Erich Städler

Leaf surface chemicals stimulating oviposition byPieris rapae (Lepidoptera: Pieridae) on cabbage

SummaryThe chemical stimulation of oviposition byPieris rapae on cabbage was investigated by leaf washing and extraction. Isolation of the stimulant by various chromatographic techniques was monitored by a bioassay using Sieva bean as a surrogate host plant. Cold water, chloroform, or chloroform followed by cold water washes failed to release the stimulant from leaf surfaces. Boiling water or chloroform followed by methanol was required. The most active stimulatory compound was identified as 3-indolylmethyl glucosinolate (glucobrassicin). Other glucosinolates were identified as sinigrin, which was only slightly active, and glucoiberin, which was completely inactive as a stimulant. The significance of the selective response ofP. rapae andP. brassicae to different glucosinolates and the implications of the binding of polar allelochemicals to leaf surfaces is discussed with respect to host utilization and perception mechanisms of pierids.

Isothiocyanates Stimulating Oviposition by the Diamondback Moth, Plutella xylostella

Recognition of cabbage as a host plant for the diamondback moth (DBM) has previously been shown to depend on compounds that are extracted by soaking intact foliage in chloroform. Analysis of such chloroform extracts by open column chromatography has now resulted in the isolation of highly active fractions that elicit oviposition on treated filter papers. Further separation of these fractions by high-performance liquid chromatography revealed the presence of two distinct groups of active compounds that may be classified as volatile and non-volatile. The two prominent volatile components were separated and identified by mass spectrometry as the isothiocyanates, iberin (3-methylsulfinylpropyl isothiocyanate) and sulforaphane (4-methylsulfinyl-3-butenyl isothiocyanate). Subsequent bioassays of a range of isothiocyanates showed that iberin and sulforaphane were the most active of those tested. Other isothiocyanates with sulfur in the side chain were also active, whereas alkyl and phenyl isothiocyanates had only limited activity. In electrophysiological experiments, electroantennograms (EAGs) indicated positive responses of moth antennae to the isothiocyanates that were most active in behavioral assays. Since sulforaphane has been identified as a major inducer of anticarcinogenic activity in mouse tissue, a synthetic analog (exo-2-acetyl-5-isothiocyanatonorbornane) that shows similar inducer activity was tested on DBM. This bicyclic analog was highly active in both behavioral and EAG assays, suggesting similarity in receptor sites for the two types of biological activity.

HOST PLANT STIMULI AFFECTING OVIPOSITION BEHAVIOR OF THE EASTERN SPRUCE BUDWORM

The host selection of the female spruce budworm Choristoneura fumiferana (Clem.), Tortricidae, for oviposition is influenced both by the shape and the composition of the substrate. Needle extracts were bioassayed using artificial twigs. The active compounds are soluble in organic solvents and steam distillable. Synthetic D‐α and L‐β pinene occurring naturally in the extracts proved to be oviposition stimulants. The optical isomer L‐α pinene did not show any activity. Ablation experiments led to the conclusion that chemoreceptors for the host attractants and the stimulants are located on the antennae and probably also on the maxillary and labial palpi.

Tarsal contact chemoreceptor response to glucosinolates and cardenolides mediating oviposition in Pieris rape

Abstract. Water‐soluble extracts of a host crucifer (Brassica oleracea L.) and non‐host crucifer (Erysimum cheiranthoides L.) and isolated pure cardenolides and glucosinolates were tested on Pieris rapae L. (Lepidoptera: Pieridae) butterflies in oviposition assays and by electrophysiological recordings from the contact‐chemoreceptor sensilla of the prothoracic tarsi.

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.

Chemoreception of host plant chemicals by ovipositing females of Delia (Hylemya) brassicae

A receptor cell sensitive to the glucosinolate sinigrin has been identified in tarsal sensory hairs designated types D, B and A. Such receptors were found neither in the corresponding tarsal hairs of males nor in the labellar setae of either of the sexes. The “glucosinolate” receptor cell of the female seemed to be different from the “sugar” receptor cell of the hairs D and A and also different from the “water” cell of the hair B. The most sensitive “sinigrin” receptors had a threshold below 10−4 M sinigrin, probably at about 10−6 M. Sinalbin, another glucosinolate, was at least as active as sinigrin. The enzymatic breakdown product of sinigrin, allylisothiocyanate was not active at 1 mM. Behavioral experiments confirmed that sinigrin on artificial leaves of filter paper does induce oviposition when synergised with the odor of allylisothiocyanate. In preliminary experiments water extracts of leaf surfaces of radishes stimulated oviposition and activated a sensory receptor cell in female tarsal D‐hairs.

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.

Secondary metabolites of the leaf surface affected by sulphur fertilisation and perceived by the diamondback moth

Summary. Summary. Oilseed rape, Brassica napus L. (cv Express), plants were grown under three different sulphur regimes: sulphur-free (S0), normal sulphur (Sn, normal field concentration) and a sulphur-rich (S+, 2 × concentration of Sn). We performed dual choice oviposition assays with the diamondback moth, Plutella xylostella, using real plants and, for the first time with this insect, artificial leaves sprayed with methanolic leaf-surface extracts. The results mirrored those of a separate study of preferences for whole plants. Females laid more eggs on surrogate leaves that were treated with Sn extracts than on S0 plants, while only a slight, not significant, difference was observed between extracts of normal and sulphur-rich plants. This shows that chemical compounds on the leaf surface mediate the oviposition preference and that the female insect can perceive the quality of the host-plants in terms of their fertilisation status.Since leaf volatiles are known to be oviposition stimulants, we investigated the effects of leaf-surface extracts on insect olfactory responses using electroantennograms (EAGs). In agreement with the behavioural data, we found that extracts of sulphur-treated plants yielded higher EAG amplitudes than the S0 extracts. Since the leaf content of the volatiles isothiocyanates is influenced by sulphur nutrition, we analysed the extracts for these compounds. Above the detection threshold of our GC-MS system, no isothiocyanates were found. Thus, other compounds present in the surface extracts must be perceived by the antenna.However, the HPLC analysis revealed 11 different glucosinolates. Progoitrin (2-Hydroxy-3-butenyl) and gluconapoleiferin (2-Hydroxy-4-pentenyl), which belong to the hydroxy-alkene class of glucosinolates, were the most abundant compounds. The total glucosinolate content sharply increased from S0 to Sn plants, whereas it was slightly lower in n versus S+ plants. Since it is known that glucosinolates can stimulate oviposition, it seems likely that the increased content we observed was influencing the insect preference in this study too.

Oviposition by Lobesia botrana is stimulated by sugars detected by contact chemoreceptors

Abstract.  The influence of glucose, fructose and sucrose on oviposition site selection by Lobesia botrana is studied by combining behavioural and electrophysiological experiments. Oviposition choice assays, using surrogate grapes treated with grape berry surface extracts of Vitis vinifera cv. Merlot at different development stages, show that L. botrana females are most stimulated by extracts of mature berries containing the highest concentrations of glucose and fructose. Choice assays reveal that the oviposition response to these sugars is dose‐dependant (with a threshold of the applied solution = 10 mm and a maximum stimulation at 1 m) and that females are more sensitive to fructose than to glucose. Tarsal contact‐chemoreceptor sensilla are unresponsive to stimulation with sugars but the ovipositor sensilla contain at least one neurone most sensitive to fructose and sucrose with a threshold of approximately 0.5 mm. Corresponding to the behavioural data, glucose is significantly less stimulatory to sensilla than fructose or sucrose. It is argued that fructose may be of special importance for herbivorous insects exploiting fruit as an oviposition site.