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The respiratory emission of CO₂ from roots is frequently proposed as an attractant that allows soil-dwelling insects to locate host plant roots, but this role has recently become less certain. CO₂ is emitted from many sources other than roots, so does not necessarily indicate the presence of host plants, and because of the high density of roots in the upper soil layers, spatial gradients may not always be perceptible by soil-dwelling insects. The role of CO₂ in host location was investigated using the clover root weevil 'Sitona lepidus' Gyllenhall and its host plant white clover ('Trifolium repens' L.) as a model system. Rhizochamber experiments showed that CO₂ concentrations were approximately 1000ppm around the roots of white clover, but significantly decreased with increasing distance from roots. In behavioural experiments, no evidence was found for any attraction by 'S. lepidus' larvae to point emissions of CO₂, regardless of emission rates. Fewer than 15% of larvae were attracted to point emissions of CO₂, compared with a control response of 17%. However, fractal analysis of movement paths in constant CO₂ concentrations demonstrated that searching by 'S. lepidus' larvae significantly intensified when they experienced CO₂ concentrations similar to those found around the roots of white clover (i.e. 1000 ppm). It is suggested that respiratory emissions of CO₂ may act as a ‘search trigger’ for 'S. lepidus', whereby it induces larvae to search a smaller area more intensively, in order to detect location cues that are more specific to their host plant. |
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