Chemical diversity rather than cultivar diversity predicts natural enemy control of herbivore pests

Effects of phytochemical diversity on multitrophic interactions

The ecological effects of plant diversity have been well studied, but the extent to which they are driven by variation in specialized metabolites is not well understood. Here we provide theoretical background on phytochemical diversity effects on herbivory and its expanded consequences for higher trophic levels. We then review empirical evidence for effects on predation and parasitism by focusing on a handful of studies that have undertaken manipulative approaches, and link back their results to theory on mechanisms. We close by summarizing key aspects for future research, building on knowledge gained thus far.

Intraspecific chemical variation of Tanacetum vulgare affects plant growth and reproductive traits in field plant communities

The study investigated the impact of intraspecific plant chemodiversity on plant growth and reproductive traits at both the plant and plot levels. It also aimed to understand how chemodiversity at stand level affects ecosystem functioning and plant-plant interactions. We describe a biodiversity experiment in which we manipulated intraspecific plant chemodiversity at the plot level using six different chemotypes of common tansy (Tanacetum vulgare L., Asteraceae). We tested the effects of chemotype identity and plot-level chemotype richness on plant growth and reproductive traits and plot-level headspace emissions. The study found that plant chemotypes differed in growth and reproductive traits and that traits were affected by the chemotype richness of the plots. Although morphological differences among chemotypes became less pronounced over time, reproductive phenology patterns persisted. Plot-level trait means were also affected by the presence or absence of certain chemotypes in a plot, and the direction of the effect depended on the specific chemotype. However, chemotype richness did not lead to overyielding effects. Lastly, chemotype blends released from plant communities were neither richer nor more diverse with increasing plot-level chemotype richness, but became more dissimilar as they became more dissimilar in their leaf terpenoid profiles. We found that intraspecific plant chemodiversity is crucial in plant-plant interactions. We also found that the effects of chemodiversity on plant growth and reproductive traits were complex and varied depending on the chemotype richness of the plots. This long-term field experiment will allow further investigation into plant-insect interactions and insect community assembly in response to intraspecific chemodiversity.

Floral volatiles evoke partially similar responses in both florivores and pollinators and are correlated with non-volatile reward chemicals

BACKGROUND

Plants often use floral displays to attract mutualists and prevent antagonist attacks. Chemical displays detectable from a distance include attractive or repellent floral volatile organic compounds (FVOCs). Locally, visitors perceive contact chemicals including nutrients but also deterrent or toxic constituents of pollen and nectar. The FVOC and pollen chemical composition can vary intra- and interspecifically. For certain pollinator and florivore species, responses to these compounds are studied in specific plant systems, yet we lack a synthesis of general patterns comparing these two groups and insights into potential correlations between FVOC and pollen chemodiversity.

SCOPE

We reviewed how FVOCs and non-volatile floral chemical displays, i.e. pollen nutrients and toxins, vary in composition and affect the detection by and behaviour of insect visitors. Moreover, we used meta-analyses to evaluate the detection of and responses to FVOCs by pollinators vs. florivores within the same plant genera. We also tested whether the chemodiversity of FVOCs, pollen nutrients and toxins is correlated, hence mutually informative.

KEY RESULTS

According to available data, florivores could detect more FVOCs than pollinators. Frequently tested FVOCs were often reported as pollinator-attractive and florivore-repellent. Among FVOCs tested on both visitor groups, there was a higher number of attractive than repellent compounds. FVOC and pollen toxin richness were negatively correlated, indicating trade-offs, whereas a marginal positive correlation between the amount of pollen protein and toxin richness was observed.

CONCLUSIONS

Plants face critical trade-offs, because floral chemicals mediate similar information to both mutualists and antagonists, particularly through attractive FVOCs, with fewer repellent FVOCs. Furthermore, florivores might detect more FVOCs, whose richness is correlated with the chemical richness of rewards. Chemodiversity of FVOCs is potentially informative of reward traits. To gain a better understanding of the ecological processes shaping floral chemical displays, more research is needed on floral antagonists of diverse plant species and on the role of floral chemodiversity in visitor responses.

Overlaps and trade-offs in the diversity and inducibility of volatile chemical profiles among diverse sympatric neotropical canopy trees

A central goal in ecology is to understand the mechanisms by which biological diversity is maintained. The diversity of plant chemical defences and the strategies by which they are deployed in nature may influence biological diversity. Trees in neotropical forests are subject to relatively high herbivore pressure. Such consistent pressure is thought to select for constitutive, non-flexible defence-related phytochemistry with limited capacity for inducible phytochemical responses. However, this has not been explored for volatile organic compounds (VOCs) that have a relatively low ratio of production costs to ecological benefits. To test this, I sampled VOCs emitted from canopy leaves of 10 phylogenetically diverse tree species (3 Magnoliids and 7 Rosids) in the Peruvian Amazon before and after induction with the phytohormone methyl jasmonate (MeJA). There was no phylogenetic signal in induction or magnitude of MeJA-induced VOC emissions from intact leaves: all trees induced VOC profiles dominated by β-ocimene, linalool, and α-farnesene of varying ratios. Moreover, overall inducibility of VOCs from intact leaves was unrelated to phytochemical diversity or richness. In contrast, experimentally wounded leaves showed considerable phylogeny-based and MeJA-independent variation the richness and diversity of constitutive wound-emitted VOCs. Moreover, VOC inducibility from wounded leaves correlated negatively with phytochemical richness and diversity, potentially indicating a tradeoff in constitutive and inducible defence strategies for non-volatile specialised metabolites but not for inducible VOCs. Importantly, there was no correlation between any chemical profile and either natural herbivory or leaf toughness. The coexistence of multiple phytochemical strategies in a hyper-diverse forest has broad implications for competitive and multitrophic interactions, and the evolutionary forces that maintain the exceptional plant biodiversity in neotropical forests.

Population Dynamics of Insect Pests and Beneficials on Different Snap Bean Cultivars

Snap bean is an important crop in the United States. Insecticides are commonly used against pests on snap bean, but many pests have developed resistance to the insecticides and beneficials are threatened by the insecticides. Therefore, host plant resistance is a sustainable alternative. Population dynamics of insect pests and beneficials were assessed on 24 snap bean cultivars every week for six weeks. The lowest number of sweetpotato whitefly (Bemisia tabaci) eggs was observed on cultivar 'Jade', and the fewest nymphs were found on cultivars 'Gold Mine', 'Golden Rod', 'Long Tendergreen', and 'Royal Burgundy'. The numbers of potato leafhopper (Empoasca fabae) and tarnished plant bug (Lygus lineolaris) adults were the lowest on cultivars 'Greencrop' and 'PV-857'. The highest numbers of adults were found in Week 1 (25 days following plant emergence) for B. tabaci and Mexican bean beetle (Epilachna varivestis); Week 3 for cucumber beetle, kudzu bug (Megacopta cribraria), and E. fabae; Weeks 3 and 4 for thrips; Week 4 for L. lineolaris; and Weeks 5 and 6 for bees. Temperature and relative humidity correlated with B. tabaci, E. varivestis, bee, and predator ladybird beetle populations. These results provide valuable information on the integrated pest management of snap beans.

Rarity begets rarity: Social and environmental drivers of rare organisms in cities

Cities are sometimes characterized as homogenous with species assemblages composed of abundant, generalist species having similar ecological functions. Under this assumption, rare species, or species observed infrequently, would have especially high conservation value in cities for their potential to increase functional diversity. Management to increase the number of rare species in cities could be an important conservation strategy in a rapidly urbanizing world. However, most studies of species rarity define rarity in relatively pristine environments where human management and disturbance is minimized. We know little about what species are rare, how many species are rare, and what management practices promote rare species in urban environments. Here, we identified which plants and species of birds and bees that control pests and pollinate crops are rare in urban gardens and assessed how social, biophysical factors, and cross-taxonomic comparisons influence rare species richness. We found overwhelming numbers of rare species, with more than 50% of plants observed classified as rare. Our results highlight the importance of women, older individuals, and gardeners who live closer to garden sites in increasing the number of rare plants within urban areas. Fewer rare plants were found in older gardens and gardens with more bare soil. There were more rare bird species in larger gardens and more rare bee species for which canopy cover was higher. We also found that in some cases, rarity begets rarity, with positive correlations found between the number of rare plants and bee species and between bee and bird species. Overall, our results suggest that urban gardens include a high number of species existing at low frequency and that social and biophysical factors promoting rare, planned biodiversity can cascade down to promote rare, associated biodiversity.

The effect of squash domestication on a belowground tritrophic interaction

The domestication of plants has commonly resulted in the loss of plant defense metabolites, with important consequences for the plants' interactions with herbivores and their natural enemies. Squash domestication started 10'000 years ago and has led to the loss of cucurbitacins, which are highly toxic triterpenes. The banded cucumber beetle (Diabrotica balteata), a generalist herbivore, is adapted to feed on plants from the Cucurbitaceae and is known to sequester cucurbitacins, supposedly for its own defense. However, the evidence for this is inconclusive. In this study we tested the impact of squash domestication on the chemical protection of D. balteata larvae against a predatory rove beetle (Dalotia coriaria). We found that cucurbitacins do not defend the larvae against this common soil dwelling predator. In fact, D. balteata larvae were less attacked when they fed on cucurbitacin-free roots of domesticated varieties compared to high-cucurbitacin roots of wild plants. This study appears to be the first to look at the consequences of plant domestication on belowground tritrophic interactions. Our results challenge the generalized assumption that sequestered cucurbitacins protect this herbivore against natural enemies, and instead reveals an opposite effect that may be due to a tradeoff between coping with cucurbitacins and avoiding predation.

Toward Unifying Evolutionary Ecology and Genomics to Understand Positive Plant-Plant Interactions Within Wild Species

In a local environment, plant networks include interactions among individuals of different species and among genotypes of the same species. While interspecific interactions are recognized as main drivers of plant community patterns, intraspecific interactions have recently gained attention in explaining plant community dynamics. However, an overview of intraspecific genotype-by-genotype interaction patterns within wild plant species is still missing. From the literature, we identified 91 experiments that were mainly designed to investigate the presence of positive interactions based on two contrasting hypotheses. Kin selection theory predicts partisan help given to a genealogical relative. The rationale behind this hypothesis relies on kin/non-kin recognition, with the positive outcome of kin cooperation substantiating it. On the other hand, the elbow-room hypothesis supports intraspecific niche partitioning leading to positive outcome when genetically distant genotypes interact. Positive diversity-productivity relationship rationalizes this hypothesis, notably with the outcome of overyielding. We found that both these hypotheses have been highly supported in experimental studies despite their opposite predictions between the extent of genetic relatedness among neighbors and the level of positive interactions. Interestingly, we identified a highly significant effect of breeding system, with a high proportion of selfing species associated with the presence of kin cooperation. Nonetheless, we identified several shortcomings regardless of the species considered, such as the lack of a reliable estimate of genetic relatedness among genotypes and ecological characterization of the natural habitats from which genotypes were collected, thereby impeding the identification of selective drivers of positive interactions. We therefore propose a framework combining evolutionary ecology and genomics to establish the eco-genomic landscape of positive GxG interactions in wild plant species.