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1. The acceptability of sucrose solutions to Formica lugubris Zett. in the field was tested by offering droplets of known concentrations to ants tending aphids.2. A probit model fitted to the data allowed the computation of the Median Effective Concentration (ED50) and its confidence limits.3. In spring and early summer ED50 was about 0.15 M sucrose, but the ED50 rose in 1 week at the end of June from 0.2 M to 0.9 M. The change was simultaneous on pine and birch trees. The EDSo fell gradually t o about 0.4 M in early November.4. The increase in ED5o is related to the production of the summer apterae in the aphids tended, Cinara pini (L.) on pines and Symydobius oblongw (von Heyden) on birch. This increases the 'quality' of food resource so that low sucrose concentrations can no longer compete for ant attendance. 5. The increase in EDso may limit the attendance of wood-ants at the nectaries of bracken, Pteridium aquifinum (L.) Kuhn. and at colonies of some other aphid species which are attended only in early summer.

Abstract. 1. A survey of 202 ha of planted forest in Langdale Forest, North Yorkshire, in 1972 discovered 326 occupied and thirty‐eight deserted nests of Formica lugubris Zetterstedt. The average density of occupied nests was therefore 1.61 ha‐1.2. Nests were associated with most crop trees and wild trees present, but were most abundant near to plantation margins and regenerating natural scrub.3. In planted areas 76% of nests were found 5 m or less from the plantation boundary. More nests were found on the south and west border of plantations than on the north and east respectively, and most nests were not shaded from the south or southeast.4. An analysis of nearest neighbour measurements showed that the dispersion of nests was not random but contagious (clumped). Nest mounds were higher in shaded situations than in unshaded.5. The diameter of nest mounds was greater in areas planted with trees before 1945 than in areas planted since 1945.6. The present population of F.lugubris appears to have spread into planted areas from nests present in marginal natural woodland which escaped the disturbance of the area at the time of planting.

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