The role of phytosterols in host plant utilization by cactophilicDrosophila

The Cactus-Drosophila Model System of the Sonoran Desert consists of four endemic species ofDrosophila (D. mojavensis, D. nigrospiracula, D. mettleri andD. pachea) and five species of columnar cacti (agria, organpipe, saguaro, cardón and senita). Extensive collection records indicate that each cactus species has only one species ofDrosophila as the primary resident. The elimination of six of the twenty possible random combinations ofDrosophila species and cactus species can be attributed directly to phytosterols.Drosophila pachea has a strict requirement for Δ⁷-sterols such as 7-cholestenol and 7-campestenol. Since Δ⁷-sterols are found only in senita cactus,D. pachea cannot use agria, organpipe, saguaro or cardón as host plants. The lipid fractions of agria and organpipe are chemically similar and contain high concentrations of several 3β,6α-dihydroxysterols. Larval viability tests using chemical constitutents of organpipe cactus demonstrate that the sterol diols are toxic toD. nigrospiracula but not to the resident, species,D. mojavensis. Agria and organpipe are therefore unsuitable as host plants forD. nigrospiracula. These results suggest that phytosterols play a major role in determining host plant utilization by cactophilicDrosophila in the Sonoran Desert. Deceased.     

Ecological aspects of cactus triterpene glycosides I. Their effect on fitness components ofDrosophila mojavensis

The effects of pentacyclic triterpene glycosides extracted from agria and organ pipe cacti on three fitness parameters of the cactophilic fruit fly,Drosophila mojavensis were tested. Triterpene glycosides from organ pipe increased development time and reduced larval viability while those from agria produced smaller adults (reduced fecundity). In addition, the microbial communities in the organ pipe saponin media were less dense than those in the media to which agria saponins had been added. The role of cactus triterpene glycosides in the ecology of thisDrosophila species is discussed.     

Combining Mutualistic Yeast and Pathogenic Virus — A Novel Method for Codling Moth Control

The combination of a pathogenic virus and mutualistic yeasts isolated from larvae of codling moth Cydia pomonella is proposed as a novel insect control technique. Apples were treated with codling moth granulovirus (CpGV) and either one of three yeasts, Metschnikowia pulcherrima, Cryptococcus tephrensis, or Aureobasidium pullulans. The combination of yeasts with CpGV significantly increased mortality of neonate codling moth larvae, compared with CpGV alone. The three yeasts were equally efficient in enhancing the activity of CpGV. The addition of brown cane sugar to yeast further increased larval mortality and the protection of fruit against larvae. In comparison, without yeast, the addition of sugar to CpGV did not produce a significant effect. A field trial confirmed that fruit injury and larval survival were significantly reduced when apple trees were sprayed with CpGV, M. pulcherrima, and sugar. We have shown earlier that mutualistic yeasts are an essential part of codling moth larval diet. The finding that yeast also enhances larval ingestion of an insect-pathogenic virus is an opportunity for the development of a novel plant protection technique. We expect the combination of yeasts and insect pathogens to essentially contribute to future insect management.     

Selective uptake and lack of dealkylation of phytosterols by cactophilic species ofDrosophila

The selective absorption of cholesterol and campesterol and discrimination against sitosterol was demonstrated withDrosophila mojavensis andDrosophila nigrospiracula, using a synthetic medium and 2 natural host cacti. Preferential absorption of trace amounts of phytosterols from a large quantity of other lipids occurred whenD. arizonensis andD. mojavensis were reared from agria cactus. The absence of dealkylation of sitosterol to cholesterol was demonstrated withD. mojavensis andD. mettleri, reared on a sterol-deficient medium supplemented with sitosterol. Arizona Agricultural Experiment Station journal article no. 3815.     

Chemical stimuli in host-habitat location byLeptopilina heterotoma (Thomson) (Hymenoptera: Eucoilidae), a parasite ofDrosophila

Chemical stimuli play an important role in the process of searching for a host habitat by parasitic wasps. Volatile compounds originating from host habitats and/or hosts are the cues that enable such a location.Leptopilina heterotoma, a larval parasite ofDrosophila, is attracted to the food of its host, baker's yeast. Analysis of the fermentation products of baker's yeast, using a mass spectrometer, and olfactometer studies indicate that three fermentation products of this yeast, the main component of the host habitat in our laboratory, attractL. heterotoma: ethanol (5%), ethyl acetate (10^–2, 10^–3%), and acetaldehyde (1%). A combination of these three compounds, however, cannot compete with baker's yeast in attracting the parasites. Thus other factors, such as different compounds, concentrations, and/or combinations, also, play a role and remain to be tested.Leptopilina heterotoma does not use host-related olfactory cues in long-distance habitat location as it cannot distinguish between host habitat and host habitat with hosts.     

Comparison of larval and adult P-450 activity levels for alkaloid metabolism in desertDrosophila

The cytochrome P-450 monooxygenase system has been implicated in plant utilization by at least three species ofDrosophila (D. nigrospiracula, D. mettleri, andD. mojavensis) that are endemic to the Sonoran Desert of the southwestern United States and northwestern Mexico. Basal and induced levels of total cytochrome P-450 were determined for third-instar and decapitated 2- to 5-day post eclosion adults of the three desert species. Total P-450 levels, both basal and induced for all species assayed, were significantly higher for adults than for larvae by up to 20-fold. On a per organism basis, the levels of in vitro metabolism of the cactus alkaloid, carnegine, and patterns of response to induction by cactus tissue for adult desertDrosophila approximated those of larvae. Induction by phenobarbital, however, resulted in levels of in vitro carnegine metabolism that were up to 5.6-fold higher in adults than in larvae.     

Premating Isolation Is Determined by Larval Rearing Substrates in CactophilicDrosophila mojavensis. III. Epicuticular Hydrocarbon Variation Is Determined by Use of Different Host Plants inDrosophila mojavensis andDrosophila arizonae

Adult epicuticular hydrocarbon profiles of male and femaleDrosophila mojavensis have been implicated as determinants of mate choice leading to premating isolation between geographically isolated populations. Hydrocarbon profiles of a Baja California and a mainland Sonora population ofDrosophila mojavensis, ayellow body mutant strain ofD. mojavensis, and a population ofD. arizonae were compared among flies that had been reared on two cactus substrates and a synthetic laboratory growth medium in order to assess the degree to which natural rearing substrates influence adult hydrocarbon composition. Twenty epicuticular hydrocarbon components, ranging from C₂₉ to C₄₁, were recovered by gas chromatography that represented major classes of alkanes, alkenes, and alkadienes. We found differences in relative amounts of epicuticular hydrocarbons among Baja and mainlandD. mojavensis, and theyellow body mutants. There were few differences betweenD. mojavensis andD. arizonae. The effects of rearing substrates were remarkable: most of the differences were due to the effects of lab food vs. cactus, but there were significant rearing substrate effects due to differences in the two cacti used. Eleven hydrocarbon components differed in abundance between males and females or showed significant sex × rearing substrate interactions from ANOVA. The effects of rearing substrates on epicuticular hydrocarbon composition inD. mojavensis are concordant with changes in the intensity of premating isolation between populations, implicating host ecology as a major determinant in patterns of mate choice in this species.     

Reaction of Mutualistic and Granivorous Ants to Ulex Elaiosome Chemicals

It has been proposed that chemicals on plant elaiosomes aid seed detection by seed-dispersing ants. We hypothesized that the chemical interaction between ants and elaiosomes is more intimate than a generic attraction, and that elaiosome chemicals will attract mutualistic but not granivorous ant species. We investigated this by using two gorse species, Ulex minor and U. europaeus, and two associated ant species from European heathlands, the mutualist Myrmica ruginodis and the granivore Tetramorium caespitum. Behavioral studies were conducted with laboratory nests and foraging arenas. Both ants will take Ulex seeds, but while M. ruginodis showed increased antennation toward ether extracts of elaiosome surface chemicals compared with controls, T. caespitum showed no response. Elaiosome extracts were separated into seven lipid fractions. M. ruginodis showed increased antennation only toward the diglyceride fractions of both Ulex species, whereas T. caespitum showed no consistent reaction. This indicates that M. ruginodis can detect the elaiosome by responding to its surface chemicals, but T. caespitum is unresponsive to these chemicals. Responses to surface chemicals could increase the rate of seed detection in the field, and so these results suggest that Ulex elaiosomes produce chemicals that facilitate attraction of mutualistic rather than granivorous ant species. This could reduce seed predation and increase Ulex fitness.     

Detoxication activity in the gypsy moth: Effects of host CO2 and NO 3 − availability

We investigated the effects of host species and resource (carbon dioxide, nitrate) availability on activity of detoxication enzymes in the gypsy moth,Lymantria dispar. Larvae were fed foliage from quaking aspen or sugar maple grown under ambient or elevated atmospheric CO₂, with low or high soil NO ₃ ^– availability. Enzyme solutions were prepared from larval midguts and assayed for activity of cytochrome P-450 monooxygenase, esterase, glutathione transferase, and carbonyl reductase enzymes. Activity of each enzyme system was influenced by larval host species, CO₂ or NO ₃ ^– availability, or an interaction of factors. Activity of all but glutathione transferases was highest in larvae reared on aspen. Elevated atmospheric CO₂ promoted all but transferase activity in larvae reared on aspen, but had little if any impact on enzyme activities of larvae reared on maple. High NO ₃ ^– availability enhanced activity of most enzyme systems in gypsy moths fed high CO₂ foliage, but the effect was less consistent for insects fed ambient CO₂ foliage. This research shows that gypsy moths respond biochemically not only to interspecific differences in host chemistry, but also to resource-mediated, intraspecific changes in host chemistry. Such responses are likely to be important for the dynamics of plantinsect interactions as they occur now and as they will be altered by global atmospheric changes in the future.     

“This is not an Apple”–Yeast Mutualism in Codling Moth

The larva of codling moth Cydia pomonella (Tortricidae, Lepidoptera) is known as the worm in the apple, mining the fruit for food. We here show that codling moth larvae are closely associated with yeasts of the genus Metschnikowia. Yeast is an essential part of the larval diet and further promotes larval survival by reducing the incidence of fungal infestations in the apple. Larval feeding, on the other hand, enables yeast proliferation on unripe fruit. Chemical, physiological and behavioral analyses demonstrate that codling moth senses and responds to yeast aroma. Female moths are attracted to fermenting yeast and lay more eggs on yeast-inoculated than on yeast-free apples. An olfactory response to yeast volatiles strongly suggests a contributing role of yeast in host finding, in addition to plant volatiles. Codling moth is a widely studied insect of worldwide economic importance, and it is noteworthy that its association with yeasts has gone unnoticed. Tripartite relationships between moths, plants, and microorganisms may, accordingly, be more widespread than previously thought. It, therefore, is important to study the impact of microorganisms on host plant ecology and their contribution to the signals that mediate host plant finding and recognition. A better comprehension of host volatile signatures also will facilitate further development of semiochemicals for sustainable insect control.     

Chemical aspects of host-plant specificity in threeLarrea-feeding grasshoppers

The host-selection behavior of three species of grasshopper feeding on creosote bush,Larrea tridentata, in southern California was investigated. The species wereBootettix argentatus, which is monophagous;Ligurotettix coquilletti, oligophagous; andCibolacris parviceps, polyphagous. The monophagous species is stimulated to bite by nordihydroguaiaretic acid (NDGA), a compound that is characteristic of the host plant and that may comprise up to 10% of the dry weight of the leaf. Host specificity ofB. argentatus is enhanced by deterrent responses to compounds present in the surface waxes of all non-host-plant species. Both the oligophagous and polyphagous species are deterred by NDGA at naturally occurring concentrations. Their association withLarrea is probably based on tolerance of the plant chemicals rather than on dependence on specific chemicals. Factors other than the chemistry of the plant probably also contribute to the specificity ofB. argentatus andL. coquilletti.     

Specific foraging kairomones used by a generalist parasitoid

In general, it is assumed that generalist natural enemies do not innately use specific cues for the location of their host or prey species. This hypothesis was tested using naïve females of the generalist parasitoid Lariophagus distinguendus Förster and two of its hosts, larvae of the lesser grain borer Rhyzopertha dominica (F.) and of the granary weevil Sitophilus granarius L., feeding in wheat grains. In a four-chamber olfactometer, female parasitoids were attracted to volatiles emanating from the feces of both host species. Chemical analysis of the volatiles from the feces of R. dominica revealed the presence of dominicalure 1 and 2, the species specific aggregation pheromones of R. dominica. The main compounds in the volatiles from feces of S. granarius were identified as chemicals related to mites that are associated with hosts of L. distinguendus. Because these mites are not specific for S. granarius but also co-occur with other hosts, the mite chemicals have to be considered as general cues. In bioassays, synthetic dominicalure was attractive to naïve L. distinguendus, explaining the attraction of feces volatiles from R. dominica. Synthetic mite chemicals and sitophilate, the aggregation pheromone of S. granarius, had no effect on naïve parasitoids. It remains to be determined which innate chemical cues from feces of S. granarius are used by L. distinguendus. In contrast to our initial hypothesis, the generalist L. distinguendus is innately using specific cues for foraging. Two ideas are provided to explain this result.     

Role of diet in host selection ofHeliothis virescens by parasitoidCampoletis sonorensis (Hymenoptera: Ichneumonidae)

Gas chromatographic analysis revealed that chemicals attractive toCampoletis sonorensis (Cameron), previously identified in cotton, were not present in wheat germ diet-rearedHeliothis virescens (F.) larvae. Diet-reared larvae fed cotton obtained the chemicals from cotton, with the consequence of enhanced kairomonal activity of the larvae and their frass toC. sonorensis. Parasitoids, presented a choice between cotton, cotton plus hosts, hosts alone, and control in an olfactometer, responded non-randomly, with the greatest number of responses to cotton plus hosts, and three times as many responses to cotton alone as to larvae alone. The role of the plant in the parasitoid-host relationship is discussed.     

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.     

Contrasting Behavioral Responses by Detritivorous and Predatory Mayflies to Chemicals Released by Injured Conspecifics and Their Predators

Larvae of the mayfly Siphlonisca are predators of the detritivorous mayfly Siphlonurus in floodplain wetlands in Maine (USA). Both mayflies are natural prey of brook trout (Salvelinus fontinalis). We exposed larvae of Siphlonurus and Siphlonisca to chemicals from injured conspecifics and their predators. Significant decreases in movement activity by Siphlonurus were elicited by chemicals released from Siphlonisca, chemicals released from brook trout fed conspecifics, and by chemicals released from injured conspecifics. A significant decrease in movement activity by Siphlonisca was elicited by chemicals released from brook trout fed either conspecifics or Siphlonurus. Movement activity by either Siphlonurus or Siphlonisca was not significantly affected by chemicals released from trout feeding on brine shrimp (Artemia). Both Siphlonurus and Siphlonisca were able to detect chemicals that provided information about past feeding behaviour by brook trout. However, their response to the chemicals used in this study was context-specific. A reduction in movement activity, a behavior that presumably reduces the probability of being consumed by visual predators, occurred only when mayflies were exposed to chemicals released by brook trout feeding on conspecific (Siphlonurus) or confamilial (Siphlonisca) prey.     

Leaf and Floral Parts Feeding by Orange Tip Butterfly Larvae Depends on Larval Position but Not on Glucosinolate Profile or Nitrogen Level

In an attempt to identify chemical signals governing the general flower and silique feeding behavior of larvae of the orange tip butterfly, Anthocharis cardamines (L.), we investigated feeding behavior and chemistry of two major host plants: Cardamine pratensis L. and Alliaria petiolata (Bieb.) Cavara & Grande (garlic mustard). Larvae reportedly feed mainly on flowers and siliques rather than leaves in nature, and did so when observed on the original host plants. Behavioral experiments, using detached A. petiolata branches, however, showed that larvae readily accepted leaves and only the final instar showed a tendency for directed movement towards floral parts. To search for semiochemicals that control plant part preference and to assess possible nutritional consequences of floral parts feeding, we determined glucosinolate profiles and total nitrogen levels of floral parts and leaves. There was only moderate difference between glucosinolate profiles of leaves and floral parts within each of two host plant species. In contrast, the profiles of floral parts differed significantly between them. A. petiolata was dominated by 2-propenyl glucosinolate, while C. pratensis was dominated by aromatic glucosinolates and branched aliphatic glucosinolates, with considerable variation among populations. Nitrogen levels tended to be higher in floral parts than in leaves in A. petiolata, but not in C. pratensis, so floral feeding could not generally be attributed to higher N content. With the exception of a tendency of last instar larvae (L5) to move to the apex and ingest flowers and upper stem, we did not find either a plant chemistry basis or larval acceptance/rejection behavior that could explain the usual feeding of floral parts by orange tip larvae of all instars. However, by artificial manipulation of vertical larval position on host plants, we found that the frequency of leaf vs. flower feeding during 24 hr depended significantly on the initial larval position. Hence, we suggest that the placement of eggs on floral parts by ovipositing female butterflies is a major explanation of orange tip feeding habits previously known from field observations.     

Experience-Induced Habituation and Preference Towards Non-Host Plant Odors in Ovipositing Females of a Moth

In phytophagous insects, experience can increase positive responses towards non-host plant extracts or induce oviposition on non-host plants, but the underlying chemical and behavioral mechanisms are poorly understood. By using the diamondback moth, Plutella xylostella, its host plant Chinese cabbage, and a non-host plant Chrysanthemum morifolium, as a model system, we observed the experience-altered olfactory responses of ovipositing females towards volatiles of the non-host plant, volatiles of pure chemicals (p-cymene and α-terpinene) found in the non-host plant, and volatiles of host plants treated with these chemicals. We assessed the experience-altered oviposition preference towards host plants treated with p-cymene. Naive females showed aversion to the odors of the non-host plant, the pure chemicals, and the pure chemical-treated host plants. In contrast, experienced females either became attracted by these non-host odors or were no longer repelled by these odors. Similarly, naive females laid a significantly lower proportion of eggs on pure chemical-treated host plants than on untreated host plants, but experienced females laid a similar or higher proportion of eggs on pure chemical-treated host plants compared to untreated host plants. Chemical analysis indicated that application of the non-host pure chemicals on Chinese cabbage induced emissions of volatiles by this host plant. We conclude that induced preference for previously repellent compounds is a major mechanism that leads to behavioral changes of this moth towards non-host plants or their extracts.     

Laboratory colonization has not reduced constitutive or induced polysubstrate monooxygenase activity in velvetbean caterpillars

We evaluated whether velvetbean caterpillars (Anticarsia gemmatalis) from a laboratory colony had reduced constitutive (basal) and/or induced activities of their polysubstrate monooxygenase (PSMO) detoxification enzyme system as a result of long-term rearing (> 100 generations) on artificial diet without introduction of field-collected individuals. Larvae from the laboratory colony and those from a recently collected field strain were fed either a standard artificial diet (control), one containing the inducing allelochemical, flavone, or foliage ofIndigofera hirsuta (a host plant of this species), and their midgut PSMO activity was assessed by measuring the in vitro rate of aldrin epoxidation. Compared with the field-strain larvae, caterpillars from the laboratory colony had 1.9-fold greater constitutive activity (standard artificial diet) and 2.3-fold greater induced activity (flavone-treated diet). In addition, the magnitude of induction was somewhat greater for the laboratory-colony larvae (induced activity was 2.0-fold greater than constitutive activity) compared with those from the field-strain (1.6-fold). In contrast, no difference in strain activity was found when larvae were fedI. hirsuta foliage. The lower PSMO activity of the field-strain larvae when fed artificial diet may have been caused by their reduced feeding and growth performance compared with laboratory-colony larvae, and it may explain their greater sensitivity to allelochemicals incorporated in the artificial diet, as we found previously. The results of this study indicate that long-term rearing of theA. gemmatalis laboratory colony on artificial diet, without the introduction of field individuals, apparently has not selected for low constitutive activity or decreased inducibility of PSMO, and thus these larvae provide a suitable model for studying xenobiotic detoxication. In addition, they suggest that using an artificial diet to evaluate resistance to pesticides or other xenobiotics in fieldcollected insects, as is frequently done, may underestimate the level of resistance if the diet, through various causes, reduces the activity of detoxification enzymes contributing to the resistance.     

Behavior and chemical disguise of cuckoo antLeptothorax kutteri in relation to its hostLeptothorax acervorum

Females of the obligately parasitic cuckoo ant,Leptothorax kutteri, a workerless inquiline, are among the only adult ants that can successfully invade ant societies and come to be accepted as a nestmate by the existing adult workers. This occurs even though the cuckoo ant is usually severely attacked by theLeptothorax acervorum workers of the colony that she is attempting to enter and parasitize. Through extensive ethogram studies of established parasites and parasitized and free-livingL. acervorum workers and queens, we show that theL. kutteri queen grooms host queens at an exceptionally high frequency. Possibly associated with this behavior, the established parasite is never attacked by theL. acervorum workers or queens she exploits. We show that there is exceptional similarity between the cuticular hydrocarbons and especially the cuticular fatty acids of the parasitic females and her nestmateL. acervorum workers, compared with nonnestmate workers and queens. We suggest that this matching of cuticular compounds may be associated with the grooming of host queens by the parasite. This in turn suggests the possibility that fatty acids have a role in colony-specific nestmate recognition in these and other ants and that grooming may serve for the dissemination of such substances throughout the colony.     

Growth and physiological responses of Black Spruce (Picea mariana) to sites dominated byLedum groenlandicum

Growth and physiological parameters of black spruce planted eight years previously were compared at sites with and withoutLedum groenlandicum to quantify the adverse effects on black spruce inLedum sites. Tree height, crown diameter, and dry weights of stems, branches, and needles of black spruce were significantly less at theLedum-dominated sites. Annual stem height, basal diameter, growth ring width, and cumulative wood volume of black spruce were significantly reduced in theLedum sites compared to those of the non-Ledum sites. Yearly growth of black spruce in the first seven years after planting was more affected in theLedum site than the eighth year's growth. Foliar nutrient analysis of black spruce in theLedum site showed an overall imbalance in nutrient accumulation. Black spruce growing in theLedum site had higher foliar concentrations of Al, Ca, Fe, Ni, and S and lower concentrations of N and Si than those growing in the non-Ledum site. Black spruce growing in theLedum site also had a higher foliar phenolic content. Preliminary experiments suggest thatLedum has the potential to leach water-soluble phenolics to its immediate organic soil.