Defensive role ofAllium sulfur compounds for leek mothAcrolepiopsis assectella Z. (Lepidoptera) against generalist predators

It has been shown previously that sulfur volatiles produced byAllium plants affect the behavior of their specialist phytophages and of their specialist entomophages. The action of these compounds in protecting the leek mothAcrolepiopsis assectella against generalist entomophages was studied in comparison to the proposed original defensive role of these compounds against generalist herbivorous insects. Two ants species,Formica selysi andF. fusca, were used as generalist predators. Six behavioral criteria of the predatory behavior of the ants were studied in presence of the last-instar caterpillars (C₅). C₅ reared on artificial diets with or without leek components were tested, as well as C₅ soaked in frass of leek-reared caterpillars or disulfide solutions. In addition, the response of the ants to pure chemicals found in leek was studied using honey solutions with or without sulfur compounds. The sulfur allelochemicals ofAllium plants have a negative action on predatory ants. Interestingly, the nonvolatile precursors of sulfur volatiles ofAllium plants seem to have a protective role for their phytophagous insects against generalist entomophages.     

Origin of kairomones in the leek moth (Acrolepiopsis assectella,Lep.) frass

Feeding leek moths on an artificial diet has shown that dimethyl and dipropyl disulfides and methyl-propyl disulfide found in frass arise from sulfur compounds specific toAllium. The addition of either propyl or methyl disulfide or their precursors to the diet leads to appearance of the three disulfides in the frass. This implies the transformation of theS-propyl moiety toS-methyl and vice versa by an as yet unknown mechanism.     

Iridoid glycosides and host-plant specificity in larvae of the buckeye butterfly,Junonia coenia (Nymphalidae)

Larvae of the buckeye,Junonia coenia (Nymphalidae) feed primarily on plants in four families: Scrophulariaceae, Plantaginaceae, Verbenaceae, and Acanthaceae. These plant families have in common the presence of a group of plant secondary compounds, the iridoid glycosides. Larvae were reared on three plant species and two artificial diets, one with and one without iridoid glycosides.Larvae grew poorly and had low survivorship on the artificial diet without iridoid glycosides, while growth and survival on the artificial diet with iridoid glycosides was comparable to that on plants. Choice tests using artificial diets with and without iridoid glycosides showed that larvae: (1) chose diets with iridoid glycosides (in the form of a crude extract or pure compound) over a diet without; (2) showed no preference between the diet with the crude extract and that with pure iridoid glycoside, and (3) preferred the artificial diet with ground leaves of the host plant,Plantago lanceolata, over the diet with pure iridoid glycosides. The artificial diet that larvae had been reared on prior to these tests had no effect on subsequent larval preferences in the choice tests.     

Increased Sulfur Precursors and Volatiles Production by the Leek <Emphasis Type="Italic">Allium porrum</Emphasis> in Response to Specialist Insect Attack

To defend themselves against herbivory, plants use a variety of direct and indirect strategies involving induced increases in secondary substances. Species of the Allium genus (Alliaceae), such as the leek Allium porrum (L.), produce nonprotein sulfur amino acids derived from cysteine, i.e., alk(en)yl-cysteine sulfoxides that are precursors of volatile thiosulfinates and disulfides. These defend most species including the specialist leek moth, Acrolepiopsis assectella. We determined by measuring the increase in the sulfur precursor propyl-cysteine sulfoxide (PCSO) if production of this precursor is induced in response to moth attack and mechanical wounding. The concentration of PCSO was determined by HPLC in 2- or 6-mo-old leeks after attacks of various intensity either by the specialist leek moth or by a generalist moth, Agrotis ipsilon. Injury-induced release of sulfur volatiles was measured by GC/MS after the attacks. Results showed an increase in the production of sulfur compounds in both the precursor and volatile form, occurring only in association with intensive attacks by leek moths. The increase in sulfur precursors also led to an increase in the release of sulfur volatiles. This induced response may provide an effective defense strategy against the plant’s main natural enemy, both directly and indirectly by attracting entomophagous insects.     

Transfer, Perception, and Activity of Male Pheromone of Acrolepiopsis assectella with Special Reference to Conspecific Male Sexual Inhibition

In the leek moth, Acrolepiopsis assectella, the male, stimulated by a calling female, produces a sexual pheromone that is active on the female. The male-produced pheromone blend contains eight alkanes previously isolated from the male hair-pencils. We used EAG techniques to study the effect of the pheromone on females reared on leek or on artificial diet and on males reared on leek. The optimal stimulation duration appeared to be 1.6 sec exposure to the pheromone or components. The concentrations tested on the antenna were of the order of 10¹² molecules/cm³. The hair-pencil extract tested was of an estimated concentration of around 10⁹ molecules/cm³. The antennal responses are expressed relative to responses to a standard, amyl acetate, but also as an absolute value. Generally, females reared on artificial diet and males reared on leek responded better than females reared on leek. Of the alkanes tested, those present in hair-pencils gave higher responses, with hexadecane always giving the strongest response. A possible inhibiting activity of male leek moth pheromone on the sexual behavior of conspecific males was investigated. The behavior of sexually stimulated males was observed in the presence of other males, hair-pencil extracts, and different compounds either pure or in a mixture. The experiments established that in this species, male pheromone inhibits wing fluttering duration of conspecific males. This inhibition was obtained not only with fluttering males as a source of pheromone but also with all the alkanes tested. The inhibition was due to hair-pencil chemicals, particularly if these were perceived by olfaction plus contact. Wing fluttering increased the inhibitory activity of male-derived alkanes.     

Effects of Sulfur-Containing Precursors of Allium Secondary Metabolites on Behavior and Physiology of Diadromus pulchellus, a Specialist Parasitoid of Acrolepiopsis assectella

Diadromus pulchellus, a monophagous parasitoid of the leek moth, Acrolepiopsis assectella, a specialist of plants in the Allium genus, was raised on four series of chrysalids coming from four diets: leek leaves (LL), an artificial diet with (D + L) or without (D – L) leek powder, and an artificial diet plus 1% methyl-cysteinesulfoxide, a precursor of characteristic sulfur volatiles (D + P). Various behavioral and physiological responses by the parent parasitoids and the progeny were studied. No differences were observed in behavior among the four series. The higher precursor concentration observed in LL and in D + P than in D + L and D – L was responsible for a lower emergence ratio and higher egg production in LL and D + P. The consequences of these variations are discussed in relation to narrowness of the tritrophic system. The fact that sulfur volatiles favorable to behavior arise from nonvolatile sulfur precursors, having negative effects on physiology, can also contribute to the maintenance of the system.     

Effects of Sequestered Iridoid Glycosides on Prey Choice of the Prairie Wolf Spider, Lycosa carolinensis

Specialist insect herbivores that sequester allelochemicals from their host plants may be unpalatable to potential predators. However, the host-plant species used may determine the degree of palatability. Spiders, including members of the family Lycosidae, are important predators of invertebrate prey. We fed buckeye caterpillars, Junonia coenia (Nymphalidae), reared on Plantago lanceolata (containing high levels of iridoid glycosides) or P. major (containing low levels of iridoid glycosides) to prairie wolf spiders, Lycosa carolinensis (Lycosidae), to determine whether the spiders found insects that sequester iridoid glycosides unpalatable. In a field experiment, spiders ate caterpillars reared on P. major significantly more often than caterpillars reared on P. lanceolata, although they attacked equal numbers of both types of prey. Spiders that bit caterpillars behind their heads or along the middle of their backs prevented caterpillars from implementing deterrent defensive strategies such as regurgitating or defecating. In a laboratory experiment, we presented spiders with P. lanceolata-reared and P. major-reared caterpillars simultaneously for eight consecutive trials. Spiders consumed P. major-reared buckeyes significantly more often than P. lanceolata-reared caterpillars. We found no evidence that the spiders learned to avoid the unpalatable prey.     

Stratospheric ozone depletion and plantinsect interactions: Effects of UVB radiation on foliage quality ofCitrus jambhiri forTrichoplusia ni

Projected decreases in stratospheric ozone may result in increases in shortwave ultraviolet (UVB) irradiation at the earth's surface. Furanocoumarins, phototoxic compounds found inCitrus jambhiri foliage, increase in concentration when these plants are grown under enhanced UVB. Survivorship schedules ofTrichoplusia ni (Lepidoptera: Noctuidae) caterpillars reared on plants in the presence and absence of enhanced UVB regimes differ significantly; larvae develop more slowly in early life when reared on plants exposed to increased UVB. This same developmental pattern is observed whenT. ni larvae are reared on artificial diets amended with ecologically appropriate amounts of furanocoumarins. Thus, anthropogenically derived changes in stratospheric ozone and concomitant changes in UV light quality at the earth's surface may influence ecological interactions between insects and their host plants by altering secondary metabolism and hence foliage quality for herbivores.     

Mechanisms of behavioral alterations of parasitoids reared in artificial systems

A high quality of mass reared parasitoids is required for successful biological control of pest insects. Although the phenomenon of behavioral deterioration of parasitoids due to rearing in artificial conditions is well known, its significance is often underestimated, and the underlying mechanisms are poorly investigated. We quantified behavioral alterations of parasitoids reared in an artificial system vs. a natural system and elucidated some of the mechanisms involved. The model systems consisted of apple fruits (natural system) or an artificial diet devoid of apple (artificial system), the herbivore Cydia pomonella, and its larval parasitoid Hyssopus pallidus, a candidate biological control agent. Two parasitoid strains, one reared for 30 generations in the natural system and one in the artificial system, were compared by using the females' ability to respond to frass from codling moth caterpillars fed on apple fruits (apple-frass). The searching response of parasitoids reared in the artificial system compared to those reared in the natural system was reduced by an average of 53.2%. Gas chromatography–mass spectrometry (GC-MS) analyses of the two types of caterpillars' food and of the two corresponding types of frass showed that 15 compounds were present only in apple fruits and apple-frass, three compounds only in artificial diet and artificial-diet-frass, while four compounds were present in both frass types but not in the food sources. This suggests the presence of a food-derived and a host-derived component in the frass. Results from both bioassays and chemical analyses indicate that the kairomonal activity of the frass is due to both apple fruit and host components. The reduced response of parasitoids reared in artificial conditions might, therefore, be due to a lack of recognition of the apple fruit component. In a further experiment, the two parasitoid strains were reared in the opposite system for one generation. While the response to the host frass was significantly reduced in parasitoids that emerged from the artificial system, it was fully restored in parasitoids that emerged from the natural system. This indicates that the behavioral alteration was related to a learning process during ontogenesis rather than to a selection exhibited over generations.     

Behavioral Effects of Carotenoid Sequestration by the Parsnip WebwormDepressaria pastinacella

The parsnip webwormDepressaria pastinacella, acquires a distinct yellow stripe when it consumes the yellow flowers of its principal host plantPastinaca sativa, the wild parsnip. Caterpillars raised on artificial diet lacking host-plant material lack this yellow coloration. By chemical characterization and comparison of caterpillars raised on parsnip flowers and on artificial diet, we were able to determine that lutein, along with smaller amounts of other xanthophylls from the host plant, is selectively sequestered in the fat body. In bioassays designed to measure avoidance of ultraviolet light, caterpillars raised on parsnip flowers or on artificial diet supplemented with lutein were less likely to avoid exposure to ultraviolet light than caterpillars raised on unaugmented artificial diet and thus lacking sequestered carotenoids. The ability to sequester xanthophylls, which are highly effective antioxidants, may confer a selective advantage on these caterpillars, whose apiaceous host plants produce large quantities of furanocoumarins, natural products that are photoactivated by light wavelengths in the ultraviolet region; such sequestered pigments may reduce not only the oxidative stress associated with ultraviolet light and diurnal foraging but also the photooxidative stress associated with ingestion of photoactive furanocoumarins.     

Accumulation of Glucosinolates by the Cabbage Aphid Brevicoryne brassicae as a Defense Against Two Coccinellid Species

Brassica nigra plants, characterized by high levels of sinigrin, and artificial aphid diets to which sinigrin was selectively added were used to rear the crucifer specialist, Brevicoryne brassicae. Aphids were provided as a food source to two species of polyphagous ladybird, Adalia bipunctata and Coccinella septempunctata. First instar A. bipunctata were unable to survive when fed with B. brassicae reared on B. nigra or diets containing 0.2% sinigrin, but when fed with aphids reared on diets containing 0% sinigrin, survival rates were high. By contrast, first instar C. septempunctata were able to survive when fed with aphids reared on B. nigra or artificial diets containing up to 1% sinigrin. However, the presence of sinigrin in the aphid diet decreased larval growth and increased the time necessary for larvae to reach second instar for this species of ladybird. These results indicate that the presence of sinigrin in the diet of B. brassicae makes this aphid unsuitable as a food source for A. bipunctata but not for C. septempunctata, although for this ladybird species, there appear to be costs associated with feeding on aphids that contain this secondary metabolite.     

The importance of sequestered iridoid glycosides as a defense against an ant predator

We reared larvae ofJunonia coenia Hubner (Nymphalidae) on artificial diets with trace concentrations of iridoid glycosides and on leaf diets with higher concentrations of iridoid glycosides. We offered these caterpillars to predacious ants and observed the effects of the following on predation: diet (artificial vs. leaf), site (ant colonies in dry vs. wet areas), instar (early vs. late), and time (changes in predation over five days). Diet and site were consistently significant predictors of the ants' propensities to reject prey and the caterpillars' abilities to escape predation. Leaf-diet caterpillars escaped more frequently than artificial-diet caterpillars, and ants from dry sites were more likely to reject prey than ants from wet sites. The percentage of iridoid glycosides found in individual caterpillars was also a good predictor of the probability of rejection by predators and prey escape. Caterpillars with higher levels of iridoids were more likely to be rejected and to escape, suggesting that sequestered iridoid glycosides are a defense against predaceous ants.     

Changes in Chemical Signature and Host Specificity from Larval Retrieval to Full Social Integration in the Myrmecophilous Butterfly Maculinea rebeli

The ant social parasite, Maculinea rebeli shows high levels of host specificity at a regional scale. While 68-88% of caterpillars in the field are adopted by nonhost Myrmica ants, 95-100% of the butterflies emerge from the natural host M. schencki the following year. While retrieval of preadoption caterpillars is specific to the genus Myrmica, it does not explain differential survival with different Myrmica species. We present survival data with host and nonhost Myrmica species suggesting that, with nonhosts (M. sabuleti and M. rubra), survival depends on the physiological state of the colony. We also compared the similarities of the epicuticular surface hydrocarbon signatures of caterpillars that were reared by host and nonhost Myrmica for 3 weeks with those from tending workers. Counterintuitively, the hydrocarbons of postadoption caterpillars were more similar (78%, 73%) to the ant colony profiles of the nonhost species than were caterpillars reared in colonies of M. schencki (42% similarity). However, caterpillars from M. schencki nests that were then isolated for 4 additional days showed unchanged chemical profiles, whereas the similarities of those from nonhost colonies fell to 52 and 56%, respectively. Six compounds, presumably newly synthesized, were detected on the isolated caterpillars that could not have been acquired from M. sabuleti and M. rubra (nor occurred on preadoption caterpillars), five of which were found on the natural host M. schencki. These new compounds may relate to the high rank the caterpillars attain within the hierarchy of M. schencki societies. The same compounds would identify the caterpillars as intruders in non-schencki colonies, where their synthesis appeared to be largely suppressed. The ability to synthesize or suppress additional compounds once adopted explains the pattern of mortalities found among fully integrated caterpillars in Myrmica colonies of different species and physiological states.     

Phenolic Metabolites in Leaves of the Invasive Shrub, Lonicera maackii, and Their Potential Phytotoxic and Anti-Herbivore Effects

Lonicera maackii is an invasive shrub in North America for which allelopathic effects toward other plants or herbivores have been suspected. We characterized the major phenolic metabolites present in methanol extracts of L. maackii leaves. In addition, we examined the effects of methanol–water extracts of L. maackii leaves on seed germination of a target plant species and on feeding preference and growth rate of a generalist insect herbivore. A total of 13 individual major and minor compounds were detected in crude leaf extracts by high-performance liquid chromatography coupled to electronspray ionization-tandem mass spectrometry (ESI-MS/MS). Extracts were dominated by two major flavones, apigenin and luteolin, and their glucoside derivatives, apigenin-7-glucoside and luteolin-7-glucoside. Quantities of these compounds, along with chlorogenic acid, varied between two sampling points. Leaf extracts that contained these compounds were inhibitory to seed germination of Arabidopsis thaliana. In addition, treatment of artificial diet with leaf extracts deterred feeding of the generalist herbivore, Spodoptera exigua, in choice experiments but had no effect on growth rate in short-term no-choice bioassays. Purified apigenin tended to deter feeding by S. exigua and inhibited seed germination of A. thaliana. We conclude that leaves of L. maackii contain phenolic compounds, including apigenin and chlorogenic acid, capable of having biological effects on other plants and insects.     

The Effects of Ants on the Entomophagous Butterfly Caterpillar Feniseca tarquinius, and the Putative Role of Chemical Camouflage in the Feniseca–Ant Interaction

Butterfly caterpillars in the lycaenid subfamily Miletinae are predators of ant-tended Homoptera, yet they lack specialized secretory and call-production organs crucial to ant association in other lycaenids. Here, we address the question of how miletine caterpillars have invaded the ant–Homoptera symbiosis through a study of the only New World miletine, Feniseca tarquinius, a predator of the wooly aphid Prociphilus tesselatus. Previous interpretations have suggested that F. tarquinius and other miletine caterpillars avoid ant aggression by concealing themselves under silken webs. In contrast, our field data indicate that F. tarquinius caterpillars are less likely to be concealed in the presence of the ants Camponotus pennsylvanicus and Formica obscuriventris than in the absence of ants, although caterpillar and ant behaviors vary between years. Chemical analysis and behavioral assays suggest that chemical camouflage, not physical concealment, is responsible for the ants’ failure to detect and remove F. tarquinius caterpillars from aphid colonies. Analyses by gas chromatography indicate that the cuticular lipid composition of caterpillars are similar to that of their aphid prey, although it varies with prey species. Behavioral assays confirm that solvent extracts of F. tarquinius caterpillars and P. tesselatus aphids evoke similar behavioral responses in C. pennsylvanicus ants. Chemical camouflage is well known in social parasites of ants, but the present study represents one of a few documented cases where chemical deceit is important to interactions with ants outside the nest.     

Reduction in diet breadth results in sequestration of plant chemicals and increases efficacy of chemical defense in a generalist grasshopper

The lubber grasshopper,Romalea guttata, is a generalist feeding on a broad diet of many herbaceous plant species and has a metathoracic defensive secretion normally containing phenolics and quinones synthesized by the insect. When insects were reared on a restricted diet of wild onion, they sequestered sulfur volatiles from the plant into their defensive secretions. These compounds were not detected by gas chromatography-mass spectroscopy in secretions of insects on an artificial diet or a natural, generalist diet of 26 plants that included wild onion as a component, nor were they present in secretions from field-collected insects. Defensive secretions of insects reared on wild onion were significantly more deterrent, by as much as an order of magnitude, to two species of ant predators than secretions from insects on either of the other two diets, despite a reduction in the concentration of autogenous defensive chemicals in secretions of insects on the onion diet. Sequestration of plant chemicals that increased defensive efficacy occurred when diet breadth was reduced. We suggest that this occurs because under conditions of specialization, plant secondary metabolites are more likely to be ingested and bioaccumulated in sufficient concentrations to have biological activity against predators. What we define as casual bioaccumulation of bioactive plant chemicals following dietary specialization may lead to evolution of sequestered defense syndromes in insects, and this process may not necessarily require specific adaptation to or coevolution with a toxic host plant.     

Larval Beetles Form a Defense from Recycled Host-Plant Chemicals Discharged as Fecal Wastes

Larvae of the leaf-feeding beetles Neolema sexpunctata and Lema trilinea carry feces on their backs that form shields. We used the generalist predatory ant, Formica subsericea, in a bioassay to determine whether shields were a physical barrier or functioned as a chemical defense. Fecal shields protected both species against ant attack. Larvae of both species reared on lettuce produced fecal shields that failed to deter ants. Commelina communis, N. sexpunctata's host, lacks noxious secondary compounds but is rich in phytol and fatty acids, metabolites of which become incorporated into the fecal defense. In contrast, the host plant of L. trilinea, Solanum dulcamara, contains steroidal glycoalkaloids and saponins, whose partially deglycosylated metabolites, together with fatty acids, appear in Lema feces. Both beetle species make modifications to host-derived precursors before incorporating the metabolites into shields. Synthetic chemicals identified as shield metabolites were deterrent when applied to baits. This study provides experimental evidence that herbivorous beetles form a chemical defense by the elimination of both primary and secondary host-derived compounds. The use of host-derived compounds in waste-based defenses may be a more widely employed strategy than was hitherto recognized, especially in instances where host plants lack elaborate secondary compounds.     

Effects of diet breadth on autogenous chemical defense of a generalist grasshopper

The lubber grasshopper,Romalea guttata, produces a metathoracic defensive secretion containing primarily phenolics and quinones. This insect feeds on a wide range of plant species. Insects reared on an artificial diet and a diet of onion,Allium canadense, had secretions that contained fewer compounds, lower concentrations of compounds, and markedly altered relative composition of components compared to insects reared on a varied diet of 26 plant species that included onion. The study demonstrates that diet breadth has a major effect on the quality and quantity of the autogenous defensive secretion of this generalist herbivore. The results are compared to diet effects known in chemically defended specialists. Two possible mechanisms explaining the effects of diet breadth are proposed: one involves changes in precursor availability with changing diet breadth; the other suggests that physiological stress due to diet restriction changes allocation of resources to chemical defense.     

Predatory Behavior of <Emphasis Type="Italic">Polistes dominulus</Emphasis> Wasps in Response to Cardenolides and Glucosinolates in <Emphasis Type="Italic">Pieris napi</Emphasis> Caterpillars

To examine how plant allelochemicals in prey affect foraging choices made by generalist predator paper wasps, Polistes dominulus (Vespidae), we compared predation on Pieris napi (Pieridae) caterpillars reared on host plants with different allelochemicals. In naturalistic behavioral choice experiments, free-flying wasps chose between living pierids reared on cabbage (Brassica oleracea), which lacks deterrent allelochemicals, or alternate host plants with potentially deterrent allelochemicals. The alternative host plants were: wormwood mustard, (Erysimum cheiranthoides: Brassicaceae), which contains cardenolides; nasturtium (Tropaeolum majus: Tropaeolaceae) with high concentrations of chlorogenic acid; and black mustard (Brassica nigra: Brassicaceae) with high concentrations of the aliphatic glucosinolate, sinigrin. Although wasps captured equal numbers of caterpillars reared on cabbage and each alternate host plant, they spent significantly longer handling prey from the alternate host plants as they selectively removed the caterpillar’s gut, which contained the plant material. This was true even if the caterpillar did not sequester toxins in its tissues, as revealed by high performance liquid chromatography (HPLC) analysis of Erysimum-reared pierids. Because handling time is longer, predators that capture pierids containing non-sequestered allelochemicals experience an overall reduction in foraging rate that may translate into a fitness cost.     

Induction of volatile emissions in maize by different larval instars of Spodoptera littoralis

Maize plants under attack by caterpillars emit a specific blend of volatiles that is highly attractive to parasitic wasps. The release of these signals is induced by elicitors in the caterpillar regurgitant. Studies suggest that plants respond differently to different herbivore species and even to different herbivore stages, thus providing parasitoids and predators with specific signals. We tested if this is the case for different larval instars of the noctuid moth Spodoptera littoralis when they feed on maize plants. Cut maize plants were incubated in diluted regurgitant from second, third, or fifth instar caterpillars. There were no differences in total amount released after these treatments, but there were small differences in the release of the minor compounds phenethyl acetate and -humulene. Regurgitant of all three instars contained the elicitor volicitin. To test the effect of actual feeding by the larvae, potted plants were infested with caterpillars of one of the three instars, and volatiles were collected the following day. The intensity of the emissions was correlated with the number of larvae feeding on a plant, and with the amount of damage inflicted, but was independent of the instar that caused the damage. We also used artificial damage to mimic the manner of feeding of each instar to test the importance of physical aspects of damages for the odor emission. The emission was highly variable, but no differences were found among the different types of damage. In olfactometer tests, Microplitis rufiventris, a parasitoid that can only successfully parasitize second and early third instar S. littoralis, did not differentiate among the odors of maize plants attacked by different instar larvae. The odor analyses as well as the parasitoid's responses indicate that maize odors induced by S. littoralis provide parasitoids with poor information on the larval developmental stage. We discuss the results in the context of variability and lack of specificity in odorous plant signals.