Interaction between individuals during the colonisation of Douglas fir needles by sistentes of Adelges cooleyi

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Abstract
A laboratory study of Adelges cooleyi sistentes emerging from diapause shows that the sistentes redistribute themselves over the needles in the form of aggregations. At low densities needle bases are preferred to the tips, this preference becoming less marked as sistentes density increases. Within each needle aggregation a certain amount of ‘spacing out’ occurs which minimises the number of sistentes settling in close proximity. Over a given period of time adult sistentes produce more eggs at the base of the needles than at the tips. The settling behaviour ensures that at low densities the most favourable parts of the needle at the base are utilised first while the ‘spacing out’ factor provides sufficient space for the development of the egg masses and minimises the possibility of over-exploitation of one part of the needle in relation to the remainder.