Silicon fertilisation affects morphological and immune defences of an insect pest and enhances plant compensatory growth

Herbivorous insects have evolved various anti-predator defences, including morphological, behavioural, and immune defences, which can make biocontrol of herbivorous pests challenging. Silicon (Si) accumulation in plants is a potent physical defence against mandibulate insects. However, it remains uncertain how Si affects the anti-predator defences of insect herbivores and plant defences following herbivory. We grew the model grass, Brachypodium distachyon, hydroponically with (+Si) or without (–Si) Si and investigated the plant-mediated effects of Si on the anti-predator defences of the cotton bollworm, Helicoverpa armigera, integrating morphological (i.e. integument resistance and thickness), behavioural, and immune defences. We also examined the effects of Si on plant compensatory growth and leaf trichome production. Larval growth, leaf consumption, and integument resistance were lower when feeding on +Si plants compared to when feeding on –Si plants. Larval integument thickness, defensive behaviours, haemocyte density, and lysozyme-like activity in the haemolymph were unaffected by Si. Larvae fed on +Si plants had higher haemolymph phenoloxidase (PO) and total-PO activities than larvae fed on –Si plants, although this did not enhance the melanisation response of larvae. Furthermore, Si supplies increased plant compensation for herbivory and constitutive trichome production, whereas herbivory induced trichome production only on –Si plants. We provide the first evidence for plant-mediated effects of Si on anti-predator defences of an insect herbivore. We suggest that the lower integument resistance of larvae when feeding on Si-supplemented plants could contribute to their vulnerability to natural enemies and that high PO activity may impose fitness costs (e.g. delayed development)

Mapping canopy nitrogen-scapes to assess foraging habitat for a vulnerable arboreal folivore in mixed-species Eucalyptus forests

Herbivore foraging decisions are closely related to plant nutritional quality. For arboreal folivores with specialized diets, such as the vulnerable greater glider (Petauroides volans), the abundance of suitable forage trees can influence habitat suitability and species occurrence. The ability to model and map foliar nitrogen would therefore enhance our understanding of folivore habitat use at finer scales. We tested whether high-resolution multispectral imagery, collected by a lightweight and low-cost commercial unoccupied aerial vehicle (UAV), could be used to predict total and digestible foliar nitrogen (N and digN) at the tree canopy level and forest stand-scale from leaf-scale chemistry measurements across a gradient of mixed-species Eucalyptus forests in southeastern Australia. We surveyed temperate Eucalyptus forests across an elevational and topographic gradient from sea level to high elevation (50–1200 m a.s.l.) for forest structure, leaf chemistry, and greater glider occurrence. Using measures of multispectral leaf reflectance and spectral indices, we estimated N and digN and mapped N and favorable feeding habitat using machine learning algorithms. Our surveys covered 17 Eucalyptus species ranging in foliar N from 0.63% to 1.92% dry matter (DM) and digN from 0.45% to 1.73% DM. Both multispectral leaf reflectance and spectral indices were strong predictors for N and digN in model cross-validation. At the tree level, 79% of variability between observed and predicted measures of nitrogen was explained. A spatial supervised classification model correctly identified 80% of canopy pixels associated with high N concentrations (≥1% DM). We developed a successful method for estimating foliar nitrogen of a range of temperate Eucalyptus species using UAV multispectral imagery at the tree canopy level and stand scale. The ability to spatially quantify feeding habitat using UAV imagery allows remote assessments of greater glider habitat at a scale relevant to support ground surveys, management, and conservation for the vulnerable greater glider across southeastern Australia

Geographic patterns of koala retrovirus genetic diversity, endogenization, and subtype distributions

Koala retrovirus (KoRV) subtype A (KoRV-A) is currently in transition from exogenous virus to endogenous viral element, providing an ideal system to elucidate retroviral–host coevolution. We characterized KoRV geography using fecal DNA from 192 samples across 20 populations throughout the koala’s range. We reveal an abrupt change in KoRV genetics and incidence at the Victoria/New South Wales state border. In northern koalas, pol gene copies were ubiquitously present at above five per cell, consistent with endogenous KoRV. In southern koalas, pol copies were detected in only 25.8% of koalas and always at copy numbers below one, while the env gene was detected in all animals and in a majority at copy numbers above one per cell. These results suggest that southern koalas carry partial endogenous KoRV-like sequences. Deep sequencing of the env hypervariable region revealed three putatively endogenous KoRV-A sequences in northern koalas and a single, distinct sequence present in all southern koalas. Among northern populations, env sequence diversity decreased with distance from the equator, suggesting infectious KoRV-A invaded the koala genome in northern Australia and then spread south. The exogenous KoRV subtypes (B to K), two novel subtypes, and intermediate subtypes were detected in all northern koala populations but were strikingly absent from all southern animals tested. Apart from KoRV subtype D, these exogenous subtypes were generally locally prevalent but geographically restricted, producing KoRV genetic differentiation among northern populations. This suggests that sporadic evolution and local transmission of the exogenous subtypes have occurred within northern Australia, but this has not extended into animals within southern Australia

Why did the koala cross the forest floor?

Koalas are fussy eaters. Ben Moore discovers why. When we shop in the supermarket, we can generally be that is both safe and nutritious. This task is made easy for us by packets that list each food’s ingredients and a raft of nutritional information, and by manufacturing processes that are regulated and carefully monitored. But life is not so straightforward for wild animals, and this is especially true for herbivores. Wild herbivores face two problems that we avoid when we shop in a supermarket

Integrating the effects of PSMs on vertebrate herbivores across spatial and temporal scales

Since Fraenkel (1959) proposed a leading role for plant secondary metabolites (PSMs) in the interactions between plants and herbivores, science has achieved broad insight into the diversity of PSMs and herbivores’ counter-adaptations to them (Freeland & Janzen, 1974; Foley et al., 1999; Foley & Moore, 2005). However, the more we learn about the distributions and functions of PSMs in natural systems, the sharper the limitations of our understanding become. In countless plant–herbivore interactions, ecologists have identified PSMs that act as feeding deterrents, toxins, digestibility reducers, feeding or oviposition cues, and signals for communicating to neighbouring plants and natural enemies of herbivores. However, most studies focus on the interaction between single species of herbivore and plant, usually with observations of captive animals fed diets containing PSMs under highly simplified conditions. Although such approaches are a necessary first step in isolating and characterising the actions of PSMs, they greatly oversimplify the complex interactions that occur between wild herbivores and plants. The next challenge for ecologists is to ‘scale up’ the roles of PSMs in plant–herbivore interactions, as we understand them from controlled experiments at small temporal scales, to predict ecological interactions at greater temporal and spatial extents. A captive herbivore may commonly eat less as PSM concentrations in its food increase, but can this predict the foraging decisions of a wild animal within its home range, or, ultimately the distributions and abundances of plant and herbivore species and genotypes? Scaling up has an obvious spatial component, because wild animals forage more extensively than do captive animals, but it also has a temporal component. Experiments usually describe plant–herbivore interactions over very short time intervals, but in nature they are continuous and the effects of PSMs can be long-lasting (Cheeke, 1998). Animal feeding preferences are dynamic and often change with season or reproductive state, or through the ongoing process of refinement of conditioned flavour aversions (Provenza, 1996). With increasing spatial extent and finer spatial grain size comes greater complexity in the interactions between plants and animals; PSMs are rarely distributed evenly throughout landscapes, and understanding how this influences plant–animal interactions requires approaches adopted from resource ecology, foraging theory and spatial ecology and often an extensive, high-resolution picture of the foodscapes within which animals forage (van Langevelde & Prins, 2008)

The Scottish Beaver Trial: Woodland Monitoring 2011

In 2008, the Scottish Government approved a licence to the Scottish Wildlife Trust (SWT) and the Royal Zoological Society of Scotland (RZSS), to undertake a five-year trial reintroduction of the European beaver (Castor fiber) to Scotland after an absence of more than 400 years. In May 2009, three beaver family groups were introduced to Loch Coille- Bharr, Loch Linne/Loch Fidhle and Creagmhor Loch/Loch Beag on land managed by Forest Enterprise Scotland (FES) at Knapdale, Argyll. Since 2009, additional releases have also taken place, and by November 2010, beaver groups were established in these three lochs and Lochan Buic. This is the third annual report that describes the effects of beavers on riparian woodland at Knapdale, and summarises effects observed up until November 2011 and attempts to identify trends that are emerging with increasing time since the reintroduction process began

Silicon defence in plants : does herbivore identity matter?

Silicon accumulation is a key defence against herbivorous pests, but may have wider detrimental impacts if plants become unpalatable for livestock. We argue that some herbivores are better adapted to silicon-rich diets than others; herbivore anatomy and physiology, and the nature of silicon deposition, are crucial to understanding these differences

Elevated atmospheric carbon dioxide concentrations alter root morphology and reduce the effectiveness of entomopathogenic nematodes

Aims: The effects of increasing atmospheric carbon dioxide (CO2) concentrations on beneficial soil fauna, such as entomopathogenic nematodes (EPNs), are poorly understood. We hence aimed to characterize how elevated CO2 (eCO2) affects maize plant (Zea mays) growth, root morphology and the effectiveness of the EPN Heterorhabditis bacteriophora. Methods: We grew plants under ambient CO2 (aCO2; 400 μmol mol-1) and eCO2 (640 μmol mol-1) and quantified shoot growth and six root traits. We simultaneously quantified the effectiveness of EPNs (mortality of insect hosts (Galleria mellonella) and EPN density within hosts) when foraging in planted and plant-free environments. Structural equation modeling (SEM) was used to model direct and indirect relationships between atmospheric CO2, root morphology and EPN effectiveness. Results: Root systems of plants grown under eCO2 grew faster, longer, denser, and larger than plants grown under aCO2. This in turn reduced EPN effectiveness as, despite no significant difference between aCO2 and eCO2 in host mortality, significantly more nematodes were recovered from hosts in the vicinity of plants grown in aCO2 environment. The SEM model revealed that this impact was indirect and mediated by the increased root morphological traits. Conclusions: We provide the first example of how changes in atmospheric CO2 indirectly reduce the effectiveness of an EPN used globally for crop protection. Other factors (e.g. plant volatile emissions) may moderate or exacerbate these patterns but our findings suggest that modifications in root traits at eCO2 negatively impact EPN effectiveness and therefore soil-dwelling insect pest management

Spatial correlations between browsing on balsam fir by white-tailed deer and the nutritional value of neighboring winter forage

Associational effects, that is, the influence of neighboring plants on herbivory suffered by a plant, are an outcome of forage selection. Although forage selection is a hierarchical process, few studies have investigated associational effects at multiple spatial scales. Because the nutritional quality of plants can be spatially structured, it might differently influence associational effects across multiple scales. Our objective was to determine the radius of influence of neighbor density and nutritional quality on balsam fir (Abies balsamea) herbivory by white-tailed deer (Odocoileus virginianus) in winter. We quantified browsing rates on fir and the density and quality of neighboring trees in a series of 10-year-old cutovers on Anticosti Island (Canada). We used cross-correlations to investigate relationships between browsing rates and the density and nutritional quality of neighboring trees at distances up to 1,000 m. Balsam fir and white spruce (Picea glauca) fiber content and dry matter in vitro true digestibility were correlated with fir browsing rate at the finest extra-patch scale (across distance of up to 50 m) and between cutover areas (300-400 m). These correlations suggest associational effects, that is, low nutritional quality of neighbors reduces the likelihood of fir herbivory (associational defense). Our results may indicate associational effects mediated by intraspecific variation in plant quality and suggest that these effects could occur at scales from tens to hundreds of meters. Understanding associational effects could inform strategies for restoration or conservation; for example, planting of fir among existing natural regeneration could be concentrated in areas of low nutritional quality

Intraspecific variation in nutritional traits of neighbouring plants generates a continuum of associational effects

Aims: When deciding whether or not to eat a plant, herbivores are influenced by the nutritional value of potential foods, but also indirectly by neighbouring plants (associational effects). We aimed to investigate how the abundance and nutritional quality of neighbours of balsam firs (Abies balsamea) affects browsing on balsam firs by white-tailed deer. We sought to distinguish the effects of conspecific and heterospecific neighbour abundance, and to evaluate whether intraspecific variation in nutritional traits produces associational effects. Location: Anticosti Island, QC, Canada. Methods: We measured the abundance of stems in 4-m2 plots centred on a focal balsam fir and evaluated nutritional value (nitrogen content and digestibility) of focal firs and neighbouring plants. We used generalized linear models to explain browsing on firs as a function of the neighbouring stem abundance (conspecific and heterospecific), and of the nutritional value of firs and neighbouring stems. Results: Fir abundance only affected browsing of firs after accounting for fir nutritional value, e.g., browsing in plots of low fir digestibility decreased as the number of fir stems increased (resource dilution effect). The associational effects of heterospecific neighbour abundance, especially birch stem and shoot abundance, were also contingent on neighbour nutritional value. For example, at low birch digestibility, browsing on the focal fir increased with the number of neighbouring birches. Browsing on fir also increased as the number of spruce stems of high nutritional value increased. Conclusions: By taking into account the abundance of all species, we could discriminate between conspecific effects and associational effects caused by heterospecifics. Our study suggests that the strength of associational effects varies in a continuous fashion in response to relative nutritional value. We propose that future studies should not only consider the identity of neighbours but also the nutritional traits of plant communities