Effect of the Presence of a Nonhost Herbivore on the Response of the Aphid Parasitoid <Emphasis Type="Italic">Diaeretiella rapae</Emphasis> to Host-infested Cabbage Plants

The vast majority of studies of plant indirect defense strategies have considered simple tritrophic systems that involve plant responses to attack by a single herbivore species. However, responses by predators and parasitoids to specific, herbivore-induced, volatile blends could be compromised when two or more different herbivores are feeding on the same plant. In Y-tube olfactometer studies, we investigated the responses of an aphid parasitoid, Diaeretiella rapae (McIntosh) (Hymenoptera: Braconidae), to odors from cabbage plants infested with the peach-potato aphid Myzus persicae (Sulzer) (Homoptera: Aphididae), in both the presence and absence of a lepidopteran caterpillar, Plutella xylostella L. (Lepidoptera: Plutellidae). Female parasitoids chose aphid-infested plants over uninfested plants but did not distinguish between caterpillar-infested and uninfested plants. When given a choice between odors from an aphid-infested plant and those from a plant infested with diamondback moth larvae, they significantly chose the former. Furthermore, the parasitoids responded equally to odors from a plant infested with aphids only and those from a plant infested with both aphids and caterpillars. The results support the hypothesis that the aphid and the caterpillar induce different changes in the volatile profile of cabbage plants and that D. rapae females readily distinguish between the two. Furthermore, the changes to the plant volatile profile induced by the caterpillar damage did not hinder the responses of the parasitoid to aphid-induced signals.     

Differential attractiveness of induced odors emitted by eight maize varieties for the parasitoid cotesia marginiventris: is quality or quantity important?

Herbivore-induced plant volatiles can function as indirect defense signals that attract natural enemies of herbivores. Several parasitoids are known to exploit these plant-provided cues to locate their hosts. One such parasitoid is the generalist Cotesia marginiventris, which is, among others, attracted to maize volatiles induced by caterpillar damage. Maize plants can be induced to produce the same blend of attractive volatiles by treating them with regurgitant of Spodoptera species. We collected and analyzed the regurgitant-induced emissions of two plant species (cowpea and maize) and of eight Mexican maize varieties and found significant differences among their volatile emissions, both in terms of total quantity and the quality of the blends. In a Y-tube olfactometer, the odors of the same artificially induced plant species and Mexican varieties were offered in dual choice experiments to naïve mated females of C. marginiventris. Wasps preferred cowpea over maize odor and, in 3 of 12 combinations with the maize varieties, they showed a preference for the odors of one of the varieties. A comparison of the odor collection with results from the behavioral assays indicates that not only the quantity of the volatile emissions, but also the quality composition of the volatile blends is important for attraction of C. marginiventris. The results are discussed in the context of the possibility of breeding crop varieties that are particularly attractive to parasitoids.     

Attraction of Spodoptera frugiperda Larvae to Volatiles from Herbivore-Damaged Maize Seedlings

Plants respond to insect attack with the induction of volatiles that function as indirect plant defenses through the attraction of natural enemies to the herbivores. Despite the fact that volatiles are induced in response to caterpillar attack, their reciprocal effects on the host location behaviors of the same foraging herbivores are poorly understood. We examined orientation responses of sixth instar fall armyworm [FAW; Spodoptera frugiperda (Smith)] to odors from herbivore-damaged and undamaged maize seedlings (Zea mays var. Golden Queen) in y-tube olfactometer bioassays. While both damaged and undamaged maize seedlings were attractive compared with air, sixth instars preferred odors from damaged maize seedlings over odors from undamaged maize seedlings. Gas chromatography–mass spectrometry analysis of plant volatiles revealed that linalool and 4,8-dimethyl-1,3,7-nonatriene were the major volatiles induced by FAW herbivory 6 hr after initial damage. Given its prominence in induced plants and established attractiveness to adult FAW, linalool was evaluated both as an individual attractant and as a supplemental component of whole plant odors. Volatile linalool was more attractive than air to sixth instar FAW over a broad range of release rates. FAW also responded selectively to different amounts of linalool, preferring the higher amount. The orientation preferences of FAW were readily manipulated through capillary release of linalool into the airstream of whole plant odors. FAW preferred linalool over undamaged plant odors, and linalool-supplemented plant odors over unsupplemented plant odors, indicating that olfactory preferences could be changed by alteration of a single volatile component. These results suggest that although many induced volatiles attract natural enemies of herbivores, these defenses may also inadvertently recruit more larval herbivores to an attacked plant or neighboring conspecifics.The use of trade, firm, or corporation names in this publication (or page) is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable.     

The “Raison D'être” of pyrrolizidine alkaloids inCynoglossum officinale: Deterrent effects against generalist herbivores

In this study we tested whether pyrrolizidine alkaloids (PAs) ofCynoglossum officinale serve as antifeedants against herbivores. Total PA N-oxide extracts of the leaves significantly deterred feeding by generalist herbivores. Specialist herbivores did not discriminate between food with high and low PA levels. Three PAs fromC. officinale, heliosupine, echinatine, and 3-acetylechinatine, equally deterred feeding by the polyphagous larvae ofSpodoptera exigua. Although the plants mainly contain PAs in their N-oxide form, reduced PAs deterred feeding byS. exigua more efficiently than PA N-oxides. On rosette plants, the monophagous weevilMogulones cruciger significantly consumed more of the youngest leaves, which had the highest PA level and the highest nitrogen percentage. Larvae ofEthmia bipunctella, which are oligophagous within the Boraginaceae, did not discriminate between leaves. All generalist herbivores tested significantly avoided the youngest leaves with the highest PA levels. In the field, the oldest leaves also were relatively more damaged by herbivores than the youngest leaves. It is hypothesized that the skewed distribution of PAs over the leaves of rosette plants reflects optimal defense distribution within the plant.     

Nitrogen Availability and Defense of Tomato Against Two-spotted Spider Mite

The aim of this work was to study how nitrogen availability affects within-plant allocation to growth and secondary metabolites and how this allocation affects host selection by herbivores. Tomato plants (Lycopersicon esculentum) were grown at six levels of nitrogen availability. When nitrogen availability increased, plant relative growth rate increased, but tissue carbon/nitrogen ratio in the second oldest true leaf and allocation to large glandular trichomes (type VI) as well as to the defense compounds rutin and chlorogenic acid decreased. Leaf protein concentration increased. Two-spotted spider mite (Tetranychus urticae) females responded significantly to these changes: in dual choice tests they preferred leaf disks from plants grown at high nitrogen availability, with a low C/N ratio. This preference persisted in an olfactometer in which the mites were offered only the odors released by leaves with damaged trichomes. We conclude that in a tomato leaf increased nitrogen availability leads to decreased allocation to defenses, and that repellent volatiles released by trichomes play a key role in affecting leaf selection by two-spotted spider mite females.     

Exogenous Application of Jasmonic Acid Induces Volatile Emissions in Rice and Enhances Parasitism of <Emphasis Type="Italic">Nilaparvata lugens</Emphasis> Eggs by theParasitoid <Emphasis Type="Italic">Anagrus nilaparvatae</Emphasis>

Jasmonate signaling pathway plays an important role in induced plant defense against herbivores and pathogens, including the emission of volatiles that serve as attractants for natural enemies of herbivores. We studied the volatiles emitted from rice plants that were wounded and treated with jasmonic acid (JA) and their effects on the host-searching behavior of the rice brown planthopper, Nilaparvata lugens (Stål), and its mymarid egg parasitoid Anagrus nilaparvatae Pang et Wang. Female adults of N. lugens significantly preferred to settle on JA-treated rice plants immediately after release. The parasitoid A. nilaparvatae showed a similar preference and was more attracted to the volatiles emitted from JA-treated rice plants than to volatiles from control plants. This was also evident from greenhouse and field experiments in which parasitism of N. lugens eggs by A. nilaparvatae on plants that were surrounded by JA-treated plants was more than twofold higher than on control plants. Analyses of volatiles collected from rice plants showed that JA treatment dramatically increased the release of volatiles, which included aliphatic aldehydes and alcohols, monoterpenes, sesquiterpenes, methyl salicylate, n-heptadecane, and several as yet unidentified compounds. These results confirm an involvement of the JA pathway in induced defense in rice plants and demonstrate that the egg parasitoid A. nilaparvatae exploits plant-provided cues to locate hosts. We explain the use of induced plant volatiles by the egg parasitoid by a reliable association between planthopper feeding damage and egg presence.     

Extending the Elemental Defense Hypothesis: Dietary Metal Concentrations Below Hyperaccumulator Levels Could Harm Herbivores

Previous work has shown that hyperaccumulator levels of some metals can defend plants against herbivores, but the possibility of defense by metal concentrations at accumulator or normal levels is unexplored. This study tested the hypothesis that metals can defend plants at low concentrations. We determined the relative toxicities of eight metals commonly acquired by plants: Cd, Co, Cr, Cu, Mn, Ni, Pb, and Zn. Larvae of the diamondback moth (Plutella xylostella), a representative crucifer specialist, were fed with artificial diet amended with concentrations of metal varying from 2 to 3,000 μg/g. Different concentration ranges were used for each of the eight metals, and larval survival at 10–14 days was calculated for each concentration. All metals were toxic to diamondback moth larvae at hyperaccumulator levels. All metals, however, were also toxic to larvae at accumulator concentrations, far below those found in hyperaccumulating plants. Five metals (Cd, Mn, Ni, Pb, and Zn) were toxic below accumulator levels, Cd and Pb were toxic near the concentration ranges of normal plants, and Zn was toxic at a concentration within the normal range. Our results indicate that uptake of certain metals may provide a defensive benefit for plants, and that elemental defenses may be effective at concentrations far lower than previously hypothesized. This study implies that elemental defenses are more widespread in plants than previously believed, and that the ecological consequences of even low levels of metal accumulation need to be explored.     

The Attraction of <Emphasis Type="Italic">Spodoptera frugiperda</Emphasis> Neonates to Cowpea Seedlings is Mediated by Volatiles Induced by Conspecific Herbivory and the Elicitor Inceptin

Neonate fall armyworms [FAW; Spodoptera frugiperda (Smith)] often encounter conspecific herbivore damage as they disperse from an egg mass to an initial feeding site. We investigated the orientation responses of dispersing neonates to herbivore damage in cowpea seedlings, specifically examining whether neonate behaviors were affected by inceptin, the primary elicitor of FAW-induced defenses in cowpea leaves. We focused on responses to damage caused by conspecific first instars, as might occur during the dispersal of siblings from an egg mass. Inceptin contents of damaging first instar FAW were controlled through their diets, with leaf-fed FAW producing inceptins in their oral secretions, and root-fed or starved FAW lacking these elicitors. In a bioassay designed to evaluate neonate dispersal off a host plant, a higher percentage of neonates remained on herbivore-induced or inceptin-treated plants than on undamaged plants, mechanically damaged plants, freshly damaged plants, or on plants damaged by FAW lacking inceptins. Further investigations of neonate responses to plant odors with a four-arm olfactometer demonstrated that neonate attraction to odors from 4-h old FAW damage was strongly dependent on previous diet of the damaging larvae. Neonates were attracted to odors from 4-h old FAW damage over odors from undamaged plants or fresh FAW damage, provided that the damaging larvae had previously ingested leaf material. In a direct comparison of odors from induced plants, plants damaged by leaf-fed FAW were as attractive as plants treated with synthetic inceptin. GC-MS analysis confirmed that (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) was the major volatile induced by FAW herbivory. While both DMNT and undamaged plant odors were more attractive than air, neonates preferred DMNT-supplemented plant odors. These results suggest that neonate FAW exploit herbivore-induced plant volatiles as host plant location and recognition cues. The use of trade, firm, or corporation names in this publication (or page) is for the information and convenience of the reader. Such use does not constitute an official endorsement or approval by the United States Department of Agriculture or the Agricultural Research Service of any product or service to the exclusion of others that may be suitable.     

Development of an Insect Herbivore and its Pupal Parasitoid Reflect Differences in Direct Plant Defense

In nature, plants defend themselves by production of allelochemicals that are toxic to herbivores. There may be considerable genetic variation in the expression of chemical defenses because of various selection pressures. In this study, we examined the development of the small cabbage butterfly, Pieris rapae, and its gregarious pupal ectoparasitoid, Pteromalus puparum, when reared on three wild populations (Kimmeridge, Old Harry, Winspit) of cabbage, Brassica oleracea, and a Brussels sprout cultivar. Wild plant populations were obtained from seeds of plants that grow naturally along the south coast of Dorset, England. Significant differences in concentrations of allelochemicals (glucosinolates) were found in leaves of plants damaged by P. rapae. Total glucosinolate concentrations in Winspit plants, the population with the highest total glucosinolate concentration, were approximately four times higher than in the cultivar, the strain with the lowest total glucosinolate concentration. Pupal mass of P. rapae and adult body mass of Pt. puparum were highest when reared on the cultivar and lowest when developing on Kimmeridge plants, the wild strain with the lowest total glucosinolate concentration. Development of male parasitoids was also more negatively affected than female parasitoids. Our results reveal that plant quality, at least for the development of ‘adapted’ oligophagous herbivores, such as P. rapae, is not based on total glucosinolate content. The only glucosinolate compound that corresponded with the performance of P. rapae was the indole glucosinolate, neoglucobrassicin. Our results show that performance of ectoparasitoids may closely reflect constraints on the development of the host.     

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.     

Variation in Herbivore and Methyl Jasmonate-Induced Volatiles Among Genetic Lines of <Emphasis Type="Italic">Datura wrightii</Emphasis>

Many plant species produce volatile organic compounds after being damaged by herbivores. The production of volatiles also may be induced by exposing plants to the plant hormone, jasmonic acid, or its volatile ester, methyl jasmonate. This study addresses the induction of the production volatile organic compounds among genetic lines of Datura wrightii. Within populations, some plants produce glandular trichomes, whereas others produce nonglandular trichomes, and trichome phenotype is controlled by a single dominant gene. Glandular trichomes not only confer resistance to some herbivorous insects, but they also inhibit many natural enemies of those herbivores. Because of the potential benefit of natural enemies that use volatile cues to find individuals of the non-glandular phenotype, it is reasonable to ask if plants of D. wrightii that differ in trichome morphology might produce different blends of volatile compounds. Volatile compounds were collected from eight genetic lines of plants that had been backcrossed for three generations. Volatiles were collected from pairs of sibling plants before and after insect damage or treatment with methyl jasmonate. Within each pair, one sib expressed glandular trichomes and the other expressed nonglandular trichomes. Overall, plants produced an array of at least 17 compounds, most of which were sesquiterpenes. Total production of volatiles increased from 3.9- to 16.2-fold among genetic lines after insect damage and from 3.6- to 32-fold in plants treated with methyl jasmonate. The most abundant compound was (E)-β-caryophyllene. This single compound comprised from 17 to 59% of the volatiles from insect-damaged plants and from 24 to 88% of the volatiles from plants treated with methyl jasmonate, depending upon genetic line. The production of (E)-β-caryophyllene by the original male parents of the eight genetic lines was significantly related to the mean production of their third-generation backcross progeny indicating that the variation in the production of (E)-β-caryophyllene was inherited. Blends did not differ qualitatively or quantitatively between sibs expressing glandular or nonglandular trichomes.     

Postpollination Changes in Floral Odor in Silene latifolia: Adaptive Mechanisms for Seed-Predator Avoidance?

Floral odor is a key trait for pollinator attraction in many plants, but may also direct antagonists like herbivores to flowers. In this study, we examined how floral scent changes after pollination in Silene latifolia, which has a specialized relationship with the seed predator Hadena bicruris. We found an overall decrease in total scent emission and considerable changes in relative amounts of scent compounds after pollination. Lilac aldehydes A and B as well as veratrole contributed most to the decrease in scent emission. These three compounds are known to be key signals for the attraction of H. bicruris to the flowers. A specific downregulation of these compounds may increase the reproductive success of the plant by reducing seed predation after pollination.     

Food deprivation affects preference of sheep for foods varying in nutrients and a toxin

We investigated how food deprivation affected preference of lambs for foods that varied in concentrations of nutrients and a toxin. Thirty lambs (10 lambs/treatment) were fed different amounts of alfalfa pellets (high in protein, marginal in energy for growth) as a basal ration (20, 40, or 60 g/kg body weight). Every morning, prior to ingesting the basal ration of alfalfa pellets, each lamb was offerend three foods for 15 min. The foods contained different amounts of energy and a toxin, depending on the addition of barley (energy) and LiCl (toxin) to alfalfa. The proportions of barley and LiCl changed every five days during the 25-day study. The results showed: (1) all lambs preferred food that was high > intermediate > low in energy (barley) in the absence of LiCl, but all lambs decreased consumption of foods high in energy as LiCl concentrations increased; (2) the greater the level of food deprivation, the lower the consumption of foods containing LiCl, even if the foods provided high levels of energy; (3) lambs moderately food deprived or fed ad libitum ingested more LiCl than lambs that were highly deprived; and (4) lambs quickly (15 min/day) regulated intake of foods in response to changes in barley and LiCl concentrations. Thus, our results suggest that the interaction between nutritional status and toxicosis plays an important role in food preference of lambs. Our findings also suggest that toxic plants may kill herbivores that lack nutritious alternative foods not only because the animals are forced to be less discriminating, but also because they are more susceptible to toxins.     

Herbivore-induced Plant Volatiles Trigger Sporulation in Entomopathogenic Fungi: The Case of <Emphasis Type="Italic">Neozygites tanajoae</Emphasis> Infecting the Cassava Green Mite

A large body of evidence shows that plants release volatile chemicals upon attack by herbivores. These volatiles influence the performance of natural enemies. Nearly all the evidence on the effect of plant volatiles on natural enemies of herbivores concerns predators, parasitoids, and entomophagous nematodes. However, other entomopathogens, such as fungi, have not been studied yet for the way they exploit the chemical information that the plant conveys on the presence of herbivores. We tested the hypothesis that volatiles emanating from cassava plants infested by green mites (Mononychellus tanajoa) trigger sporulation in three isolates of the acaropathogenic fungus Neozygites tanajoae. Tests were conducted under climatic conditions optimal to fungal conidiation, such that the influence of the plant volatiles could only alter the quantity of conidia produced. For two isolates (Altal.brz and Colal.brz), it was found that, compared with clean air, the presence of volatiles from clean, excised leaf discs suppressed conidia production. This suppressive effect disappeared in the presence of herbivore-damaged leaves for the isolate Colal.brz. For the third isolate, no significant effects were observed. Another experiment differing mainly in the amount of volatiles showed that two isolates produced more conidia when exposed to herbivore-damaged leaves compared with clean air. Taken together, the results show that volatiles from clean plants suppress conidiation, whereas herbivore-induced plant volatiles promote conidiation of N. tanajoae. These opposing effects suggest that the entomopathogenic fungus tunes the release of spores to herbivore-induced plant signals indicating the presence of hosts.     

Ozone Degrades Common Herbivore-Induced Plant Volatiles: Does This Affect Herbivore Prey Location by Predators and Parasitoids?

Inducible terpenes and lipoxygenase pathway products, e.g., green-leaf volatiles (GLVs), are emitted by plants in response to herbivory. They are used by carnivorous arthropods to locate prey. These compounds are highly reactive with atmospheric pollutants. We hypothesized that elevated ozone (O₃) may affect chemical communication between plants and natural enemies of herbivores by degrading signal compounds. In this study, we have used two tritrophic systems (Brassica oleracea–Plutella xylostella–Cotesia plutellae and Phaseolus lunatus–Tetranychus urticae–Phytoseiulus persimilis) to show that exposure of plants to moderately enhanced atmospheric O₃ levels (60 and 120 nl l⁻¹) results in complete degradation of most herbivore-induced terpenes and GLVs, which is congruent with our hypothesis. However, orientation behavior of natural enemies was not disrupted by O₃ exposure in either tritrophic system. Other herbivore-induced volatiles, such as benzyl cyanide, a nitrile in cabbage, and methyl salicylate in lima bean, were not significantly reduced in reactions with O₃. We suggest that more atmospherically stable herbivore-induced volatile compounds can provide important long-distance plant-carnivore signals and may be used by natural enemies of herbivores to orientate in O₃-polluted environments.     

Response of Predatory Mites to a Herbivore-Induced Plant Volatile: Genetic Variation for Context-Dependent Behaviour

Plants infested with herbivores release specific volatile compounds that are known to recruit natural enemies. The response of natural enemies to these volatiles may be either learned or genetically determined. We asked whether there is genetic variation in the response of the predatory mite Phytoseiulus persimilis to methyl salicylate (MeSa). MeSa is a volatile compound consistently produced by plants being attacked by the two-spotted spider mite, the prey of P. persimilis. We predicted that predators express genetically determined responses during long-distance migration where previously learned associations may have less value. Additionally, we asked whether these responses depend on odors from uninfested plants as a background to MeSa. To infer a genetic basis, we analyzed the variation in response to MeSa among iso-female lines of P. persimilis by using choice-tests that involved either (1) MeSa presented as a single compound or (2) MeSa with background-odor from uninfested lima bean plants. These tests were conducted for starved and satiated predators, i.e., two physiological states, one that approximates migration and another that mimics local patch exploration. We found variation among iso-female lines in the responses to MeSa, thus showing genetic variation for this behavior. The variation was more pronounced in the starved predators, thus indicating that P. persimilis relies on innate preferences when migrating. Background volatiles of uninfested plants changed the predators’ responses to MeSa in a manner that depended on physiological state and iso-female line. Thus, it is possible to select for context-dependent behavioral responses of natural enemies to plant volatiles.     

Lonicera Implexa Leaves Bearing Naturally Laid Eggs of the Specialist Herbivore Euphydryas Aurinia have Dramatically Greater Concentrations of Iridoid Glycosides than other Leaves

We tested in the field the hypothesis that the specialist butterfly Euphydryas aurinia (Lepidoptera: Nymphalidae, Melitaeinae) lays eggs on leaves of Lonicera implexa (Caprifoliaceae) plants with greater iridoid concentrations. We conducted our investigations in a Mediterranean site by analyzing leaves with and without naturally laid egg clusters. There were no significant differences in iridoid glycoside concentrations between leaves from plants that did not receive eggs and the unused leaves from plants receiving eggs, a fact that would seem to indicate that E. aurinia butterflies do not choose plants for oviposition by their iridoid content. However, the leaves of L. implexa that bore egg clusters had dramatically greater (over 15-fold) concentrations of iridoid glycosides than the directly opposite leaves on the same plant. These huge foliar concentrations of iridoids (15% leaf dry weight) may provide specialist herbivores with compounds that they either sequester for their own defense or use as a means of avoiding competition for food from generalist herbivores. Nevertheless, it may still be possible that these high concentrations are detrimental to the herbivore, even if the herbivore is a specialist feeder on the plant.     

Comparative Detoxification of Plant (Magnolia virginiana) Allelochemicals by Generalist and Specialist Saturniid Silkmoths

The foliage of sweetbay magnolia (Magnolia virginana) contains at least two biologically active phenylpropanoid compounds (magnolol and a biphenyl ether) that are toxic to a number of generalist insect herbivores. These compounds have little effect on caterpillars of the sweetbay silkmoth, C. securifera, which is a specialist on sweetbay, but they are toxic to two closely related silkmoths, C. angulifera and C. promethea. To understand the influence of phytochemistry on the evolution of host use and feeding specialization in Callosamia, the detoxification capability of C. securifera was compared with that of C. angulifera and C. promethea. Degradation of magnolol and the biphenyl ether by midgut homogenate of the sweetbay specialist was NADPH-dependent and inhibited by piperonyl butoxide, suggesting the involvement of cytochrome P-450 detoxification enzymes. Both were degraded three times faster in the specialist compared to the unadapted herbivores. Higher rates of degradation could not be induced in the polyphagous C. promethea by a mixture of magnolol and the biphenyl ether or by the P-450 inducer pentamethylbenzene, nor did activity vary significantly when larvae were reared on different host plants. Use of sweetbay by Callosamia silkmoths appears to be dependent on their ability to degrade host toxins rapidly via midgut detoxification enzymes. Moreover, the intraspecific comparisons contradict the common prediction that higher levels of cytochrome P-450 activity are found in more polyphagous species; instead, P-450 activity is more closely associated with specific chemical attributes of the herbivores' host plants.     

Reduction of rodent fertility by plant consumption

The present study investigates the possibility that plants used in traditional medicine for birth control may also reduce reproduction in their natural herbivores. Ten species of plants utilized by Bedouins for birth control were selected. These were dried, ground, and mixed with the standard diet and offered to female laboratory rats. Six plant species (60%) were found to reduce reproduction rate in white female rats, and the shoots and fruit of one of the effective species,Ziziphus spina-christi, when offered to its natural herbivore,Meriones tristrami, at the level of 35% of the standard diet, postponed female puberty and significantly reduced offspring survival. Plants that are known to be an effective factor in human birth control may have similar effects on their natural mammalian herbivores. In such cases, when the seeds of the plant are part of the herbivore diet, a certain percentage of the seeds will be dispersed and germinate, while the resulting population control of the animal achieved by its consumption of the seeds will prevent overgrazing, thus maintaining a mutual balance. It is suggested that ethnopharmacological data may assist in uncovering plants that, under the following conditions, have the potential to regulate reproduction in mammalian herbivores (with implications for human reproduction): (1) the plant is an important component of the animal's diet and (2) the active secondary metabolites of the plant directly interact with the physiological systems governing reproduction in the herbivore.     

Intraspecific Variation in Plant Defense Alters Effects of Root Herbivores on Leaf Chemistry and Aboveground Herbivore Damage

Root herbivores can indirectly affect aboveground herbivores by altering the food quality of the plant. However, it is largely unknown whether plant genotypes differ in their response to root herbivores, leading to variable defensive phenotypes. In this study, we investigated whether root-feeding insect larvae (Agriotes sp. larvae, wireworms) induce different responses in Plantago lanceolata plants from lines selected for low and high levels of iridoid glycosides (IG). In the absence of wireworms, plants of the “high-IG line” contained approximately twofold higher levels of total IG and threefold higher levels of catalpol (one of the IG) in leaves than plants from the “low-IG line,” whereas both lines had similar levels of IG in roots. In response to wireworms, roots of plants from both lines showed increased concentrations of catalpol. Leaves of “low-IG line” plants increased catalpol concentrations in response to wireworms, whereas catalpol concentrations of leaves of “high-IG line” plants decreased. In contrast, glucose concentrations in roots of “low-IG” plants decreased, while they increased in “high-IG” plants after feeding by wireworms. The leaf volatile profile differed between the lines, but was not affected by root herbivores. In the field, leaf damage by herbivores was higher in wireworm-induced compared to noninduced “low-IG” plants and lower in wireworm-induced compared to noninduced “high-IG” plants, despite induction of catalpol in leaves of the “low-IG” plants and reduction in “high-IG” plants. This pattern might arise if damage is caused mainly by specialist herbivores for which catalpol may act as feeding stimulant rather than as deterrent. The present study documents for the first time that intraspecific variation in plant defense affects the outcome of plant-mediated interactions between root and shoot herbivores.