Feeding and Growth of Plutella xylostella and Spodoptera eridania on Brassica juncea with Varying Glucosinolate Concentrations and Myrosinase Activities

We measured feeding behavior, feeding damage, and larval growth of the crucifer specialist, Plutella xylostella and the generalist, Spodoptera eridania, on the cotyledons of 14 homozygous lines of Brassica juncea differing in myrosinase activity and glucosinolate profiles. The proportion of time feeding and area damaged by P. xylostella were lower on lines with high myrosinase activities [0.49–0.73 nmol glucose released/mg tissue(fresh weight, FW)/min] than on lines with low myrosinase activities [0.20–0.31 nmol glucose released/mg tissue(FW)/min]. In contrast, the proportion of time feeding and area damaged by S. eridania were not related to myrosinase activity, but were lower on cotyledons of lines with high glucosinolate concentrations [6.8–21.3 g/g(FW)] than on lines with low glucosinolate concentrations [0.09–0.61 g/g(FW)]. Relative growth rates (RGR) of both insect species were lower on lines with high glucosinolate concentrations, but were not related to myrosinase activity in the lines. In toxicity experiments that used artificial diets, allyl isothiocyanate, but not allyl glucosinolate, was lethally toxic to neonate P. xylostella (LC₅₀s of 1.54 mol/g, and 100 mol/g, respectively), whereas isothiocyanate and the glucosinolate were lethally toxic to neonate S. eridania (LC₅₀s of 3.42 and 6.73 mol/g, respectively). We interpret these results to indicate that myrosinase activity might be more important for plant defense against specialist insects that have adaptations to intact glucosinolates, but less important for defense against generalists, which are susceptible to the intact glucosinolates.     

Damage-Induced Accumulation of Phytoecdysteroids in Spinach: A Rapid Root Response Involving the Octadecanoic Acid Pathway

Some plant defenses are known to be rapidly induced following attack by phytophagous insects. Plant-produced insect molting hormones, termed phytoecdysteroids, are believed to aid plant resistance; however, their dynamics are poorly understood. Using spinach (Spinacia oleracea) as a model system, we examined the inducibility of phytoecdysteroids, primarily 20-hydroxyecdysone (20E), in an effort to characterize potential interactions with herbivorous insects. Rapid phytochemical induction was investigated using damage treatments and applications of defense-related plant-signal analogs, specifically methyl jasmonate (MJ) and methyl salicylate (MSA). Within two days, mechanically damaged roots exhibited two to three fold increases in phytoecdysteroid concentrations. Four days after root damage, small increases in shoot levels were also detectable. Unlike roots, foliar 20E concentrations were unaltered over a range of shoot treatments including insect herbivory (Spodoptera exigua), mechanical damage, and MJ applications. Additions of MJ (12.5–50 g/liter) to the root systems of hydroponically grown plants stimulated accumulations of root phytoecdysteroids in a dose-dependent manner, similar in magnitude to the response induced by root damage. Under identical conditions, MSA did not affect the accumulation of 20E when added to the hydroponic solutions of undamaged plants. Moreover, MSA inhibited the induction of 20E in wounded roots, but did not interfere with the action of applied MJ. In contrast to mechanical damage, roots did not induce 20E levels when challenged with two different fungal pathogens (Pythium aphanidermatum and Phytophthora capsici).We propose that wound-induced accumulations of 20E are generated in the roots, signaled via endogenous jasmonates, and may confer enhanced resistance against subterranean herbivorous insects.     

Behavioral Effects and Sensory Detection of Drimane Deterrents in Myzus persicae and Aphis gossypii Nymphs

Eleven synthetic drimane compounds were tested for their deterrency to nymphs of Myzus persicae (Sulzer) and Aphis gossypii (Glover) (Homoptera: Aphididae). In general, A. gossypii nymphs were less sensitive to the drimanes than M. persicae nymphs. Warburganal (1) and polygodial (2), with a -dialdehyde configuration and double bond at C7–C8, were highly active as deterrents and/or feeding inhibitors against both species. One of the synthetic compounds (10) showed relatively high activity against A. gossypii, while it was not active at all against M. persicae, indicating that synthetic analogs of natural deterrents can be highly selective feeding deterrents. From 24-hr interval observations and ablation studies, it is concluded that nymphs of both M. persicae and A. gossypii detect polygodial and possibly the other drimanes tested with contact chemosensilla at the tips of their antennae. The ablation studies also showed that in both species no tarsal, labial, or epipharyngeal sensilla are involved in detecting polygodial within 24 hr.     

Distribution of the carcinogenic terpene ptaquiloside in bracken fronds,rhizomes (Pteridium aquilinum), and litter in Denmark

The distribution of ptaquiloside (PTA) was studied in four Danish bracken populations in order to evaluate the transfer of PTA from ferns to soil. Populations showed statistically significant differences in PTA contents of fronds and rhizomes despite large in-site variations. The highest concentrations were encountered in fronds with concentrations ranging between 213 and 2145 g/g, while rhizomes had concentrations between 11 and 902 g/g. PTA was present in soil materials in amounts of 0.22–8.49 g/g but apparently with no correlation with PTA contents of fronds or rhizomes. Laboratory tests showed that water could leach PTA from bracken fronds, which is in support of the high soil contents observed at sites exposed to heavy showers just before sampling. The observed soil contents correspond to estimated soil solution concentrations of 200–8500 g/liter, demonstrating a substantial risk of PTA contamination of surface water and groundwater.     

Comparison of electroantennograms from female and male cabbage looper moths (Trichoplusia ni) of different ages and for various pheromone concentrations

The electroantennogram (EAG) responses of both male and femaleTrichoplusia ni to two components of the female-released sex pheromone were determined over a range of concentrations of chemical and age of insect. The EAG was at a peak at a dose of 100 g for both dodecyl acetate and (Z)-7-dodecenyl acetate (Z7–12OAc) for both sexes. The EAG responses varied with age of the insect. Responsiveness to both components was at a maximum at three days postemergence for males followed by a fairly rapid senescence, and for females it peaked at three days followed by a rapid senescence forZ7–12OAc but not for dodcecyl acetate.     

Oxidizable Phenolic Concentrations Do Not Affect Development and Survival of <Emphasis Type="Italic">Paropsis Atomaria</Emphasis> Larvae Eating <Emphasis Type="Italic">Eucalyptus</Emphasis> Foliage

Insect folivores can cause extensive damage to plants. However, different plant species, and even individuals within species, can differ in their susceptibility to insect attack. Polyphenols that readily oxidize have recently gained attention as potential defenses against insect folivores. We tested the hypothesis that variation in oxidizable phenolic concentrations in Eucalyptus foliage influences feeding and survival of Paropsis atomaria (Eucalyptus leaf beetle) larvae. First we demonstrated that oxidizable phenolic concentrations vary both within and between Eucalyptus species, ranging from 0 to 61 mg.g^?1 DM (0 to 81% of total phenolics), in 175 samples representing 13 Eucalyptus species. Foliage from six individuals from each of ten species of Eucalyptus were then offered to batches of newly hatched P. atomaria larvae, and feeding, instar progression and mortality of the first and second instar larvae were recorded. Although feeding and survival parameters differed dramatically between individual plants, they were not influenced by the oxidizable phenolic concentration of leaves, suggesting that P. atomaria larvae may have effective mechanisms to deal with oxidizable phenolics. Larvae feeding on plants with higher nitrogen (N) concentrations had higher survival rates and reached third instar earlier, but N concentrations did not explain most of the variation in feeding and survival. The cause of variation in eucalypt herbivory by P. atomaria larvae is therefore still unknown, although oxidizable phenolics could potentially defend eucalypt foliage against other insect herbivores.     

Threshold hypothesis for pheromone perception

Field results have shown that male moths of some species are not always trapped by the ratio of pheromone components produced by the female moths. For cases involving a binary mixture of geometrical or positional isomers, this phenomenon may be explained by use of a threshold diagram in which the isomer ratio in the mixture is plotted against release rate (concentration). In this diagram an attraction area is bounded by the threshold for flight activation over the full range of binary mixtures and by the threshold for alteration of in-flight behavior (disorientation) by each pheromone component. A low release rate of the natural mixture may fall within this attraction area, or a high release rate of mixtures in certain other ratios may fall within this area and would be expected to attract male moths. This hypothesis has been used to explain heretofore anomalous trapping data with some moth species, and it can be useful in future studies on defining pheromone blends, species specificity, and potentially disruptive mixtures for insect control.     

Feeding Response to Host and Nonhost Compounds by Males and Females of the Spruce Bark Beetle <Emphasis Type="Italic">Ips typographus</Emphasis> in a Tunneling Microassay

Research on host selection by bark and wood boring insects has concentrated on flight orientation behavior. Less is known of the factors that govern the steps successive to host landing. Here, we discuss chemical factors involved in host acceptance by bark beetles and a new microassay. Adult males and females of Ips typographus were offered an artificial diet treated with various concentrations of different plant-derived compounds (host terpenes and nonhost compounds) in a no-choice mode. Beetles were tested individually in a glass tube for 4 hr, and the length of feeding was measured and compared to a control (diet with only solvent). The first effect was diet rejection, especially when nonhost compounds were tested at high concentrations. Most compounds reduced feeding, in proportion to concentration. Females were fed more readily than males after addition of both host and nonhost compounds. Diet removal was significantly affected by all the tested factors (sex, compound, dose) as well as by their interactions. With increased concentrations, males were more responsive than females to antifeedants, as all compounds (except juglone) showed clear sex differences of diet consumption. 3-Octanol, 1-hexanol, and a Green Leaf Volatile (GLV)-blend (three C6 alcohols) showed the strongest antifeedant effects, which started at a low dose (0.1%) and had a low Effective Dose 50 (ED50, 0.3–1%). In contrast, host monoterpenes, limonene and -pinene, inhibited feeding at high doses (10–30%) only with ED50 > 10%. The highest Antifeedant Indexes (AFIs) were shown by verbenone, carvone, and 1-hexanol (0.90–1.00). Both host and nonhost compounds inhibited feeding at some concentration. No significant stimulation of feeding by any host compound at concentrations reported in the literature as optimal were found, with the possible exception of -pinene at low concentrations in females.     

Larval exposure to oviposition deterrents alters subsequent oviposition behavior in generalist,Trichoplusia ni and specialist,Plutella xylostella moths

The present study was undertaken to determine the effects of larval feeding experience on subsequent oviposition behavior of the resulting moths. Larvae of the cabbage looper (Trichoplusia ni, Noctuidae) and the diamondback moth (Plutella xylostella, Plutellidae) were exposed to the phenylpropanoid allelochemical trans-anethole (at 100 ppm fw in artificial diet) or the limonoid allelochemical toosendanin (10 ppm sprayed on cabbage leaves). Both compounds had been shown to deter oviposition in naïve moths in previous choice tests. Moths developing from experienced larvae (both sexes) showed a decrease in oviposition deterrence response when given a choice between control and treated leaves, unlike naïve moths. This phenomenon, analogous to habituation to feeding deterrents in lepidopteran larva, occurred irrespective of duration of feeding on the deterrent compound. We also observed that F₁larvae resulting from experienced moths (previously exposed to toosendanin as larvae) grew as well on toosendanin-treated foliage as on control foliage. In contrast, growth of F₁larvae from naïve moths was significantly impaired by toosendanin. These results demonstrate that host-selection behavior in cabbage looper (a generalist) and diamondback moth (a specialist) may be shaped by feeding experience according to Hopkins' Host Selection Principle in addition to chemical legacy.     

Insect-Induced Synthesis of Phytoecdysteroids in Spinach, Spinacia oleracea

Spinach (Spinacia oleracea) foliage is known to synthesize and accumulate insect molting hormones, predominantly in the form of 20-hydroxyecdysone (20E). We previously demonstrated that root 20E accumulation is increased following root damage. We designed two further experiments to address root responses to both mechanical and insect damage. In plants grown hydroponically, removal of 35% or less of the root mass did not result in changes in root 20E levels. However, removal of 70% of the root mass stimulated 6.0- and 1.5-fold increases in the root and shoot 20E concentrations, respectively. The effects of insect damage on soil-grown plants were investigated by infesting plant roots with black vine weevil (BVW: Otiorhynchus sulcatus) larvae and allowing them to feed for seven days. Decreases in root mass occurred in young plants; however, no changes were detected in mature plants. In all cases, root herbivory resulted in at least a 3.0-fold increase in root 20E concentrations. Our previous experiments implicated jasmonic acid and the analog methyl jasmonate (MJ) in signaling the damage-induced accumulation of root 20E levels. We investigated the activity of other phytohormones and growth regulators (GRs) on the 20E accumulation patterns of young plants as a means of examining the significance of jasmonates in the induction response. Hydroponic additions of MJ (0.5 M) and the synthetic auxin, 1-naphthaleneacetic acid (NAA; 0.5 M), resulted in significant increases in root 20E levels. At the concentrations tested, indole-3-acetic acid (IAA), gibberellic acid (GA₃), abscisic acid (ABA), and trans-zeatin (Z) had no effects on root 20E concentrations. However, both NAA (0.5–5.0 M) and Z (5.0 M) treatments caused increases in the root/shoot dry mass ratios, indicating shifts in resource allocation to the roots. Treatments involving ABA (5.0 M) and Z (0.5–5.0 M) caused significant increases in shoot 20E concentrations. No other hormone treatments altered shoot accumulation patterns. The mechanisms underlying the root 20E induction phenomena were investigated through the incorporation of [2-¹⁴C]mevalonic acid ([¹⁴C]MVA). Within one day, excised roots readily incorporated radioactivity into 20E from [¹⁴C]MVA. In intact plants, [¹⁴C]MVA absorbed by the roots was rapidly incorporated into root 20E pools following damage and MJ treatments. This implies that the wound-induced root 20E accumulation is the result of increased de novo 20E synthesis in the root.     

Effect of fungal metabolite peramine and analogs on feeding and development of argentine stem weevil (Listronotus bonariensis)

Peramine, a pyrrolopyrazine alkaloid produced by the fungal endophyte of perennial ryegrassAcremonium lolii, deterred the feeding of both adults and larvae of the graminacious herbivore, the Argentine stem weevil (Listronotus bonariensis), at 0.1 g/g and 10 g/g, respectively. In a no-choice test fewer stem weevil larvae fed and developed on diet containing as little as 2 g/g peramine. The proportion of larvae which did not develop beyond the first instar was higher on diet containing peramine and appeared to be due to a higher proportion of larvae which did not feed. For larvae which fed on the peramine-containing diet, feeding scores and times to pupation were not significantly different from those of controls. A number of simple peramine analogues showed feeding-deterrent activity against adult weevils, indicating the importance of the pyrrolopyrazine ring system of peramine in determining feeding-deterrent activity.     

Insecticidal chromenes from the volatile oil ofHemizonia fitchii

Based on field observations of the effects of the resinous tarweedHemizonia fitchii A. Gray (Asteraceae) on mosquito populations in California, the volatile oil of this plant was investigated for insecticidal activity. Analysis of thé oil by TLC and capillary GC-MS showed the presence of five major constituent which were identified as the monoterpenoid 1,8-cineole, and the chromenes encecalin, eupatoriochromene (desmethylencecalin), 6-vinyl-7-methoxy-2,2-dimethylchromene, and desmethoxyencecalin. Trace amounts of several volatile fatty acids, alkanes,p-coumarate derivatives, additional chromene derivatives, and numerous mono- and sesquiterpenoids were also detected and identified by GC-MS. Fractionation of the oil by preparative TLC and column chromatography afforded the major chromenes, the identities of which were confirmed by NMR and IR spectral data. The chromenes exhibited weak to moderate toxicity againstCulex pipiens (house mosquito) larvae andOncopeltus fasciatus (large milkweed bug) nymphs. However, no antijuvenile hormone activity was observed for any of the compounds tested against these insect species.Part I in the series Biologically Active Constituents of North American Plants.Presented at the 24th Annual Meeting of the American Society of Pharmacognosy, The University of Mississippi, Oxford Campus, July 1983.     

Interactions amongHeliothis virescens larvae,cotton condensed tannin and the CryIA(c) δ-endotoxin ofBacillus thuringiensis

The potential interactions among a plant-produced allelochemical, a phytophagous insect, and an endotoxin produced byBacillus thuringiensis were investigated using purified cotton condensed tannins, the CryIA(c)-endotoxin fromB. thuringiensis subsp.kurstaki strain HD-73, and larvae ofHeliothis virescens. Purified condensed tannin from cotton fed to neonateH. virescens reduced feeding and mortality caused by insecticidal crystals ofB. thuringiensis. In fifth instars, tannin reduced relative growth rate (RGR), relative consumption rate (RCR), but antagonized the effects of the crystal-endotoxin. Tannin did not deter feeding of fifth instars in choice tests with cellulose-ester disks. Masking tannin from interacting with the dietary ingredients of artificial diets and crystal protein by encapsulation in alginate gel suggested that tannin adversely affected feeding after ingestion.These results suggest that insect control tactics that employ-endotoxins in microbial insecticides and transgenic cotton plants may not be compatible when used in conjunction with plants containing high tannin concentrations.     

Chemical Defenses Against Fish Predation in Three Brazilian Octocorals: 11β,12β-Epoxypukalide as a Feeding Deterrent in Phyllogorgia dilatata

The feeding-deterrence properties of crude extracts of three Brazilian octocoral species, Neospongodes atlantica Kükenthal (Alcyonacea, Nephtheidae), Plexaurella regia Castro (Gorgonacea, Plexauridae), and Phyllogorgia dilatata Esper (Gorgonacea, Gorgoniidae), were investigated. All the extracts were incorporated into food strips at the concentrations occurring in the living organisms. Crude extract and its ethyl acetate fraction obtained from P. dilatata collected in Armação dos Búzios (Rio de Janeiro State), when incorporated into artificial diets and tested in the habitat of origin, reduced consumption of food strips by fishes, relative to controls. Crude extracts from two octocoral species collected at the National Marine Park of Abrolhos (Bahia State), N. atlantica and P. regia, had no apparent feeding-deterrence properties; in fact, they seemed to stimulate feeding. Bioassay-guided fractionation of the bioactive P. dilatata crude extract revealed that the deterrent property was restricted to a medium polarity fraction. Field palatability experiments with two pure compounds isolated from this fraction revealed that the furanocembranolide 11,12-epoxypukalide is a potent feeding deterrent produced by P. dilatata against fish. Apparently, furanocembranolides are a particular class of compounds with feeding deterrent properties, protecting some octocorals from potential fish predator species in both tropical and temperate environments.     

Sequestration of Furostanol Saponins by <Emphasis Type="Italic">Monophadnus</Emphasis> Sawfly Larvae

Sawfly larvae of the tribe Phymatocerini (Hymenoptera: Tenthredinidae), which are specialized on toxic plants in the orders Liliales and Ranunculales, exude a droplet of deterrent hemolymph upon attack by a predator. We investigated whether secondary plant metabolites from Ranunculaceae leaves are sequestered by phymatocerine Monophadnus species, i.e., Monophadnus alpicola feeding upon Pulsatilla alpina and Monophadnus monticola feeding upon Ranunculus lanuginosus. Moreover, two undescribed Monophadnus species were studied: species A collected from Helleborus foetidus and species B collected from Helleborus viridis. Comparative high-performance liquid chromatographic–photodiode array detection–electrospray ionization–mass spectrometric analyses of plant leaf and insect hemolymph extracts revealed the presence of furostanol saponins in all samples. Larvae of species A and B actively sequestered (25R)-26-[(α-l-rhamnopyranosyl)oxy]-22α-methoxyfurost-5-en-3β-yl O-β-d-glucopyranosyl-(1→3)-O-[6-acetyl-β-d-glucopyranosyl-(1→3)]-O-β-d-glucopyranoside (compound 1). This compound occurred at a 65- to 200-fold higher concentration in the hemolymph of the two species (1.6 and 17.5 μmol/g FW, respectively) than in their host plant (0.008 and 0.268 μmol/g FW, respectively). In M. monticola, compound 1 was found at a concentration (1.2 μmol/g FW) similar to that in the host plant (1.36 μmol/g FW). The compound could not be detected consistently in M. alpicola larvae where, however, a related saponin may be present. Additional furostanol saponins were found in H. foetidus and H. viridis, but not in the two Monophadnus species feeding on them, indicating that sequestration of compound 1 is a highly specific process. In laboratory bioassays, crude hemolymph of three Monophadnus species showed a significant feeding deterrent activity against a potential predator, Myrmica rubra ant workers. Isolated furostanol saponins were also active against the ants, at a concentration range similar to that found in the hemolymph. Thus, these compounds seem to play a major role for chemical defense of Monophadnus larvae, although other plant secondary metabolites (glycosylated ecdysteroids) were also detected in their hemolymph. Physiological and ecological implications of the sequestered furostanol saponins are discussed. Dedicated to the memory of Professor Ivano Morelli (1940–2005)     

Field response of spruce bark beetle,Ips typographus,to aggregation pheromone candidates

Six compounds previously identified from hindguts of unmated maleIps typographus (L.) during host colonization: 2-methyl-3-buten-2-ol (MB),cis-verbenol (cV),trans-verbenol (tV), myrtenol (Mt),trans-myrtanol (tM), and 2-phenylethanol (PE), were tested for their attractivity in the field with a subtractive method. The amounts of MB and cV released from a pipe trap were similar to those given off from the commercial bait Ipslure as well as that from a Norway spruce tree,Picea abies (L.) Karst., under mass attack. The blend of the compounds became nonattractive when either MB or cV was subtracted, while subtraction of any of the other four compounds had no effect. Addition of ipsdienol (Id) to the blend did not significantly increase the attraction. In a second comparative test, the addition of three compounds as a group (tV + Mt + PE) to MB + cV again had no effect on the attraction, but the addition of Id increased the catch somewhat. Addition of host logs to a bait releasing MB + cV at a rate lower than in previous experiments did not influence the attraction to pipe traps. Sticky traps containing natural pheromone sources (50 males in a log), which released 1–5 mg/day of MB as determined by aerations with deuterated MB as internal standard, were less attractive than a synthetic source releasing similar amounts of MB.Coleoptera: Scolytidae.This study was made within the Swedish project Odour Signals for Control of Pest Insects.     

Sequestration of host plant-derived compounds by geometrid moth, Milionia basalis, toxic to a predatory stink bug, Eocanthecona furcellata

A predatory stink bug, Eocanthecona furcellata, died after feeding on Milionia basalis larvae. The compounds toxic to E. furcellata were isolated from the hemolymph of M. basalis larvae and identified as inumakilactone A, nagilactone C, and nagilactone C glucoside. The concentrations of inumakilactone A, nagilactone C, and nagilactone C glucoside in the hemolymph of the final instar larvae were 130, 50, and 770 g/ml, respectively. Nagilactone C showed the highest insecticidal activity against second-instar nymphs of E. furcellata, while nagilactone C glucoside showed the lowest, one twentieth of that of nagilactone C. When mixed compounds were given at the same concentrations as those in hemolymph of M. basalis, all nymphs of E. furcellata died with in three days. Inumakilactone A and nagilactone C were found to be in the leaves of podocarp, Podocarpus macrophyllus, the only host plant of M. basalis, at concentrations of 13 and 175 g/g fresh weight, respectively. However, no nagilactone C glucoside was detected in the leaves of this species. These results suggested that M. basalis may transform nagilactone C to its glucoside.     

Up in smoke: I. Smoke-derived germination cues for postfire annual,Nicotiana attenuata torr. Ex. Watson

Some postfire annuals with dormant seeds use heat or chemical cues from charred wood to synchronize their germination with the postfire environment. We report that wood smoke and polar extracts of wood smoke, but not the ash of burned wood, contain potent cue(s) that stimulate germination in the postfire annual plant,Nicotiana attenuata. We examined the responses of seeds from six populations of plants from southwest Utah to extracts of smoke and found the proportion of viable seeds that germinated in the presence of smoke cues to vary between populations but to be consistent between generations. With the most dormant genotypes, we examine three mechanisms by which smoke-derived chemical cues may stimulate germination (chemical scarification of the seed coat and nutritive- and signal-mediated stimulation of germination) and report that the response is consistent with the signal-mediated mechanism. The germination cue(s) found in smoke are produced by the burning of hay, hardwood branches, leaves, and, to a lesser degree, cellulose. Moreover, the cues are found in the common food condiment, liquid smoke, and we find no significant differences between brands. With a bioassay-driven fractionation of liquid smoke, we identified 71 compounds in active fractions by GC-MS and AA spectrometry. However, when these compounds were tested in pure form or in combinations that mimicked the composition of active fractions over a range of concentrations, they failed to stimulate germination to the same degree that smoke fractions did. Moreover, enzymatic oxidation of some of these compounds also failed to stimulate germination. In addition, we tested 43 additional compounds also reported from smoke, 85 compounds that were structurally similar to those reported from smoke and 34 compounds reported to influence germination in other species. Of the 233 compounds tested, 16 proved to inhibit germination at the concentrations tested, and none reproduced the activity of wood smoke. By thermally desorbing smoke produced by cellulose combustions that was trapped on Chromosorb 101, we demonstrate that the cue is desorbed between 125 and 150°C. We estimate that the germination cues are active at concentrations of less than 1 pg/seed and, due to their chromatographic behavior, infer that a number of different chemical structures are active. In separate experiments, we demonstrate that cues remain active for at least 53 days in soil under greenhouse conditions and that the application of aqucous extracts of smoke to soil containing seeds results in dramatic increases in germination of artificial seed banks. Hence, although the chemical nature of the germination cue remains elusive, the stability of the germination cues, their water-solubility, and their activity in low concentrations suggest that these cues could serve as powerful tools for the examination of dormant seed banks and the selective factors thought to be important in the evolution of postfire plant communities.     

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--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.     

Comparative effects of two plant secondary metabolites on host-parasitoid association

Two plant-derived allelochemicals, berberine and -terthienyl (- T), were tested for their effects on the European corn borer,Ostrinia nubilalis, and its endoparasitoidDiadegma terebrans. The compounds were administered to the host insect in meridic diets, and the responses of the host larvae and parasitoids reared from treated hosts were measured in terms of growth parameters and survival. InO. nubilalis, survival to pupation and adult emergence were reduced significantly by the inclusion of berberine and -T in larval diets at a concentration of 100 g/g. However, in the parasitoid, adverse effects were much more apparent with the -T treatment than with the berberine treatment. -T and one of its metabolites were found in host larvae and in emerged adult parasitoids and their cocoons. Berberine residues were not detected. The implications of these responses to compounds of widely differing physiological properties are discussed with reference to host-plant resistance and biological control.