Plant nutrient status: implications for pest control

Understanding how insect pests such as phloem feeding aphids interact with their host plants is crucial for understanding their population dynamics and developing effective methods for their control. A series of interrelated topics including host plant selection, plant defence mechanisms, nutritional quality of different plant species and the impact of elevated temperature and CO₂  have been investigated in the aphid - plant relationship. Precise quantification of the nutrient concetration in the phloem sap on which the aphids feed has seldom been achieved. Consequently, the way in which environmental factors such as temperature and photoperiod affect the nutritional quality of the phloem sap to aphids has been largely overlooked.

Our studies study aim to answer a range of questions using a variety of techniques now established at Birmingham. Both crop and weed hosts are grown hydroponically under a range of environmental conditions. Phloem sap is collected from the plants using aphid stylectomy. The sap is then analysed for dissolved solutes (osmometry), amino acids (high performance liquid chromatography), inorganics (X-ray microanalysis) and sugars. The quality and quantity of honeydew produced can also be analysed using the same techniques as for phloem sap. Aphid behaviour on different plants is monitored by electrical penetration graphs, identifying the frequency of feeding and the source tissue (phloem or xylem). Aphid performance is measured by monitoring the developmental rate and fecundity of individual aphids caged to plants. In addition, the effect of phloem sap composition on the overwintering ability of aphids and the contribution of the aphids' symbionts in impoving the quality of phloem sap of differing quality will be investigated.

Insect – plant interactions and climate change: the importance of water relations.

Although the responses of plants, as individuals, populations and assemblages, to climate change have been the subject of considerable research, the effects on their associated insect herbivores are little understood. Since 1993, a manipulative field experiment on Upper Seeds, Wytham (a TIGER flagship site) has been addressing the impact of climate change on a calcareous grassland community. Two climate change scenarios were imposed: warmer winters with either wetter or dryer summers. The warming of 3°C is applied from November – April by heating cables at the soil surface; complete summer drought is applied from July – August by mobile, non-invasive rainshelters; ambient summer rainfall is supplemented by 20% from June – September by a deionised spray. The focus of this experiment has been the vegetation and the inveretebrates. Manipulations began in 1993/94 and the experiment is on-going. Total sward cover has followed a consistent pattern, being greatest under rainfall supplementation and lowest under drought. Winter warming has a transient effect that is soil moisture dependent. The treatments are having direct and indirect effects on the invertebrates. Winter warming has a direct effect on insect overwintering strategies, activity and phenological associations with host plants. The rainfall manipulations, however, have an indirect effect. Supplementation leads to a lusher sward that supports a higher herbivore abundance. Drought, however, leads to a deporporate sward cover, but insect herbivores have consistently high populations. For example, the Auchenorrhyncha (plant and leaf hoppers), where 37 species have been recorded to date from the treatments, shows a population increase under drought (Masters et al. 1998). These sucking insects may be utilising increased nutrient levels (as a consequence of drought) within the plant’s vascular bundles and mesophyll cells, thus leading to higher numbers as a consequence of increased performance related to food quality.

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