K. Sachdev-Gupta

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.

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.

Isolation and identification of oviposition deterrents to cabbage butterfly,Pieris rapae, fromErysimum cheiranthoides.

Avoidance of some crucifer species by the crucifer specialist,Pieris rapae, has been attributed to the presence of oviposition deterrents in these plants. Studies on one such unacceptable plant,Erysimum cheiranthoides, have resulted in the isolation ofn-butanol-soluble deterrents from the alcoholic extract of foliage. The active fraction contained three cardiac glycosides, which were isolated by reversed-phase HPLC and by open column chromatography on silica gel. Chemical and spectral evidence (UV, [1H]NMR, and FAB-MS) led to the characterization of these compounds as erysimoside (1), erychroside (2), and erycordin (3). Erysimoside and erychroside were strongly deterrent toPieris rapae, but erycordin was inactive. Both active compounds have the same aglycone, strophanthidin (5) and the inner sugar in both cases is a 2,6-dideoxy hexose to which the outer sugar is attached at position C-4. These structural features, which are absent in the inactive compound (3), may represent specific requirements for oviposition deterrent activity.

Chemical constituents ofErysimum cheiranthoides deterring oviposition by the cabbage butterfly,Pieris rapae

Avoidance ofErysimum cheiranthoides for oviposition byPieris rapae has been attributed to the presence of water-soluble deterrents. The active material was extracted inton-butanol and isolated by a series of HPLC separations. TLC of the active fraction and visualization of individual constituents with Keddes reagent indicated that cardenolides are responsible for deterring oviposition. UV spectra were also characteristic of cardenolides. Bioassays of selected known cardenolides revealed a general lack of activity, except for cymarin, which was as strongly deterrent as the most prominent cardenolide isolated in pure form fromE. cheiranthoides. The results suggest that cardenolides in this plant can explain its escape from cabbage butterflies, but specific structural features of the glycosides are necessary for oviposition-deterring activity.

Cardenolides fromErysimum cheiranthoides: Feeding deterrents toPieris rapae larvae

Larvae of the cabbage butterfly,Pieris rapae, refuse to feed on the wild mustard,Erysimum cheiranthoides, due to the presence of alcoholextractable deterrents. The active components were extracted inton-BuOH, and this extract was separated into four fractions (I–IV) by reverse-phase HPLC. Fractions III and IV retained the feeding deterrent activity. The activity of fraction III was found to be due to the cardenolide diglycosides 1 and 2, which were previously reported as oviposition deterrents for gravidP. rapae butterflies. Three active compounds were isolated from fraction IV by column chromatography on silica gel followed by reverse-phase HPLC. These compounds were identified as a monoglycoside, digitoxigenin 3-O-β-D-glucoside (4), and two diglycosides, glucodigigulomethyloside (5) and glucodigifucoside (6). An additional cardenolide isolated from fraction II was identified as cheirotoxin (7). All compounds were identified by UV, NMR (1H and13C), and mass spectrometry, as well as hydrolysis experiments. The feeding deterrent activity of these compounds was compared with that of related commercially available chemicals and other compounds isolated fromE. cheiranthoides.

Oviposition stimulants inBarbarea vulgaris forPieris rapae andP. napi oleracea: isolation, identification and differential activity.

The closely related butterflies,Pieris rapae andP. napi oleracea, readily laid eggs onBarbarea vulgaris in greenhouse cages. When offered a choice between cabbage andB. vulgaris, P. rapae showed no preference, butP. napi oleracea preferredB. vulgaris. Bioassays of extracts ofB. vulgaris foliage revealed the presence of oviposition deterrent(s) in l-butanol extracts as well as stimulants in the postbutanol water extracts. However, the deterrent effect was apparently outweighed by the strong stimulatory effect in the whole plants. The postbutanol water extract was preferred over an equivalent cabbage extract by both species, but more significantly in the case ofP. napi oleracea. The stimulants were isolated by open column chromatography and HPLC, and the activity was associated with three glucosinolates.P. napi oleracea was more sensitive thanP. rapae to the natural concentration of compounds1 and3, whereas both species were strongly stimulated to oviposit by natural concentrations of compound2. Compounds1 and2 were identified as (2R)-glucobarbarin and (2S)-glucobarbarin, respectively, and3 was identified as glucobrassicin, on the basis of their UV, mass, and NMR spectra. When the pure compounds were tested at the same concentrations applied to bean plants, the (2R)-glucobarbarin at 0.2 mg/plant was preferred over a standard cabbage extract by both butterfly species. However, at a dose of 0.02 mg/plant,P. rapae preferred the cabbage extract whereasP. napi oleracea still preferred the (2R)-glucobarbarin. No such difference in response of the two species to the same two concentrations of (2S)-glucobarbarin was obtained. The results indicate a distinct difference in sensitivity of these butterflies to the epimers of glucobarbarin, and the differences in behavioral responses of the two butterfly species depend to a large extent on the concentration of stimulant present.

Antifeedant activity of cucurbitacins from Iberis amara against larvae of Pieris rapae

Abstract Two cucurbitacins isolated from Iberis amara foliage were identified by UV and NMR spectroscopy and acid catalysed hydrolysis as 2- O -β- d -glucopyranosyl cucurbitacin E and 2- O -β- d -glucopyranosyl cucurbitacin I. The first compound was found to be a potent antifeedant to the larvae of Pieris rapae in a choice test.

Oviposition stimulants and deterrents regulating differential acceptance ofIberis amara byPieris rapae andP. napi oleracea

Iberis amara (Cruciferae) contains both stimulants and deterrents that are involved in regulating oviposition byPieris rapae andP. napi oleracea. The most active deterrents toP. rapae isolated from butanol extracts of the plant were found to be 2-O-Β-d-glucosyl cucurbitacin I and 2-O-Β-d-glucosyl cucurbitacin E. However,P. napi oleracea was behaviorally insensitive to these compounds and was only weakly deterred by other individual fractions of the butanol extract. Stimulant activity of the postbutanol water extract ofI. amara was associated with glucosinolates. The most abundant of these was identified as sinigrin, and a relatively minor component was shown to be glucoiberin. The isolated sinigrin was more stimulatory toP. rapae than was the glucoiberin-containing fraction, butP. napi oleracea was stimulated as strongly by the glucoiberin fraction, even though the concentration of this compound was much lower. The contrasting responses of the twoPieris species to the deterrents and stimulants inI. amara can explain the differential acceptance of the plant by these butterflies.

A chemical basis for differential acceptance ofErysimum cheiranthoides by twoPieris species.

Wormseed mustard,Erysimum cheiranthoides, is unacceptable as a host for the cabbage butterfly,Pieris rapae. However, it is preferred for oviposition byPieris napi oleracea in the greenhouse. Isolation and identification of the oviposition stimulants toP. napi oleracea were accomplished by C18 open-column chromatography, TLC, ion-exchange chromatography, HPLC, UV, and NMR spectroscopy. Glucoiberin and glucocheirolin were identified as the most active stimulants. The extracted glucoiberin was as stimulatory as glucocheirolin, although its concentration in theErysimum plants was about 10 times lower than that of glucocheirolin. These glucosinolates were only weak stimulants toP. rapae. Furthermore,P. rapae was strongly deterred by the cardenolides, erysimoside and erychroside, fromE. cheiranthoides, andP. napi oleracea was less sensitive to these compounds. No other deterrent toP. napi oleracea was detected in this plant species. The results explain the differential acceptance ofE. cheiranthoides by these twoPieris species.

Chemical constituents of an unacceptable crucifer,Erysimum cheiranthoides, deter feeding byPieris rapae

The wild cruciferErysimum cheiranthoides was found to contain extractable constituents that deterred feeding by larvae of the crucifer specialistPieris rapae when applied to cabbage leaf disks in both choice and nochoice bioassays. High-performance liquid chromatography was used to separate the extract into several fractions, two of which retained the feeding deterrent activity of the extract. UV-absorption spectra of the fractions suggested that one contained cardenolides similar or identical to those reported to deter oviposition byP. rapae onE. cheiranthoides. The other active fraction evidently contains a compound that deters larval feeding but not adult oviposition. The results suggest that the chemical defense ofE. cheiranthoides depends on two types of compounds acting on separate developmental stages of the insect.