Induced plant responses to multiple damagers: differential effects on an herbivore and its parasitoid

Phloem-feeding insects create parasitoid-free space for caterpillars

Seemingly small ecological changes can have large, ramifying effects that defy expectations. Such are keystone effects in ecosystems. Phloem-feeding insect herbivores can act as keystone species by altering community structure and species interactions via plant-mediated or ant-mediated mechanisms. Plant responses triggered by phloem feeders can disrupt tri-trophic interactions induced by leaf-chewing herbivores, while ants that tend phloem feeders can deter or prey on other arthropods. Here, we investigate how phloem-feeding herbivores change caterpillar-parasitoid interactions on Quercus alba (white oak) trees in natural forests. We factorially manipulated the presence of phloem-feeding insects as well as ant access on Q. alba branches over multiple years and sites and measured parasitism rates of co-occurring caterpillars. While 19.3% of caterpillars were parasitized when phloem feeders were removed, the presence of phloem feeders completely suppressed parasitism of caterpillars (0%). This stark pattern was consistent across the diverse community of phloem feeders and caterpillars. Our manipulation of ant access had no effect on parasitism of caterpillars, implicating a plant-mediated mechanism. We further assessed the mechanistic hypothesis that phloem feeders suppress plant emission of caterpillar-induced volatile compounds, which could disrupt host-location behavior by parasitoids of caterpillars. Phloem feeders indeed reduced concentrations of four volatile compounds, consistent with the putative plant volatile-mediated mechanism. Given the important role of parasitoids in controlling herbivore populations, this keystone effect of phloem feeders offers novel insight into community dynamics in forests and potentially other terrestrial ecosystems.

Volatile-mediated oviposition preference for healthy over root-infested plants by the European corn borer

The selection of oviposition sites by female moths is crucial in shaping their progeny performance and survival, and consequently in determining insect fitness. Selecting suitable plants that promote the performance of the progeny is referred to as the Preference-Performance hypothesis (or 'mother-knows-best'). While root infestation generally reduces the performance of leaf herbivores, little is known about its impact on female oviposition. We investigated whether maize root infestation by the Western corn rootworm (WCR) affects the oviposition preference and larval performance of the European corn borer (ECB). ECB females used leaf volatiles to select healthy plants over WCR-infested plants. Undecane, a compound absent from the volatile bouquet of healthy plants, was the sole compound to be upregulated upon root infestation and acted as a repellent for first oviposition. ECB larvae yet performed better on plants infested below-ground than on healthy plants, suggesting an example of 'bad motherhood'. The increased ECB performance on WCR-infested plants was mirrored by an increased leaf consumption, and no changes in the plant primary or secondary metabolism were detected. Understanding plant-mediated interactions between above- and below-ground herbivores may help to predict oviposition decisions, and ultimately, to manage pest outbreaks in the field.

Assessing the impact of piercing-sucking pests on greenhouse-grown industrial hemp (Cannabis sativa L.)

Cannabis sativa or hemp, defined as <0.3% total tetrahydrocannabinol (THC), is a specialty crop in the United States, of particular interest among growers in the southeastern United States to replace tobacco production. Tetranychus urticae (twospotted spider mite), Aculops cannabicola (hemp russet mite), Polyphagotarsonemus latus (broad mites), and Phorodon cannabis (cannabis aphids) are considered the most significant pests in greenhouse grown hemp. Mite and aphid injury can cause cupping and yellowing of leaves, resulting in leaf drop, and reduced flower and resin production. We sought to understand the effects of feeding by T. urticae and Myzus persicae (green peach aphid), as a proxy for P. cannabis, on the concentration of economically significant cannabinoids through a series of experiments on greenhouse grown plants. First, we compared the variability of chemical concentrations in samples collected from individual plants versus pooled samples from 5 plants, and found that chemical concentrations in single plants were similar to those in pooled plant samples. Next, we compared chemical concentrations prior to arthropod infestation and post infestation. When evaluating the mite feeding damage in 2020, cannabinoids in plants infested with high densities of T. urticae increased more slowly than in uninfested control plants or plants infested with low T. urticae densities. In 2021, the concentration of tetrahydrocannabinol did not differ significantly between treatments. Cannabidiol increased more slowly in plants with low T. urticae densities when compared to uninfested controls but did not differ from the high T. urticae densities 14 days after infestation.

The Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), Influences Nilaparvata lugens Population Growth Directly, by Preying on Its Eggs, and Indirectly, by Inducing Defenses in Rice

The fall armyworm (FAW), Spodoptera frugiperda, has become one of the most important pests on corn in China since it invaded in 2019. Although FAW has not been reported to cause widespread damage to rice plants in China, it has been sporadically found feeding in the field. If FAW infests rice in China, the fitness of other insect pests on rice may be influenced. However, how FAW and other insect pests on rice interact remains unknown. In this study, we found that the infestation of FAW larvae on rice plants prolonged the developmental duration of the brown planthopper (BPH, Nilaparvata lugens (Stål)) eggs and plants damaged by gravid BPH females did not induce defenses that influenced the growth of FAW larvae. Moreover, co-infestation by FAW larvae on rice plants did not influence the attractiveness of volatiles emitted from BPH-infested plants to Anagrus nilaparvatae, an egg parasitoid of rice planthoppers. FAW larvae were able to prey on BPH eggs laid on rice plants and grew faster compared to those larvae that lacked available eggs. Studies revealed that the delay in the development of BPH eggs on FAW-infested plants was probably related to the increase in levels of jasmonoyl-isoleucine, abscisic acid and the defensive compounds in the rice leaf sheaths on which BPH eggs were laid. These findings indicate that, if FAW invades rice plants in China, the population density of BPH may be decreased by intraguild predation and induced plant defenses, whereas the population density of FAW may be increased.

Generalist herbivore response to volatile chemical induction varies along a gradient in soil salinization

Elevated soil salinity directly modifies plant physiology and indirectly alters the biotic interactions that shape plant performance. However, it is unclear how soil salinization interacts with plant defenses to alter patterns of leaf consumption or herbivore survival, development, and performance. In this study, we carried out laboratory feeding trials and a common garden experiment to investigate how gradients in soil salinization interact with plant induction status (modified via exogenous application of methyl jasmonate [MeJA]) to influence feeding consumption and performance of the generalist herbivore Spodoptera exigua on tomato (Solanum lycoperscium) plants. Our results showed that S. exigua consumed less leaf tissue from tomatoes treated with ≥ 50 mM NaCl; at these higher salinity treatments, these herbivores were less likely to pupate and died more quickly. Treatment with MeJA only reduced leaf consumption in the 0 mM NaCl treatment. Our common garden study demonstrated that natural populations of leaf chewing herbivores were less likely to damage tomatoes treated with > 50 mM NaCl solutions. Treatment with MeJA in the common garden reduced damage from natural populations of herbivores, but only for salt treatments at the 50 mM NaCl concentration level and we did observe variation in herbivore damage between cohorts in common garden trials. These results suggest that both soil salinization and volatile jasmonate signals may generate complementary shifts in decreased plant quality for herbivores. Overall, our study concludes that soil salinization could be a potential driver in spatial patterns of variation in both herbivory and herbivore demography.

Plasticity in induced resistance to sequential attack by multiple herbivores in Brassica nigra

In nature, plants interact with multiple insect herbivores that may arrive simultaneously or sequentially. There is extensive knowledge on how plants defend themselves against single or dual attack. However, we lack information on how plants defend against the attack of multiple herbivores that arrive sequentially. In this study, we investigated whether Brassica nigra L. plants are able to defend themselves against caterpillars of the late-arriving herbivore Plutella xylostella L., when plants had been previously exposed to sequential attack by four other herbivores (P. xylostella, Athalia rosae, Myzus persicae and Brevicoryne brassicae). We manipulated the order of arrival and the history of attack by four herbivores to investigate which patterns in sequential herbivory determine resistance against the fifth attacker. We recorded that history of sequential herbivore attack differentially affected the capability of B. nigra plants to defend themselves against caterpillars of P. xylostella. Caterpillars gained less weight on plants attacked by a sequence of four episodes of attack by P. xylostella compared to performance on plants that were not previously damaged by herbivores. The number of times the plant was attacked by herbivores of the same feeding guild, the identity of the first attacker, the identity and the guild of the last attacker as well as the order of attackers within the sequence of multiple herbivores influenced the growth of the subsequent herbivory. In conclusion, this study shows that history of sequential attack is an important factor determining plant resistance to herbivores.

Global change calls for novel plant protection: reviewing the potential of omnivorous plant-inhabiting arthropods as predators and plant defence inducers

Global change poses new challenges for pest management. Omnivorous predatory arthropods play an important role in pest management, yet their potential has not been fully explored. Not only do they consume prey, but their plant-feeding induces plant defences that decrease herbivores' performance, and increases production of volatiles that attract natural enemies. Growing evidence from different plant-arthropod systems indicates the generality of plant defence induction following omnivore plant-feeding. Furthermore, these responses appear to affect other organisms (e.g. plant viruses), altering multi-trophic interactions. Here, we review the dual role of omnivores (as predators and plant inducers), identify knowledge gaps and provide future perspectives to increase our understanding of omnivores' multiple functions, and how this can be applied to advance plant protection strategies.

Wild strawberry shows genetic variation in tolerance but not resistance to a generalist herbivore

Plants' defenses against herbivores usually include both resistance and tolerance mechanisms. Their deployment has predominantly been studied in either single-plant genotypes or multiple genotypes exposed to single herbivores. In natural situations, however, most plants are attacked by multiple herbivores. Therefore, aims of this study were to assess and compare the effects of single and multiple herbivores on plant resistance and tolerance traits, and the consequences for overall plant performance. For this, we exposed multiple genotypes of wild woodland strawberry (Fragaria vesca) to jasmonic acid (JA), to mimic chewing herbivory and induce the plants' defense responses, and then introduced the generalist herbivore Spodoptera littoralis to feed on them. We found that woodland strawberry consistently showed resistance to S. littoralis herbivory, with no significant genetic variation between the genotypes. By contrast, the studied genotypes showed high variation in tolerance, suggesting evolutionary potential in this trait. Prior JA application did not alter these patterns, although it induced an even higher level of resistance in all tested genotypes. The study provides novel information that may be useful for breeders seeking to exploit tolerance and resistance mechanisms to improve strawberry crops' viability and yields, particularly when multiple herbivores pose significant threats.

Previous Aphid Infestation Induces Different Expression Profiles of Genes Associated with Hormone-Dependent Responses in Near-Isogenic Winter Wheat Lines

Hormone-dependent responses in host plants induced by herbivore infestation have species-specific effects. This study focused on determining the relative expression profiles of the genes associated with hormone-dependent pathways in two near-isogenic wheat lines when attacked by cereal aphids. Infestation with Rhopalosiphum padi Linnaeus (Hemiptera: Aphididae) and/or Sitobion avenae Fabricius (Hemiptera: Aphididae) significantly upregulated the expression of marker genes related to the salicylic acid (SA)- and jasmonic acid (JA)-dependent pathways in the tested lines. In the resistant line 35-E4, previous infestation with R. padi significantly increased the relative expression of plant pathogenesis-related protein 1 at all sampling times but did not have a significant effect on the expression of the phenylalanine ammonia-lyase (PAL) gene. In addition, the expression levels of the lipoxygenase (LOX) and allene oxide synthase (AOS) genes immediately increased after the aphid attack. In susceptible line 35-A20, infestation with either R. padi or S. avenae led to significantly increased expression levels of the AOS and PAL genes. Moreover, sequential aphid infestation induced higher expression of AOS compared with a single-species aphid infestation, whereas the expression of the PAL gene was antagonistically affected by sequential aphid infestation. Overall, these results showed that aphid infestation induced SA- and JA-dependent responses in host plants. However, the expression profiles of these genes in resistant and susceptible host lines were significantly different.

Cross-seasonal legacy effects of arthropod community on plant fitness in perennial plants

In perennial plants, interactions with other community members during the vegetative growth phase may influence community assembly during subsequent reproductive years and may influence plant fitness. It is well-known that plant responses to herbivory affect community assembly within a growing season, but whether plant-herbivore interactions result in legacy effects on community assembly across seasons has received little attention. Moreover, whether plant-herbivore interactions during the vegetative growing season are important in predicting plant fitness directly or indirectly through legacy effects is poorly understood.Here, we tested whether plant-arthropod interactions in the vegetative growing season of perennial wild cabbage plants, Brassica oleracea, result in legacy effects in arthropod community assembly in the subsequent reproductive season and whether legacy effects have plant fitness consequences. We monitored the arthropod community on plants that had been induced with either aphids, caterpillars or no herbivores in a full-factorial design across 2 years. We quantified the plant traits 'height', 'number of leaves' and 'number of flowers' to understand mechanisms that may mediate legacy effects. We measured seed production in the second year to evaluate plant fitness consequences of legacy effects.Although we did not find community responses to the herbivory treatments, our data show that community composition in the first year leaves a legacy on community composition in a second year: predator community composition co-varied across years. Structural equation modelling analyses indicated that herbivore communities in the vegetative year correlated with plant performance traits that may have caused a legacy effect on especially predator community assembly in the subsequent reproductive year. Interestingly, the legacy of the herbivore community in the vegetative year predicted plant fitness better than the herbivore community that directly interacted with plants in the reproductive year. Synthesis. Thus, legacy effects of plant-herbivore interactions affect community assembly on perennial plants across growth seasons and these processes may affect plant reproductive success. We argue that plant-herbivore interactions in the vegetative phase as well as in the cross-seasonal legacy effects caused by plant responses to arthropod herbivory may be important in perennial plant trait evolution such as ontogenetic variation in growth and defence strategies.

Herbivore performance and plant defense after sequential attacks by inducing and suppressing herbivores

It is well known that herbivore-induced plant defenses alter host plant quality and can affect the behavior and performance of later arriving herbivores. Effects of sequential attacks by herbivores that either suppress or induce plant defenses are less well studied. We sequentially infested leaves of tomato plants with a strain of the phytophagous spider mite Tetranychus urticae that induces plant defenses and the closely related Tetranychus evansi, which suppresses plant defenses. Plant quality was quantified through oviposition of both spider mite species and by measuring proteinase inhibitor activity using plant material that had been sequentially attacked by both herbivore species. Spider-mite oviposition data show that T. evansi could suppress an earlier induction of plant defenses by T. urticae, and T. urticae could induce defenses in plants previously attacked by T. evansi in 1 day. Longer attacks by the second species did not result in further changes in oviposition. Proteinase inhibitor activity levels showed that T. evansi suppressed the high activity levels induced by T. urticae to constitutive levels in 1 day, and further suppressed activity to levels similar to those in plants attacked by T. evansi alone. Attacks by T. urticae induced proteinase inhibitor activity in plants previously attacked by T. evansi, eventually to similar levels as induced by T. urticae alone. Hence, plant quality and plant defenses were significantly affected by sequential attacks and the order of attack does not affect subsequent performance, but does affect proteinase inhibitor activity levels. Based on our results, we discuss the evolution of suppression of plant defenses.

Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites

Tomato plants are attacked by diverse herbivorous arthropods, including by cell-content-feeding mites, such as the extreme generalist Tetranychus urticae and specialists like Tetranychus evansi and Aculops lycopersici. Mite feeding induces plant defense responses that reduce mite performance. However, T. evansi and A. lycopersici suppress plant defenses via poorly understood mechanisms and, consequently, maintain a high performance on tomato. On a shared host, T. urticae can be facilitated by either of the specialist mites, likely due to the suppression of plant defenses. To better understand defense suppression and indirect plant-mediated interactions between herbivorous mites, we used gene-expression microarrays to analyze the transcriptomic changes in tomato after attack by either a single mite species (T. urticae, T. evansi, A. lycopersici) or two species simultaneously (T. urticae plus T. evansi or T. urticae plus A. lycopersici). Additionally, we assessed mite-induced changes in defense-associated phytohormones using LC-MS/MS. Compared to non-infested controls, jasmonates (JAs) and salicylate (SA) accumulated to higher amounts upon all mite-infestation treatments, but the response was attenuated after single infestations with defense-suppressors. Strikingly, whereas 8 to 10% of tomato genes were differentially expressed upon single infestations with T. urticae or A. lycopersici, respectively, only 0.1% was altered in T. evansi-infested plants. Transcriptome analysis of dual-infested leaves revealed that A. lycopersici primarily suppressed T. urticae-induced JA defenses, while T. evansi dampened T. urticae-triggered host responses on a transcriptome-wide scale. The latter suggests that T. evansi not solely down-regulates plant gene expression, but rather directs it back towards housekeeping levels. Our results provide valuable new insights into the mechanisms underlying host defense suppression and the plant-mediated facilitation of competing herbivores.

Modulation of plant-mediated interactions between herbivores of different feeding guilds: Effects of parasitism and belowground interactions

Herbivory affects subsequent herbivores, mainly regulated by the phytohormones jasmonic (JA) and salicylic acid (SA). Additionally, organisms such as soil microbes belowground or parasitoids that develop inside their herbivorous hosts aboveground, can change plant responses to herbivory. However, it is not yet well known how organisms of trophic levels other than herbivores, below- and above-ground, alter the interactions between insect species sharing a host plant. Here, we investigated whether the parasitoid Aphidius colemani and different soil microbial communities (created through plant-soil feedbacks) affect the JA and SA signalling pathways in response to the aphid Myzus persicae and the thrips Frankliniella occidentalis, as well as subsequent thrips performance. Our results show that the expression of the JA-responsive gene CaPINII in sweet pepper was more suppressed by aphids than by parasitised aphids. However, parasitism did not affect the expression of CaPAL1, a biosynthetic gene of SA. Furthermore, aphid feeding enhanced thrips performance compared with uninfested plants, but this was not observed when aphids were parasitised. Soils where different plant species were previously grown, did not affect plant responses or the interaction between herbivores. Our study shows that members of the third trophic level can modify herbivore interactions by altering plant physiology.

Aboveground herbivory induced jasmonates disproportionately reduce plant reproductive potential by facilitating root nematode infestation

Different plant feeders, including insects and parasitic nematodes, can influence each other by triggering systemic changes in their shared host plants. In most cases, however, the underlying mechanisms are unclear, and the consequences for plant fitness are not well understood. We studied the interaction between leaf feeding Manduca sexta caterpillars and root parasitic nematodes in Nicotiana attenuata. Simulated M. sexta attack increased the abundance of root parasitic nematodes in the field and facilitated Meloidogyne incognita reproduction in the glasshouse. Intact jasmonate biosynthesis was found to be required for both effects. Flower counts revealed that the jasmonate-dependent facilitation of nematode infestation following simulated leaf attack reduces the plant's reproductive potential to a greater degree than would be expected from the additive effects of the individual stresses. This work reveals that jasmonates mediate the interaction between a leaf herbivore and root parasitic nematodes and illustrates how plant-mediated interactions can alter plant's reproductive potential. The selection pressure resulting from the demonstrated fitness effects is likely to influence the evolution of plant defense traits in nature.

Caterpillars induce jasmonates in flowers and alter plant responses to a second attacker

In nature, herbivorous insects and plant pathogens are generally abundant when plants are flowering. Thus, plants face a diversity of attackers during their reproductive phase. Plant responses to one attacker can interfere with responses to a second attacker, and phytohormones that orchestrate plant reproduction are also involved in resistance to insect and pathogen attack. We quantified phytohormonal responses of flowering plants exposed to single or dual attack and studied resistance mechanisms of plants in the flowering stage. Flowering Brassica nigra were exposed to either a chewing caterpillar, a phloem-feeding aphid or a bacterial pathogen, and plant hormonal responses were compared with dual attack situations. We quantified phytohormones in inflorescences and leaves, and determined the consequences of hormonal changes for components of direct and indirect plant resistance. Caterpillars were the main inducers of jasmonates in inflorescences, and the phytohormonal profile of leaves was not affected by either insect or pathogen attack. Dual attack increased plant resistance to caterpillars, but compromised resistance to aphids. Parasitoid performance was negatively correlated with the performance of their hosts. We conclude that plants prioritize resistance of reproductive tissues over vegetative tissues, and that a chewing herbivore species is the main driver of responses in flowering B. nigra.

The Ratio between Field Attractive and Background Volatiles Encodes Host-Plant Recognition in a Specialist Moth

Volatiles emitted by plants convey an array of information through different trophic levels. Animals such as host-seeking herbivores encounter plumes with filaments from both host and non-host plants. While studies showed a behavioral effect of non-host plants on herbivore host location, less information is available on how a searching insect herbivore perceives and flies upwind to a host-plant odor plume within a background of non-host volatiles. We hypothesized here that herbivorous insects in search of a host-plant can discriminate plumes of host and non-host plants and that the taxonomic relatedness of the non-host have an effect on finding the host. We also predicted that the ratio between certain plant volatiles is cognized as host-plant recognition cue by a receiver herbivorous insect. To verify these hypotheses we measured the wind tunnel response of the moth Argyresthia conjugella to the host plant rowan, to non-host plants taxonomically related (Rosaceae, apple and pear) or unrelated to the host (Pinaceae, spruce) and to binary combination of host and non-host plants. Volatiles were collected from all plant combinations and delivered to the test insect via an ultrasonic sprayer as an artificial plume. While the response to the rowan as a plant was not affected by the addition of any of the non-host plants, the attraction to the corresponding sprayed headspace decreased when pear or apple but not spruce were added to rowan. A similar result was measured toward the odor exiting a jar where freshly cut plant material of apple or pear or spruce was intermixed with rowan. Dose-response gas-chromatography coupled to electroantennography revealed the presence of seven field attractive and seven background non-attractive antennally active compounds. Although the abundance of field attractive and of some background volatiles decreased in all dual combinations in comparison with rowan alone, an increased amount of the background compounds (3E)-4,8-Dimethyl-1,3,7-nonatriene ((E)-DMNT) and (Z)-3-hexenyl acetate was found in the rowan-apple and rowan-pear but not in the rowan-spruce headspace. A higher ratio between the abundance of each field attractive component and that of (E)-DMNT and (Z)-3-hexenyl acetate was measured for rowan and rowan-spruce in contrast to rowan-pear and rowan-apple headspaces. Our result suggests that the ratio between field attractive and background antennaly active volatiles encodes host-plant recognition in our study system.

Terpenoid biosynthesis in Arabidopsis attacked by caterpillars and aphids: effects of aphid density on the attraction of a caterpillar parasitoid

One of the responses of plants to insect attack is the production of volatile organic compounds that mediate indirect defence of plants by attracting natural enemies of the attacking herbivores. Herbivore-induced plant volatiles (HIPVs) include terpenoids that play key roles in the attraction of natural enemies. Crosstalk between phytohormonal signalling pathways is well known to affect the regulation of plant defences, including the emission of HIPVs. Thus, simultaneous feeding on the same plant by caterpillars and aphids, can affect the attraction of parasitoids by the plant compared to single insect attack. The role of aphid density in the regulation of HIPV emission by plants under dual attack has not been studied previously. Here, we investigated the attraction of Diadegma semiclausum, a parasitoid of the Diamondback moth Plutella xylostella, to volatiles emitted by Arabidopsis thaliana plants, simultaneously attacked by host caterpillars, and by the non-host aphid Brevicoryne brassicae. Our study shows that the effect of aphid infestation on parasitoid attraction is influenced by the density of the aphids. Biosynthesis and emission of (E,E)-α-farnesene could be linked to the observed preference of D. semiclausum parasitoids for the HIPV blend emitted by plants dually infested by caterpillars and aphids at a high density compared to dually infested plants with a low aphid density. Parasitoids such as D. semiclausum are important enemies of herbivorous insects and a better understanding of how plants express indirect defence mechanisms in response to multiple insect attack will provide important knowledge on plant-herbivore-parasitoid interactions under multiple stress conditions.

Response of Brassica oleracea to temporal variation in attack by two herbivores affects preference and performance of a third herbivore

1. Plants are frequently under attack by multiple insect herbivores, which may interact indirectly through herbivore-induced changes in the plant's phenotype. The identity, order, and timing of herbivore arrivals may influence the outcome of interactions between two herbivores. How these aspects affect, in turn, subsequently arriving herbivores that feed on double herbivore-induced plants has not been widely investigated. 2. This study tested whether the order and timing of arrival of two inducing herbivores from different feeding guilds affected the preference and performance of a subsequently arriving third herbivore, caterpillars of Mamestra brassicae L. (Lepidoptera: Noctuidae). Aphids [Brevicoryne brassicae L. (Hemiptera: Aphididae)] and caterpillars [Plutella xylostella L. (Lepidoptera: Yponomeutidae)] were introduced onto wild Brassica oleracea L. (Brassicaceae) plants in different sequences and with different arrival times. The effects of these plant treatments on M. brassicae caterpillars were assessed in pairwise preference tests and no-choice performance tests. 3. The caterpillars of M. brassicae preferred to feed from undamaged plants rather than double herbivore-induced plants. Compared with undamaged plants, they preferred plant material on which aphids had arrived first followed by caterpillars, whereas they avoided plant material with the reverse order of herbivore arrival. Performance of the caterpillars increased with increasing arrival time between herbivore infestations in double herbivore-induced plants. Although M. brassicae grew faster on plants induced by aphids than on those induced by caterpillars alone, its performance was not affected by the order of previous herbivore arrival. 4. These results imply that the timing of colonisation by multiple herbivores determines the outcome of plant-mediated herbivore-herbivore interactions.

Attraction of Three Mirid Predators to Tomato Infested by Both the Tomato Leaf Mining Moth Tuta absoluta and the Whitefly Bemisia tabaci

Plants emit volatile compounds in response to insect herbivory, which may play multiple roles as defensive compounds and mediators of interactions with other plants, microorganisms and animals. Herbivore-induced plant volatiles (HIPVs) may act as indirect plant defenses by attracting natural enemies of the attacking herbivore. We report here the first evidence of the attraction of three Neotropical mirid predators (Macrolophus basicornis, Engytatus varians and Campyloneuropsis infumatus) toward plants emitting volatiles induced upon feeding by two tomato pests, the leaf miner Tuta absoluta and the phloem feeder Bemisia tabaci, in olfactometer bioassays. Subsequently, we compared the composition of volatile blends emitted by insect-infested tomato plants by collecting headspace samples and analyzing them with GC-FID and GC-MS. Egg deposition by T. absoluta did not make tomato plants more attractive to the mirid predators than uninfested tomato plants. Macrolophus basicornis is attracted to tomato plants infested with either T. absoluta larvae or by a mixture of B. tabaci eggs, nymphs and adults. Engytatus varians and C. infumatus responded to volatile blends released by tomato plants infested with T. absoluta larvae over uninfested plants. Also, multiple herbivory by T. absoluta and B. tabaci did not increase the attraction of the mirids compared to infestation with T. absoluta alone. Terpenoids represented the most important class of compounds in the volatile blends and there were significant differences between the volatile blends emitted by tomato plants in response to attack by T. absoluta, B. tabaci, or by both insects. We, therefore, conclude that all three mirids use tomato plant volatiles to find T. absoluta larvae. Multiple herbivory did neither increase, nor decrease attraction of C. infumatus, E. varians and M. basicornis. By breeding for higher rates of emission of selected terpenes, increased attractiveness of tomato plants to natural enemies may improve the effectiveness of biological control.

Phytophagy of omnivorous predator Macrolophus pygmaeus affects performance of herbivores through induced plant defences

Plants possess various inducible defences that result in synthesis of specialized metabolites in response to herbivory, which can interfere with the performance of herbivores of the same and other species. Much less is known of the effects of plant feeding by omnivores. We found that previous feeding of the omnivorous predator Macrolophus pygmaeus on sweet pepper plants significantly reduced reproduction of the two-spotted spider mite Tetranychus urticae and western flower thrips Frankliniella occidentalis on the same plants, also on leaves that had not been exposed to the omnivore. In contrast, no effect was found on the reproduction of the green peach aphid Myzus persicae. Juvenile survival and developmental time of T. urticae and M. persicae, and larval survival of F. occidentalis were not affected by plant feeding by M. pygmaeus. Larvae of F. occidentalis feeding on leaves previously exposed to M. pygmaeus required longer to develop into adults. Defence-related plant hormones were produced locally and systemically after exposure to M. pygmaeus. The concentrations of 12-oxo-phytodienoic acid and jasmonic acid–isoleucine in the attacked leaves were significantly higher than in the corresponding leaves on the uninfested plants, and jasmonic acid concentrations showed the same trend, suggesting that jasmonic-acid-related defence pathways were activated. In contrast, similar concentrations of salicylic acid were found in the attacked leaves of M. pygmaeus-infested plants and uninfested plants. Our results show that plant feeding by omnivorous predators decreases the performance of herbivores, suggesting that it induces plant defences.

Effects of single and dual species herbivory on the behavioral responses of three thrips species to cotton seedlings

This study investigated the olfactory responses of 3 thrips species [Frankliniella schultzei Trybom, F. occidentalis Pergrande and Thrips tabaci Lindeman (Thysanoptera: Thripidae)] to cotton seedlings [Gossypium hirsutum L. (Malvales: Malvaceae)] simultaneously damaged by different combinations of herbivores. Cotton seedlings were damaged by foliar feeding Tetranychus urticae Koch (Trombidiforms: Tetranychidae), Helicoverpa armigera Hübner (Lepidoptera: Noctuidae), Aphis gossypii Glover (Hemiptera: Aphididae) or root feeding Tenebrio molitor L. (Coleoptera: Tenebrionidae). Thrips responses to plants simultaneously damaged by 2 species of herbivore were additive and equivalent to the sum of the responses of thrips to plants damaged by single herbivore species feeding alone. For example, F. occidentalis was attracted to T. urticae damaged plants but more attracted to undamaged plants than to plants damaged by H. armigera. Plants simultaneously damaged by low densities of T. urticae and H. armigera repelled F. occidentalis but as T. urticae density increased relative to H. armigera density, F. occidentalis attraction to coinfested plants increased proportionally. Thrips tabaci did not discriminate between undamaged plants and plants damaged by H. armigera but were attracted to plants damaged by T. urticae alone or simultaneously damaged by T. urticae and H. armigera. Olfactometer assays showed that simultaneous feeding by 2 herbivores on a plant can affect predator-prey interactions. Attraction of F. occidentalis to plants damaged by its T. urticae prey was reduced when the plant was simultaneously damaged by H. armigera, T. molitor, or A. gossypii and F. schultzei was more attracted to plants simultaneously damaged by T. urticae and H. armigera than to plants damaged by T. urticae alone. We conclude that plant responses to feeding by 1 species of herbivore are affected by responses to feeding by other herbivores. These plant-mediated interactions between herbivore complexes affect the behavioral responses of thrips which vary between species and are highly context dependent.

Does Aphid Infestation Interfere with Indirect Plant Defense against Lepidopteran Caterpillars in Wild Cabbage?

Attraction of parasitoids to plant volatiles induced by multiple herbivory depends on the specific combinations of attacking herbivore species, especially when their feeding modes activate different defense signalling pathways as has been reported for phloem feeding aphids and tissue feeding caterpillars. We studied the effects of pre-infestation with non-host aphids (Brevicoryne brassicae) for two different time periods on the ability of two parasitoid species to discriminate between volatiles emitted by plants infested by host caterpillars alone and those emitted by plants infested with host caterpillars plus aphids. Using plants originating from three chemically distinct wild cabbage (Brassica oleracea) populations, Diadegma semiclausum switched preference for dually infested plants to preference for plants infested with Plutella xylostella hosts alone when the duration of pre-aphid infestation doubled from 7 to 14 days. Microplitis mediator, a parasitoid of Mamestra brassicae caterpillars, preferred dually-infested plants irrespective of aphid-infestation duration. Separation of the volatile blends emitted by plants infested with hosts plus aphids or with hosts only was poor, based on multivariate statistics. However, emission rates of individual compounds were often reduced in plants infested with aphids plus hosts compared to those emitted by plants infested with hosts alone. This effect depended on host caterpillar species and plant population and was little affected by aphid infestation duration. Thus, the interactive effect of aphids and hosts on plant volatile production and parasitoid attraction can be dynamic and parasitoid specific. The characteristics of the multi-component volatile blends that determine parasitoid attraction are too complex to be deduced from simple correlative statistical analyses.

Qualitative and Quantitative Differences in Herbivore-Induced Plant Volatile Blends from Tomato Plants Infested by Either Tuta absoluta or Bemisia tabaci

Plants release a variety of volatile organic compounds that play multiple roles in the interactions with other plants and animals. Natural enemies of plant-feeding insects use these volatiles as cues to find their prey or host. Here, we report differences between the volatile blends of tomato plants infested with the whitefly Bemisia tabaci or the tomato borer Tuta absoluta. We compared the volatile emission of: (1) clean tomato plants; (2) tomato plants infested with T. absoluta larvae; and (3) tomato plants infested with B. tabaci adults, nymphs, and eggs. A total of 80 volatiles were recorded of which 10 occurred consistently only in the headspace of T. absoluta-infested plants. Many of the compounds detected in the headspace of the two herbivory treatments were emitted at different rates. Plants damaged by T. absoluta emitted at least 10 times higher levels of many compounds compared to plants damaged by B. tabaci and intact plants. The multivariate separation of T. absoluta-infested plants from those infested with B. tabaci was due largely to the chorismate-derived compounds as well as volatile metabolites of C₁₈-fatty acids and branched chain amino acids that had higher emission rates from T. absoluta-infested plants, whereas the cyclic sesquiterpenes α- and β-copaene, valencene, and aristolochene were emitted at significantly higher levels from B. tabaci-infested plants. Our findings imply that feeding by T. absoluta and B. tabaci induced emission of volatile blends that differ quantitatively and qualitatively, providing a chemical basis for the recently documented behavioral discrimination by two generalist predatory mirid species, natural enemies of T. absoluta and B. tabaci employed in biological control.

Can plant-natural enemy communication withstand disruption by biotic and abiotic factors?

The attraction of natural enemies towards herbivore-induced plant volatiles is a well-documented phenomenon. However, the majority of published studies are carried under optimal water and nutrient regimes and with just one herbivore. But what happens when additional levels of ecological complexity are added? Does the presence of a second herbivore, microorganisms, and abiotic stress interfere with plant-natural enemy communication? or is communication stable enough to withstand disruption by additional biotic and abiotic factors?Investigating the effects of these additional levels of ecological complexity is key to understanding the stability of tritrophic interactions in natural ecosystems and may aid to forecast the impact of environmental disturbances on these, especially in climate change scenarios, which are often associated with modifications in plant and arthropod species distribution and increased levels of abiotic stress.This review explores the literature on natural enemy attraction to herbivore-induced volatiles when, besides herbivory, plants are challenged by additional biotic and abiotic factors.The aim of this review was to establish the impact of different biotic and abiotic factors on plant-natural enemy communication and to highlight critical aspects to guide future research efforts.

Effects of plant intraspecific diversity across three trophic levels: Underlying mechanisms and plant traits

PREMISE OF STUDY

Although there is increasing recognition of the effects of plant intraspecific diversity on consumers, the mechanisms by which such effects cascade-up to higher trophic levels remain elusive.

METHODS

We evaluated the effects of plant (lima bean, Phaseolus lunatus) intraspecific diversity on a suite of insect herbivores (leaf-chewers, aphids, and seed-eating beetles) and their third trophic-level associates (parasitoids and aphid-tending ants). We established plots of three plants, classified as monocultures of one population source or polycultures with mixtures of three of the four population sources (N = 16 plots per level of diversity). Within each plot, plants were individually placed in pots and canopy contact was prevented, therefore eliminating diversity effects on consumers arising from changes in plant traits due to plant physical interactions.

KEY RESULTS

Plant diversity reduced damage by leaf-chewers as well as aphid abundance, and the latter effect in turn reduced ant abundance. In contrast, plant diversity increased the abundance of seed-eating beetles, but did not influence their associated parasitoids. There were no effects of diversity on seed traits potentially associated with seed predation, suggesting that differences in early season herbivory between monocultures and polycultures (a likely mechanism of diversity effects on plants since plant interactions were prevented) did not drive concomitant changes in plant traits.

CONCLUSIONS

This study emphasizes that effects of plant intraspecific diversity on consumers are contingent upon differences in associate responses within and among higher trophic levels and suggests possible mechanisms by which such effects propagate up this food web.

Stimulation of the Salicylic Acid Pathway Aboveground Recruits Entomopathogenic Nematodes Belowground

Plant defense pathways play a critical role in mediating tritrophic interactions between plants, herbivores, and natural enemies. While the impact of plant defense pathway stimulation on natural enemies has been extensively explored aboveground, belowground ramifications of plant defense pathway stimulation are equally important in regulating subterranean pests and still require more attention. Here we investigate the effect of aboveground stimulation of the salicylic acid pathway through foliar application of the elicitor methyl salicylate on belowground recruitment of the entomopathogenic nematode, Steinernema diaprepesi. Also, we implicate a specific root-derived volatile that attracts S. diaprepesi belowground following aboveground plant stimulation by an elicitor. In four-choice olfactometer assays, citrus plants treated with foliar applications of methyl salicylate recruited S. diaprepesi in the absence of weevil feeding as compared with negative controls. Additionally, analysis of root volatile profiles of citrus plants receiving foliar application of methyl salicylate revealed production of d-limonene, which was absent in negative controls. The entomopathogenic nematode S. diaprepesi was recruited to d-limonene in two-choice olfactometer trials. These results reinforce the critical role of plant defense pathways in mediating tritrophic interactions, suggest a broad role for plant defense pathway signaling belowground, and hint at sophisticated plant responses to pest complexes.

Interrelated responses of tomato plants and the leaf miner Tuta absoluta to nitrogen supply

Plant-insect interactions are strongly modified by environmental factors. This study evaluates the influence of nitrogen fertilisation on the tomato (Solanum lycopersicum L.) cv. Santa clara and the leafminer (Tuta absoluta (Meyrick), Lepidoptera: Gelechiidae). Greenhouse-grown tomato plants were fed hydroponically on a complete nutrient solution containing either a high nitrogen concentration (HN) sustaining maximum growth or a low nitrogen concentration (LN) limiting plant growth. Insect-free plants were compared with plants attacked by T. absoluta. Seven and 14 days after artificial oviposition leading to efficacious hatching and larvae development, we measured total carbon, nitrogen and soluble protein as well as defence compounds (phenolics, glycoalkaloids, polyphenol oxidase activity) in the HN versus LN plants. Only in the HN treatment did T. absoluta infestation slightly impair leaf growth and induce polyphenol oxidase (PPO) activity in the foliage. Neither the concentration of phenolic compounds and proteins nor the distribution of nitrogen within the plant was affected by T. absoluta infestation. In contrast, LN nutrition impaired T. absoluta-induced PPO activity. It decreased protein and total nitrogen concentration of plant organs and enhanced the accumulation of constitutive phenolics and tomatine. Moreover, LN nutrition impaired T. absoluta development by notably decreasing pupal weight and lengthening the development period from egg to adult. Adjusting the level of nitrogen nutrition may thus be a means of altering the life cycle of T. absoluta. This study provides a comprehensive dataset concerning interrelated responses of tomato plants and T. absoluta to nitrogen nutrition.

Eco-evolutionary dynamics of plant-herbivore communities: incorporating plant phenotypic plasticity

The interplay between evolution and ecological communities is critical for the integration of different levels of biological organization. Recent work has begun to unveil the importance of plant phenotypic plasticity and plant-herbivore (co)evolution to link plant evolution and associated insect communities. Specifically, herbivore-induced plant traits (i.e., plastic phenotypes) have significant effects on the structure and diversity of herbivore communities, which can in turn promote the evolution of not only the focal plant but also insect community members. Here, I will provide a conceptual framework on the eco-evolutionary dynamics of plant-herbivore communities to understand how biological organizations are integrated in plant-insect interactions. Research on eco-evolutionary dynamics of plant-herbivore communities will undoubtedly enrich understanding of a wide range of plant-insect interactions.

Tomato yellow leaf curl virus differentially influences plant defence responses to a vector and a non-vector herbivore

Plants frequently engage in simultaneous interactions with diverse classes of biotic antagonists. Differential induction of plant defence pathways by these antagonists, and interactions between pathways, can have important ecological implications; however, these effects are currently not well understood. We explored how Tomato yellow leaf curl virus (TYLCV) influenced the performance of its vector (Bemisia tabaci) and a non-vector herbivore (Tetranychus urticae) occurring separately or together on tomato plants (Solanum lycopersicum). TYLCV enhanced the performance of B. tabaci, although this effect was statistically significant only in the absence of T. urticae, which adversely affected B. tabaci performance regardless of infection status. In contrast, the performance of T. urticae was enhanced (only) by the combined presence of TYLCV and B. tabaci. Analyses of phytohormone levels and defence gene expression in wild-type tomatoes and various plant-defence mutants indicate that the enhancement of herbivore performance (for each species) entails the disruption of downstream defences in the jasmonic acid (JA) pathway. For T. urticae, this disruption appears to involve antagonistic effects of salicylic acid (SA), which is cumulatively induced to high levels by B. tabaci and TYLCV. In contrast, TYLCV was found to suppress JA-mediated responses to B. tabaci via mechanisms independent of SA.

A conserved pattern in plant-mediated interactions between herbivores

Plant‐mediated interactions between herbivores are important determinants of community structure and plant performance in natural and agricultural systems. Current research suggests that the outcome of the interactions is determined by herbivore and plant identity, which may result in stochastic patterns that impede adaptive evolution and agricultural exploitation. However, few studies have systemically investigated specificity versus general patterns in a given plant system by varying the identity of all involved players. We investigated the influence of herbivore identity and plant genotype on the interaction between leaf‐chewing and root‐feeding herbivores in maize using a partial factorial design. We assessed the influence of leaf induction by oral secretions of six different chewing herbivores on the response of nine different maize genotypes and three different root feeders. Contrary to our expectations, we found a highly conserved pattern across all three dimensions of specificity: The majority of leaf herbivores elicited a negative behavioral response from the different root feeders in the large majority of tested plant genotypes. No facilitation was observed in any of the treatment combinations. However, the oral secretions of one leaf feeder and the responses of two maize genotypes did not elicit a response from a root‐feeding herbivore. Together, these results suggest that plant‐mediated interactions in the investigated system follow a general pattern, but that a degree of specificity is nevertheless present. Our study shows that within a given plant species, plant‐mediated interactions between herbivores of the same feeding guild can be stable. This stability opens up the possibility of adaptations by associated organisms and suggests that plant‐mediated interactions may contribute more strongly to evolutionary dynamics in terrestrial (agro)ecosystems than previously assumed.

Biochar-macrofauna interplay: Searching for new bioindicators

Biochar incorporation in agricultural soils has been proposed as a climate change mitigation strategy and has proved to substantially increase crop productivity via physical, chemical and biological mechanisms. The changes induced in soil properties are known to have a direct impact on soil ecosystem with consequences for soil biota community that, in turn, can influence biochar aging in soil. Despite several studies investigated in the interplay between biochar and soil microbiology, there is a clear lack of information on groups that live in the most superficial ground layers: soil meso and macro fauna. These groups are of great importance if we consider that biochar application should ideally be located in the soil's surface layer (0-30 cm). Our study is the first attempt to investigate the interactions between biochar soil amendments and aboveground soil macro-meso fauna in a field crop. This was made setting-up a randomized-block experiment on a processing tomato crop in northern Italy, using three different biochar types and periodically monitoring soil parameters and fauna abundances along the crop growing cycle in summer 2013. Results show that the impact of biochar application on soil fauna as a whole is small when compared to that of agricultural management, suggesting that this amendment does not have short-term ecological interferences. Nevertheless, ants exhibited variations in abundances and distribution connected to properties of amended soils such as temperature, pH and humidity, proving that they can be effectively used as a target group in the study of interactions between biochar and soil biota.

Specificity of induced defenses, growth, and reproduction in lima bean (Phaseolus lunatus) in response to multispecies herbivory

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PREMISE OF THE STUDY

Following herbivore attack, plants can either reduce damage by inducing defenses or mitigate herbivory effects through compensatory growth and reproduction. It is increasingly recognized that such induced defenses in plants are herbivore-specific, but less is known about the specificity of compensatory responses. Damage by multiple herbivores may also lead to synergistic effects on induction and plant fitness that differ from those caused by a single herbivore species. Although largely unstudied, the order of arrival and damage by different herbivore species might also play an important role in the impacts of herbivory on plants.•

METHODS

We investigated the specificity of defense induction (phenolics) and effects on growth (number of stems and leaves) and reproduction (number of seeds, seed mass, and germination rate) from feeding by two generalist leaf-chewing herbivores (Spodoptera eridania and Diabrotica balteata) on Phaseolus lunatus plants and evaluated whether simultaneous attack by both herbivores and their order of arrival influenced such dynamics.•

KEY RESULTS

Herbivory increased levels of leaf phenolics, but such effects were not herbivore-specific. In contrast, herbivory enhanced seed germination in an herbivore-specific manner. For all variables measured, the combined effects of both herbivore species did not differ from their individual effects. Finally, the order of herbivore arrival did not influence defense induction, plant growth, or seed number but did influence seed mass and germination.•

CONCLUSIONS

Overall, this study highlights novel aspects of the specificity of plant responses induced by damage from multiple species of herbivores and uniquely associates such effects with plant lifetime fitness.

Herbivores with similar feeding modes interact through the induction of different plant responses

Plants respond to attacks by herbivores with various defences, which are mounted through the activation of different biochemical pathways that are known to interact. Thus, the attack of a plant by one herbivore species may result in changes in the performances of other species on the same plant. It has been suggested that species with comparable feeding modes induce similar plant defences and such herbivores are therefore expected to have a negative effect on each other’s performance. We studied two closely related phytophagous mite species with identical feeding modes. Yet, one of the species (Tetranychus urticae) induces tomato plant defences, whereas the other (T. evansi) reduces them. We found that the “inducing” species benefits from the downregulation of defences by the “reducing” species, which, in turn, suffers from the induction of defences by the inducing species. Moreover, the performances of the two mite species on leaves that were previously attacked by both species simultaneously were intermediate between that on leaves previously attacked by each of the mites separately. The activity of proteinase inhibitor, a defensive compound, was not found to be intermediate in leaves attacked by both species simultaneously—it was almost as high as the activity seen in leaves with defences induced by T. urticae. Oviposition rates of T. urticae showed a nonlinear correlation with inhibitor activity, suggesting that it is potentially problematic to use this activity as an indicator of the level of plant defence. Our results show that herbivores with similar feeding modes have opposite effects on plant defence and differentially affect each other’s performance on co-infested plants.

Density-dependent interference of aphids with caterpillar-induced defenses in Arabidopsis: involvement of phytohormones and transcription factors

In nature, plants are exposed to attacks by multiple herbivore species at the same time. To cope with these attacks, plants regulate defenses with the production of hormones such as salicylic acid (SA) and jasmonic acid (JA). Because herbivore densities are dynamic in time, this may affect plant-mediated interactions between different herbivores attacking at the same time. In Arabidopsis thaliana, feeding by Brevicoryne brassicae aphids interferes with induced defenses against Plutella xylostella caterpillars. This is density dependent: at a low aphid density, the growth rate of P. xylostella was increased, whereas caterpillars feeding on plants colonized by aphids at a high density have a reduced growth rate. Growth of P. xylostella larvae was unaffected on sid2-1 or on dde2-2 mutant plants when feeding simultaneously with a low or high aphid density. This shows that aphid interference with caterpillar-induced defenses requires both SA and JA signal transduction pathways. Transcriptional analysis revealed that simultaneous feeding by caterpillars and aphids at a low density induced the expression of the SA transcription factor gene WRKY70 whereas expression of WRKY70 was lower in plants induced with both caterpillars and a high aphid density. Interestingly, the expression of the JA transcription factor gene MYC2 was significantly higher in plants simultaneously attacked by aphids at a high density and caterpillars. These results indicate that a lower expression level of WRKY70 leads to significantly higher MYC2 expression through SA-JA cross-talk. Thus, plant-mediated interactions between aphids and caterpillars are density dependent and involve phytohormonal cross-talk and differential activation of transcription factors.

The timing of herbivore-induced volatile emission in black poplar (Populus nigra) and the influence of herbivore age and identity affect the value of individual volatiles as cues for herbivore enemies

BACKGROUND

The role of herbivore-induced plant volatiles as signals mediating the attraction of herbivore enemies is a well-known phenomenon. Studies with short-lived herbaceous plant species have shown that various biotic and abiotic factors can strongly affect the quantity, composition and timing of volatile emission dynamics. However, there is little knowledge on how these factors influence the volatile emission of long-lived woody perennials. The aim of this study was to investigate the temporal dynamics of herbivore-induced volatile emission of black poplar (Populus nigra) through several day-night cycles following the onset of herbivory. We also determined the influence of different herbivore species, caterpillars of the gypsy moth (Lymantria dispar) and poplar hawkmoth (Laothoe populi), and different herbivore developmental stages on emission.

RESULTS

The emission dynamics of major groups of volatile compounds differed strikingly in response to the timing of herbivory and the day-night cycle. The emission of aldoximes, salicyl aldehyde, and to a lesser extent, green leaf volatiles began shortly after herbivore attack and ceased quickly after herbivore removal, irrespective of the day-night cycle. However, the emission of most terpenes showed a more delayed reaction to the start and end of herbivory, and emission was significantly greater during the day compared to the night. The identity of the caterpillar species caused only slight changes in emission, but variation in developmental stage had a strong impact on volatile emission with early instar L. dispar inducing more nitrogenous volatiles and terpenoids than late instar caterpillars of the same species.

CONCLUSIONS

The results indicate that only a few of the many herbivore-induced black poplar volatiles are released in tight correlation with the timing of herbivory. These may represent the most reliable cues for herbivore enemies and, interestingly, have been shown in a recent study to be the best attractants for an herbivore enemy that parasitizes gypsy moth larvae feeding on black poplar.

Odor uniformity among tomato individuals in response to herbivore depends on insect species

Plants produce specific volatile organic compound (VOC) blends in response to herbivory. Herbivore-induced blends may prime the plant for future attack or attract carnivorous insects; these responses have been considered adaptive for plants. If herbivores differentially modify the VOC emission among individuals within a group of plants they feed upon, then plant responses to herbivores will not only produce specific blends but also variation in odor among individuals, i.e. individuals smell the same, then having a uniform odor. We investigated the VOC emission variation or uniformity among tomato individuals (Solanum lycopersicum L. cv. Castlemart) in response to moderate wounding by (1) nymphs of the psyllid Bactericera cockerelli (Sulc.) (TP); (2) Lepidoptera chewing-feeding larvae of Fall Armyworm (Spodoptera frugiperda Smith) (FAW) and (3) of Cabbage Looper (Trichoplusia ni Hübner) (CL), and (4) mechanical damage (MD). We used a ratio-based analysis to compare the fold-change in concentration from constitutive to induced VOC emission. We also used size and shape analysis to compare the emission of damaged and non-damaged individuals. Aside of finding herbivore-specific blends in line with other studies, we found patterns not described previously. We detected constitutive and induced odor variation among individuals attacked by the same herbivore, with the induced odor uniformity depending on the herbivore identity. We also showed that the fold-change of VOCs from constitutive to induced state differed among individuals independently of the uniformity of the blends before herbivore attack. We discuss our findings in the context of the ecological roles of VOCs in plant-plant and plant-carnivore insects’ interactions.

Herbivore-induced plant volatiles to enhance biological control in agriculture

Plants under herbivore attack synthetize defensive organic compounds that directly or indirectly affect herbivore performance and mediate other interactions with the community. The so-called herbivore-induced plant volatiles (HIPVs) consist of odors released by attacked plants that serve as important cues for parasitoids and predators to locate their host/prey. The understanding that has been gained on the ecological role and mechanisms of HIPV emission opens up paths for developing novel strategies integrated with biological control programs with the aim of enhancing the efficacy of natural enemies in suppressing pest populations in crops. Tactics using synthetic HIPVs or chemically/genetically manipulating plant defenses have been suggested in order to recruit natural enemies to plantations or help guiding them to their host more quickly, working as a "synergistic" agent of biological control. This review discusses strategies using HIPVs to enhance biological control that have been proposed in the literature and were categorized here as: (a) exogenous application of elicitors on plants, (b) use of plant varieties that emit attractive HIPVs to natural enemies, (c) release of synthetic HIPVs, and (d) genetic manipulation targeting genes that optimize HIPV emission. We discuss the feasibility, benefits, and downsides of each strategy by considering not only field studies but also comprehensive laboratory assays that present an applied approach for HIPVs or show the potential of employing them in the field.

Spatiotemporal patterns of induced resistance and susceptibility linking diverse plant parasites

Induced defenses mediate interactions between parasites sharing the same host plant, but the outcomes of these interactions are challenging to predict because of spatiotemporal variation in plant responses and differences in defense pathways elicited by herbivores or pathogens. Dissecting these mediating factors necessitates an approach that encompasses a diversity of parasitic feeding styles and tracks interactions over space and time. We tested indirect plant-mediated relationships across three tomato (Solanum lycopersicum) consumers: (1) the fungal pathogen-powdery mildew, Oidium neolycopersici; (2) a sap-feeding insect-silverleaf whitefly, Bemisia tabaci; and (3) a chewing insect-the leaf miner, Tuta absoluta. Further, we evaluated insect/pathogen responses on local vs. systemic leaves and over short (1 day) vs. long (4 days) time scales. Overall, we documented: (1) a bi-directional negative effect between O. neolycopersici and B. tabaci; (2) an asymmetrical negative effect of B. tabaci on T. absoluta; and (3) an asymmetrical positive effect of T. absoluta on O. neolycopersici. Spatiotemporal patterns varied depending on the species pair (e.g., whitefly effects on leaf miner performance were highly localized to the induced leaf, whereas effects on pathogen growth were both local and systemic). These results highlight the context-dependent effects of induced defenses on a diverse community of tomato parasites. Notably, the outcomes correspond to those predicted by phytohormonal theory based on feeding guild differences with key implications for the recent European invasion by T. absoluta.

How predictable are the behavioral responses of insects to herbivore induced changes in plants? Responses of two congeneric thrips to induced cotton plants

Changes in plants following insect attack are referred to as induced responses. These responses are widely viewed as a form of defence against further insect attack. In the current study we explore whether it is possible to make generalizations about induced plant responses given the unpredictability and variability observed in insect-plant interactions. Experiments were conducted to test for consistency in the responses of two congeneric thrips, Frankliniella schultzei Trybom and Frankliniella occidentalis Pergrande (Thysanoptera: Thripidae) to cotton seedlings (Gossypium hirsutum Linneaus (Malvales: Malvaceae)) damaged by various insect herbivores. In dual-choice experiments that compared intact and damaged cotton seedlings, F. schultzei was attracted to seedlings damaged by Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), Tetranychus urticae (Koch) (Trombidiforms: Tetranychidae), Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae), F. schultzei and F. occidentalis but not to mechanically damaged seedlings. In similar tests, F. occidentalis was attracted to undamaged cotton seedlings when simultaneously exposed to seedlings damaged by H. armigera, T. molitor or F. occidentalis. However, when exposed to F. schultzei or T. urticae damaged plants, F. occidentalis was more attracted towards damaged plants. A quantitative relationship was also apparent, F. schultzei showed increased attraction to damaged seedlings as the density of T. urticae or F. schultzei increased. In contrast, although F. occidentalis demonstrated increased attraction to plants damaged by higher densities of T. urticae, there was a negative relationship between attraction and the density of damaging conspecifics. Both species showed greater attraction to T. urticae damaged seedlings than to seedlings damaged by conspecifics. Results demonstrate that the responses of both species of thrips were context dependent, making generalizations difficult to formulate.