Thermal physiology components of the grasshopper Cornops aquaticum (Brüner, 1906) and its impact on establishment in South Africa

Abstract

Water hyacinth, Pontederia crassipes (Commelinales:Pontederiaceae), remains one of the worlds’ worst invasive aquatic plants, causing high environmental damage and economic costs. In South Africa its damage has been reduced through the introduction of nine biological control agents, however complete control has yet to be achieved. Biological control efforts are hindered by eutrophic water bodies, but also a lack of effective biological control agents, potentially the result of climatic unsuitability between the agent and the South African climate. Despite numerous release attempts since 2011, the establishment of the water hyacinth grasshopper Cornops aquaticum (Orthoptera: Acrididae) has not been achieved. Sixteen years under quarantine laboratory before release may have selected for a thermally incompatible agent. Cornops aquaticum have critical thermal minima (6.15 ± 0.12 ºC) higher than other water hyacinth biological control agents, limiting their potential for establishment at many water hyacinth field sites. The grasshopper is capable of acclimation to lower critical thermal limits (5.53 ± 0.15 ºC), provided adequate time for acclimation is given. Using flow-through respirometry, metabolic rates of adults were found to be significantly lower at 15 ºC compared to 25 ºC and 35 ºC. Immature C. aquaticum showed significantly lower metabolic rates at 15 ºC and 25 ºC. Cornops aquaticum adults show a high mass-specific resting metabolic rate (1139.5 ± 89.97μl.hr-1.g-1) at 25 ºC. Outdoor experiments showed high winter mortality (>60 %), potential problems with thermoregulation during colder conditions and an increased likelihood of adults to be found off their host plant at colder conditions. Higher temperatures (≥ 25 ºC) were associated with higher feeding damage, body mass, oviposition rates, egg and instar survival and faster developmental times. Using STELLA 9.1.4, a stage-structured C. aquaticum population model on a temperature dependent water hyacinth model is described based on temperature data from two water hyacinth field sites. Overall, the C. aquaticum population released in South Africa shows sensitivity to lower temperatures. This is probably due to selection imposed by more than 80 generations spent in temperature-controlled quarantine. Release at only the warmest sites is recommended, with release efforts focused on a single site at a time. It is suggested that the population be “renaturalised”, to increase genetic diversity and thermal tolerance