Effects of comsumption of high and low nicotine tobacco byManduca sexta (Lepidoptera: Sphingidae) on survival of gregarious endoparasitoidCotesia congregata (Hymenoptera: Braconidae)

Metabolism of plant-derived toxins from its insect host increases the success of the entomopathogenic fungus Beauveria bassiana

Beauveria bassiana is a soil fungus that parasitizes a large number of arthropod species, including numerous crop pests, causing white muscardine disease and is therefore used as a biological insecticide. However, some insects, such as the cabbage aphid (Brevicoryne brassicae), defend themselves chemically by sequestering dietary pro-toxins (glucosinolates) from their Brassicales host plants. Glucosinolates are accumulated by cabbage aphids and activated to form toxic isothiocyanates when under attack. While isothiocyanate formation protects aphids against most attackers, B. bassiana is still able to infect the cabbage aphid under natural conditions. We therefore investigated how this fungus is able to circumvent the chemical defense system of the cabbage aphid. Here, we describe how B. bassiana infection activates the cabbage aphid defense system, but the resulting toxins are metabolized by B. bassiana via the mercapturic acid pathway, of which the first step is catalyzed by glutathione-S-transferases of low substrate specificity. This detoxification pathway enhances B. bassiana growth when isothiocyanates are present in natural concentrations, and so appears to be an important factor in fungal parasitization of these chemically defended aphids.

Dietary Challenges for Parasitoid Wasps (Hymenoptera: Ichneumonoidea); Coping with Toxic Hosts, or Not?

Many insects defend themselves against predation by being distasteful or toxic. The chemicals involved may be sequestered from their diet or synthesized de novo in the insects' body tissues. Parasitoid wasps are a diverse group of insects that play a critical role in regulating their host insect populations such as lepidopteran caterpillars. The successful parasitization of caterpillars by parasitoid wasps is contingent upon their aptitude for locating and selecting suitable hosts, thereby determining their efficacy in parasitism. However, some hosts can be toxic to parasitoid wasps, which can pose challenges to their survival and reproduction. Caterpillars employ a varied array of defensive mechanisms to safeguard themselves against natural predators, particularly parasitoid wasps. These defenses are deployed pre-emptively, concurrently, or subsequently during encounters with such natural enemies. Caterpillars utilize a range of strategies to evade detection or deter and evade attackers. These tactics encompass both measures to prevent being noticed and mechanisms aimed at repelling or eluding potential threats. Post-attack strategies aim to eliminate or incapacitate the eggs or larvae of parasitoids. In this review, we investigate the dietary challenges faced by parasitoid wasps when encountering toxic hosts. We first summarize the known mechanisms through which insect hosts can be toxic to parasitoids and which protect caterpillars from parasitization. We then discuss the dietary adaptations and physiological mechanisms that parasitoid wasps have evolved to overcome these challenges, such as changes in feeding behavior, detoxification enzymes, and immune responses. We present new analyses of all published parasitoid-host records for the Ichneumonoidea that attack Lepidoptera caterpillars and show that classically toxic host groups are indeed hosts to significantly fewer species of parasitoid than most other lepidopteran groups.

Manduca sexta experience high parasitoid pressures in the field but minor fitness costs of consuming plant secondary compounds

Plant-herbivore coevolutionary interactions have led to a range of plant defenses that minimize insect damage and a suite of counter adaptations that allow herbivores to feed on defended plants. Consuming plant secondary compounds results in herbivore growth and developmental costs but can have beneficial effects such as deterrence or harm of parasitoid enemies. Therefore, the role of secondary compounds on herbivore fitness must be considered in the context of the abundance and level of harm from natural enemies and the costs herbivores incur feeding on plant secondary compounds.In this study, I combined field measurements of Cotesia congregata wasp parasitism pressure with detailed measurements of the costs of plant secondary compounds across developmental stages in the herbivore host, Manduca sexta.I show that C. congregata parasitoids exert large negative selective pressures, killing 31%-57% of M. sexta larvae in the field. Manduca sexta developed fastest during instars most at risk for parasitoid oviposition but growth was slowed by consumption of plant secondary compounds. The negative effects of consuming plant secondary compounds as larvae influenced adult size traits but there were no immune, survival, or fecundity costs.These results suggest that developmental costs experienced by M. sexta herbivores consuming defensive compounds are minor in comparison to the strong negative survival pressures from abundant parasitoid enemies.

TOC1 in Nicotiana attenuata regulates efficient allocation of nitrogen to defense metabolites under herbivory stress

The circadian clock contextualizes plant responses to environmental signals. Plants use temporal information to respond to herbivory, but many of the functional roles of circadian clock components in these responses, and their contribution to fitness, remain unknown. We investigate the role of the central clock regulator TIMING OF CAB EXPRESSION 1 (TOC1) in Nicotiana attenuata's defense responses to the specialist herbivore Manduca sexta under both field and glasshouse conditions. We utilize 15 N pulse-labeling to quantify nitrogen incorporation into pools of three defense compounds: caffeoylputrescine (CP), dicaffeoyl spermidine (DCS) and nicotine. Nitrogen incorporation was decreased in CP and DCS and increased in nicotine pools in irTOC1 plants compared to empty vector (EV) under control conditions, but these differences were abolished after simulated herbivory. Differences between EV and irTOC1 plants in nicotine, but not phenolamide production, were abolished by treatment with the ethylene agonist 1-methylcyclopropene. Using micrografting, TOC1's effect on nicotine was isolated to the root and did not affect the fitness of heterografts under field conditions. These results suggest that the circadian clock contributes to plant fitness by balancing production of metabolically expensive nitrogen-rich defense compounds and mediating the allocation of resources between vegetative biomass and reproduction.

Detoxification of plant defensive glucosinolates by an herbivorous caterpillar is beneficial to its endoparasitic wasp

Plant chemical defences impact not only herbivores, but also organisms in higher trophic levels that prey on or parasitize herbivores. While herbivorous insects can often detoxify plant chemicals ingested from suitable host plants, how such detoxification affects endoparasitoids that use these herbivores as hosts is largely unknown. Here, we used transformed plants to experimentally manipulate the major detoxification reaction used by Plutella xylostella (diamondback moth) to deactivate the glucosinolate defences of its Brassicaceae host plants. We then assessed the developmental, metabolic, immune, and reproductive consequences of this genetic manipulation on the herbivore as well as its hymenopteran endoparasitoid Diadegma semiclausum. Inhibition of P. xylostella glucosinolate metabolism by plant-mediated RNA interference increased the accumulation of the principal glucosinolate activation products, the toxic isothiocyanates, in the herbivore, with negative effects on its growth. Although the endoparasitoid manipulated the excretion of toxins by its insect host to its own advantage, the inhibition of herbivore glucosinolate detoxification slowed endoparasitoid development, impaired its reproduction, and suppressed the expression of genes of a parasitoid-symbiotic polydnavirus that aids parasitism. Therefore, the detoxification of plant glucosinolates by an herbivore lowers its toxicity as a host and benefits the parasitoid D. semiclausum at multiple levels.

Effects of soybean resistance on variability in life history traits of the higher trophic level parasitoid Meteorus pulchricornis (Hymenoptera: Braconidae)

To extrapolate the influence of plant cultivars varying in resistance levels to hosts on parasitoid life history traits, we estimated variation in parasitoid developmental and reproductive performances as a function of resistance in soybean cultivars, which were randomly chosen from a line of resistant genotypes. Our study showed that the parasitoid Meteorus pulchricornis varied widely in offspring survival and lifetime fecundity, but varied slightly in development time and adult body size, in response to the soybean cultivars that varied in resistance to the host Spodoptera litura. Furthermore, the variability in survival and lifetime fecundity was different between attacking the 2nd and the 4th instar host larvae, varying more in survival but less in lifetime fecundity when attacking the 4th than 2nd instar larvae. Our study provides further evidence supporting that plant resistance to herbivorous hosts have variable effects on different life history traits of higher trophic level parasitoids.

Possible Synergistic Effects of Thymol and Nicotine Against Crithidia bombi Parasitism in Bumble Bees

Floral nectar contains secondary compounds with antimicrobial properties that can affect not only plant-pollinator interactions, but also interactions between pollinators and their parasites. Although recent work has shown that consumption of plant secondary compounds can reduce pollinator parasite loads, little is known about the effects of dosage or compound combinations. We used the generalist pollinator Bombus impatiens and its obligate gut parasite Crithidia bombi to study the effects of nectar chemistry on host-parasite interactions. In two experiments we tested (1) whether the secondary compounds thymol and nicotine act synergistically to reduce parasitism, and (2) whether dietary thymol concentration affects parasite resistance. In both experiments, uninfected Bombus impatiens were inoculated with Crithidia and then fed particular diet treatments for 7 days, after which infection levels were assessed. In the synergism experiment, thymol and nicotine alone and in combination did not significantly affect parasite load or host mortality. However, the thymol-nicotine combination treatment reduced log-transformed parasite counts by 30% relative to the control group (P = 0.08). For the experiment in which we manipulated thymol concentration, we found no significant effect of any thymol concentration on Crithidia load, but moderate (2 ppm) thymol concentrations incurred a near-significant increase in mortality (P = 0.054). Our results tentatively suggest the value of a mixed diet for host immunity, yet contrast with research on the antimicrobial activity of dietary thymol and nicotine in vertebrate and other invertebrate systems. We suggest that future research evaluate genetic variation in Crithidia virulence, multi-strain competition, and Crithidia interactions with the gut microbe community that may mediate antimicrobial activities of secondary compounds.

Adaptation to nicotine in the facultative tobacco-feeding hemipteran Bemisia tabaci

BACKGROUND

Plant defensive metabolites such as nicotine can provide barriers to host-range expansion by generalist herbivores. Nicotine is one of the most abundant and toxic plant secondary metabolites in nature and is defined by high toxicity to plant-feeding insects. There is significant variation in nicotine tolerance among Bemisia tabaci (tobacco whitefly) isolates. Some nicotine-tolerant B. tabaci strains can consume 40-fold higher nicotine levels than susceptible strains, and also show cross-resistance to neonicotinoid insecticides. In this study, biological and molecular assays were used to investigate the responses of B. tabaci strains that differ in their ability to tolerate dietary nicotine.

RESULTS

Egg laying and honeydew secretion bioassays as well as gene expression microarrays were used to measure B. tabaci biological parameters and gene transcripts misregulated in response to nicotine in resistant and susceptible strains. The resistant B. tabaci strain laid significantly fewer eggs and excreted more honeydew on a tobacco strain with high levels of nicotine, suggesting a fitness cost effect. The molecular response was drastic in the susceptible strain, while the resistant strain exhibited moderate response. Higher expression of the previously identified CYP6CM1 P450 monooxygenase gene related to the resistance to neonicotinoids, as well as other P450s and metabolic genes, was identified in the resistant and susceptible strains after exposure to nicotine.

CONCLUSIONS

Nicotine is a very toxic plant natural compound, and its mode of action resembles that of synthetic neonicotinoids. The biological and molecular responses observed in this study suggest that nicotine may play an important role in providing barriers for host-plant expansion by generalists, and may act as a natural factor that contributes to the development of insect populations resistant to synthetic pesticides.

Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46's role in a nicotine-mediated antipredator herbivore defense

Manduca sexta (Ms) larvae are known to efficiently excrete ingested nicotine when feeding on their nicotine-producing native hostplant, Nicotiana attenuata. Here we describe how ingested nicotine is co-opted for larval defense by a unique mechanism. Plant-mediated RNAi was used to silence a midgut-expressed, nicotine-induced cytochrome P450 6B46 (CYP6B46) in larvae consuming transgenic N. attenuata plants producing MsCYP6B46 dsRNA. These and transgenic nicotine-deficient plants were planted into native habitats to study the phenotypes of larvae feeding on these plants and the behavior of their predators. The attack-behavior of a native wolf spider (Camptocosa parallela), a major nocturnal predator, provided the key to understanding MsCYP6B46's function: spiders clearly preferred CYP6B46-silenced larvae, just as they had preferred larvae fed nicotine-deficient plants. MsCYP6B46 redirects a small amount (0.65%) of ingested nicotine from the midgut into hemolymph, from which nicotine is exhaled through the spiracles as an antispider signal. CYP6B46-silenced larvae were more susceptible to spider-attack because they exhaled less nicotine because of lower hemolymph nicotine concentrations. CYP6B46-silenced larvae were impaired in distributing ingested nicotine from midgut to hemolymph, but not in the clearing of hemolymph nicotine or in the exhalation of nicotine from hemolymph. MsCYP6B46 could be a component of a previously hypothesized pump that converts nicotine to a short-lived, transportable, metabolite. Other predators, big-eyed bugs, and antlion larvae were insensitive to this defense. Thus, chemical defenses, too toxic to sequester, can be repurposed for defensive functions through respiration as a form of defensive halitosis, and predators can assist the functional elucidation of herbivore genes.

Plant allelochemicals and insect parasitoids effects of nicotine onCotesia congregata (say) (Hymenoptera: Braconidae) andHyposoter annulipes (Cresson) (Hymenoptera: Ichneumonidae)

Parasitoids developing within tobacco hornworms or fall army-worms exhibit significant differences in development and survival depending on whether their hosts fed on nicotine-free or nicotine-containing diets. The effects of nicotine were more severe on the relatively less adapted parasitoid,H. annulipes than the specialist parasitoid,Cotesia congregata. Labeled alkaloid originally placed in herbivore diet was incorporated in several parasitoid tissues. These results suggest that interactions between plant allelochemicals and parasitoids should be considered in the development of theory on insect herbivory and plant defense.

Fate of quinolizidine alkaloids through three trophic levels:Laburnum anagyroides (Leguminosae) and associated organisms

The quinolizidine alkaloids (QA) of golden rain,Laburnum anagyroides, and those of phytophagous insects associated with the plant, as well as of parasitoids of the latter, were analyzed by capillary GLC and GLC-MS. The alkaloid content in samples of vegetative plant parts was high at the beginning of the season, then decreased, while that of reproductive organs was high throughout flowering, pod formation, and maturation. The analyses showed that the QA of the plant passed through two higher trophic levels (herbivorous insects and their parasitoids) and that the alkaloid pattern changed little during the passage. The alkaloids were present in two phytophagous insect species associated with golden rain: the predispersal seed predator,Bruchidius villosus [5-13μg/g fresh weight (fw)], andAphis cytisorum (182-1012μg/g fw), an aphid that feeds on shoots, leaves, and inflorescences. Braconid and chalcidoid parasitoids emerging from the bruchid host also contained alkaloids (1.3-3μg/g fw), as did three foraging ant species,Lasius niger, Formica rufibarbis, andF. cunicularia (45μg/g fw), that visited the aphid colonies or honeydew-covered leaves of aphid-infested plants. The hypothesis that developing bruchid larvae and/or the plant "manipulate" QA supply to infested seeds was not supported, because QA content of leftover endosperm in seeds after bruchid development was similar to that of uninfested seeds. The frass of developing bruchid larvae was rich in QA (31μg/ g dry weight). While aphids sequestered, the bruchid larvae took up and eliminated QA with the frass without chemical transformation.

Effect of DIMBOA, an aphid resistance factor in wheat, on the aphid predatorEriopis connexa Germar (Coleoptera: Coccinellidae)

DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one), a secondary metabolite found in cereal extracts, confers resistance in wheat to aphids. Its effect on beneficial organisms was tested on larvae of the aphid predatorEriopis connexa Germar. Larvae were fed until pupation on artificial diets to which different concentrations of DIMBOA (2-200μg/g diet) were added, as well as on aphids that had been feeding on wheat seedlings with different DIMBOA levels (140-440 μg/g fresh tissue). In diets, the effect of DIMBOA was greatest on survival of third-instar larvae and on the duration of the second and fourth instars. When aphids were provided as food, those that had fed on a wheat cultivar with an intermediate DIMBOA level led to a significantly longer larval duration in the predator than did those that fed on either low or high DIMBOA cultivars. Shortest predator development times were obtained with aphid prey that had fed on high DIMBOA seedlings. Higher DIMBOA levels in the plant appear to reduce aphid feeding rates (and rates of DIMBOA ingestion), decreasing aphid survival and minimizing the effect of the toxin on the predator.

Population genetic structure of two primary parasitoids of Spodoptera frugiperda (Lepidoptera), Chelonus insularis and Campoletis sonorensis (Hymenoptera): to what extent is the host plant important?

Plant chemistry can strongly influence interactions between herbivores and their natural enemies, either by providing volatile compounds that serve as foraging cues for parasitoids or predators, or by affecting the quality of herbivores as hosts or prey. Through these effects plants may influence parasitoid population genetic structure. We tested for a possible specialization on specific crop plants in Chelonus insularis and Campoletis sonorensis, two primary parasitoids of the fall armyworm, Spodoptera frugiperda. Throughout Mexico, S. frugiperda larvae were collected from their main host plants, maize and sorghum and parasitoids that emerged from the larvae were used for subsequent comparison by molecular analysis. Genetic variation at eight and 11 microsatellites were respectively assayed for C. insularis and C. sonorensis to examine isolation by distance, host plant and regional effects. Kinship analyses were also performed to assess female migration among host-plants. The analyses showed considerable within population variation and revealed a significant regional effect. No effect of host plant on population structure of either of the two parasitoid species was found. Isolation by distance was observed at the individual level, but not at the population level. Kinship analyses revealed significantly more genetically related--or kin--individuals on the same plant species than on different plant species, suggesting that locally, mothers preferentially stay on the same plant species. Although the standard population genetics parameters showed no effect of plant species on population structure, the kinship analyses revealed that mothers exhibit plant species fidelity, which may speed up divergence if adaptation were to occur.

Leaf herbivory and nutrients increase nectar alkaloids

Correlations between traits may constrain ecological and evolutionary responses to multispecies interactions. Many plants produce defensive compounds in nectar and leaves that could influence interactions with pollinators and herbivores, but the relationship between nectar and leaf defences is entirely unexplored. Correlations between leaf and nectar traits may be mediated by resources and prior damage. We determined the effect of nutrients and leaf herbivory by Manduca sexta on Nicotiana tabacum nectar and leaf alkaloids, floral traits and moth oviposition. We found a positive phenotypic correlation between nectar and leaf alkaloids. Herbivory induced alkaloids in nectar but not in leaves, while nutrients increased alkaloids in both tissues. Moths laid the most eggs on damaged, fertilized plants, suggesting a preference for high alkaloids. Induced nectar alkaloids via leaf herbivory indicate that species interactions involving leaf and floral tissues are linked and should not be treated as independent phenomena in plant ecology or evolution.

Plant chemistry and natural enemy fitness: effects on herbivore and natural enemy interactions

Tremendous strides have been made regarding our understanding of how host plant chemistry influences the interactions between herbivores and their natural enemies. While most work has focused on plant chemistry effects on host location and acceptance by natural enemies, an increasing number of studies examine negative effects. The tritrophic role of plant chemistry is central to several aspects of trophic phenomena including top-down versus bottom-up control of herbivores, enemy-free space and host choice, and theories of plant defense. Furthermore, tritrophic effects of plant chemistry are important in assessing the degree of compatibility between biological control and plant resistance approaches to pest control. Additional research is needed to understand the physiological effects of plant chemistry on parasitoids. Explicit tests are required to determine whether natural enemies can act as selective forces on plant defense. Finally, further studies of natural systems are crucial to understanding the evolution of multitrophic relationships.

Nicotine's defensive function in nature

Plants produce metabolites that directly decrease herbivore performance, and as a consequence, herbivores are selected for resistance to these metabolites. To determine whether these metabolites actually function as defenses requires measuring the performance of plants that are altered only in the production of a certain metabolite. To date, the defensive value of most plant resistance traits has not been demonstrated in nature. We transformed native tobacco(Nicotiana attenuata) with a consensus fragment of its two putrescine N-methyl transferase (pmt) genes in either antisense or inverted-repeat (IRpmt) orientations. Only the latter reduced (by greater than 95%) constitutive and inducible nicotine. With D(4)-nicotinic acid (NA), we demonstrate that silencing pmt inhibits nicotine production, while the excess NA dimerizes to form anatabine. Larvae of the nicotine-adapted herbivore Manduca sexta (tobacco hornworm) grew faster and, like the beetle Diabrotica undecimpunctata, preferred IRpmt plants in choice tests. When planted in their native habitat, IRpmt plants were attacked more frequently and, compared to wild-type plants, lost 3-fold more leaf area from a variety of native herbivores, of which the beet armyworm, Spodoptera exigua, and Trimerotropis spp. grasshoppers caused the most damage. These results provide strong evidence that nicotine functions as an efficient defense in nature and highlights the value of transgenic techniques for ecological research.

Effect of iridoid glycoside content on oviposition host plant choice and parasitism in a specialist herbivore

The Glanville fritillary butterfly Melitaea cinxia feeds upon two host plant species in Aland, Finland, Plantago lanceolata and Veronica spicata, both of which produce iridoid glycosides. Iridoids are known to deter feeding or decrease the growth rate of many generalist insect herbivores, but they often act as oviposition cues to specialist butterflies and are feeding stimulants to their larvae. In this study, two iridoid glycosides (aucubin and catalpol) were analyzed by micellar electrokinetic capillary chromatography. We measured the spatial and temporal variation of iridoid glycosides in natural populations of the host plants of M. cinxia. We also analyzed the aucubin and catalpol content in plants in relation to their use by ovipositing females, and in relation to the incidence of parasitism of M. cinxia larvae in natural populations. The mean concentrations of aucubin and catalpol were higher in P. lanceolata than in V. spicata, and catalpol concentrations were higher than aucubin concentrations in both host species. Plantago lanceolata individuals that were used for oviposition by M. cinxia had higher aucubin concentrations than random plants and neighboring plants. Additionally, oviposition and random plants had higher catalpol concentrations than neighboring plants, indicating that ovipositing females select for high iridoid glycoside plants or that oviposition induces iridoid glycoside production in P. lanceolata. Parasitism by the specialist parasitoid wasp Cotesia melitaearum occurred most frequently in larval groups that were feeding on plants with low concentrations of catalpol, irrespective of year, population, and host plant species. Therefore, parasitoids appear to avoid or perform poorly in host larvae with high catalpol content.

Insect-resistant transgenic plants in a multi-trophic context

So far, genetic engineering of plants in the context of insect pest control has involved insertion of genes that code for toxins, and may be characterized as the incorporation of biopesticides into classical plant breeding. In the context of pesticide usage in pest control, natural enemies of herbivores have received increasing attention, because carnivorous arthropods are an important component of insect pest control. However, in plant breeding programmes, natural enemies of herbivores have largely been ignored, although there are many examples that show that plant breeding affects the effectiveness of biological control. Negative influences of modified plant characteristics on carnivorous arthropods may induce population growth of new, even more harmful pest species that had no pest status prior to the pesticide treatment. Sustainable pest management will only be possible when negative effects on non-target, beneficial arthropods are minimized. In this review, we summarize the effects of insect-resistant crops and insect-resistant transgenic crops, especially Bt crops, from a food web perspective. As food web components, we distinguish target herbivores, non-target herbivores, pollinators, parasitoids and predators. Below-ground organisms such as Collembola, nematodes and earthworms should also be included in risk assessment studies, but have received little attention. The toxins produced in Bt plants retain their toxicity when bound to the soil, so accumulation of these toxins is likely to occur. Earthworms ingest the bound toxins but are not affected by them. However, earthworms may function as intermediaries through which the toxins are passed on to other trophic levels. In studies where effects of insect-resistant (Bt) plants on natural enemies were considered, positive, negative and no effects have been found. So far, most studies have concentrated on natural enemies of target herbivores. However, Bt toxins are structurally rearranged when they bind to midgut receptors, so that they are likely to lose their toxicity inside target herbivores. What happens to the toxins in non-target herbivores, and whether these herbivores may act as intermediaries through which the toxins may be passed on to the natural enemies, remains to be studied.

Molecular interactions between the specialist herbivore Manduca sexta (Lepidoptera, Sphingidae) and its natural host Nicotiana attenuata. IV. Insect-Induced ethylene reduces jasmonate-induced nicotine accumulation by regulating putrescine N-methyltransferase transcripts

Attack by the specialist herbivore, Manduca sexta, on its native host Nicotiana attenuata Torr. ex Wats. produces a dramatic ethylene release, a jasmonate burst, and a suppression of the nicotine accumulation that results from careful simulations of the herbivore's damage. Methyl-jasmonate (MeJA) treatment induces nicotine biosynthesis. However, this induction can be suppressed by ethylene as pretreatment of plants with 1-methylcyclopropene (1-MCP), a competitive inhibitor of ethylene receptors, restores the full MeJA-induced nicotine response in herbivore attacked plants (J. Kahl, D.H. Siemens, R.J. Aerts, R. Gäbler, F. Kühnemann, C.A. Preston, I.T. Baldwin [2000] Planta 210: 336-342). To understand whether this herbivore-induced signal cross-talk occurs at the level of transcript accumulation, we cloned the putrescine methyltransferase genes (NaPMT1 and NaPMT2) of N. attenuata, which are thought to represent the rate limiting step in nicotine biosynthesis, and measured transcript accumulations by northern analysis after various jasmonate, 1-MCP, ethephon, and herbivory treatments. Transcripts of both root putrescine N-methyltransferase (PMT) genes and nicotine accumulation increased dramatically within 10 h of shoot MeJA treatment and immediately after root treatments. Root ethephon treatments suppressed this response, which could be reversed by 1-MCP pretreatment. Moreover, 1-MCP pretreatment dramatically amplified the transcript accumulation resulting from both wounding and M. sexta herbivory. We conclude that attack from this nicotine-tolerant specialist insect causes N. attenuata to produce ethylene, which directly suppresses the nitrogen-intensive biosynthesis of nicotine.

Potential side effects of insect-resistant transgenic plants on arthropod natural enemies

Engineering genes encoding insecticidal proteins into crop plants offers numerous benefits to agriculture. However, like many conventional insecticides, this new technology has the potential to disrupt natural biological control through both direct and indirect side effects of the plants on the fitness or behaviour of arthropod predators and parasitoids. Interactions between transgenic plants and these beneficial insects are being assessed to avoid incompatibility.

Behavioral responses to host foodplants of two populations of the insect parasitoid Cotesia congregata (Say)

To test the hypothesis that natural enemy populations differ in their behavioral responses to plants or to plant allelochemicals, we compared two populations of the gregarious larval endoparasitoid, Cotesia congregata (Say) (Hymenoptera: Braconidae) that differed in their historical and present exposure to tobacco. The major hosts for both populations were Manduca sexta L. and M. quinquemaculata (Haworth) (Lepidoptera: Sphingidae), but these hosts were typically encountered on tobacco by parasitoids in one population (Upper Marlboro) and on tomato by parasitoids in another population (Wye). Early in the season, Wye parasitoids preferred to oviposit in M. sexta on tomato rather than on tobacco and Upper Marlboro parasitoids showed no preference; neither population showed any preference later in the season. Neither of the strains originating from the two populations showed a landing preference for tobacco or tomato in flight chamber trials, but Upper Marlboro parasitoids searched longer on tobacco than on tomato, and Wye parasitoids searched longer on tomato. When nicotine solutions were applied to tobacco leaf, searching responses of Upper Marlboro parasitoids were enhanced by 0.001-1.0% nicotine, and searching responses of Wye parasitoids were decreased by 0.01-1.0% nicotine. We speculate that population differences in searching responses to tobacco and nicotine may explain the differential parasitism responses found early in the season.