J. Alan A. Renwick

The chemical world of crucivores: lures, treats and traps

The host ranges of several insects that are specialists on crucifers (Brassicaceae) are closely linked to the presence of glucosinolates in these plants. These glycosides often serve as stimulants for oviposition and/or feeding, while their volatile hydrolysis products may be attractants for several species. However, many crucifers produce additional secondary compounds that act as repellents, deterrents or toxins, which protect them from these insects. The widely different responses of the various crucifer specialists to these compounds reflect different degrees of adaptation to the plant defenses. Thus native insects are often unable to survive on introduced plants, although the ubiquitous glucosinolates may trigger oviposition ‘mistakes’. The success of highly invasive cruciferous weeds may be due in part to a lack of local herbivore adaptation to unique chemical constituents of these plants. However, the concentrations of secondary chemicals vary with season, environmental conditions, and geographical location. This could mean that windows of opportunity exist for utilization of introduced plants. Recent studies with garlic mustard, Alliaria petiolata, and wintercress, Barbarea vulgaris, in the USA have shown that these introduced plants are resistant to the native butterfly, Pieris napi oleracea. The combined effects of a flavone glycoside and a unique butenenitrile glycoside in the garlic mustard appear to be responsible for blocking feeding by this insect. Barbarea vulgaris is also resistant to the diamondback moth, Plutella xylostella, in North America and to the flea beetle, Phyllotreta nemorum, in Europe. Comparative studies indicate that common resistance mechanisms are involved and bioassays have been developed to elucidate the chemical nature of this resistance.

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.

Host recognition by the tobacco hornworm is mediated by a host plant compound.

It is generally believed that animals make decisions about the selection of mates, kin or food on the basis of pre-constructed recognition templates. These templates can be innate or acquired through experience. An example of an acquired template is the feeding preference exhibited by larvae of the moth, Manduca sexta. Naive hatchlings will feed and grow successfully on many different plants or artificial diets, but once they have fed on a natural host they become specialist feeders. Here we show that the induced feeding preference of M. sexta involves the formation of a template to a steroidal glycoside, indioside D, that is present in solanaceous foliage. This compound is both necessary and sufficient to maintain the induced feeding preference. The induction of host plant specificity is at least partly due to a tuning of taste receptors to indioside D. The taste receptors of larvae fed on host plants show an enhanced response to indioside D as compared with other plant compounds tested.

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.

A Saponin Correlated with Variable Resistance of Barbarea vulgaris to the Diamondback Moth Plutella xylostella

Two types of Barbarea vulgaris var. arcuata, the G-type and the P-type, differed in resistance to larvae of the diamondback moth (DBM) Plutella xylostella. Rosette plants of the G-type were fully resistant to the DBM when grown in a greenhouse or collected in the summer season, but leaves collected during the late fall were less resistant, as previously found for flea beetle resistance. The P-type was always susceptible. Extracts of resistant leaflets inhibited larval growth in a bioassay, and a growth-inhibiting fraction was isolated by activity-guided fractionation. A triterpenoid saponin (1) was isolated from this fraction and identified as 3-O-β-cellobiosyloleanolic acid from spectroscopic data and analysis of hydrolysis products. The decrease in resistance of the G-type in the fall was correlated with a decrease in the level of 1, from 0.6–0.9 to <0.2 μmol/g dry wt. Compound 1 was not detected in the susceptible P-type. We conclude that 1 is correlated with the variable resistance of B. vulgaris foliage to the DBM.

Sequestration of Glucosinolates by Harlequin Bug Murgantia histrionica

Murgantia histrionica, the harlequin bug, is an aposematic pentatomid that feeds on toxic crucifer plants. By performing predator trials, we found that the bugs are distasteful to several species of bird predators. Given this, we tested the hypothesis that the bugs sequester toxins from the crucifer plants they feed on for use in defense against predation. We used high-pressure liquid chromatography for analyses and tested if M. histrionica sequesters toxic chemicals from its crucifer diet. We found that M. histrionica sequesters mustard oil glycosides, precursors to zootoxic mephitic nitriles, and that sequestration is characteristic of the plant species fed upon. Glucosinolate titers in M. histrionica bodies were 20–30 times higher than in their guts. We found that cabbage-fed M. histrionica had higher titers of cabbage glucosinolates than bugs that were fed on a cabbage diet and then switched to a diet of garden nasturtium. This indicates that M. histrionica immediately sequesters chemicals from whichever plant it feeds upon. The study shows that M. histrionica can sequester glucosinolates from its host plants for use in defense against predation and that the bugs can retain the glucosinolates for an extended period of time.

Variable diets and changing taste in plant-insect relationships.

The host ranges of phytophagous insects are determined to a large degree by plant chemistry. Specialist insects are often closely associated with plants that produce characteristic chemicals, which may act as attractants or stimulants to aid in finding or recognizing a host. Generalist insects are generally believed to rely on the presence of repellents or deterrents to ensure avoidance of unsuitable plants. However, the chemistry of any plant can be highly variable, as a result of growth characteristics, genetic variation, or environmental factors. Such variable chemistry may provide windows of opportunity for nonadapted insects to utilize a plant or for a plant to become resistant to a normally adapted herbivore. Differences in insect responses to plant constituents may also result from genetic variation or environmental factors. In particular, dietary experience has been found to influence the ability of insects to taste plant chemicals that may serve as signals of suitability or unsuitability. Certain dietary constituents appear to suppress the development of taste sensitivity to deterrents in an insect, whereas the presence of specific stimulants in the diet may result in the development of dependence on these compounds. These findings further emphasize the fact that the dynamics of plant biochemistry along with plasticity in the sensory system of insects might be expected to play a major role in the evolution of new plant–insect relationships.

DUAL CHEMICAL BARRIERS PROTECT A PLANT AGAINST DIFFERENT LARVAL STAGES OF AN INSECT

The host plants of the native American butterfly, Pieris napi oleracea, include most wild mustards. However, garlic mustard, Alliaria petiolata, a highly invasive weed that was introduced from Europe, appears to be protected from this insect. Although adults will oviposit on the plant, most larvae of P. n. oleracea do not survive on garlic mustard. We used feeding bioassays with different larval stages of the insect to monitor the isolation and identification of two bioactive constituents that could explain the natural resistance of this plant. A novel cyanopropenyl glycoside (1), alliarinoside, strongly inhibits feeding by first instars, while a flavone glycoside (2), isovitexin-6″-D-β-glucopyranoside, deters later instars from feeding. Interestingly, the first instars are insensitive to 2, and the late instars are little affected by 1. Furthermore, differential effects of dietary experience on insect responses suggest that 1 acts through a mechanism of post-ingestive inhibition, whereas 2 involves gustatory deterrence of feeding.

Isovitexin 6″-O-β-d-glucopyranoside: A feeding deterrent to Pieris napi oleracea from Alliaria petiolata

Abstract Pieris napi oleracea, an indigenous butterfly to North America lays eggs on Alliaria petiolata, an invasive weed introduced from Europe, but the larvae generally do not survive. A new apigenin glycoside, isovitexin 6″-O-glucoside has been isolated from the leaves A. petiolata and identified as a feeding deterrent for P. napi fourth instar larvae. The structure was elucidated by UV, MS and NMR spectroscopy.

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.