Bracken, Ants and Extrafloral Nectaries. IV. Do Wood Ants (Formica lugubris) Protect the Plant Against Insect Herbivores?

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Abstract
(1) The influence of wood ants (Formica lugubris Zett.) on populations of bracken herbivores was studied at sites in North Yorkshire, England. (2) Ant-exclusion experiments at one site, using a grease-banding technique, produced no significant changes in the abundances of the insect herbivores present. (3) In contrast, experimental introductions and encounters between ants and herbivores revealed that F. lugubris can readily find and remove most external feeders within a few hours. Only sawfly larvae, Aneugmenus spp., showed any significant immunity to predation by wood ants. They exhibited reflex bleeding when attacked by ants and were apparently highly distasteful. (4) A survey of the herbivore faunas on bracken at matched sites with and without ants revealed consistently lower populations of external feeders at sites where F. lugubris was present, but the only significant difference was for the guild of sucking insects. Only two species, Aneugmenus spp. and an aphid (Macrosiphum ptericolens), were apparently immune to the ants, as were all internally feeding miners and gall formers. (5) This is the first demonstration that ants can have any, even weak, effects on populations of common bracken herbivores in contemporary time. The effectiveness of these aggressive wood ants as protectors of bracken is discussed in relation to the evolution of extrafloral nectaries and the possible mutualistic role of ants.