Parameters affecting plant defense pathway mediated recruitment of entomopathogenic nematodes

Entomopathogenic nematodes are natural enemies and effective biological control agents of subterranean insect herbivores. Interactions between herbivores, plants, and entomopathogenic nematodes are mediated by plant defense pathways. These pathways can induce release of volatiles and recruit entomopathogenic nematodes. Stimulation of these plant defense pathways for induced defense against belowground herbivory may enhance biological control in the field. Knowledge of the factors affecting entomopathogenic nematode behaviour belowground is needed to effectively implement such strategies. To that end, we explore the effect of elicitor, elicitor dose, mechanical damage, and entomopathogenic nematode release distance on recruitment of entomopathogenic nematode infective juveniles to corn seedlings. Increasing doses of methyl jasmonate and methyl salicylate elicitors recruited more entomopathogenic nematodes as did mechanical damage. Recruitment of entomopathogenic nematodes was higher at greater release distances. These results suggest entomopathogenic nematodes are highly tuned to plant status and present a strategy for enhancing biological control using elicitor-stimulated recruitment of entomopathogenic nematodes.     

Simultaneous feeding by aboveground and belowground herbivores attenuates plant-mediated attraction of their respective natural enemies

Herbivore-damaged plants emit volatile organic compounds that attract natural enemies of the herbivores. This form of indirect plant defence occurs aboveground as well as belowground, but it remains unclear how simultaneous feeding by different herbivores attacking leaves and roots may affect the production of the respective defence signals. We employed a setup that combines trapping of volatile organic signals and simultaneous measurements of the attractiveness of these signals to above and belowground natural enemies. Young maize plants were infested with either the foliar herbivore Spodoptera littoralis , the root herbivore Diabrotica virgifera virgifera , or with both these important pest insects. The parasitic wasp Cotesia marginiventris and the entomopathogenic nematode Heterorhabditis megidis were strongly attracted if their respective host was feeding on a plant, but this attraction was significantly reduced if both herbivores were on a plant. The emission of the principal root attractant was indeed reduced due to double infestation, but this was not evident for the leaf volatiles. The parasitoid showed an ability to learn the differences in odour emissions and increased its response to the odour of a doubly infested plant after experiencing this odour during an encounter with hosts. This first study to measure effects of belowground herbivory on aboveground tritrophic signalling and vice-versa reemphasizes the important role of plants in bridging interactions between spatially distinct components of the ecosystem.     

Interactions between root and shoot herbivores of Ammophila arenaria in the laboratory do not translate into correlated abundances in the field

Over the past decades a growing body of literature has presented proof of the possible interactions between foliar and root herbivores. These effects can be positive, negative or neutral in either direction, depending on the species and the involved mechanism. Most of these studies however concern experiments under controlled conditions. Whether these interactions affect the distribution of herbivores under natural conditions still largely remains an open question. This study examined interactions between root feeding nematodes and shoot feeding aphids on Ammophila arenaria in the laboratory. We subsequently addressed the question whether expectations from this experiment are reflected in correlations between plant related variables and the abundance of both herbivores in the field. We demonstrated that nematodes and aphids can negatively affect each other in a controlled microcosm. In the field however no significant correlations between nematode and aphid abundances could be detected. There, shorter plants with a more vital leaf set and a higher root density supported the highest numbers of aphids. Plants with a lower root density and higher root vitality held more migratory endoparasitic nematodes, while more nematode cysts were found among roots with a low vitality. A certain plant property can furthermore affect above‐ and belowground herbivores in the opposite direction, such as root density in this case. This study suggests that effects of root herbivores on foliar herbivores or vice versa seem to be blurred in a field situation where other variables related to plant vitality and water content structure the herbivore populations. Therefore, caution should be used in generalising the prevalence of these interactions between the above‐ and belowground fauna, based solely on laboratory experiments.     

Influence of drought on plant performance through changes in belowground tritrophic interactions

Climate change is predicted to increase the risk of drought in many temperate agroecosystems. While the impact of drought on aboveground plant‐herbivore‐natural enemy interactions has been studied, little is known about its effects on belowground tritrophic interactions and root defense chemistry. We investigated the effects of low soil moisture on the interaction between maize, the western corn rootworm (WCR, Diabrotica virgifera), and soil‐borne natural enemies of WCR. In a manipulative field experiment, reduced soil moisture and WCR attack reduced plant performance and increased benzoxazinoid levels. The negative effects of WCR on cob dry weight and silk emergence were strongest at low moisture levels. Inoculation with entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora) was ineffective in controlling WCR, and the EPNs died rapidly in the warm and dry soil. However, ants of the species Solenopsis molesta invaded the experiment, were more abundant in WCR‐infested pots and predated WCR independently of soil moisture. Ant presence increased root and shoot biomass and was associated with attenuated moisture‐dependent effects of WCR on maize cob weight. Our study suggests that apart from directly reducing plant performance, drought can also increase the negative effects of root herbivores such as WCR. It furthermore identifies S. molesta as a natural enemy of WCR that can protect maize plants from the negative impact of herbivory under drought stress. Robust herbivore natural enemies may play an important role in buffering the impact of climate change on plant‐herbivore interactions.     

Effects of decomposers and herbivores on plant performance and aboveground plant-insect interactions

Most ecologists acknowledge that plants are subject to complex interactions between both below- and aboveground dwelling animals. However, these complex interactions are seldomly investigated simultaneously. In a factorial common garden experiment we tested single and combined effects of decomposers, root herbivores and leaf herbivores on the growth, flower visitation, and abundance of naturally colonizing aphids and parasitoids on wild mustard ( Sinapis arvensis ). We found that the individual presence of either root herbivores or decomposers resulted in increased aphid abundance, demonstrating that the same aboveground plant–insect interaction can be released by different belowground processes. Enhanced aphid densities caused higher numbers of parasitoids. Furthermore, decomposers increased plant growth and plant fitness (measured as the number of seeds produced), indicating that mustard may benefit from nutrients provided by decomposers, regardless whether plants are attacked by root herbivores or leaf herbivores, or both simultaneously. More flower visits were observed in plants attacked by root herbivores but without leaf herbivores than in plants with both herbivores, suggesting that root herbivory can modify flower attractivity to pollinators. Our results suggest that patterns in plant–insect interactions above the ground are not only affected by aboveground factors but also by a wealth of different belowground processes mediated by the plant.     

Above- and belowground insect herbivores differentially affect soil nematode communities in species-rich plant communities

Interactions between above‐ and belowground invertebrate herbivores alter plant diversity, however, little is known on how these effects may influence higher trophic level organisms belowground. Here we explore whether above‐ and belowground invertebrate herbivores which alter plant community diversity and biomass, in turn affect soil nematode communities. We test the hypotheses that insect herbivores 1) alter soil nematode diversity, 2) stimulate bacterial‐feeding and 3) reduce plant‐feeding nematode abundances. In a full factorial outdoor mesocosm experiment we introduced grasshoppers (aboveground herbivores), wireworms (belowground herbivores) and a diverse soil nematode community to species‐rich model plant communities. After two years, insect herbivore effects on nematode diversity and on abundance of herbivorous, bacterivorous, fungivorous and omni‐carnivorous nematodes were evaluated in relation to plant community composition. Wireworms did not affect nematode diversity despite enhanced plant diversity, while grasshoppers, which did not affect plant diversity, reduced nematode diversity. Although grasshoppers and wireworms caused contrasting shifts in plant species dominance, they did not affect abundances of decomposer nematodes at any trophic level. Primary consumer nematodes were, however, strongly promoted by wireworms, while community root biomass was not altered by the insect herbivores. Overall, interaction effects of wireworms and grasshoppers on the soil nematodes were not observed, and we found no support for bottom‐up control of the nematodes. However, our results show that above‐ and belowground insect herbivores may facilitate root‐feeding rather than decomposer nematodes and that this facilitation appears to be driven by shifts in plant species composition. Moreover, the addition of nematodes strongly suppressed shoot biomass of several forb species and reduced grasshopper abundance. Thus, our results suggest that nematode feedback effects on plant community composition, due to plant and herbivore parasitism, may strongly depend on the presence of insect herbivores.     

After‐life effects: living and dead invertebrates differentially affect plants and their associated above‐ and belowground multitrophic communities

Above–belowground (AG–BG) studies typically focus on plant‐mediated effects inflicted by living organisms. However, animal cadavers may also play an important role in AG–BG interactions. Here, we explore whether living and dead foliar‐feeding and soil‐dwelling invertebrates differentially affect plants and their associated AG and BG multitrophic communities. In a mesocosm study we separated effects of living and dead locusts (AG herbivores) and earthworms (BG detritivores) on experimental multitrophic communities consisting of eight plant species, an AG aphid and parasitoid community and a BG nematode community. We measured root and shoot biomass and determined plant community composition and densities of aphids, parasitoids and nematodes. Living locusts decreased total shoot and root biomass in the mesocosms, whereas living earthworms enhanced total root biomass. Cadavers of both invertebrates strongly increased total root and shoot biomass, and changed the plant community composition mainly via enhanced growth of grasses. Earthworm cadavers affected plant biomass and community composition more strongly than their living counterparts, while this was reversed for locusts. Structural equation models showed that aphids and parasitoids were influenced via changes in plant community composition. Nematode densities in the soil, especially those of bacterivorous and entomopathogenic nematodes, were strongly increased by dead invertebrates, but unaffected by living ones. We conclude that effects of invertebrates on plant growth and densities of AG and BG organisms strongly depend on whether the invertebrates are dead or alive. Remarkably, invertebrate cadavers may inflict even stronger effects than their living counterparts. Hence, our study reveals an important, but often neglected, role of animal cadavers in AG–BG studies.     

Plant-mediated interactions between shoot-feeding aphids and root-feeding nematodes depend on nitrate fertilization

The plant metabolite composition is modulated by various abiotic and biotic factors including nutrient availability and herbivory. In turn, induced changes in plant quality can affect herbivore performance and mediate indirect interactions between spatially separated herbivores sharing a host. Studies on plant-mediated herbivore interactions have been carried out at single fertilization regimes only, but we hypothesized that nutrient availability modifies these interactions. Therefore, we studied the interactions between two vascular tissue herbivores, the aboveground feeding aphid Brevicoryne brassicae and the belowground infesting nematode Heterodera schachtii, on Arabidopsis thaliana grown under two nitrate fertilization conditions (varying by 33 %). Furthermore, we investigated plant growth and primary metabolic responses to fertilization and herbivore treatments, which could potentially mediate these interactions, as the herbivores may act as metabolic sinks. Whereas nematodes had no effects on aphids, aphid presence influenced nematodes in opposite directions, depending on fertilization: at low nitrate supply, aphids had a promoting effect on nematodes, whereas at high nitrate fertilization they lowered the nematode infestation compared to control plants. Plants produced significantly more biomass under high nitrate supply but C and N contents were not altered. Primary metabolite profiles differed only marginally between roots of both fertilization treatments in plants with and without aphids, indicating that nematodes may respond to these or other metabolic modifications, which are caused by minute environmental changes, in a sensitive way. Our results highlight the need to consider the importance of plant nutrient availability on the outcome of interactions between co-occurring herbivores in future studies.     

Toward a mechanistic understanding of competition in vascular-feeding herbivores: an empirical test of the sink competition hypothesis

Recent evidence suggests that competitive interactions among herbivores are mostly indirect and mediated by plant responses to herbivory. Most studies, however, emphasize chewing insects and secondary chemistry, thus ignoring the diverse group of vascular-parasites that may be more likely to compete through induced changes in phytonutrients. Using an aboveground phloem-feeding aphid (Myzus persicae) and a belowground gall-forming nematode (Meloidogyne incognita) on tobacco plants, we assessed the importance of competition via induced host–plant sinks. In a series of experimental trials, nematode root herbivory caused 55 and 72% declines in the growth and fecundity of aphids, respectively. Aphids, on the other hand, did not impact nematode performance. Therefore, we predicted that nematodes out-compete M. persicae by attenuating the magnitude of aphid-induced sinks. Through a combination of invertase enzyme measurements and stable isotope (¹³C and ¹⁵N) enrichment, we found evidence that both herbivores act as mobilizing sinks. Aphids attracted photoassimilates to feeding aggregations on leaves and nematode galls accumulated resources in the roots. Levels of invertase enzymes, for example, were more than fourfold higher in nematode galls than in surrounding root tissue. Yet we found no evidence supporting a sink competition model for aphid–nematode interactions. The strength of aphid-induced leaf sinks was entirely unaffected by nematode presence, and vice versa. Thus, induced host–plant sinks appear to be a common strategy employed by vascular parasites to manipulate the physiology of their host, but multi-sink competition may be limited to herbivores that co-occur on the same tissue type and/or plants under growth-limited abiotic conditions.     

Sequential effects of root and foliar herbivory on aboveground and belowground induced plant defense responses and insect performance

Plants are often simultaneously or sequentially attacked by multiple herbivores and changes in host plants induced by one herbivore can influence the performance of other herbivores. We examined how sequential feeding on the plant Plantago lanceolata by the aboveground herbivore Spodoptera exigua and the belowground herbivore Agriotes lineatus influences plant defense and the performance of both insects. Belowground herbivory caused a reduction in the food consumption by the aboveground herbivore independent of whether it was initiated before, at the same time, or after that of the aboveground herbivore. By contrast, aboveground herbivory did not significantly affect belowground herbivore performance, but significantly reduced the performance of later arriving aboveground conspecifics. Interestingly, belowground herbivores negated negative effects of aboveground herbivores on consumption efficiency of their later arriving conspecifics, but only if the belowground herbivores were introduced simultaneously with the early arriving aboveground herbivores. Aboveground–belowground interactions could only partly be explained by induced changes in an important class of defense compounds, iridoid glycosides (IGs). Belowground herbivory caused a reduction in IGs in roots without affecting shoot levels, while aboveground herbivory increased IG levels in roots in the short term (4 days) but only in the shoots in the longer term (17 days). We conclude that the sequence of aboveground and belowground herbivory is important in interactions between aboveground and belowground herbivores and that knowledge on the timing of exposure is essential to predict outcomes of aboveground–belowground interactions.     

Legacy effects of aboveground-belowground interactions

Root herbivory can greatly affect the performance of aboveground insects via changes in plant chemistry. These interactions have been studied extensively in experiments where aboveground and belowground insects were feeding on the same plant. However, little is known about how aboveground and belowground organisms interact when they feed on plant individuals that grow after each other in the same soil. We show that feeding by aboveground and belowground insect herbivores on ragwort (Jacobaea vulgaris) plants exert unique soil legacy effects, via herbivore-induced changes in the composition of soil fungi. These changes in the soil biota induced by aboveground and belowground herbivores of preceding plants greatly influenced the pyrrolizidine alkaloid content, biomass and aboveground multitrophic interactions of succeeding plants. We conclude that plant-mediated interactions between aboveground and belowground insects are also important when they do not feed simultaneously on the same plant.     

Effects of above- and belowground herbivory on growth,pollination, and reproduction in cucumber

Plants experience unique challenges due to simultaneous life in two spheres, above- and belowground. Interactions with other organisms on one side of the soil surface may have impacts that extend across this boundary. Although our understanding of plant–herbivore interactions is derived largely from studies of leaf herbivory, belowground root herbivores may affect plant fitness directly or by altering interactions with other organisms, such as pollinators. In this study, we investigated the effects of leaf herbivory, root herbivory, and pollination on plant growth, subsequent leaf herbivory, flower production, pollinator attraction, and reproduction in cucumber (Cucumis sativus). We manipulated leaf and root herbivory with striped cucumber beetle (Acalymma vittatum) adults and larvae, respectively, and manipulated pollination with supplemental pollen. Both enhanced leaf and root herbivory reduced plant growth, and leaf herbivory reduced subsequent leaf damage. Plants with enhanced root herbivory produced 35% fewer female flowers, while leaf herbivory had no effect on flower production. While leaf herbivory reduced the time that honey bees spent probing flowers by 29%, probing times on root-damaged plants were over twice as long as those on control plants. Root herbivory increased pollen limitation for seed production in spite of increased honey bee preference for plants with root damage. Leaf damage and hand-pollination treatments had no effect on fruit production, but plants with enhanced root damage produced 38% fewer fruits that were 25% lighter than those on control plants. Despite the positive effect of belowground damage on honey bee visitation, root herbivory had a stronger negative effect on plant reproduction than leaf herbivory. These results demonstrate that the often-overlooked effects of belowground herbivores may have profound effects on plant performance.     

Bacterial phytopathogen infection disrupts belowground plant indirect defense mediated by tritrophic cascade

Plants can defend themselves against herbivores through activation of defensive pathways and attraction of third‐trophic‐level predators and parasites. Trophic cascades that mediate interactions in the phytobiome are part of a larger dynamic including the pathogens of the plant itself, which are known to greatly influence plant defenses. As such, we investigated the impact of a phloem‐limited bacterial pathogen, Candidatus Liberibacter asiaticus (CLas), in cultivated citrus rootstock on a well‐studied belowground tritrophic interaction involving the attraction of an entomopathogenic nematode (EPN), Steinernema diaprepesi, to their root‐feeding insect hosts, Diaprepes abbreviatus larvae. Using belowground olfactometers, we show how CLas infection interferes with this belowground interaction by similarly inducing the release of a C12 terpene, pregeijerene, and disconnecting the association of the terpene with insect presence. D. abbreviatus larvae that were not feeding but in the presence of a CLas‐infected plant were more likely to be infected by EPN than those near uninfected plants. Furthermore, nonfeeding larvae associated with CLas‐infected plants were just as likely to be infected by EPN as those near noninfected plants with D. abbreviatus larval damage. Larvae of two weevil species, D. abbreviatus and Pachnaeus litus, were also more attracted to plants with infection than to uninfected plants. D. abbreviatus larvae were most active when exposed to pregeijerene at a concentration of 0.1 μg/μl. We attribute this attraction to CLas‐infected plants to the same signal previously thought to be a herbivore‐induced plant volatile specifically induced by root‐feeding insects, pregeijerene, by assessing volatiles collected from the roots of infected plants and uninfected plants with and without feeding D. abbreviatus. Synthesis. Phytopathogens can influence the structuring of soil communities extending to the third trophic level. Field populations of EPN may be less effective at host‐finding using pregeijerene as a cue in citrus grove agroecosystems with high presence of CLas infection.     

Quantifying the impact of above‐ and belowground higher trophic levels on plant and herbivore performance by modeling1

Growing empirical evidence suggests that aboveground and belowground multitrophic communities interact. However, investigations that comprehensively explore the impacts of above‐ and belowground third and higher trophic level organisms on plant and herbivore performance are thus far lacking. We tested the hypotheses that above‐ and belowground higher trophic level organisms as well as decomposers affect plant and herbivore performance and that these effects cross the soil–surface boundary. We used a well‐validated simulation model that is individual‐based for aboveground trophic levels such as shoot herbivores, parasitoids, and hyperparasitoids while considering belowground herbivores and their antagonists at the population level. We simulated greenhouse experiments by removing trophic levels and decomposers from the simulations in a factorial design. Decomposers and above‐ and belowground third trophic levels affected plant and herbivore mortality, root biomass, and to a lesser extent shoot biomass. We also tested the effect of gradual modifications of the interactions between different trophic level organisms with a sensitivity analysis. Shoot and root biomass were highly sensitive to the impact of the fourth trophic level. We found effects that cross the soil surface, such as aboveground herbivores and parasitoids affecting root biomass and belowground herbivores influencing aboveground herbivore mortality. We conclude that higher trophic level organisms and decomposers can strongly influence plant and herbivore performance. We propose that our modelling framework can be used in future applications to quantitatively explore the possible outcomes of complex above‐ and belowground multitrophic interactions under a range of environmental conditions and species compositions.     

Root herbivore identity matters in plant-mediated interactions between root and shoot herbivores

Plants are simultaneously attacked by a multitude of herbivores that affect plant responses and plant-mediated interactions in a variety of ways. So far, studies on indirect interactions between below- and aboveground herbivores have almost exclusively focused on interactions between only one root and one shoot herbivore species at the same time. Since these studies show a variety of outcomes, we test the hypothesis that root herbivore identity matters in below-/aboveground interactions. We studied the combined effects root-feeding nematodes (Pratylenchus penetrans) and wireworms (Agriotes lineatus larvae) on Plantago lanceolata and on the performance of aboveground phloem-feeding aphids (Myzus persicae) and chewing caterpillars (Chrysodeixis chalcites larvae). Since root herbivores may also affect resource availability and the microbial community in the rhizosphere, we examined resource utilization by soil microorganisms using BIOLOG EcoPlates™.

Wireworms decreased root biomass by 13%, but led to compensatory shoot growth. Nematodes and the aboveground herbivores did not affect the biomass of Plantago lanceolata. Feeding by C. chalcites larvae enhanced the concentration of aucubin in leaves, which might explain the high mortality of the caterpillars. Aphids and the belowground herbivores did not change iridoid glycoside levels in the leaves. However, the number of aphid offspring was reduced by 44% when nematodes had been added to the soil, whereas wireworms had no effect. We observed higher utilization of BIOLOG carbon sources by the soil microorganisms only in the presence of Pratylenchus penetrans. Our results suggest that the outcome of below–aboveground interactions highly depends on herbivore identity.     

Above‐ and belowground insect herbivores mediate the impact of nitrogen eutrophication on the soil food web in a grassland ecosystem

Insect herbivores are important drivers of ecosystem processes in grasslands, and can mediate the grassland's response to environmental change. For example, recent evidence shows that above‐ and belowground herbivory, individually and in combination, can modify how a plant community responds to nitrogen (N) eutrophication, an important driver of global change. However, knowledge about how such effects extend to the associated soil food web is lacking. In a mesocosm experiment, we investigated how communities of soil nematodes – an abundant and functionally important group of soil organisms – responded to above‐ and belowground insect herbivory at contrasting N levels. We found that the strongest influence of above‐ and belowground herbivory on the nematode community appeared at elevated N. The abundance of root‐feeding nematodes increased when either above‐ or belowground insect herbivores were present at elevated N, but when applied together the two herbivore types cancelled out one another's effect. Additionally, at elevated N aboveground herbivory increased the abundance of fungal‐feeders relative to bacterial‐feeders, which indicates changes in decomposition pathways induced by N and herbivory. Belowground herbivory increased the abundance of omnivorous nematodes. The shifts in both the herbivorous and detrital parts of the soil food web demonstrate that above‐ and belowground herbivory does not only mediate the response of the plant community to N eutrophication, but in extension also the soil food web sustained by the plant community. We conclude that feedbacks between effects of above‐ and belowground herbivory mediate the response of the grassland ecosystem to N eutrophication.     

Linking aboveground and belowground interactions via induced plant defenses

Plants have a variety of chemical defenses that often increase in concentration following attack by herbivores. Such induced plant responses can occur aboveground, in the leaves, and also belowground in the roots. We show here that belowground organisms can also induce defense responses aboveground and vice versa. Indirect defenses are particularly sensitive to interference by induced feeding activities in the other compartment, and this can disrupt multitrophic interactions. Unravelling the involvement of induced plant responses in the interactions between aboveground and belowground communities associated with plants is likely to benefit from comprehensive metabolomic analyses. Such analyses are likely to contribute to a better understanding of the costs and benefits involved in the selection for induced responses in plants.     

Latitudinal variation in soil nematode communities under climate warming‐related range‐expanding and native plants

Current climate change has led to latitudinal and altitudinal range expansions of numerous species. During such range expansions, plant species are expected to experience changes in interactions with other organisms, especially with belowground biota that have a limited dispersal capacity. Nematodes form a key component of the belowground food web as they include bacterivores, fungivores, omnivores and root herbivores. However, their community composition under climate change‐driven intracontinental range‐expanding plants has been studied almost exclusively under controlled conditions, whereas little is known about actual patterns in the field. Here, we use novel molecular sequencing techniques combined with morphological quantification in order to examine nematode communities in the rhizospheres of four range‐expanding and four congeneric native species along a 2,000 km latitudinal transect from South‐Eastern to North‐Western Europe. We tested the hypotheses that latitudinal shifts in nematode community composition are stronger in range‐expanding plant species than in congeneric natives and that in their new range, range‐expanding plant species accumulate fewest root‐feeding nematodes. Our results show latitudinal variation in nematode community composition of both range expanders and native plant species, while operational taxonomic unit richness remained the same across ranges. Therefore, range‐expanding plant species face different nematode communities at higher latitudes, but this is also the case for widespread native plant species. Only one of the four range‐expanding plant species showed a stronger shift in nematode community composition than its congeneric native and accumulated fewer root‐feeding nematodes in its new range. We conclude that variation in nematode community composition with increasing latitude occurs for both range‐expanding and native plant species and that some range‐expanding plant species may become released from root‐feeding nematodes in the new range.     

Induced carbon reallocation and compensatory growth as root herbivore tolerance mechanisms

Upon attack by leaf herbivores, many plants reallocate photoassimilates below ground. However, little is known about how plants respond when the roots themselves come under attack. We investigated induced resource allocation in maize plants that are infested by the larvae Western corn rootworm Diabrotica virgifera virgifera. Using radioactive ¹¹CO₂, we demonstrate that root‐attacked maize plants allocate more new ¹¹C carbon from source leaves to stems, but not to roots. Reduced meristematic activity and reduced invertase activity in attacked maize root systems are identified as possible drivers of this shoot reallocation response. The increased allocation of photoassimilates to stems is shown to be associated with a marked thickening of these tissues and increased growth of stem‐borne crown roots. A strong quantitative correlation between stem thickness and root regrowth across different watering levels suggests that retaining photoassimilates in the shoots may help root‐attacked plants to compensate for the loss of belowground tissues. Taken together, our results indicate that induced tolerance may be an important strategy of plants to withstand belowground attack. Furthermore, root herbivore‐induced carbon reallocation needs to be taken into account when studying plant‐mediated interactions between herbivores.     

Interactions between above- and belowground insect herbivores as mediated by the plant defense system

Plants are frequently attacked by both above- and belowground arthropod herbivores. Nevertheless, studies rarely consider root and shoot herbivory in conjunction. Here we provide evidence that the root-feeding insect Agriotes lineatus reduces the performance of the foliage feeding insect Spodoptera exigua on cotton plants. In a bioassay, S. exigua larvae were allowed to feed on either undamaged plants, or on plants that had previously been exposed to root herbivory, foliar herbivory, or a combination of both. Previous root herbivory reduced the relative growth rates as well as the food consumption of S. exigua by more than 50% in comparison to larvae feeding on the undamaged controls. We found no effects in the opposite direction, as aboveground herbivory by S. exigua did not affect the relative growth rates of root-feeding A. lineatus . Remarkably, neither did the treatment with foliar herbivory affect the food consumption and relative growth rate of S. exigua in the bioassay. However, this treatment did result in a significant change in the distribution of S. exigua feeding. Plants that had been pre-exposed to foliar herbivory suffered significantly less damage on their young terminal leaves. While plant growth and foliar nitrogen levels were not affected by any of the treatments, we did find significant differences between treatments with respect to the level and distribution of plant defensive chemicals (terpenoids). Exposure to root herbivores resulted in an increase in terpenoid levels in both roots as well as in mature and immature foliage. Foliar damage, on the other hand, resulted in high terpenoid levels in young, terminal leaves only. Our results show that root-feeding herbivores may change the level and distribution of plant defenses aboveground. Our data suggest that the reported interactions between below- and aboveground insect herbivores are mediated by induced changes in plant secondary chemistry.