Robert Baur

Identification of glucosinolates on the leaf surface of plants from the Cruciferae and other closely related species.

Leaf-surface extracts prepared from 18 non-cultivated (wild) plant species, derived from the Capparidaceae, Cruciferae, Resedaceae and Tropaeolaceae were ranked for their ability to stimulate oviposition by the cabbage root fly, and analysed for glucosinolates. A total of 28 different glucosinolates were identified. A clear relationship was detected between the indolyl-, benzyl- and the total glucosinolate composition on the leaf surface and oviposition preference by cabbage root fly females. However, as the results are not fully explained by differences in leaf surface glucosinolates, other important oviposition deterrents and stimuli on the leaf surface of these wild crucifers must also be present.

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.

Oviposition stimulants for the cabbage root fly: isolation from cabbage leaves

Abstract Two compounds present on the surface of Brassica oleracea cv. botrytis leaves have been isolated and identified which stimulate very effectively oviposition in the cabbage root fly, Delia radicum and which are perceived by a specific receptor neuron in the tarsal sensillum C5 of the female fly. Activity of extracts and chromatographic fractions were bioassayed, using oviposition experiments and mainly electrophysiological recordings from the C5 tarsal contact chemoreceptor sensillum of female flies. Spectroscopic data indicate that the main compound is 1,2-dihydro-3-thia-4,10,10b-triaza-cyclopenta[.a.]fluorene-1-carboxylic acid, a novel compound related to Brassica phytoalexins like brassicanal C. It is accompanied by its glycine conjugate.

Oviposition stimulants for the black swallowtail butterfly: Identification of electrophysiologically active compounds in carrot volatiles.

Headspace volatiles were collected from undamaged foliage of carrot,Daucus carota, a host-plant species of the black swallowtail butterfly,Papilio polyxenes. The volatiles were fractionated over silica on an open column, and the fractions were tested in behavioral assays withP. polyxenes females in laboratory experiments. The polar fractions, as well as the total mixture of volatiles, increased the landing frequency and the number of eggs laid on model plants with leaves bearing contact-oviposition stimulants. The nonpolar fraction, containing the most abundant compounds in carrot odor, was not stimulatory. Gas Chromatographic (GC) separation of the fractions was coupled with electroantennogram (EAG) recordings to identify the compounds perceived byP. polyxenes females. The EAG activity corresponded to the behavioral activity of the fractions. None of the nonpolar compounds, identified as various monoterpenes, evoked a major EAG response, but several constituents of the polar fractions elicited high EAG responses. Sabinene hydrate (both stereoisomers), 4-terpineol, bomyl acetate, and (Z)-3-hexenyl acetate were identified by GC-MS as active compounds.

Phytoalexins from Brassica (Cruciferae) as oviposition stimulants for the cabbage root fly, Delia radicum

Summary. Bacteria species known to induce the biosynthesis of crucifer-specific phytoalexins have earlier been shown to be associated with Delia flies. Eleven crucifer-specific phytoalexins and related synthetic compounds were applied on surrogate paper leaves and offered to cabbage root flies in oviposition assays. Since three of these compounds (methoxybrassinin, cyclobrassinin, brassitin) proved to be significantly stimulatory whereas the remaining metabolites had no effect, the reaction of the fly appears to be structure-specific. Inactive phytoalexins had no inhibitory effect on oviposition. 100 μg of methoxybrassinin per surrogate leaf was as stimulatory as 0.05 gle (gram leaf equivalent) of a methanolic host-leaf (Brassica oleracea) extract. Thus the three active phytoalexins can explain only part of the activity of host-pant extracts but might induce a preference for infected plants.

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual‐choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1‐17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.

Root damage by conspecific larvae induces preference for oviposition in cabbage root flies

In nature, herbivorous insects probably rarely encounter host plants that are free of other herbivores. These herbivores may have altered the suitability of a plant for the development of further herbivores, e.g. by induction of chemical, physical, or phenological responses of the plant (Karban & Myers, 1989). Sensory and behavioural mechanisms to detect and avoid intra- or interspecific competitors on a potential host plant are known for several insects. For instance, Pieris brassicae, P. rapae and Danaus plexippus all prefer plants for oviposition which are free of conspecific eggs or larvae (Rothschild & Schoonhoven, 1977). Similarly, oviposition by Trichoplusia ni on cabbage leaves is deterred by the presence of feeding conspecific larvae or their frass (Renwick & Radke, 1980). For Delia radicum (Dipt., Anthomyiidae), oviposition is deterred by the presence of frass from the caterpillars of Evergestis forficalis on host leaves (Jones & Finch, 1987).

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.

Optimisation of the pheromone blend of the swede midge, Contarinia nasturtii, for monitoring

BACKGROUND The swede midge, Contarinia nasturtii Kieffer, is a serious pest in crucifers. Its pheromone is a blend of (2S,9S)-diacetoxyundecane, (2S,10S)-diacetoxyundecane and (2S)-acetoxyundecane. The pheromone is used in monitoring traps, and this study examines possible ways to optimise the traps. RESULTS Two dispenser types were compared: polyethylene dispensers and cotton dispensers. Polyethylene dispensers attracted male C. nasturtii for more than 6 weeks, whereas cotton dispensers were attractive for only 2 weeks. All three pheromone components were important for attraction of male midges in the field. The importance of the stereoisomeric compositions of the pheromone compounds was also tested-both in the wind tunnel and in the field. In the case of 2,9-diacetoxyundecane and 2-acetoxyundecane, the non-natural stereoisomers did not inhibit male C. nasturtii attraction, whereas one or both of the stereoisomers of 2,10-diacetoxyundecane did. CONCLUSION Pheromone traps with the synthetic pheromone in a 1:2:0.02 ratio emitted from PE dispensers were highly effective and long lasting. As the mixture of stereoisomers of 2,10-diacetoxyundecane strongly inhibited attraction of male C. nasturtii while those of 2,9-diacetoxyundecane and 2-acetoxyundecane did not have any inhibitory effect, it is possible to produce traps that are effective and long lasting but cheaper to produce and maintain.

Comparative electrophysiological analysis of plant odor perception in females of threePapilio species

SummaryAntennae of femalePapilio butterflies perceive many volatile plant constituents with widely differing, constituent-specific sensitivities. We compared the responses of threePapilio species to volatiles from host and non-host plants to assess species-specificity and the degree of evolutionary conservatism in olfactory responses.Since previous studies had demonstrated that the polar constituents in odor fromDaucus carota stimulate oviposition behavior inPapilio polyxenes, we collected headspace volatiles fromD. carota, Pastinaca sativa (both Apiaceae) andArtemisia dracunculus (Asteraceae) and separated the polar fraction of these volatiles by gas chromatography. GC-coupled electroantennograms (GC-EAG) were recorded from the speciesPapilio polyxenes, P. machaon hippocrates andP. troilus. In addition, the responses of the three species to five compounds known as generally occurring constituents of plant odor were recorded. The relative sensitivities for these compounds were nearly identical in all threePapilio species. The response spectra to the separated plant volatiles also showed considerable similarities among the species.From the limited set of GC peaks evoking a response in one of the species, 64% (D. carota), 44% (P. sativa) and 29% (A. dracunculus) also evoked a response in both of the other species. The responses of the two closely related Apiaceae feeders (P. polyxenes, P. m. hippocrates) to volatiles fromD. carota were more similar to each other than was either to the response ofP. troilus, which feeds on Lauraceae. However, this was not true for the responses to volatiles fromP. sativa. The least congruence among the three species was found in the responses to volatiles fromA. dracunculus, a non-host for all of them. The differences and similarities found in the response profiles of the threePapilio species are discussed with respect to evolutionary adaptation to host odor versus evolutionary conservatism in adaptation of olfactory receptors.