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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. A pair of ventro‐medial C sensilla on die fifth tarsomere respond strongly to a novel compound called tentatively ‘cabbage identification factor’ (CIF), but not to sucrose, glucose, fructose and proline. CDF is a new non‐glucosinolate oviposition stimulant. A single neurone in each sensillum is activated by this compound and the same is true for glucosinolates. In some flies a mixture of bom types of stimuli evoked an apparent mononeural spike train, whereas in odiers spikes of two separate cells were activated. The significance of this variability is not yet clear. The new stimulant, CIF, does not evoke responses in glucosinolate receptors in the D sensilla. The involvement of the C3 sensilla in the detection of host‐specific compounds constitutes the first known function for C sensilla in D. radicum. CIF appears to be present in leaf surface extracts from the host‐plant Brassica oleracea in quantities as low as 1 ng per gram leaf. In spite of this low level, it stimulates oviposition significantly better than glucobrassicin at higher concentrations, which up till now was known as the most powerful stimulant for D.radicum.

Coupled gas chromatographic-electroantennographic detection analyses of ovipositor extracts of calling Contarinia nasturtii females revealed two compounds that elicited responses from antennae of male midges. Using synthetic reference samples, these components were identified by gas chromatography mass spectrometry and enantioselective GC as (2S,9S)-diacetoxyundecane and (2S,10S)-diacetoxyundecane. In addition, trace amounts of 2-acetoxyundecane were found in ovipositor extracts, and the (S)-enantiomer was synthesized. When tested in the wind tunnel, a blend of 5 ng (2S,9S)-diacetoxyundecane and 10 ng (2S,10S)-diacetoxyundecane (mimicking the ratio found in the extracts) did not attract any of the males tested, but when 0.1 ng (S)-2-acetoxyundecane was added to the blend, 86.8% of the males were attracted to the bait. Three-component blends with lower or higher relative concentrations than 1% of (S)-2-acetoxyundecane [relative to (2S,10S)-diacetoxyundecane] were less attractive. In a field trapping experiment with released laboratory-reared C. nasturtii adults, traps baited with 500:1000:10 ng of (2S,9S)-diacetoxyundecane/(2S,10S)-diacetoxyundecane/(S)-2-acetoxyundecane applied to rubber septa or dental cotton rolls were tested. Traps without dispensers were used as controls. All three treatments were tested at 20 and 50 cm above ground. The estimated recapture rate was 30-50%, and 81.9% of the recaptured males were caught in traps positioned at 20 cm above ground, and 88.4% in traps with dental cotton rolls as dispensers.

In Brassica crops differences in susceptibility to root fly attack can be largely attributed to antixenotic resistance. Plants of four genotypes (two swedes and two kales) with widely differing resistance in field trials, were compared in laboratory choice assays for their susceptibility to oviposition by the root flies Delia radicum (L.) and D. floralis (Fallen) (Diptera, Anthomyiidae). For both species the preference among the genotypes corresponded to the susceptibility of the genotypes in the field. The preference ranking in response to surrogate leaves treated with methanolic surface extracts of the four genotypes was identical to the preference among potted plants, demonstrating that chemical factors on the leaf surface mediate host preference for oviposition in these species. For both species of fly, glucosinolates are major oviposition stimulants and for D. radicum an additional, non‐glucosinolate oviposition stimulant, presently called CIF, is known. We describe a procedure for chromatographic separation of glucosinolates from CIF in leaf surface extracts. In oviposition‐choice assays with D. radicum, the CIF‐fractions of the two swede genotypes applied to surrogate leaves received a 1.8 and 4.6 times higher proportion of eggs than the respective glucosinolate‐fractions, confirming the major importance of CIF as an oviposition stimulant. The genotype of swede that was preferred by both fly species in tests with plants and methanolic leaf surface extracts, also stimulated oviposition more in tests with the glucosinolate‐fractions or the CIF‐fractions derived from the surface extracts, respectively. Thus, glucosinolates and CIF together account for the observed preference among the genotypes and may also be responsible for their susceptibility under field conditions. In the two kale genotypes the preference for plants or surface extracts differed from the preference among the corresponding glucosinolate‐ and CIF‐fractions, indicating that additional, as yet unknown chemical factors may also be involved. For both groups of stimulants tarsal chemoreceptors allow electrophysiological monitoring of glucosinolate‐ and CIF‐activity in fractionated surface extracts. For D. radicum the chemosensory activity of both glucosinolate‐ and CIF‐fractions corresponded to the respective behavioural activity in the oviposition preference tests, suggesting that preference for oviposition among genotypes can be predicted from the electrophysiological activity of their fractions. The chemosensory response of D. floralis, in particular to the CIF‐fractions, was less pronounced than the response of D. radicum, indicating interspecific differences in the perception of the major oviposition stimulants. We discuss the potential application of electrophysiological techniques in support of other screening methods used in breeding for root fly resistance in Brassica crops.

Two compounds present on the surface of Brassica oleracea cv. botrytis leaves have been isolated and identi®ed which stimulate very eectively oviposition in the cabbage root¯y, Delia radicum and which are perceived by a speci®c receptor neuron in the tarsal sensillum C 5 of the female¯y. Activity of extracts and chromatographic fractions were bioassayed, using oviposition experiments and mainly electrophysiological recordings from the C 5 tarsal contact chemoreceptor sensillum of femalē ies. Spectroscopic data indicate that the main compound is 1,2-dihydro-3-thia-4,10,10b-triaza-cyclopenta[.a.]¯uorene-1carboxylic acid, a novel compound related to Brassica phytoalexins like brassicanal C. It is accompanied by its glycine conjugate.

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Abstract. Behavioural events during host selection by ovipositing monarch butterflies (Danaus plexippus (L.), Danainae, Nymphalidae) include tapping the leaf surface with fore‐tarsi and touching this surface with mid‐tarsi (‘drumming’) and antennae. Flavonoids identified from host plant extracts are known to stimulate oviposition. Scanning electron microscopy revealed the presence of contact‐chemoreceptor sensilla on all appendages that contact the leaf surface. This electrophysiological study was conducted to identify the contact chemoreceptors that are sensitive to the known oviposition stimuli and are therefore probably involved in host recognition. Receptor cells of conspicuous sensilla grouped in clusters on fore‐tarsi of females were sensitive to the behaviourally active butanol fraction of host plant (Asclepias curassavica) extract. However, these receptors generally had low sensitivity to three oviposition‐stimulating flavonoids identified from this fraction, but they were also sensitive to the butanol fraction of a non‐host (Brassica oleracea). Chemoreceptors in sensilla of the tarsomers 2–4 of the mid‐legs also responded to the behaviourally active fraction of host plant extract and showed some sensitivity to two of the flavonoids that stimulate oviposition. Similar results were obtained from receptor cells in sensilla on the tip of the antennae. Most of these sensilla had cells responding to the butanol fraction of A. curassavica extract but only 25% of them were also sensitive to one of the behaviourally active flavonoids. These electrophysiological results, in combination with behavioural observations, suggest that host selection in monarch butterflies relies on a complex pattern of peripheral sensory information from several types of tarsal and antennal contact chemoreceptors.

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.