromone, ipsdienol, and the aggregation inhibitor, ipsenol

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Receptor discrimination of enantiomers of the aggregation pheromone ipsdienol, in two species ofIps

Specialization of olfactory cells to insect-and host-produced volatiles in the bark beetleIps pini (say)

Responses of single receptor cells in the pine engraver beetle, Ips pini (SAY) (Coleoptera: Scolytidae) to its aggregation pheromone, ipsdienol, and the aggregation inhibitor, ipsenol