Kinuyo Yoneya

Can insect egg deposition 'warn' a plant of future feeding damage by herbivorous larvae?

Plant anti-herbivore defence is inducible by both insect feeding and egg deposition. However, little is known about the ability of insect eggs to induce defences directed not against the eggs themselves, but against larvae that subsequently hatch from the eggs. We studied how oviposition (OP) by the sawfly Diprion pini on Pinus sylvestris foliage affects the plants defensive potential against sawfly larvae. Larvae that initiated their development on P. sylvestris twigs on which they hatched from eggs gained less weight and suffered higher mortality than those fed on egg-free twigs. The poor performance of these larvae also affected the next herbivore generation since fecundity of resulting females was lower than that of females which spent their larval development on egg-free pine. Transcript levels of P. sylvestris sesquiterpene synthases (PsTPS1, PsTPS2) were increased by D. pini OP, reached their highest levels just before larval hatching, and decreased when larvae started to feed. However, concentrations of terpenoid and phenolic metabolites presumed to act as feeding deterrents or toxins for herbivores did not change significantly after OP and feeding. Nevertheless, our performance data suggest that insect egg deposition may act to ‘warn’ a plant of upcoming feeding damage by larvae.

Can herbivore-induced plant volatiles inform predatory insect about the most suitable stage of its prey?

The leaf beetle Plagiodera versicolora (Coleoptera: Chrysomelidae) is a specialist herbivore, all of whose mobile stages feed on the leaves of salicaceous plants. Both the larval and adult stages of the ladybird Aiolocaria hexaspilota (Coleoptera: Coccinellidae) are dominant natural enemies of the larvae of the leaf beetle. To clarify the role of plant volatiles in prey‐finding behaviour of A. hexaspilota, the olfactory responses of the ladybird in a Y‐tube olfactometer are studied. The ladybird adults show no preference for willow plants Salix eriocarpa that are infested by leaf beetle adults (nonprey) over that for intact plants but move more to the willow plants infested by leaf beetle larvae (prey) than to intact plants. Moreover, ladybird larvae show no preference for willow plants infested by leaf beetle larvae or adults over intact plants. Using gas chromatography‐mass spectrometry, six volatile compounds are released in larger amounts in the headspace of willow plants infested by leaf beetle larvae than in the headspace of willow plants infested by leaf beetle adults. In addition, the total amount of volatiles emitted from willow plants that are either intact or infested by leaf beetle adults is much smaller than that from willow plants infested by leaf beetle larvae. These results indicate that volatiles from S. eriocarpa infested by P. versicolora inform A. hexaspilota adults about the presence of the most suitable stage of their prey, whereas A. hexaspilota larvae do not use such information.

Egg laying of cabbage white butterfly (Pieris brassicae) on Arabidopsis thaliana affects subsequent performance of the larvae.

Plant resistance to the feeding by herbivorous insects has recently been found to be positively or negatively influenced by prior egg deposition. Here we show how crucial it is to conduct experiments on plant responses to herbivory under conditions that simulate natural insect behaviour. We used a well-studied plant – herbivore system, Arabidopsis thaliana and the cabbage white butterfly Pieris brassicae, testing the effects of naturally laid eggs (rather than egg extracts) and allowing larvae to feed gregariously as they do naturally (rather than placing single larvae on plants). Under natural conditions, newly hatched larvae start feeding on their egg shells before they consume leaf tissue, but access to egg shells had no effect on subsequent larval performance in our experiments. However, young larvae feeding gregariously on leaves previously laden with eggs caused less feeding damage, gained less weight during the first 2 days, and suffered twice as high a mortality until pupation compared to larvae feeding on plants that had never had eggs. The concentration of the major anti-herbivore defences of A. thaliana, the glucosinolates, was not significantly increased by oviposition, but the amount of the most abundant member of this class, 4-methylsulfinylbutyl glucosinolate was 1.8-fold lower in larval-damaged leaves with prior egg deposition compared to damaged leaves that had never had eggs. There were also few significant changes in the transcript levels of glucosinolate metabolic genes, except that egg deposition suppressed the feeding-induced up-regulation of FMOGS-OX2, a gene encoding a flavin monooxygenase involved in the last step of 4-methylsulfinylbutyl glucosinolate biosynthesis. Hence, our study demonstrates that oviposition does increase A. thaliana resistance to feeding by subsequently hatching larvae, but this cannot be attributed simply to changes in glucosinolate content.

Interaction–information networks mediated by plant volatiles: a case study on willow trees

Abstract Volatiles from uninfested plants as well as those from plants infested by herbivores have been reported to potentially contain information that can be used by herbivorous arthropods, their carnivorous natural enemies, and plants. In this context, tritrophic interaction–information networks are expected. Here, as a case study of such a volatile-mediated network, we reviewed our recent studies on a naturally occurring tritrophic system of a willow tree (Salix eriocarpa), a leaf beetle (Plagiodera versicolora), and a predatory ladybird (Aiolocaria hexaspilota) mediated by volatiles from uninfested and infested willow trees. Ecological functions of uninfested and infested willow-shoot volatiles depended on receivers (i.e. leaf beetle adults, leaf beetle larvae, ladybirds, and conspecific tree). By studying such multifunctional aspects of plant volatiles in different natural willow fields, we would acquire a more comprehensive understanding of interaction–information networks.

Do adult leaf beetles (Plagiodera versicolora) discriminate between odors from intact and leaf-beetle-infested willow shoots?

Abstract We investigated how adults of willow leaf beetle Plagiodera versicolora find shoots with new leaves that were suitable food resources for them by focusing on shoot odor. Female and male beetles (starved and satiated) preferred the odor from intact shoots of Salix eriocarpa to clean air. Starved females preferred odor from shoots with leaves infested by conspecifics to odor from intact shoots. However, satiated females as well as starved and satiated males showed no significant discrimination between the two odors. Pooling of the preference data revealed that starved individuals of females and males preferred odor from infested shoots to an extent that was significantly different from satiated individuals, while the distribution of starved and satiated females was not significantly different from that of males. These olfactory responses would provide an explanation for the host plant finding behavior of this specialist herbivore in the field.

An Apparent Trade-Off between Direct and Signal-Based Induced Indirect Defence against Herbivores in Willow Trees

Signal-based induced indirect defence refers to herbivore-induced production of plant volatiles that attract carnivorous natural enemies of herbivores. Relationships between direct and indirect defence strategies were studied using tritrophic systems consisting of six sympatric willow species, willow leaf beetles (Plagiodera versicolora), and their natural predators, ladybeetles (Aiolocaria hexaspilota). Relative preferences of ladybeetles for prey-infested willow plant volatiles, indicating levels of signal-based induced indirect defence, were positively correlated with the vulnerability of willow species to leaf beetles, assigned as relative levels of direct defence. This correlation suggested a possible trade-off among the species, in terms of resource limitation between direct defence and signal-based induced indirect defence. However, analyses of volatiles from infested and uninfested plants showed that the specificity of infested volatile blends (an important factor determining the costs of signal-based induced indirect defence) did not affect the attractiveness of infested plant volatiles. Thus, the suggested trade-off in resource limitation was unlikely. Rather, principal coordinates analysis showed that this ‘apparent trade-off’ between direct and signal-based induced indirect defence was partially explained by differential preferences of ladybeetles to infested plant volatiles of the six willow species. We also showed that relative preferences of ladybeetles for prey-infested willow plant volatiles were positively correlated with oviposition preferences of leaf beetles and with the distributions of leaf beetles in the field. These correlations suggest that ladybeetles use the specificity of infested willow plant volatiles to find suitable prey patches.

Co‐evolution of foraging behaviour in herbivores and their natural enemies predicts multifunctionality of herbivore‐induced plant volatiles

Summary Herbivorous arthropods often induce phenotypic changes in plants of terrestrial systems, which include increased direct resistance to herbivores. In addition, infested plants release quantitatively and qualitatively different volatiles from those of uninfested plants, which are so-called herbivore-induced plant volatiles (HIPVs). Due to HIPVs, plants are more conspicuous to herbivorous arthropods and their natural enemies (carnivorous arthropods) than uninfested plants, thus acting as foraging cues in tri-trophic systems. Intriguingly, herbivore and natural enemy responses to HIPVs are highly diverse. Whether they are attracted or repelled by HIPVs depend on the plant–herbivore–natural enemy species combination, suggesting a HIPV multifunctionality. We hypothesized that co-evolutionary diversification of foraging strategies in herbivores and natural enemies, where arthropods adapted to diverse plant ecological traits, served to explain HIPV multifunctionality. We developed a food chain model of plant–herbivore–natural enemy to test our hypothesis, where plants exhibited three different states and different apparency (uninfested highly infested). The shifts between plant states were mediated by herbivore life cycle, interactions among herbivores, natural enemies and plant reproduction. The natural enemy evolutionary stable strategy (ESS) was predicted to choose the lightly and heavily infested plants in broad ranges of induced direct resistance and plant apparency (HIPV levels), implying that HIPVs acted as the induced indirect resistance. The corresponding herbivore ESS was to choose (or avoid) HIPVs when HIPV levels were low (or high). When the induced direct resistance was very high and thus herbivore quality was low, natural enemies tended to avoid increased HIPVs with increasing herbivory levels. This was associated with herbivore ESS choice for HIPVs, leading to intraspecific enemy-free space. These predicted ESS diversities explained observed herbivore and natural enemy responses to HIPVs in several contrasting plant systems, including willow trees, lima bean and wild tobacco. Our findings suggest that non-lethal herbivory and variations in induced plant responses are key mechanisms for evolutionary diversification of animal foraging behaviour and thus structure of ecological networks.

Herbivore-constructed leaf shelters on Salix eriocarpa shoots affect arthropod communities

We observed the abundance of leaf shelters, aphids, other herbivores, and predators on willow trees, Salix eriocarpa, from May to October 2003. There was a positive correlation between the growth rate of aphids and the number of ants per shoot, suggesting ant attendance to aphids. Although the mean abundance of leaf shelters per shoot was rather low (1.7–2.2) throughout the observation period, aphids preferred to use shoots with leaf shelters compared with those without leaf shelters. The abundance of ants was positively influenced by the presence of leaf shelters and aphids from May to August. The abundance of other herbivores was positively influenced by leaf shelters, but negatively influenced by aphid presence from May to August. Furthermore, leaf shelters had a positive effect on the abundance of predators from July to October. These data suggest that a relatively low abundance of naturally occurring leaf shelters per shoot influenced the arthropod communities on S. eriocarpa, and the effect of those leaf shelters on each type of arthropod varied according to the season.

Statistical recipe for quantifying microbial functional diversity from EcoPlate metabolic profiling

EcoPlate quantifies the ability of a microbial community to utilize 31 distinct carbon substrates, by monitoring color development of microplate wells during incubation. Well color patterns represent metabolic profiles. Previous studies typically used color patterns representing average values of three technical replicates on the final day of the incubation and did not consider substrate chemical diversity. However, color fluctuates during incubation and color varies between replicates, undermining statistical power to distinguish differences among samples in microbial functional composition and diversity. Therefore, we developed a protocol to improve statistical power with two approaches. First, we optimized data treatment for color development during incubation and technical replicates. Second, we incorporated chemical structural information for the 31 carbon substrates into the computation. Our framework implemented as the protocol in the R environment is able to compare the statistical power among different calculation methods. When we applied it to data from aquatic microcosm and forest soil systems, we observed substantial improvement in statistical power when we incorporated temporal patterns during incubation instead of using only endpoint data. Using maximum or minimum values of technical replicates also sometimes gave better results than averages. Incorporating chemical structural information based on fuzzy set theory could improve statistical power but only when relative color density information was considered; it was not seen when the pattern was first binarized into the presence or absence of metabolic activity. Finally, we discuss research directions to improve these approaches and offer some practical considerations for applying our methods to other datasets.

Parasitoid wasps’ exposure to host-infested plant volatiles affects their olfactory cognition of host-infested plants

Using Cotesia vestalis, a parasitoid wasp of diamondback moth larvae and three crucifer plant species (cabbage, komatsuna, and Japanese radish), we examined the effects of exposure to host-infested plant volatiles from one plant species on a newly emerged wasp’s subsequent olfactory cognition of host-infested plant volatiles from the same or different plant species. The preference of C. vestalis between infested and uninfested plant volatiles was tested in a choice chamber. Volatile-inexperienced wasps significantly preferred infested cabbage and infested radish volatiles, but not infested komatsuna volatiles. After exposure to infested cabbage volatiles, wasps showed a significant preference for infested cabbage volatiles, while the significant preference for infested radish volatiles that had been observed in inexperienced wasps was no longer observed. After exposure to infested komatsuna volatiles, wasps significantly preferred infested komatsuna volatiles, and the pre-exposure significant preferences for infested cabbage volatiles and infested radish volatiles remained. After exposure to infested radish volatiles, the significant preferences for infested cabbage and infested radish volatiles did not change. Furthermore, wasps showed a significant preference for infested komatsuna volatiles. The compound compositions of the volatile blends from the three infested plant species were grouped separately in a principal coordinates analysis. The experience-based cognition of C. vestalis for host-infested plant volatiles of three plant species is discussed.