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.
Summary
1. The identification of dispersal mechanisms which facilitate particular biological invasions is paramount for the successful management of invasive species. If the dispersal mechanism promotes high propagule pressure, the probability of successful establishment and spread is enhanced.
2. Invasive species may enter mainland Australia from Papua New Guinea via the Torres Strait islands, and their dispersal through the region may be assisted by wind. The island sugarcane planthopper Eumetopina flavipes is of particular concern to Australian quarantine authorities. Long‐distance, wind‐assisted immigration from Papua New Guinea may be responsible for the continued presence of E. flavipes in the Torres Strait islands and on the tip of mainland Australia. Simulation was used to predict E. flavipes wind‐assisted migration potential from Papua New Guinea into the Torres Strait islands and mainland Australia. Field studies were used to test the predictions.
3. Wind‐assisted immigration from Papua New Guinea was predicted to occur widely throughout the Torres Strait islands and the tip of mainland Australia, especially in the presence of tropical depressions and cyclones. Simulation showed potential for a definite, seasonal immigration which reflected variation in the onset, length and cessation of the summer monsoon.
4. In general, simulation predictions did not explain E. flavipes observed infestations. The discrepancy suggests that post‐colonization processes such as the temporal and spatial availability of host may be equally or more important than possible wind‐assisted immigration in determining population establishment, persistence and viability.
5. Despite the potential for wide‐spread, annual immigration throughout the Torres Strait islands and the tip of mainland Australia, E. flavipes control may be possible by managing the cultivation of host plants on an ongoing annual basis to avoid recolonization, especially prior to or during critical immigration periods.
6. Synthesis and applications. Wind may promote significant incursions of E. flavipes from Papua New Guinea into northern Australia. Management strategies should consider the relative importance of both pre‐ and post‐invasion processes in determining establishment success, so that response measures can be implemented at the appropriate stage of invasion. In this way, successful control may be enhanced, serving to reduce the overall cost of invasion.
Lepidopteran stemborers are among the most damaging agricultural pests worldwide, able to reduce crop yields by up to 40%. Sugarcane is the world’s most prolific crop, and several stemborer species from the families Noctuidae, Tortricidae, Crambidae and Pyralidae attack sugarcane. Australia is currently free of the most damaging stemborers, but biosecurity efforts are hampered by the difficulty in morphologically distinguishing stemborer species. Here we assess the utility of DNA barcoding in identifying stemborer pest species. We review the current state of the COI barcode sequence library for sugarcane stemborers, assembling a dataset of 1297 sequences from 64 species. Sequences were from specimens collected and identified in this study, downloaded from BOLD or requested from other authors. We performed species delimitation analyses to assess species diversity and the effectiveness of barcoding in this group. Seven species exhibited <0.03 K2P interspecific diversity, indicating that diagnostic barcoding will work well in most of the studied taxa. We identified 24 instances of identification errors in the online database, which has hampered unambiguous stemborer identification using barcodes. Instances of very high within-species diversity indicate that nuclear markers (e.g. 18S, 28S) and additional morphological data (genitalia dissection of all lineages) are needed to confirm species boundaries.
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