Risky roots and careful herbivores: Sustained herbivory by a root‐feeding herbivore attenuates indirect plant defences

Western corn rootworm resistance in maize persists in the absence of jasmonic acid

MAIN CONCLUSION

Larva growth, survival, and development speed were not affected by the absence of jasmonic acid (JA) indicating that JA does not have a direct role in maize resistance to western corn rootworm. Jasmonic acid (JA) is a plant hormone that regulates multiple physiological processes including defense against herbivory by chewing insects. Previous research showed its importance for resistance to aboveground herbivory. While few studies have investigated the role of JA in resistance to belowground root-feeding herbivores, none has directly tested the role of JA in such resistance. In this study, we used an opr7opr8 double mutant to directly test the role of JA in resistance to western corn rootworm (Diabrotica virgifera virgifera, WCR), a devastating and specialist pest of maize. The opr7opr8 double mutant is deficient in JA accumulation as we found that it does not accumulate JA nor JA-Ile independently of exposure to WCR. We found no significant difference in growth (body mass), survival, and development of WCR larvae in response to JA deficiency, suggesting that disruption of JA biosynthesis does not impact resistance in maize roots to WCR. Additionally, we observed no significant effect on loss of root tissue caused by WCR associated with JA deficiency, while we found a reduction in shoot growth (mass) associated with WCR herbivory in the opr7opr8 mutant that was not observed in the wildtype. This suggested a role for JA in aboveground growth response to WCR herbivory rather than resistance to WCR.

Simulation of early season herbivory via mechanical damage affects flower production in pumpkin (Cucurbita pepo ssp. pepo)

BACKGROUND

Damage from insect herbivores can elicit a wide range of plant responses, including reduced or compensatory growth, altered volatile profiles, or increased production of defence compounds. Specifically, herbivory can alter floral development as plants reallocate resources towards defence and regrowth functions. For pollinator-dependent species, floral quantity and quality are critical for attracting floral visitors; thus, herbivore-induced developmental effects that alter either floral abundance or attractiveness may have critical implications for plant reproductive success. Based on past work on resource trade-offs, we hypothesize that herbivore damage-induced effects are stronger in structural floral traits that require significant resource investment (e.g. flower quantity), as plants reallocate resources towards defence and regrowth, and weaker in secondary floral traits that require less structural investment (e.g. nectar rewards).

METHODS

In this study, we simulated early-season herbivore mechanical damage in the domesticated jack-o-lantern pumpkin Cucurbita pepo ssp. pepo and measured a diverse suite of floral traits over a 60-d greenhouse experiment.

KEY RESULTS

We found that mechanical damage delayed the onset of male anthesis and reduced the total quantity of flowers produced. Additionally, permutational multivariate analysis of variance (PERMANOVA) indicated that mechanical damage significantly impacts overall floral volatile profile, though not output of sesquiterpenoids, a class of compounds known to recruit specialized cucumber beetle herbivores and squash bee pollinators.

CONCLUSIONS

We show that C. pepo spp. pepo reduces investment in male flower production following mechanical damage, and that floral volatiles do exhibit shifts in production, indicative of damage-induced trait plasticity. Such reductions in male flower production could reduce the relative attractiveness of damaged plants to foraging pollinators in this globally relevant cultivated species.

Arbuscular mycorrhizal fungi influence belowground interactions between a specialist root-feeder and its natural enemy

As primary producers, plants play a central role in mediating interactions across trophic levels. Although plants are the primary food source for herbivorous insects, they can protect themselves from herbivore damage. Many plants produce toxic compounds that directly reduce herbivore feeding, but plants also protect themselves indirectly by attracting natural enemies of the attacking herbivore through volatile signaling. These so-called tri-trophic interactions have historically been documented aboveground in aerial plant parts but are also known to occur belowground in root systems. In addition to herbivores, plants directly interact with other organisms, which can influence the outcomes of tri-trophic interactions. Arbuscular mycorrhizal fungi (AMF) are symbiotic soil microbes that colonize the roots of plants and facilitate nutrient uptake. These microbes can alter plant chemistry and subsequent resistance to herbivores. Few studies, however, have shown how AMF affect tri-trophic interactions above- or belowground. This study examines how AMF colonization affects the emission of root volatiles when plants are under attack by western corn rootworm, a problematic pest of corn, and subsequent attraction of entomopathogenic nematodes, a natural enemy of western corn rootworm. Mycorrhizal fungi increased rootworm survival but decreased larval weight. Differences were detected across root volatile profiles, but there was not a clear link between volatile signaling and nematode behavior. Nematodes were more attracted to non-mycorrhizal plants without rootworms and AMF alone in soil, suggesting that AMF may interfere with cues that are used in combination with volatiles which nematodes use to locate prey.

Navigational Signals for Insect and Slug Parasitic Nematodes: The Role of Ascorbate-Glutathione System and Volatiles Released by Insect-Damaged Sweet Pepper Roots

This study of underground multitrophic communication, involving plant roots, insects, and parasitic nematodes, is an emerging field with significant implications for understanding plant-insect-nematode interactions. Our research investigated the impact of wireworm (Agriotes lineatus L. [Coleoptera: Elateridae]) infestations on the ascorbate-glutathione system in sweet pepper (Capsicum annuum L.) plants in order to study the potential role in root-exudate-mediated nematode chemotaxis. We observed that an A. lineatus infestation led to a decrease in leaf ascorbate levels and an increase in root ascorbate, with corresponding increases in the glutathione content in both roots and leaves. Additionally, a pigment analysis revealed increased carotenoid and chlorophyll levels and a shift towards a de-epoxidized state in the xanthophyll cycle. These changes suggest an individual and integrated regulatory function of photosynthetic pigments accompanied with redox modifications of the ascorbate-glutathione system that enhance plant defense. We also noted changes in the root volatile organic compound (VOC). Limonene, methyl salicylate, and benzyl salicylate decreased, whereas hexanal, neoisopulegol, nonanal, phenylethyl alcohol, m-di-tert-butylbenzene, and trans-β-ionone increased in the roots of attacked plants compared to the control group. Most notably, the VOC hexanal and amino acid exudate cysteine were tested for the chemotaxis assay. Nematode responses to chemoattractants were found to be species-specific, influenced by environmental conditions such as temperature. This study highlights the complexity of nematode chemotaxis and suggests that VOC-based biological control strategies must consider nematode foraging strategies and environmental factors. Future research should further explore these dynamics to optimize nematode management in agricultural systems.

Aphid-Induced Volatiles and Subsequent Attraction of Natural Enemies Varies among Sorghum Cultivars

The production of herbivore-induced plant volatiles (HIPVs) is a type of indirect defense used by plants to attract natural enemies and reduce herbivory by insect pests. In many crops little is known about genotypic variation in HIPV production or how this may affect natural enemy attraction. In this study, we identified and quantified HIPVs produced by 10 sorghum (Sorghum bicolor) cultivars infested with a prominent aphid pest, the sorghum aphid (Melanaphis sorghi Theobald). Volatiles were collected using dynamic headspace sampling techniques and identified and quantified using GC-MS. The total amounts of volatiles induced by the aphids did not differ among the 10 cultivars, but overall blends of volatiles differed significantly in composition. Most notably, aphid herbivory induced higher levels of methyl salicylate (MeSA) emission in two cultivars, whereas in four cultivars, the volatile emissions did not change in response to aphid infestation. Dual-choice olfactometer assays were used to determine preference of the aphid parasitoid, Aphelinus nigritus, and predator, Chrysoperla rufilabris, between plants of the same cultivar that were un-infested or infested with aphids. Two aphid-infested cultivars were preferred by natural enemies, while four other cultivars were more attractive to natural enemies when they were free of aphids. The remaining four cultivars elicited no response from parasitoids. Our work suggests that genetic variation in HIPV emissions greatly affects parasitoid and predator attraction to aphid-infested sorghum and that screening crop cultivars for specific predator and parasitoid attractants has the potential to improve the efficacy of biological control.

Belowground insect herbivory induces systemic volatile emissions that strengthen neighbouring plant resistance aboveground

Plants transmit ecologically relevant messages to neighbouring plants through chemical cues. For instance, insect herbivory triggers the production of herbivore-induced plant volatiles (HIPVs), which can enhance neighbouring plant defences. HIPVs are emitted from directly damaged plant tissues and from systemic, nondamaged tissues. Although volatile-mediated interplant interactions have been observed both above- and belowground, it remains unknown whether belowground herbivory induces systemic HIPVs aboveground that influence neighbouring plants. To explore how belowground herbivory affects interplant interactions aboveground, we characterised systemic HIPVs from squash induced by belowground striped cucumber beetle (Acalymma vittatum) larval herbivory. We exposed squash 'receiver plants' to systemic HIPVs or volatiles from nondamaged plants. We then measured herbivore resistance by challenging 'receiver plants' with aboveground-feeding herbivores: adult beetles (A. vittatum) or squash bugs (Anasa tristis). We discovered belowground-damaged plants emitted more (E)-β-ocimene, a key volatile from the systemic HIPV blend, than nondamaged controls, and that exposure to systemic HIPVs enhanced neighbouring plant resistance to aboveground squash bugs, but not adult beetles. Further investigations into the mechanism of interplant interaction revealed β-ocimene alone can elicit plant resistance against squash bugs. Overall, our findings reveal a novel form of volatile-mediated interactions between plants spanning across aboveground-belowground plant systems.

The Role of Ascorbate-Glutathione System and Volatiles Emitted by Insect-Damaged Lettuce Roots as Navigation Signals for Insect and Slug Parasitic Nematodes

The effect of wireworm-damaged lettuce roots on the antioxidative defense system (ascorbate-glutathione cycle, photosynthetic pigments) and movement of insect/slug parasitic nematodes towards determined root exudates was studied in a glasshouse experiment. Lettuce seedlings were grown in a substrate soil in the absence/presence of wireworms (Elateridae). The ascorbate-glutathione system and photosynthetic pigments were analyzed by HPLC, while volatile organic compounds (VOC) emitted by lettuce roots were investigated by GC-MS. Herbivore-induced root compounds, namely 2,4-nonadienal, glutathione, and ascorbic acid, were selected for a chemotaxis assay with nematodes Steinernema feltiae, S. carpocapsae, Heterorhabditis bacteriophora, Phasmarhabditis papillosa, and Oscheius myriophilus. Root pests had a negative effect on the content of photosynthetic pigments in the leaves of infested plants, indicating that they reacted to the presence of reactive oxygen species (ROS). Using lettuce as a model plant, we recognized the ascorbate-glutathione system as a redox hub in defense response against wireworms and analyzed its role in root-exudate-mediated chemotaxis of nematodes. Infected plants also demonstrated increased levels of volatile 2,4-nonadienal. Entomopathogenic nematodes (EPNs, S. feltiae, S. carpocapsae, and H. bacteriophora) proved to be more mobile than parasitic nematodes O. myriophilus and P. papillosa towards chemotaxis compounds. Among them, 2,4-nonadienal repelled all tested nematodes. Most exudates that are involved in belowground tritrophic interactions remain unknown, but an increasing effort is being made in this field of research. Understanding more of these complex interactions would not only allow a better understanding of the rhizosphere but could also offer ecologically sound alternatives in the pest management of agricultural systems.

(-)-Loliolide is a general signal of plant stress that activates jasmonate-related responses

The production of defensive metabolites in plants can be induced by signaling chemicals released by neighboring plants. Induction is mainly known from volatile aboveground signals, with belowground signals and their underlying mechanisms largely unknown. We demonstrate that (-)-loliolide triggers defensive metabolite responses to competitors, herbivores, and pathogens in seven plant species. We further explore the transcriptional responses of defensive pathways to verify the signaling role of (-)-loliolide in wheat and rice models with well-known defensive metabolites and gene systems. In response to biotic and abiotic stressors, (-)-loliolide is produced and secreted by roots. This, in turn, induces the production of defensive compounds including phenolic acids, flavonoids, terpenoids, alkaloids, benzoxazinoids, and cyanogenic glycosides, regardless of plant species. (-)-Loliolide also triggers the expression of defense-related genes, accompanied by an increase in the concentration of jasmonic acid and hydrogen peroxide (H₂ O₂ ). Transcriptome profiling and inhibitor incubation indicate that (-)-loliolide-induced defense responses are regulated through pathways mediated by jasmonic acid, H₂ O₂ , and Ca ²⁺ . These findings argue that (-)-loliolide functions as a common belowground signal mediating chemical defense in plants. Such perception-dependent plant chemical defenses will yield critical insights into belowground signaling interactions.

Root volatile profiles and herbivore preference are mediated by maize domestication, geographic spread, and modern breeding

Domestication affected the abundances and diversity of maize root volatiles more than northward spread and modern breeding, and herbivore preference for roots was correlated with volatile diversity and herbivore resistance. Studies show that herbivore defenses in crops are mediated by domestication, spread, and breeding, among other human-driven processes. They also show that those processes affected chemical communication between crop plants and herbivores. We hypothesized that (i) preference of the herbivore (Diabrotica virgifera virgifera) larvae for embryonic roots of maize (Zea mays mays) would increase and (ii) root volatile diversity would decrease with the crop's domestication, northward spread to present-day USA, and modern breeding. We used Balsas teosinte (Zea mays parviglumis), Mexican and USA landrace maizes, and US inbred maize lines to test these hypotheses. We found that herbivore preference and volatile diversity increased with maize domestication and northward spread but decreased with modern breeding. Additionally, we found that the abundances of single volatiles did not consistently increase or decrease with maize domestication, spread, and breeding; rather, volatiles grouped per their abundances were differentially affected by those processes, and domestication had the greatest effects. Altogether, our results suggested that: the herbivore's preference for maize roots is correlated with volatile diversity and herbivore resistance; changes in abundances of individual volatiles are evident at the level of volatile groups; and maize domestication, but not spread and breeding, affected the abundances of some green leaf volatiles and sesquiterpenes/sesquiterpenoids. In part, we discussed our results in the context of herbivore defense evolution when resources for plant growth and defense vary across environments. We suggested that variability in relative abundance of volatiles may be associated with their local, functional relevance across wild and agricultural environments.

Undercover operation: Belowground insect herbivory modifies systemic plant defense and repels aboveground foraging insect herbivores

Plants attacked by insects may induce defenses locally in attacked plant tissues and/or systemically in non-attacked tissues, such as aboveground herbivory affecting belowground roots or belowground herbivory modifying aboveground tissues (i.e., cross-compartment systemic defense). Through induced systemic plant defenses, above-and belowground insect herbivores indirectly interact when feeding on a shared host plant. However, determining the systemic effects of herbivory on cross-compartment plant tissues and cascading consequences for herbivore communities remains underexplored. The goal of this study was to determine how belowground striped cucumber beetle (Acalymma vittatum) larval herbivory alters aboveground zucchini squash (Cucurbita pepo subsp. pepo) defenses and interactions with herbivores, including adult cucumber beetles and squash bugs (Anasa tristis). To explore this question, field and laboratory experiments were conducted to compare responses of aboveground herbivores to belowground larvae-damaged plants and non-damaged control plants. We also characterized changes in defensive chemicals and nutritional content of aboveground plant structures following belowground herbivory. We discovered belowground herbivory enhanced aboveground plant resistance and deterred aboveground foraging herbivores. We also found that larvae-damaged plants emitted higher amounts of a key volatile compound, (E)-β-ocimene, compared to non-damaged controls. Further investigation suggests that other mechanisms, such as plant nutrient content, may additionally contribute to aboveground herbivore foraging decisions. Collectively, our findings underscore connections between above-and belowground herbivore communities as mediated through induced systemic defenses of a shared host plant. Specifically, these findings indicate that belowground larval herbivory systemically enhances plant defenses and deters a suite of aboveground herbivores, suggesting larvae may manipulate aboveground plant defenses for their own benefit, while plants may benefit from enhanced systemic defenses against multi-herbivore attack.

Deadly scents: Exposure to plant volatiles increases mortality of entomopathogenic nematodes during infection

Plants attacked by insects commonly mobilize various defense mechanisms, including the biosynthesis and release of so-called herbivore-induced plant volatiles (HIPVs). Entomopathogenic nematodes (EPNs) can be attracted to these belowground HIPVs, which can enhance biocontrol services from EPNs. However, recent research has also demonstrated that HIPVs can induce and initiate insect immune responses, decreasing the insect’s susceptibility to pathogens and parasites. Therefore, experiments were conducted to test the impact of HIPVs on insects and EPNs during the initial stage of EPN infection. Compounds that can impact EPN attraction and infectivity such as pregeijerene, β-caryophyllene, and α-pinene, and compounds that have been determined to increase or decrease susceptibility of insects to pathogens, such as (Z)-3-hexenyl acetate, linalool, and β-ocimene, were selected. Exposure of Galleria mellonella larvae to pregeijerene, linalool, β-ocimene and α-pinene during invasion significantly increased mortality of Steinernema diaprepesi and Heterorhabditis bacteriophora after 48 h. Larval treatment with β-caryophyllene only increased mortality for Heterorhabditis bacteriophora. (Z)-3-hexenyl acetate did not cause differential mortality from the controls for either nematode species. In additional experiments, we found that EPNs exposed to α-pinene and linalool were more readily recognized by the insects’ immune cells compared to the control treatment, thus the observed increased mortality was likely due to HIPVs-EPN interactions with the insect’s immune system. These results show that the presence of HIPVs can impact EPN survival in the model host, G. mellonella.

Improving Natural Enemy Selection in Biological Control through Greater Attention to Chemical Ecology and Host-Associated Differentiation of Target Arthropod Pests

Host-associated differentiation (HAD) refers to cases in which genetically distinct populations of a species (e.g., herbivores or natural enemies) preferentially reproduce or feed on different host species. In agroecosystems, HAD often results in unique strains or biotypes of pest species, each attacking different species of crops. However, HAD is not restricted to pest populations, and may cascade to the third trophic level, affecting host selection by natural enemies, and ultimately leading to HAD within natural enemy species. Natural enemy HAD may affect the outcomes of biological control efforts, whether classical, conservation, or augmentative. Here, we explore the potential effects of pest and natural enemy HAD on biological control in agroecosystems, with emphases on current knowledge gaps and implications of HAD for selection of biological control agents. Additionally, given the importance of semiochemicals in mediating interactions between trophic levels, we emphasize the role of chemical ecology in interactions between pests and natural enemies, and suggest areas of consideration for biological control. Overall, we aim to jump-start a conversation concerning the relevance of HAD in biological control by reviewing currently available information on natural enemy HAD, identifying challenges to incorporating HAD considerations into biological control efforts, and proposing future research directions on natural enemy selection and HAD.

A Beneficial Plant-Associated Fungus Shifts the Balance toward Plant Growth over Resistance, Increasing Cucumber Tolerance to Root Herbivory

Plants allocate their limited resources toward different physiological processes, dynamically adjusting their resource allocation in response to environmental changes. How beneficial plant-associated microbes influence this allocation is a topic that continues to interest plant biologists. In this study, we examined the effect of a beneficial fungus, Phialemonium inflatum, on investment in growth and anti-herbivore resistance traits in cucumber plants (Cucumis sativus). We inoculated cucumber seeds with P. inflatum spores and measured several growth parameters, including germination rate, above and belowground biomass, and number of flowers. We also examined plant resistance to adult and larval striped cucumber beetles (Acalymma vitattum), and quantified levels of defense hormones in leaves and roots. Our results indicate that P. inflatum strongly enhances cucumber plant growth and reproductive potential. Although fungus treatment did not improve plant resistance to cucumber beetles, inoculated plants were more tolerant to root herbivory, experiencing less biomass reduction. Together, these findings document how a beneficial plant-associated fungus shifts plant investment in growth over herbivore resistance, highlighting the importance of microbes in mediating plant-herbivore interactions. These findings also have important implications for agricultural systems, where beneficial microbes are often introduced or managed to promote plant growth or enhance resistance.

Belowground plant-microbe communications via volatile compounds

Volatile compounds play important roles in rhizosphere biological communications and interactions. The emission of plant and microbial volatiles is a dynamic phenomenon that is affected by several endogenous and exogenous signals. Diffusion of volatiles can be limited by their adsorption, degradation, and dissolution under specific environmental conditions. Therefore, rhizosphere volatiles need to be investigated on a micro and spatiotemporal scale. Plant and microbial volatiles can expand and specialize the rhizobacterial niche not only by improving the root system architecture such that it serves as a nutrient-rich shelter, but also by inhibiting or promoting the growth, chemotaxis, survival, and robustness of neighboring organisms. Root volatiles play an important role in engineering the belowground microbiome by shaping the microbial community structure and recruiting beneficial microbes. Microbial volatiles are appropriate candidates for improving plant growth and health during environmental challenges and climate change. However, some technical and experimental challenges limit the non-destructive monitoring of volatile emissions in the rhizosphere in real-time. In this review, we attempt to clarify the volatile-mediated intra- and inter-kingdom communications in the rhizosphere, and propose improvements in experimental design for future research.

Defense Suppression through Interplant Communication Depends on the Attacking Herbivore Species

In response to herbivory, plants emit volatile compounds that play important roles in plant defense. Herbivore-induced plant volatiles (HIPVs) can deter herbivores, recruit natural enemies, and warn other plants of possible herbivore attack. Following HIPV detection, neighboring plants often respond by enhancing their anti-herbivore defenses, but a recent study found that herbivores can manipulate HIPV-interplant communication for their own benefit and suppress defenses in neighboring plants. Herbivores induce species-specific blends of HIPVs and how these different blends affect the specificity of plant defense responses remains unclear. Here we assessed how HIPVs from zucchini plants (Cucurbita pepo) challenged with different herbivore species affect resistance in neighboring plants. Volatile "emitter" plants were damaged by one of three herbivore species: saltmarsh caterpillars (Estigmene acrea), squash bugs (Anasa tristis), or striped cucumber beetles (Acalymma vittatum), or were left as undamaged controls. Neighboring "receiver" plants were exposed to HIPVs or control volatiles and then challenged by the associated herbivore species. As measures of plant resistance, we quantified herbivore feeding damage and defense-related phytohormones in receivers. We found that the three herbivore species induced different HIPV blends from squash plants. HIPVs induced by saltmarsh caterpillars suppressed defenses in receivers, leading to greater herbivory and lower defense induction compared to controls. In contrast, HIPVs induced by cucumber beetles and squash bugs did not affect plant resistance to subsequent herbivory in receivers. Our study shows that herbivore species identity affects volatile-mediated interplant communication in zucchini, revealing a new example of herbivore defense suppression through volatile cues.

Chemical Cues from Entomopathogenic Nematodes Vary Across Three Species with Different Foraging Strategies, Triggering Different Behavioral Responses in Prey and Competitors

Chemical cues play important roles in predator-prey interactions. Semiochemicals can aid predator foraging and alert prey organisms to the presence of predators. Previous work suggests that predator traits differentially influence prey behavior, however, empirical data on how prey organisms respond to chemical cues from predator species with different hunting strategies, and how foraging predators react to cues from potential competitors, is lacking. Furthermore, most research in this area has focused on aquatic and aboveground terrestrial systems, while interactions among belowground, soiling-dwelling organisms have received relatively little attention. Here, we assessed how chemical cues from three species of entomopathogenic nematodes (EPNs), each with a different foraging strategy, influenced herbivore (cucumber beetle) and natural enemy (EPN) foraging behavior. We predicted these cues could serve as chemical indicators of increased predation risk, prey availability, or competition. Our findings revealed that foraging cucumber beetle larvae avoided chemical cues from Heterorhabditis bacteriophora (active-foraging cruiser EPNs), but not Steinernema carpocapsae (ambusher EPNs) or Steinernema riobrave (intermediate-foraging EPNs). In contrast, foraging H. bacteriophora EPNs were attracted to cues produced by the two Steinernema species but not conspecific cues. Notably, the three EPN species produced distinct blends of olfactory cues, with only a few semi-conserved compounds across species. These results indicate that a belowground insect herbivore responds differently to chemical cues from different EPN species, with some EPN species avoiding prey detection. Moreover, the active-hunting EPNs were attracted to heterospecific cues, suggesting these cues indicate a greater probability of available prey, rather than strong interspecific competition.

Herbivore-induced plant volatiles mediate defense regulation in maize leaves but not in maize roots

Plant leaves that are exposed to herbivore-induced plant volatiles (HIPVs) respond by increasing their defenses, a phenomenon referred to as priming. Whether this phenomenon also occurs in the roots is unknown. Using maize plants, Zea mays, whose leaves respond strongly to leaf HIPVs, we measured the impact of belowground HIPVs, emanating from roots infested by the banded cucumber beetle, Diabrotica balteata, on constitutive and herbivore-induced levels of defense-related gene expression, phytohormones, volatile and non-volatile primary and secondary metabolites, growth and herbivore resistance in roots of neighbouring plants. HIPV exposure did not increase constitutive or induced levels of any of the measured root traits. Furthermore, HIPV exposure did not reduce the performance or survival of D. balteata on maize or its ancestor teosinte. Cross-exposure experiments between HIPVs from roots and leaves revealed that maize roots, in contrast to maize leaves, neither emit nor respond strongly to defense-regulating HIPVs. Together, these results demonstrate that volatile-mediated defense regulation is restricted to the leaves of maize. This finding is in line with the lower diffusibility of volatiles in the soil and the availability of other, potentially more efficient, information conduits below ground.

In Planta Localization of Endophytic Cordyceps fumosorosea in Carrizo Citrus

Entomopathogenic fungi can be a useful resource for controlling insect vectors of citrus plant pathogens, such as the Asian citrus psyllid (Diaphorina citri) associated with huanglongbing or the citrus root weevil (Diaprepes abbreviatus) associated with the spread of Phytophtora spp. In this study, Cordyceps fumosorosea (Cfr) was investigated in planta as a potential endophytic entomopathogenic fungus and various inoculation techniques were used to determine if it would colonize the Carrizo citrange (Citrus × insitorum) seeds and plants. The four inoculation methodologies evaluated were seed soaking, stem injection, foliar spray, and soil drench. Seed immersion trials demonstrated that the roots of the Carrizo citrange plant can be inoculated successfully with Cfr. Stem injection, foliar spray, and soil drench also provided successful inoculation of Cfr. However, this fungus was only endophytic in the plant stem. Sand cores indicated that Cfr moved down through the sand column and was able to inoculate the roots. Given the prevalence of Cfr in the soil during the drench experiment, and that the fungus was able to colonize Carrizo citrange roots through seed immersion, this finding provides evidence of the potential endophytism of this fungus when applied to citrus plant species.

The Scent of Life: Phoretic Nematodes Use Wasp Volatiles and Carbon Dioxide to Choose Functional Vehicles for Dispersal

Hitchhikers (phoretic organisms) need vehicles to disperse out of unsuitable habitats. Therefore, finding vehicles with the right functional attributes is essential for phoretic organisms. To locate these vehicles, phoretic organisms employ cues within modalities, ranging from visual to chemical senses. However, how hitchhikers discriminate between individual vehicles has rarely been investigated. Using a phoretic nematode community associated with an obligate fig-fig wasp pollination mutualism, we had earlier established that hitchhiking nematodes make decisions based on vehicle species identity and number of conspecific hitchhikers already present on the vehicle. Here we investigate if hitchhikers can differentiate between physiological states of vehicles. We asked whether phoretic nematodes choose between live or dead vehicles present in a chemically crowded environment and we investigated the basis for any discrimination. We conducted two-choice and single-choice behavioral assays using single nematodes and found that plant- and animal-parasitic nematodes preferred live over dead vehicles and used volatiles as a sensory cue to make this decision. However, in single-choice assays, animal-parasitic nematodes were also attracted towards naturally dead or freeze-killed wasps. The volatile profile of the wasps was dominated by terpenes and spiroketals. We examined the volatile blend emitted by the different wasp physiological states and determined a set of volatiles that the phoretic nematodes might use to discriminate between these states which is likely coupled with respired CO₂. We determined that CO₂ levels emitted by single wasps are sufficient to attract nematodes, demonstrating the high sensitivity of nematodes to this metabolic product.

Parasitoids, Nematodes, and Protists in Populations of Striped Cucumber Beetle (Coleoptera: Chrysomelidae)

The striped cucumber beetle, Acalymma vittatum (Fabricius), is an important pest of cucurbit production in the eastern United States, where most commercial producers rely on insecticides to control this pest species. Biological control provides an alternative to insecticide use, but for A. vittatum, top-down control has not been well developed. In the northeastern United States, two parasitoid species, Celatoria setosa (Coquillett) (Diptera: Tachinidae) and Centistes diabroticae (Gahan) (Hymenoptera: Braconidae) have been reported from A. vittatum, but their distribution is poorly known. To determine whether these parasitoid species are attacking A. vittatum in Pennsylvania and the amount of mortality they provide, we characterized the parasitoid dynamics in two distinct efforts. First, we reared parasitoids from beetles captured at two research farms. Second, we focused on one of these farms and dissected beetles to quantify both parasitoid and parasite species attacking A. vittatum. Both efforts confirmed Cl. setosa and Cn. diabroticae, and parasitism rates varied widely between locations and among years (4-60%). Unexpectedly, our dissections revealed that a potentially undescribed nematode species (Howardula sp.) as the most common parasite in the community. We also discovered gregarine protists. Despite being smaller than females, males were more commonly attacked by parasitic species, but we detected no relationship between the size of beetles and abundance of parasitic species in A. vittatum. This work provides a baseline understanding of the parasitoid and parasite community attacking A. vittatum and advances opportunities for conservation biological control using these natural-enemy species.