Possible host location mechanisms in the chalcid wasp Rhopalicus tutela (Walker) (Hym., Pteromalidae), a parasitoid of the eight‐spined spruce bark beetle, Ips typographus (L.) (Col., Scolytidae), were examined. This was carried out in order to repeat and complement former studies on parallel parasitoid–scolytid systems that had contradictory results. Morphological examinations of the parasitoid antennae were made using both scanning and transmission electron microscopy. Possible functions of the sensilla placodea (multiporous plate sensillum), and other sensilla present on the antennae, have been indicated. For the first time, the placoid sensilla in a pteromalid parasitoid have shown porous walls and numerous innervations, which are typical characteristics for chemoreceptors. Previously the placoid sensilla have been suggested to be an infrared receptor. In order to test the chemoreceptive ability of R. tutela females and males, a synthetic reference blend was analysed by combined gas chromatography and electroantennographic detector (GC‐EAD). Their sensitivity to host‐related volatiles (such as certain pheromone components and oxygenated monoterpenes) was significantly greater than that for host‐tree‐related compounds (monoterpene hydrocarbons). Employing an infrared thermo‐scanner, the current study failed to detect ‘hot spots’ associated with susceptible hosts beneath the bark. Results from electrophysiology and electron microscopy revealed clear odour‐perceptive functions of the parasitoid antennae. These results strongly support the major importance of volatiles in host location by the bark beetle parasitoid R. tutela.

Plant volatiles mediate host finding in insect herbivores and lead to host fidelity and habitat‐specific mating, generating premating reproductive isolation and facilitating sympatric divergence. The apple fruit moth, Argyresthia conjugella Zeller (Lepidoptera: Argyresthiidae), is a particularly suitable species to study the cues and behavioural mechanisms leading to colonization of a new host: it recurrently oviposits on the non‐host plant, apple Malus domestica Borkh. (Rosaceae), where the larvae cannot complete their development. The larval host of the apple fruit moth (Lepidoptera, Argyresthiidae), is rowan Sorbus aucuparia L. (Rosaceae). Fruit setting in rowan, however, fluctuates strongly over large areas in Scandinavia. Every 2–4 years, when too few rowanberries are available for egg laying in forests, apple fruit moth females oviposit instead on apple in nearby orchards, but not on other fruits, such as pear or plum. This poses the question of which cues mediate attraction to rowan and apple, and how apple fruit moth discriminates rowan from apple. Chemical analysis and antennal recordings showed that 11 out of 15 rowan volatiles eliciting an antennal response in A. conjugella females co‐occur in rowan and apple headspace, in a different proportion. In the field, A. conjugella was attracted to several of these plant volatiles, especially to 2‐phenyl ethanol, methyl salicylate, and decanal. Addition of anethole to 2‐phenyl ethanol had a strong synergistic effect, the 1 : 1 blend is a powerful attractant for A. conjugella males and females. These results confirm that volatiles common to both plants may account for a host switch in A. conjugella from rowan to apple. Some of the most attractive compounds, including 2‐phenyl ethanol, anethole, and decanal, which have been found in several apple cultivars, were not present in the headspace of the apple cultivar, Aroma, which is also susceptible to attack by A. conjugella. This supports the idea that the odour signal from apple is suboptimal for attraction of A. conjugella, but is nonetheless sufficient for attraction, during times when rowan is not available for egg laying.

Chrysomeline larvae respond to disturbance and attack by everting dorsal glandular reservoirs, which release defensive secretions. The ancestral defense is based on the de novo synthesis of monoterpene iridoids. The catabolization of the host-plant O-glucoside salicin into salicylaldehyde is a character state that evolved later in two distinct lineages, which specialized on Salicaceae. By using two species producing monoterpenes (Hydrothassa marginella and Phratora laticollis) and two sequestering species (Chrysomela populi and Phratora vitellinae), we studied the molecular basis of sequestration by feeding the larvae structurally different thioglucosides resembling natural O-glucosides. Their accumulation in the defensive systems demonstrated that the larvae possess transport systems, which are evolutionarily adapted to the glycosides of their host plants. Minor structural modifications in the aglycon result in drastically reduced transport rates of the test compounds. Moreover, the ancestral iridoid-producing leaf beetles already possess a fully functional import system for an early precursor of the iridoid defenses. Our data confirm an evolutionary scenario in which, after a host-plant change, the transport system of the leaf beetles may play a pivotal role in the adaptation on new hosts by selecting plant-derived glucosides that can be channeled to the defensive system.

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