Functional Ecology 2008, 22, 801– 807 doi: 10.1111/j.1365-2435.2008.01440.x © 2008 The Authors. Journal compilation © 2008 British Ecological Society Blackwell Publishing Ltd Simulated herbivory induces extrafloral nectary production in Prunus avium C. E. Pulice and A. A. Packer* Department of Biology, Susquehanna University, 514 University Avenue, Selinsgrove, PA 17870, USA Summary 1. Induced chemical responses to herbivory have been widely documented, however less attention has been given to the induced morphological defences, particularly those that mediate protection mutualisms. One of the most widely studied mutualisms is the ant–plant protection mutualism, in which ants and other predatory arthropods defend plants against herbivores in exchange for food or housing. 2. Chemical and morphological traits that mediate this protection mutualism attract beneficial partners to plants. Thus induction of traits can lead to greater protection by mutualists when herbivores are present. In this study we examined whether extrafloral nectaries (EFNs) are induced in response to simulated herbivory in Prunus avium. 3. In a greenhouse experiment, continuous damage to seedlings was simulated in two treatments intended to mimic different types of herbivores: (i) 50% of the area of each leaf was removed using a paper hole punch (e.g. insect herbivore simulation), and (ii) 50% of the area of each leaf was removed using scissors (e.g. browsing mammal simulation). Seedlings in the control group were not damaged. 4. Post-treatment, damaged plants produced significantly more EFNs per leaf on pre-existing leaves, as well as those that emerged following the onset of damage, than plants in the control group. Regardless of treatment, leaves emerging earlier supported fewer EFNs than leaves emerging later in the experiment. 5. Allocation to defence by seedlings, which have limited resources available, suggests the protection mutualism could be important to plant fitness in the field. Further study of patterns of EFN induction in P. avium seedlings is needed to better understand the significance of this induced response. Key-words: artificial damage, extrafloral nectaries, induced response, mutualism, Prunus avium, simulated herbivory Introduction Plants have evolved a wide variety of morphological and chemical traits in response to selective pressures imposed by herbivores and other natural enemies. Such traits may directly protect plants from herbivores (e.g. spines, trichomes or tannins), or they may offer indirect protection, by attracting the herbivores’ enemies to the plant (e.g. domatia, herbivore- induced release of volatile chemicals, or extrafloral nectaries (EFNs)). Regardless of whether defence mechanisms are direct or indirect, defence against natural enemies is assumed to require costly investments on the part of the plant (McKey 1974; Rhoades 1979; Zangerl & Bazzaz 1992). Thus in the absence of natural enemies, better defended plants may have lower fitness (Baldwin 1998). Because herbivores are spatially and temporally variable within and between communities (Lowman 1985), the damage they inflict on plants can be unpredictable. When faced with unpredictable threats, induced defences (those which can be initiated following damage) may be favoured by selection (Zangerl & Rutledge 1996; Harvell & Tollrian 1999). Unlike constitutive defences, induced defences require that the plant invest in defensive traits only when there is a risk of damage. In many species, induced defences are initiated only after the onset of an attack (Karban & Meyer 1989; Karban & Baldwin 1997), although induced defences have also been documented in undamaged plants that have detected damage to neighbouring plants via airborne chemical cues (Baldwin & Schultz 1983; Karban et al. 2000; Kost & Heil 2006). *Correspondence author. E-mail: packer@susqu.edu