Peter Roessingh

Foliar form, colour and surface characteristics influence oviposition behaviour in the cabbage root fly Delia radicum

The effects of foliar form, colour and surface characteristics on the oviposition behaviour of the cabbage fly were investigated by presenting females with various leaf models made of paper. The models differed in shape, size, colour, surface coating, the presence of 3 dimensional folds and a stem. It was shown that in the presence of host leaf surface extracts physical factors can strongly influence oviposition behaviour. Females laid the most eggs around the base of bright green or yellow models having a stem, vertical folds and covered with a thin layer of paraffin. The vertical folds in the surface increased the chance of a behavioural transition from leaf run to stem run.

Tarsal chemoreceptors and oviposition behaviour of the cabbage root fly (Delia radicum) sensitive to fractions and new compounds of host-leaf surface extracts

Abstract. Contact chemoreception plays a decisive role in host selection and oviposition behaviour of the cabbage root fly, Delia radicum L. (Diptera, Anthomyiidae). Glucosinolates (mustard oil glucosides) are known to be perceived by the flies, and when sprayed on paper leaf‐models induce oviposition. Recently it has become clear that other non‐volatile types of compounds must also be involved in host selection.

Tarsal contact chemoreceptors of the black swallowtail butterfly Papilio polyxenes: responses to phytochemicals from host- and non-host plants

Abstract Tarsal contact chemoreceptors of the black swallowtail butterfly Papilio polyxenes (Lepidoptera: Papilionidae) were stimulated with leaf‐surface extracts and ethanolic extracts of whole leaves of a host‐plant (Daucus carota) and a non‐host (Brassica oleracea). Both leaf extracts evoked large numbers of spikes but stimulated different receptor neurones. It is concluded that a large difference exists between the sensory responses to host and non‐host extracts. Two chemicals, luteolin 7‐0‐(6“‐0‐malonyl)‐β‐D‐glucopyranoside and trans‐chlorogenic acid, known to be present in the host and known oviposi‐tion stimulants for P.polyxenes, were also tested and shown to be active. No responses were found to luteolin 7‐O‐β‐D‐glucoside or to luteolin 7‐O‐β‐D‐glucuronide. These flavonoids occur in D.carota foliage, but do not stimulate oviposition.

Chemoreception of oviposition inhibiting terpenoids in the diamondback moth Plutella xylostella

The effects of six terpenoids and two terpenoid containing extracts of the neem tree (Azadirachta indica A. Juss.) on oviposition by the diamondback moth (Plutella xylostella L., Yponomeutidae: Lepidoptera) were tested. Two drimane terpenoids, the sesquiterpenoid polygodial and the neem extract Margosan‐O exerted significant inhibitory effects at the dosages tested. Ablation experiments showed that both antennae and fore‐tarsi contributed to mediation of the inhibition by a drimane. Location of chemosensilla on prothoracic tarsi and ovipositor was examined by scanning electronmicroscopy. Electrophysiological recordings from ovipositor and tarsal taste sensilla showed that distilled water produced distinct responses from one neuron. In tarsal sensilla, ethanol and drimane solutions produced responses from two neurons, one of which might be the water cell that fired at a reduced rate. A drimane significantly decreased the responses of tarsal chemoreceptors to a cabbage leaf extract, which is a possible sensory mechanism leading to behavioural avoidance of this compound.

Olfactory receptors on the maxillary palps of small ermine moth larvae: Evolutionary history of benzaldehyde sensitivity

In lepidopterous larvae the maxillary palps contain a large portion of the sensory equipment of the insect. Yet, knowledge about the sensitivity of these cells is limited. In this paper a morphological, behavioral, and electrophysiological investigation of the maxillary palps of Yponomeuta cagnagellus (Lepidoptera: Yponomeutidae) is presented. In addition to thermoreceptors, CO2 receptors, and gustatory receptors, evidence is reported for the existence of two groups of receptor cells sensitive to plant volatiles. Cells that are mainly sensitive to (E)-2-hexenal and hexanal or to (Z)-3-hexen-1-ol and 1-hexanol were found. Interestingly, a high sensitivity for benzaldehyde was also found. This compound is not known to be present in Euonymus europaeus, the host plant of the monophagous Yponomeuta cagnagellus, but it is a prominent compound in Rosaceae, the presumed hosts of the ancestors of Y. cagnagellus. To elucidate the evolutionary history of this sensitivity, and its possible role in host shifts, feeding responses of three Yponomeuta species to benzaldehyde were investigated. The results confirm the hypothesis that the sensitivity to benzaldehyde evolved during the ancestral shift from Celastraceae to Rosaceae and can be considered an evolutionary relict, retained in the recently backshifted Celastraceae-specialist Y. cagnagellus.

The trail following behaviour of Yponomeuta cagnagellus

In this study the trail following behaviour of the caterpillar Yponomeuta cagnagellus (Hübner) (Lepidoptera, Yponomeutidae) is investigated. It is demonstrated that these caterpillars follow trails made by conspecifics. Ablation experiments show that at least part of this behaviour is directed by the tactile senses, but additional chemical cues cannot be excluded. In choice experiments using trails from different species, Y. cagnagellus strongly preferred conspecific trails over those from Malacosoma neustria, but did not prefer conspecific over other Yponomeuta trails. This lack of species‐specificity within Yponomeuta is discussed and it is concluded that trail following is probably of little help in the elucidation of the evolutionary history of the genus.

Ultrastructure of a tarsal sensillum of Delia radicum L. (Diptera : Anthomyiidae) sensitive to important host-plant compounds

Abstract The ultrastructure of a pair of tarsal “C” sensilla located on the ventromedial side near the distal margin of the 5th tarsomere of the female cabbage root fly, Delia radicum (L.) (Diptera : Anthomyiidae) was investigated by electron microscopy and electrophysiological recordings. This “C” sensillum is a typical gustatory sensillum, consisting of a uniporous hair-shaft inserted in a specialized socket and innervated by 5 sensory neurons (i.e. one mechanosensitive and 4 chemosensitive). One of the chemoreceptor cells is sensitive to host-plant compounds, stimulating oviposition. Non-host-plant (carrot) leaf extracts and sucrose did not stimulate any of the receptor cells. Direct contacts between sensory cell somata were observed, and the possibility of peripheral neural interaction at the sensillum level is discussed.

Pollination of Specklinia by nectar-feeding Drosophila: the first reported case of a deceptive syndrome employing aggregation pheromones in Orchidaceae.

BACKGROUND AND AIMSnThe first documented observation of pollination in Pleurothallidinae was that of Endrés, who noticed that the viscid sepals of Specklinia endotrachys were visited by a small fly. Chase would later identify the visiting flies as being members of the genus Drosophila. This study documents and describes how species of the S. endotrachys complex are pollinated by different Drosophila species.nnnMETHODSnSpecimens of Specklinia and Drosophila were collected in the field in Costa Rica and preserved in the JBL and L herbaria. Flies were photographed, filmed and observed for several days during a 2-year period and were identified by a combination of non-invasive DNA barcoding and anatomical surveys. Tissue samples of the sepals, petals and labellum of Specklinia species were observed and documented by SEM, LM and TEM. Electroantennogram experiments were carried out on Drosophila hydei using the known aggregation pheromones ethyl tiglate, methyl tiglate and isopropyl tiglate. Floral compounds were analysed by gas chromatography-mass spectometry using those same pheromones as standards.nnnKEY RESULTSnFlowers of S. endotrachys, S. pfavii, S. remotiflora and S. spectabilis are visited and pollinated by several different but closely related Drosophila species. The flies are arrested by aggregation pheromones, including ethyl tiglate, methyl tiglate and isopropyl tiglate, released by the flowers, and to which at least D. hydei is very sensitive. Visible nectar drops on the adaxial surface of sepals are secreted by nectar-secreting stomata, encouraging male and female Drosophila to linger on the flowers for several hours at a time. The flies frequently show courtship behaviour, occasionally copulating. Several different Drosophila species can be found on a single Specklinia species.nnnCONCLUSIONSnSpecies of the S. endotrachys group share a similar pollination syndrome. There seem to be no species-specific relationships between the orchids and the flies. It is not expected that Specklinia species will hybridize naturally as their populations do not overlap geographically. The combination of pheromone attraction and nectar feeding is likely to be a generalized pollination syndrome in Pleurothallidinae.

Evolution of gustatory sensitivity in Yponomeuta caterpillars: sensitivity to the stereo-isomers dulcitol and sorbitol is localised in a single sensory cell

Abstract The cellular localisation of the sensitivity to two host-plant specific larval phagostimulants was determined in Yponomeuta evonymellus and in inter-specific hybrids of Y. cagnagellus and Y. padellus. The combined results of cross-adaptation experiments, analysis of mixture responses and inter-spike interval distributions indicated that the sensitivity for sorbitol and dulcitol is localised in the same cell in the investigated species. This result suggests that during the evolution of the genus the main alteration in the mechanism of sugar-alcohol detection occurred at the level of the receptor proteins. Relatively simple modification of this kind can, however, have large effects, since larvae with modified receptor proteins will directly respond to the new stimulus while all central processing and resulting behaviour can remain unmodified.

Sympatric speciation in Yponomeuta: no evidence for host plant fidelity

According to sympatric speciation theory, adaptation to different host plants is expected to pleiotropically lead to assortative mating, an important factor in the reduction of gene flow between the diverging subpopulations. This scenario predicts mating on and oviposition preference for the respective hosts in both the diverging subpopulations and recently originated species. Here, we test both predictions in the oligophagous Yponomeuta padellus (L.) and the monophagous Yponomeuta cagnagellus (Hübner) (Lepidoptera: Yponomeutidae), two closely related small ermine moth species from the western European clade of Yponomeuta for which speciation in sympatry has been proposed. Mating location and adult host acceptance were evaluated under both semi‐field (in a large outdoor cage with a choice of host and non‐host plants) and field conditions. In the semi‐field experiment, only Y. cagnagellus showed some preference for mating on its own host (16% of all mating pairs) over non‐host plants (3% of all mating pairs). However, in both species, more than 80% of the mating pairs were not formed on a plant but instead on the cage itself. Further examination of the mating site of Y. cagnagellus in the field revealed no preference for host plants over non‐host plants in the two consecutive years of observation. Yponomeuta padellus females, collected from and reared on Prunus spinosa L. (Rosaceae), showed an oviposition preference for the alternative host Crataegus monogyna Jacq. (Rosaceae) in the semi‐field experiment. We thus found no evidence that host‐plant fidelity (in terms of mating site) has been the driving force in the speciation process of these Yponomeuta species, nor did we find evidence of host race formation in the tested population of the oligophagous Y. padellus.