Adam Frew

Trade-Offs between Silicon and Phenolic Defenses may Explain Enhanced Performance of Root Herbivores on Phenolic-Rich Plants

Phenolic compounds play a role in plant defense against herbivores. For some herbivorous insects, particularly root herbivores, host plants with high phenolic concentrations promote insect performance and tissue consumption. This positive relationship between some insects and phenolics, however, could reflect a negative correlation with other plant defenses acting against insects. Silicon is an important element for plant growth and defense, particularly in grasses, as many grass species take up large amounts of silicon. Negative impact of a high silicon diet on insect herbivore performance has been reported aboveground, but is unreported for belowground herbivores. It has been hypothesized that some silicon accumulating plants exhibit a trade-off between carbon-based defense compounds, such as phenolics, and silicon-based defenses. Here, we investigated the impact of silicon concentrations and total phenolic concentrations in sugarcane roots on the performance of the root-feeding greyback canegrub (Dermolepida albohirtum). Canegrub performance was positively correlated with root phenolics, but negatively correlated with root silicon. We found a negative relationship in the roots between total phenolics and silicon concentrations. This suggests the positive impact of phenolic compounds on some insects may be the effect of lower concentrations of silicon compounds in plant tissue. This is the first demonstration of plant silicon negatively affecting a belowground herbivore.

Belowground Ecology of Scarabs Feeding on Grass Roots: Current Knowledge and Future Directions for Management in Australasia

Many scarab beetles spend the majority of their lives belowground as larvae, feeding on grass roots. Many of these larvae are significant pests, causing damage to crops and grasslands. Damage by larvae of the greyback cane beetle (Dermolepida albohirtum), for example, can cause financial losses of up to AU

The Importance of Testing Multiple Environmental Factors in Legume-Insect Research: Replication, Reviewers, and Rebuttal

40 million annually to the Australian sugarcane industry. We review the ecology of some scarab larvae in Australasia, focusing on three subfamilies; Dynastinae, Rutelinae, and Melolonthinae, containing key pest species. Although considerable research on the control of some scarab pests has been carried out in Australasia, for some species, the basic biology and ecology remains largely unexplored. We synthesize what is known about these scarab larvae and outline key knowledge gaps to highlight future research directions with a view to improve pest management. We do this by presenting an overview of the scarab larval host plants and feeding behavior; the impacts of abiotic (temperature, moisture, and fertilization) and biotic (pathogens, natural enemies, and microbial symbionts) factors on scarab larvae and conclude with how abiotic and biotic factors can be applied in agriculture for improved pest management, suggesting future research directions. Several host plant microbial symbionts, such as arbuscular mycorrhizal fungi and endophytes, can improve plant tolerance to scarabs and reduce larval performance, which have shown promise for use in pest management. In addition to this, several microbial scarab pathogens have been isolated for commercial use in pest management with particularly promising results. The entomopathogenic fungus Metarhizium anisopliae caused a 50% reduction in cane beetle larvae while natural enemies such as entomopathogenic nematodes have also shown potential as a biocontrol. Key abiotic factors, such as soil water, play an important role in affecting both scarab larvae and these control agents and should therefore feature in future multi-factorial experiments. Continued research should focus on filling knowledge gaps including host plant preferences, attractive trap crops, and naturally occurring pathogens that are locally adapted, to achieve high efficacy in the field.

Arbuscular mycorrhizal fungi promote silicon accumulation in plant roots, reducing the impacts of root herbivory

Investigating the impacts of predicted changes in our atmosphere and climate change on insect–plant interactions is a widely pursued area of research. To date, the majority of experimental studies have tested the impacts of single environmental factors on insect–plant interactions, but meta-analyses have clearly illustrated the importance of investigating multiple factors in tandem (Zvereva and Kozlov, 2006; Robinson et al., 2012). In particular, environmental change factors often interact with each other which can either strengthen or mitigate the effects of environmental factors acting alone (Robinson et al., 2012). For example, the positive effects of elevated atmospheric carbon dioxide concentrations (e [CO2]) on plant growth are stronger under high nitrogen (N) conditions compared to low N conditions (+ 32 and+ 19%, respectively; Robinson et al., 2012). Likewise, from the limited number of studies available, Robinson et al.(2012) showed that e [CO2] had different impacts on plant nitrogen, plant biomass, and secondary metabolites under elevated air temperature (eT) conditions. This does not invalidate single factor studies, of which we have published numerous examples, but this is an important consideration for making realistic predictions about how plants and insects will respond to future climates (Facey et al., 2014).

Silicon-induced root nodulation and synthesis of essential amino acids in a legume is associated with higher herbivore abundance

AimsStudies have shown that arbuscular mycorrhizal (AM) fungi can reduce the performance of typically detrimental root feeding insects, yet the mechanisms remain unclear. This study aimed to investigate the effects of different sources of AM inocula on plant resistance to a root feeding insect in two different soils with different silicon (Si) concentrations.MethodsSugarcane (Saccharum spp. hybrid) was grown in high or low Si soil; plants were treated with either an inoculum comprising the native AM fungi, a commercial AM fungal inoculum or with no AM fungi. Root herbivore (Dermolepida albohirtum) performance was measured in a feeding assay.ResultsIn the low Si soil AM fungi increased root Si concentrations and reduced root herbivore performance. Both commercial and native AM treatments increased root Si and also reduced root herbivore growth rates by 107% and 81%, respectively. AM colonisation positively correlated with root Si concentrations. Distinct from this, in the high Si soil AM fungi had no impact on root Si or root herbivore growth. However, root consumption was reduced; a response independent of Si concentrations.ConclusionsOur study suggests AM fungi can enhance Si based plant defences against root herbivores, but also highlights that interactions between AM fungi and root herbivores involves multiple mechanisms requiring further research.

Benefits from below: silicon supplementation maintains legume productivity under predicted climate change scenarios

Summary Ecologists have become increasingly aware that silicon uptake by plants, especially the Poaceae, can have beneficial effects on both plant growth and herbivore defence. The effects of silicon on other plant functional groups, such as nitrogen-fixing legumes, have been less well studied. Silicon could, however, indirectly promote herbivore performance in this group if reported increases in N2-fixation caused improvements in host plant quality for herbivores. We tested how silicon supplementation in the legume Medico sativa affected plant growth rates, root nodulation and foliage quality (silicon content and amino acid profiles) for an insect herbivore (Acyrthosiphon pisum). Plants supplemented with silicon (Si+) grew three times as quickly as those without supplementation (Si-), almost entirely in shoot mass. While root growth was unaffected by silicon uptake, root nodules containing nitrogen-fixing bacteria were 44% more abundant on Si+ plants. Aphid abundance was twice as high on Si+ plants compared to Si- plants and was positively correlated with silicon-stimulated plant growth. Si+ plants accumulated more than twice as much silicon as Si- plants, but did not have higher silicon concentrations because of dilution effects linked to the rapid growth of Si+ plants. Si+ plants showed a 65% increase in synthesis of essential foliar amino acids, probably due to increased levels of root nodulation. These results suggest that increased silicon supply makes M. sativa more susceptible to A. pisum, mainly because of increased plant growth and resource availability (i.e. essential amino acids). While silicon augmentation of the Poaceae frequently improves herbivore defence, the current study illustrates that this cannot be assumed for other plant families where the beneficial effects of silicon on plant growth and nutrition may promote herbivore performance in some instances. This article is protected by copyright. All rights reserved.

Dryland management regimes alter forest habitats and understory arthropod communities

Many studies demonstrate that elevated atmospheric carbon dioxide concentrations (eCO2) can promote root nodulation and biological nitrogen fixation (BNF) in legumes such as lucerne (Medicago sativa). But when elevated temperature (eT) conditions are applied in tandem with eCO2, a more realistic scenario for future climate change, the positive effects of eCO2 on nodulation and BNF in M. sativa are often much reduced. Silicon (Si) supplementation of M. sativa has also been reported to promote root nodulation and BNF, so could potentially restore the positive effects of eCO2 under eT. Increased nitrogen availability, however, could also increase host suitability for aphid pests, potentially negating any benefit. We applied eCO2 (+240 ppm) and eT (+4°C), separately and in combination, to M. sativa growing in Si supplemented (Si+) and un-supplemented soil (Si-) to determine whether Si moderated the effects of eCO2 and eT. Plants were either inoculated with the aphid Acyrthosiphon pisum or insect-free. In Si- soils, eCO2 stimulated plant growth by 67% and nodulation by 42%, respectively, whereas eT reduced these parameters by 26 and 48%, respectively. Aphids broadly mirrored these effects on Si- plants, increasing colonization rates under eCO2 and performing much worse (reduced abundance and colonization) under eT when compared to ambient conditions, confirming our hypothesized link between root nodulation, plant growth, and pest performance. Examined across all CO2 and temperature regimes, Si supplementation promoted plant growth (+93%), and root nodulation (+50%). A. pisum abundance declined sharply under eT conditions and was largely unaffected by Si supplementation. In conclusion, supplementing M. sativa with Si had consistent positive effects on plant growth and nodulation under different CO2 and temperature scenarios. These findings offer potential for using Si supplementation to maintain legume productivity under predicted climate change scenarios without making legumes more susceptible to insect pests.

Do eucalypt plantation management practices create understory reservoirs of scarab beetle pests in the soil

Dryland forests, those characterised as having low precipitation and soil nutrients, account for over a quarter of forests globally. Increasing their productivity often relies on irrigation and fertilisation, but the impacts on the wider habitat are largely unknown. Understory invertebrates, in particular, play key roles in forest systems (e.g. nutrient cycling), but their responses to dryland forest management practices are untested. We investigated the impacts of irrigation, fertilisation and a combination of both on soil chemistry, understory vegetation, tree growth and understory arthropod communities in a Eucalyptus plantation to establish linkages between dryland management and ecosystem responses. Fertilisation increased all soil nutrients (N, NO3N, P and K) with similar effects on the chemical composition of understory grasses. Fertilisation also caused declines in foliar silicon concentrations, an important herbivore defence in grasses. Irrigation increased growth of both understory plants (+90%) and trees (+68%). Irrigation increased the abundance of ground‐dwelling arthropods by over 480% relative to control plots, but depressed higher level taxon arthropod diversity by 15%, declining by a further 7% (−22%) in combined treatment plots. Irrigation also caused a surge in the abundance of Collembola (+1300%) and Isopoda (+323%). Fertilisation drove increases in the abundance of Isopoda (+196%) and Diptera (+63%), whereas fertilisation combined with irrigation increased populations of Thysanoptera (+166%) and Acarina (+328%). Airborne arthropods were less affected, but fertilisation increased the abundance of Apocrita (+95%) and depressed populations of Thysanoptera (−77%). Diptera abundance was positively related to understory vegetation growth, whereas the abundance of other groups (Collembola, Isopoda, Thysanoptera and Acarina) correlated positively with tree growth. We proposed that the large increases in populations of key detritivores, Collembola and Isopoda, were linked to increased leaf litter from enhanced tree growth in irrigated and combined treatment plots. Our findings suggest that dryland management can increase both plant productivity and abundance of arthropods, but cause arthropod diversity at the higher taxon level to decline overall.