SUMMARY In contrast to previous studies of rapid cold-hardening (RCH), which have investigated the responses of insects maintained under `summer conditions'(20° to 25°C), this study focuses on the ability of low-temperature acclimated insects to undergo RCH. When the grain aphid Sitobion avenae Fabricus was low-temperature acclimated by rearing for three generations at 10°C, the discriminating temperatures (temperature that results in approximately 20% survival after direct transfer from the rearing temperature to a sub-zero temperature for a period of 3 h), of first instar nymphs and adult aphids were –11.5° and –12°C,respectively. Maximum rapid cold-hardening was induced by cooling aphids at 0°C for 2 h (nymphs) or 30 min (adults), resulting in survival at the respective discriminating temperatures increasing from 26% to 96% (nymphs) and 22% to 70% (adults). Cooling from 10° to 0°C at 1°, 0.1° and 0.05°C min-1 significantly increased survival of nymphs at the discriminating temperature, but not of adults. There were no `ecological costs' associated with rapid cold-hardening at 0°C, or with exposure of rapidly cold-hardened aphids to the discriminating temperatures; fecundity and longevity, in both nymphs and adults were either similar to control aphids or significantly increased. The study demonstrates that rapid cold-hardening ability is retained in aphids that have already undergone cold-acclimation, as would be the case in overwintering aphids. Both rapid cold-hardening and subsequent exposure at previously lethal temperatures can enhance fitness in surviving individuals.
The effect of long-term (seasonal) acclimation and rapid cold hardening is investigated on the cold torpor temperature ( CT min ) of adult grain aphids, Sitobion avenae, reared at 20 or 10 °C for more than 6 months before experimentation. Rapid cold hardening is induced by exposing aphids reared at 20 to 0 °C for 3 h and aphids reared at 10 to 0 °C for 30 min (acclimation regimes previously found to induce maximum rapid cold hardening). The effect of cooling aphids from the same rearing regimes from 10 to −10 °C at 1, 0.5 and 0.1 °C min −1 is also investigated. In the 20 °C acclimated population, rapid cold hardening and cooling at 0.1 °C min −1 both produce a significant decrease in CT min from 1.5 ± 0.3 to -0.9 ± 0.3 and -1.3 ± 0.3 °C, respectively. Rapid cold hardening also results in a significant reduction in CT min of the population reared at 10 °C from 0.8 ± 0.1 to -0.9 ± 0.2 °C. However, none of the cooling regimes tested reduces the CT min of the winter-acclimated (10 °C) population. The present study demonstrates that rapid cold-hardening induced during the cooling phase of natural diurnal temperature cycles could lower the movement threshold of S. avenae , allowing insects to move and continue feeding at lower temperatures than would otherwise be possible.
1. When first instar nymphs and adults of the grain aphid Sitobion avenae (Fabricius) (Hemiptera: Aphidiae) were maintained in long‐term cultures (>6 months) at 20 °C and 10 °C, the LT50 decreased from −8 and −8.8 °C to −16.0 and −13.5 °C, respectively. 2. When aphids from the 20 °C culture were transferred to 10 °C, there was a progressive increase in cold tolerance through three successive generations. Transfer of newly moulted pre‐reproductive adults reared at 10 °C for three generations back to 20 °C resulted in a rapid loss of cold hardiness in their nymphal offspring. 3. In all generations reared at 10 °C, first born nymphs were more cold hardy than those born later in the birth sequence. The LT50 of nymphs produced on the first day of reproduction in the first, second and third generations maintained at 10 °C were −14.8, −17.0 and −16.6 °C, respectively. Thereafter, nymphal cold hardiness decreased over the subsequent 14 days of reproduction in each generation at 10 °C with mean LT50 values of −10.3, −12.6 and −14.8 °C, respectively. By contrast, the cold tolerance of first born nymphs of aphids reared continuously at 20 °C did not differ in comparison with later born siblings. The LT50 of adult aphids was also unaffected by ageing. 4. The ecological relevance of these findings is discussed in relation to the overwintering survival of aphids such as S. avenae.
1. Shallow groundwater aquifers regularly support drought refuges for water-dependent ecosystems. However, many aquifers are impacted by over-extraction and pollution, potentially degrading their ability to support groundwater-fed drought refuges. 2. We investigated the response of groundwater-connected riverine forests to a drought considered equivalent in intensity to those predicted under severe climate change for 2030. The drought's impact was investigated in an area where shallow groundwater resources are heavily exploited and polluted by salinization. 3. We used remotely sensed vegetation productivity (enhanced vegetation index) data from a long-term data set (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011) at 475 riverine forest sites in the Campaspe catchment, southeastern Australia. Generalized additive mixed models and boosted regression trees were used to model the relationship between groundwater and other environmental covariates with forest change during drought. 4. Models explained up to 44% of the variation in forest change during drought. Forests underwent the greatest declines in areas of high salinity (>6000 lS cm À1 ) associated with shallow groundwater depths (0-5 m). Conversely, forests in areas of lowest salinity (<2000 lS cm À1 ) and groundwater depths of more than 7Á5 m showed the least decline during drought. 5. In landscapes where groundwater quality is not compromised, previous studies have shown that shallow groundwater provides important drought refuges and refugia. Here, we show that when groundwater salinization has occurred, forests connected to shallow groundwater are more vulnerable to drought. In effect, salinization reduces the capacity of groundwater-connected habitats to function as drought refuges. 6. Synthesis and applications. Currently, there is an emphasis on managing environmental flows to support freshwater ecosystems and associated forests under water stress. However, delivery of environmental water is restricted to areas within a linear stream network and there is often limited capacity to deliver environmental flows during drought. Alternatively, a focus on drought refuges and refugia and processes important for maintaining groundwater quality (e.g. catchment revegetation to reduce shallow groundwater salinization) may better allow drought effects to be managed across a catchment, without directly focusing on highly contested surface water resources.
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