Interactions between aphids and their host plants under drought stress
Simpson, Katrina Lynn Scott
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Interactions between host plants and aphids under water stress conditions were examined. Two controlled environment room studies, a Veld experiment and phenological modelling analysis were used to investigate three different plant-aphid relationships. The aim of these studies was to determine the results of water shortage on plant and pest populations, which are likely to result under climate change scenarios. The central hypothesis was that mild drought stress would lead to greater aphid populations through a beneficial increase of small nitrogenous molecules in the phloem sap providing improved diet quality — the Plant Stress Hypothesis (White, 1969). Myzus persicae (the peach–potato aphid) is a broad-range feeder with a preference for senescent foliage. Contrary to expectations, on young cabbage plants, Brassica oleracea var capitata, it was found to increase faster and to a greater extent on well-watered hosts. Aphids were also found to prevent osmoregulation in droughted plants. Despite this, the aphid infestation led to a drought-like response in all plants irrespective of watering regime, causing significantly reduced growth. Metopolophium dirhodum (the rose–grain aphid) overwinters as eggs on Rosa spp. but most of its life-cycle is spent on grasses and cereal crops. As with M. persicae, aphid infestation caused drought-like symptoms even in well-watered plants. The mechanism of reduction in plant health is most likely through aphid damage to the efficiency of Photosystem a within 7 days of infestation. On young barley plants, Hordeum vulgare, the rose–grain aphid was found to increase faster and to greater extent on droughted hosts. In contrast to M. persicae, the study strongly supported the Plant Stress Hypothesis, Finding that even mild water stress of approximately −0.3 MPa could lead to conditions favourable to rapid aphid population increase. The difference in the response of different species of aphids may be associated more with the levels of soluble nitrogen in their diet than specifically host plant water status. Drepanosiphum platanoidis is an aphid specific to sycamore, Acer pseudoplatanus and spends its entire life-cycle on that species. In the Veld, the total number of aphids supported by a single tree was found to correlate most strongly with soluble leaf nitrogen. In addition, the number of aphids on individual leaves was found to correlate strongly and negatively with stomatal conductance and leaf water potential. Finally, analysis of historical aphid suction-trap data was carried out. UK Rothamsted suction-traps are 12.2 m tall and sample air at 0.75 m3 s-1, with daily aphid sampling between April and November. The traps are designed to catch the largest proportion of aphids in the total insect sample and to reduce the effect of surrounding land-use on the sample. The area represented by a trap may be as much as 700 km2 (Harrington and Woiwod, 2007). The dataset is the longest and most geographically extensive for aphid populations, comprising 46 sites across Europe as of 2011. In this analysis, aphid and meteorological data from the Edinburgh East Craigs site were used. This also enabled comparison with D. platanoidis numbers collected in the Veld, which showed a strongly significant positive relationship between the two. Since the data collected in suction-traps are correlated with numbers in the Veld, aphid population analysis can be improved with use of these longer datasets, without reliance on shorter-term Veld surveys. The various population characteristics examined were found to be strongly correlated with each other, but were unconnected to data from previous years. The date of first catch was found to be significantly advancing over time, with strong influences from daily minimum temperatures and vapour pressure deficits of the air within the year. Higher temperatures with lower humidity tend to bring forward the date of first catch. Lower tends to depress the population in the summer. The overall hypothesis that mild drought would benefit aphid populations cannot be accepted, but in two cases of the experiment sections of the work there were strong relationships between aphid numbers and both leaf temperature and soil moisture. The meteorological data analysis indicates that climatic changes towards a hotter, drier climate could substantially alter population dynamics. Harrington, R. and Woiwod, I. (2007). Foresight from hindsight: The Rothamsted Insect Survey. Outlooks on Pest Management, 18, 9–14. White, T. (1969). An index to measure weather-induced stress of trees associated with outbreaks of psyllids in Australia. Ecology, 50, 905–909.