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
