Low temperature mortality of the peach-potato aphid Myzus persicae

ABSTRACT.

• 1 The mean supercooling points of first instar and adult Myzus persicae (Sulzer) maintained at 20°C and cooled at 1°C min^?1 were ?26.6 and ?25.0°C respectively.
• 2 The LT₅₀ (temperature) of the same age groups drawn from the same population and cooled at the same rate were ?8.1 and ?6.9°C, indicating extensive pre-freeze mortality in M.persicae under laboratory conditions.
• 3 Acclimation at 10 and 5°C did not affect supercooling but depressed the LT₅₀ of both first instars and adult aphids.
• 4 Freezing of leaves during feeding did not increase mortality above that expected from the direct effects of low temperature.
• 5 The level of cold in different winters can be expressed in terms of the total number of frost days, and the frequency of abnormally cold days. Winter temperatures differ markedly in a vertical profile from the soil to the soil or grass surface, and then to the air (and foliage) above.
• 6 The time of the first record of M.persicae in suction trap samples is correlated with January and February temperatures except in the west of England and Wales. Further north December and January temperatures are relatively more important.
• 7 Winter temperatures and the resultant aphid mortality is a primary determinant of the timing of the spring migration.

     

Differential cold hardiness in adults and nymphs of the peach-potato aphid Myzus persicae

The LT₅₀ (lethal temperature) of first instar and adult stages of the peach-potato aphid Myzus persicae was lowered following long term acclimation at low temperatures.
First instars consistently showed greater cold hardiness than adult stages at each acclimation temperature, with the differential increasing as the temperature was lowered. When maintained at 5°C (the lowest acclimation regime) nymphs and adults had dLT₅₀8.3°C and 4.7°C respectively lower than those for non-acclimated individuals.
When 10°C acclimated adults were returned to 20°C, the acclimation effect was retained in full for 6 days but complete deacclimation occurred by day 10. In contrast the LT₅₀ of their progeny increased gradually from the first day of adult deacclimation towards the level of the unacclimated control over a period of 10 days.
A change in cold hardiness was observed in first instars according to their position in the birth sequence. The LT₅₀ of first-born nymphs (day 1 of reproduction) from 20°C parents was - 15.9°C rising to - 8.3°C by day 4 and remaining at this level until the end of the reproductive period.
The differential mortality between nymphs and adults observed in the laboratory was supported by the results of a field experiment. Adult aphids kept in clip-cages on a crop of oilseed rape showed greater mortality compared with those introduced as nymphs when the minimum temperature fell below -4°C for the first time in winter. At - 10°C mortality of aphids introduced as adults approached 100% whereas more than 50% of those introduced as nymphs were still alive at this temperature.     

Semiempirical modeling of abiotic and biotic factors controlling ecosystem respiration across eddy covariance sites

In this study we examined ecosystem respiration (R_ECO) data from 104 sites belonging to FLUXNET, the global network of eddy covariance flux measurements. The goal was to identify the main factors involved in the variability of R_ECO: temporally and between sites as affected by climate, vegetation structure and plant functional type (PFT) (evergreen needleleaf, grasslands, etc.). We demonstrated that a model using only climate drivers as predictors of R_ECO failed to describe part of the temporal variability in the data and that the dependency on gross primary production (GPP) needed to be included as an additional driver of R_ECO. The maximum seasonal leaf area index (LAI_MAX) had an additional effect that explained the spatial variability of reference respiration (the respiration at reference temperature T_ref=15 °C, without stimulation introduced by photosynthetic activity and without water limitations), with a statistically significant linear relationship (r²=0.52, P<0.001, n=104) even within each PFT. Besides LAI_MAX, we found that reference respiration may be explained partially by total soil carbon content (SoilC). For undisturbed temperate and boreal forests a negative control of total nitrogen deposition (N_depo) on reference respiration was also identified. We developed a new semiempirical model incorporating abiotic factors (climate), recent productivity (daily GPP), general site productivity and canopy structure (LAI_MAX) which performed well in predicting the spatio‐temporal variability of R_ECO, explaining >70% of the variance for most vegetation types. Exceptions include tropical and Mediterranean broadleaf forests and deciduous broadleaf forests. Part of the variability in respiration that could not be described by our model may be attributed to a series of factors, including phenology in deciduous broadleaf forests and management practices in grasslands and croplands.     

Photosynthesis and photoprotection in mangroves under field conditions

Net CO₂ exchange and in vivo chlorophyll fluorescence were studied in mangrove (Rhizophora stylosa) leaves at a field site in Western Australia, and leaf samples were collected for the analysis of enzymes and substrates potentially involved in anti-oxidant photoprotection. Photosynthesis saturated at 900 μmol quanta m^?2 s^?1 and at no more than 7.5 μmol CO₂ m^?2 s^?1. However, fluorescence analysis indicated no chronic photoinhibition: F_v:F_m was 0.8 shortly after sunset, and quantum efficiencies of PSII were high up to 500 μmol quanta m^?2 s^?1. Electron flow through PSII was more than 3 times higher than electron consumption through Calvin cycle activity, however, even with photorespiration and temperature-dependent Rubisco specificities taken into account. Acknowledging the growing body of literature attributing a role to antioxidant systems in photoprotection, we also assayed the activities of superoxide dismutase (SOD) and several enzymes potentially involved in H₂O₂ metabolism. Their levels of maximal potential activity were compared with those in greenhouse-grown mangroves (R. mangle), and growth chamber-grown peas. Monodehydroascorbate reductase activities were similar in all species, and glutathione reductase was lower, and ascorbate peroxidase ～40% higher, in the mangroves. SOD activities in field-grown mangroves were more than 40 times those in peas. Our results support the hypothesis that O₂ may be a significant sink for photochemically derived electrons under field conditions, and suggest an important role for O₂^? scavenging in photoprotection. However, when relative patterns are compared between species, imbalances between SOD and the other enzymes in the mangroves suggest that more components of the system (e.g. phenolics or peroxidases) are yet to be identified.     

Comparison of measured and EF5-r-derived N2O fluxes from a spring-fed river

There is considerable uncertainty in the estimates of indirect N₂O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N₂O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N₂O budget. The objectives of this research were to measure the N₂O fluxes from a spring‐fed river, relate these fluxes to the dissolved N₂O concentrations and NO₃‐N loading of the river, and to try to define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N₂O L^?1. River NO₃‐N and dissolved N₂O concentrations ranged from 2.5 to 5.3 mg L^?1 and 0.4 to 1.9 μg N₂O‐N L^?1, respectively, with N₂O saturation reaching 404%. Floating headspace chambers were used to sample N₂O fluxes. N₂O‐N fluxes were significantly related to dissolved N₂O‐N concentrations (r²=0.31) but not to NO₃‐N concentrations. The N₂O‐N fluxes ranged from 38 to 501 μg m^?2 h^?1, averaging 171 μg m^?2 h^?1 (±SD 85) overall. The measured N₂O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N₂O present in the groundwater that fed the river.     

Effects of an experimental drought on the functioning of a cacao agroforestry system,Sulawesi, Indonesia

Agroforestry systems may play a critical role in reducing the vulnerability of farmers' livelihood to droughts as tree‐based systems provide several mechanisms that can mitigate the impacts from extreme weather events. Here, we use a replicated throughfall reduction experiment to study the drought response of a cacao/Gliricidia stand over a 13‐month period. Soil water content was successfully reduced down to a soil depth of at least 2.5 m. Contrary to our expectations we measured only relatively small nonsignificant changes in cacao (?11%) and Gliricidia (?12%) sap flux densities, cacao leaf litterfall (+8%), Gliricidia leaf litterfall (?2%), soil carbon dioxide efflux (?14%), and cacao yield (?10%) during roof closure. However, cacao bean yield in roof plots was substantially lower (?45%) compared with control plots during the main harvest following the period when soil water content was lowest. This indicates that cacao bean yield was more sensitive to drought than other ecosystem functions. We found evidence in this agroforest that there is complementary use of soil water resources through vertical partitioning of water uptake between cacao and Gliricidia. This, in combination with acclimation may have helped cacao trees to cope with the induced drought. Cacao agroforests may thus play an important role as a drought‐tolerant land use in those (sub‐) tropical regions where the frequency and severity of droughts is projected to increase.     

Spatio-temporal patterns of fish assemblages in a large regulated alluvial river

1. The River Durance, the last alpine tributary of the River Rhône, is a large, braided alluvial hydrosystem. Following large-scale regulation, flow downstream of the Serre-Ponçon dam has been maintained at 1/40th of previous annual mean discharge. To assess the effects of historical disturbances, fish assemblages and habitat use were analysed during five summers in a representative reach of the middle Durance.
2. Habitat availability and use were assessed with a multi-scale approach including the variables water depth, current velocity, roughness height of substratum, amount of woody debris and lateral/longitudinal location. Eighteen fish species were sampled by electrofishing in 289 habitat sample units.
3. Partial least square (PLS) regression showed that taxa were mainly distributed according to relationships between their total length and water depth/velocity variables. Fish assemblage composition was also related to roughness height as well as distance from the bank or to the nearest large woody debris. However, PLS regression revealed no significant differences in habitat selection between two periods of varying hydromorphological stability.
4. Fish distribution patterns and density were related to proximity to the bank and cover, indicating that local scale variables need to be considered in conservation and restoration programmes.     

Comparison of measured and EF5-r-derived N2O fluxes from a spring-fed river

There is considerable uncertainty in the estimates of indirect N₂O emissions as defined by the intergovernmental panel on climate change's (IPCC) methodology. Direct measurements of N₂O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N₂O budget. The objectives of this research were to measure the N₂O fluxes from a spring‐fed river, relate these fluxes to the dissolved N₂O concentrations and NO₃‐N loading of the river, and to try and define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N₂O L^?1. River NO₃‐N and dissolved N₂O concentrations ranged from 2.5 to 5.3 mg L^?1 and 0.4 to 1.9 μg N₂O‐N L^?1, respectively, with N₂O saturation reaching 404%. Floating headspace chambers were used to sample N₂O fluxes. N₂O‐N fluxes were significantly related to dissolved N₂O‐N concentrations (r²=30.6) but not to NO₃‐N concentrations. The N₂O‐N fluxes ranged from 38–501 μg m^?2 h^?1, averaging 171 μg m^?2 h^?1 (±SD 85) overall. The measured N₂O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N₂O present in the groundwater that fed the river.     

Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux

Soil CO₂ efflux was measured in clear‐cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO₂ effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear‐cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kg_C m^?2 yr^?1 in intact and clear‐cut plots, respectively. During the first year following harvest, residues have lost 0.79 kg_C m^?2 yr^?1. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear‐cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear‐cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear‐cut plots was estimated at 1.18 kg_C m^?2 yr^?1 whereas it was only 0.65 kg_C m^?2 yr^?1 in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.