How the soil environment affects root feeding scarabs with particular emphasis on the canegrub

The interactions between insect herbivores and their host plants have fascinated scientists for generations. There is a vast diversity of these insects, which have a variety of different feeding strategies and diet breadth. Many have become significant pests of managed ecosystems, such as forest and crop production systems. The majority of research has focussed on aboveground insect defoliators rather than root feeding insects. This is surprising as root feeding insects are some of the most economically damaging and difficult to control. Several scarab species (Coleoptera: Scarabaeidae) are among the most significant insect pests to agriculture, particularly during the root feeding larval stages. The soil environment in which scarab larvae feed and develop involves several abiotic and biotic factors which can be highly influential in shaping the relationship between these root feeders and their host plants. Soil nutrients including nitrogen (N), phosphorus (P) and potassium (K), along with soil moisture, are important to plant growth and quality, and thereby are important to the belowground insects which feed on their roots. Silicon (Si) is known to impact plant growth, but can also act as a plant defence mechanism against insect herbivores, although this remains untested in root feeding insects. Biotic soil influences include arbuscular mycorrhizal (AM) fungi, which associate with the majority of land plants, and can alter plant quality and defences, while negatively impacting root herbivore performance, although the mechanisms remain unclear. This work addresses how these different factors within the soil environment affect root feeding scarab larvae. This is initially investigated at the community level, then subsequently using the grass crop plant sugarcane, Saccharum species hybrids, and the canegrub, Dermolepida albohirtum. Recent literature on the ecology of scarab larval pests within Australasia and the effects of different soil factors on scarab larvae are synthesised in chapters one and two, respectively. These chapters highlight the paucity of knowledge surrounding the ecology of root feeding scarab larvae and the importance of factors such as soil nutrients (N,P,K and Si) and microbial communities (AM fungi) on plant-herbivore interactions belowground. Chapter three investigates how fertilisation and irrigation practices, which alter soil nutrients (N,P,K) and moisture, impact host plant communities, scarab larval communities and their natural enemies (entomopathogenic nematodes). Scarab larval communities were positively affected by fertilisation, increasing their abundance by 52%. While irrigation did not impact scarab communities, there was an increase in entomopathogenic nematode presence by 78%, suggesting scarab populations were suppressed by their natural enemies. Chapters four and five focus on the effects of Si on sugarcane and the canegrub. Specifically, chapter four investigates previous observations of positive responses by root feeding insects to phenolic compounds and a suggested trade-off between carbon and Si based plant defences. Canegrub performance positively correlated with root phenolics, while correlating negatively with root Si. A negative correlation between phenolics and Si suggested positive responses by root feeding insects to high phenolic concentrations may be a response to low Si concentrations. This was the first example of plant Si negatively impacting a root feeding insect. Chapter five looks at the impacts of silicon on sugarcane and canegrub performance under ambient and elevated atmospheric carbon dioxide concentrations (eCO2). Elevated CO2 decreased sugarcane root nutritional value while increasing canegrub growth rate and root consumption by 116% and 57%, respectively. Silicon decreased performance of the canegrub under both ambient and eCO2, highlighting the potential role of Si in future pest management strategies. Chapter six investigated the impacts of two AM fungal communities on sugarcane and canegrub performance within different soil types, known to have different concentrations of Si. Both AM communities had the same effect on sugarcane and canegrub responses. Arbuscular mycorrhizal fungi promoted sugarcane growth and photosynthesis by 81% and 39%, respectively, while also increasing root Si concentrations, but only in soil with low Si concentrations. Similarly, AM fungi decreased canegrub performance, but only within the low Si soil. This suggested that AM fungi promote Si accumulation within Si depleted soil environments, negatively impacting canegrub performance. Chapter seven concluded this research, building on the observations from chapter five, by directly testing the effects of Si and AM fungi on plant growth alongside their impacts on canegrub performance, root consumption and immune function within two different sugarcane varieties. Si decreased canegrub performance and consumption, while AM fungi decreased canegrub performance when Si was not applied and only on one plant variety. AM fungi increased canegrub immune function by 62%, a response that was not explainable by any measured plant trait. Canegrub immune function negatively correlated with canegrub mass, suggesting a trade-off between growth and immunity. The results of this PhD research contribute to the understanding of (a) the impacts of management practices altering soil factors such as N,P,K and moisture on belowground pest communities; (b) the potential trade-off between carbon and Si in plants and the impacts of Si on root feeding insects; (c) the importance of plant Si defences against root feeding insects under climate change; (d) the interactions between AM fungi, host plants and their root feeding insects and the importance of soil Si availability to this relationship; (e) the role of AM fungi and Si on the growth and immunity of root feeding insects. This research has shown how the impacts of common agricultural management practices can potentially exacerbate scarab pest problems. This work has demonstrated that Si and AM fungi can promote plant growth and reduce canegrub performance, although the effects of AM fungi can be context dependent, specifically on soil Si availability and plant variety. In terms of applied implications, this suggests that future pest management strategies should look to exploit plant Si defences through targeted application of Si fertiliser in Si depleted soils. Practices that encourage native AM communities also hold potential in reducing soil pest persistence, though mechanisms including increased Si uptake, or perhaps even through direct interactions with soil insects

‘Soil probiotics’ promise bigger, healthier crops, but there’s a downside

More than half the world’s plant-derived energy intake comes from just three crops: rice, wheat and maize. These crops, like most land plants, live in an evolutionarily ancient partnership with a certain type of fungus, called arbuscular mycorrhizal fungi. These fungi penetrate plants’ roots, even entering the root cells themselves. In a win-win relationship, the fungi provide the plants with crucial nutrients and the plant provides the fungi with sugar. By helping plants take up nutrients from the soil, these fungi can enhance crop yields, increase pest resistance, and reduce the need for fertiliser. So it’s hardly surprising that there has been a long-held interest in harnessing these soil-dwelling fungi for agriculture. But our research shows that in some cases these fungi can harm crops instead of helping them. This means we need to proceed with caution in pursuing the benefits of using these fungi as fertilisers

Contrasting effects of commercial and native arbuscular mycorrhizal fungal inoculants on plant biomass allocation, nutrients, and phenolics

As the global population increases, the need to feed more people must be met while simultaneously conserving the long‐term sustainability of our agroecosystems. There is mounting interest and discussion around the application of arbuscular mycorrhizal fungal (AMF) inoculants to enhance crop growth, nutrient uptake, and pest resistance. However, the effects of AMF inoculation are variable and context dependent. This study found that a multi‐species AMF inoculant had a stronger effect on plant biomass allocation and chemistry than a single AMF species inoculant, however, neither of these had a stronger effect than re‐inoculating plants with a field‐sourced native AMF community

Silicon accumulation suppresses arbuscular mycorrhizal fungal colonisation in the model grass Brachypodium distachyon

Purpose Silicon (Si) accumulation by grasses alleviates diverse biotic and abiotic stresses. Despite this important functional role, we have limited understanding of how root microbial symbionts, such as arbuscular mycorrhizal (AM) fungi, affect Si uptake and even less about how Si supply and accumulation affect AM fungal colonisation. Our objective was to determine the nature of this two–way interaction in the model grass, Brachypodium distachyon. Methods We grew B. distachyon with five levels of Si supplementation using wild-type plants and a mutant (Bdlsi1-1) that has little capacity for Si uptake. Half of the plants were colonised by AM fungi; half were free of AM fungi. We measured Si accumulation, AM fungal colonisation, leaf carbon (C), nitrogen (N) and phosphorus (P) concentrations. Results AM fungi did not affect Si accumulation, although small increases occurred when root mass was included as a covariate. Si supplemented soil promoted plant growth and P uptake. Si accumulation suppressed colonisation by AM fungi and C concentrations in wild type but not in Bdlsi1-1 plants. Si concentrations were negatively correlated with C and N concentrations, with correlations being stronger in wild-type plants than Bdlsi1-1 plants. Conclusions Our results indicate that Si accumulation in the plant, rather than Si availability in the soil, underpinned reduced AMF colonisation. We propose that Si accumulation is unlikely to be impacted by AM fungi in plants with inherently high Si accumulation, but Si accumulation may suppress AM fungal colonisation in such plants

40Ar/39Ar geochronology of Burdigalian paleobotanical localities in the central Paratethys (south Slovakia)

The Lipovany and Mučín paleobotanical localities contain important floral associations within the tuff horizons, which were used for determination of subtropical to tropical climatic conditions during the Early Miocene. Based on the combination of results from plagioclase and biotite 40Ar/39Ar dating, the age of the tuff deposition is around 17.3Ma. For the Lipovany locality, single-grain 40Ar/39Ar convergent ages of 17.49±0.54Ma and 17.28±0.06Ma, for plagioclase and biotite were obtained, respectively. The Mučín locality only provide an imprecise convergent age of 16.5±1.4Ma due to the small size of the analyzed plagioclase crystals. The results thus allowed to include the fossil subtropical flora of the studied localities in the late Ottnangian regional stage (upper part of the Burdigalian). Additionally, these age data indicate that deposition of the overlaying Salgótarján Formation starts much later than originally thought (during Ottnangian-Karpatian boundary)

How fungi’s knack for networking boosts ecological recovery after bushfires

The unprecedented bushfires that struck the east coast of Australia this summer killed an estimated one billion animals across millions of hectares. Scorched landscapes and animal corpses brought into sharp relief what climate-driven changes to wildfire mean for Australia’s plants and animals. Yet the effects of fire go much deeper, quite literally, to a vast and complex underground world that we know stunningly little about, including organisms that might be just as vulnerable to fire, and vital to Australia’s ecological recovery: the fungi

New frontiers in belowground ecology for plant protection from root-feeding insects

Herbivorous insect pests living in the soil represent a significant challenge to food security given their persistence, the acute damage they cause to plants and the difficulties associated with managing their populations. Ecological research effort into rhizosphere interactions has increased dramatically in the last decade and we are beginning to understand, in particular, the ecology of how plants defend themselves against soil-dwelling pests. In this review, we synthesise information about four key ecological mechanisms occurring in the rhizosphere or surrounding soil that confer plant protection against root herbivores. We focus on root tolerance, root resistance via direct physical and chemical defences, particularly via acquisition of silicon-based plant defences, integration of plant mutualists (microbes and entomopathogenic nematodes, EPNs) and the influence of soil history and feedbacks. Their suitability as management tools, current limitations for their application, and the opportunities for development are evaluated. We identify opportunities for synergy between these aspects of rhizosphere ecology, such as mycorrhizal fungi negatively affecting pests at the root-interface but also increasing plant uptake of silicon, which is also known to reduce herbivory. Finally, we set out research priorities for developing potential novel management strategies

Understanding Racial HIV/STI Disparities in Black and White Men Who Have Sex with Men: A Multilevel Approach

Background: The reasons for black/white disparities in HIV epidemics among men who have sex with men have puzzled researchers for decades. Understanding reasons for these disparities requires looking beyond individual-level behavioral risk to a more comprehensive framework. Methods and Findings: From July 2010-Decemeber 2012, 803 men (454 black, 349 white) were recruited through venuebased and online sampling; consenting men were provided HIV and STI testing, completed a behavioral survey and a sex partner inventory, and provided place of residence for geocoding. HIV prevalence was higher among black (43%) versus white (13% MSM (prevalence ratio (PR) 3.3, 95% confidence interval (CI): 2.5–4.4). Among HIV-positive men, the median CD4 count was significantly lower for black (490 cells/mL) than white (577 cells/mL) MSM; there was no difference in the HIV RNA viral load by race. Black men were younger, more likely to be bisexual and unemployed, had less educational attainment, and reported fewer male sex partners, fewer unprotected anal sex partners, and less non-injection drug use. Black MSM were significantly more likely than white MSM to have rectal chlamydia and gonorrhea, were more likely to have racially concordant partnerships, more likely to have casual (one-time) partners, and less likely to discuss serostatus with partners. The census tracts where black MSM lived had higher rates of poverty and unemployment, and lower median income. They also had lower proportions of male-male households, lower male to female sex ratios, and lower HIV diagnosis rates. Conclusions: Among black and white MSM in Atlanta, disparities in HIV and STI prevalence by race are comparable to those observed nationally. We identified differences between black and white MSM at the individual, dyadic/sexual network, and community levels. The reasons for black/white disparities in HIV prevalence in Atlanta are complex, and will likely require a multilevel framework to understand comprehensively

Discovery of Precursor LBV Outbursts in Two Recent Optical Transients: The Fitfully Variable Missing Links UGC 2773-OT and SN 2009ip

We present progenitor-star detections, light curves, and optical spectra of SN2009ip and the 2009 optical transient in UGC2773 (U2773-OT), which were not genuine SNe. Precursor variability in the decade before outburst indicates that both of the progenitor stars were LBVs. Their pre-outburst light curves resemble the S Doradus phases that preceded giant eruptions of eta Carinae and SN1954J (V12 in NGC2403), with intermediate progenitor luminosities. HST detections a decade before discovery indicate that the SN2009ip and U2773-OT progenitors were supergiants with likely initial masses of 50-80 Msun and \ga20 Msun, respectively. Both outbursts had spectra befitting known LBVs, although in different physical states. SN 2009ip exhibited a hot LBV spectrum with characteristic speeds of 550 km/s, plus faster material up to 5000 km/s, resembling the slow Homunculus and fast blast wave of eta Carinae. U2773-OT shows a forest of narrow absorption and emission lines comparable to that of S Dor in its cool state, plus [CaII] emission and an IR excess indicative of dust, similar to SN2008S and N300-OT. [CaII] emission is probably tied to a dusty pre-outburst environment, and not the outburst mechanism. SN2009ip and U2773-OT may provide a critical link between historical LBV eruptions, while U2773-OT may provide a link between LBVs and SN2008S and N300-OT. Future searches will uncover more examples of precursor LBV variability of this kind, providing key clues that may help unravel the instability driving LBVs.Comment: 18 pages, 13 Figures, accepted AJ. added significant material while revising after referee repor

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

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 (Medicago 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