Floral Volatiles Controlling Ant Behaviour
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Functional Ecology 2009, 23, 888–900 doi: 10.1111/j.1365-2435.2009.01632.x FLORAL SCENT IN A WHOLE-PLANT CONTEXT Floral volatiles controlling ant behaviour Pat G. Willmer*,1, Clive V. Nuttman1, Nigel E. Raine2, Graham N. Stone3, Jonathan G. Pattrick1, Kate Henson1, Philip Stillman1, Lynn McIlroy1, Simon G. Potts4 and Jeffe T. Knudsen5 1School of Biology, University of St Andrews, Fife KY16 9TS, Scotland, UK; 2Research Centre for Psychology, School of Biological & Chemical Sciences, Queen Mary University of London, Mile End Road, London, E1 4NS, UK; 3Institute of Evolutionary Biology, School of Biology, University of Edinburgh, Kings Buildings, Edinburgh EH9 3JT, Scotland, UK; 4Centre for Agri-Environmental Research, University of Reading, Reading, RG6 6AR, UK; and 5Department of Ecology, Lund University, Solvegatan 37, SE-223 62 Lund, Sweden Summary 1. Ants show complex interactions with plants, both facultative and mutualistic, ranging from grazers through seed predators and dispersers to herders of some herbivores and guards against others. But ants are rarely pollinators, and their visits to flowers may be detrimental to plant fitness. 2. Plants therefore have various strategies to control ant distributions, and restrict them to foliage rather than flowers. These ‘filters’ may involve physical barriers on or around flowers, or ‘decoys and bribes’ sited on the foliage (usually extrafloral nectaries - EFNs). Alternatively, volatile organic compounds (VOCs) are used as signals to control ant behaviour, attracting ants to leaves and ⁄ or deterring them from functional flowers. Some of the past evidence that flowers repel ants by VOCs has been equivocal and we describe the shortcomings of some experimental approaches, which involve behavioural tests in artificial conditions. 3. We review our previous study of myrmecophytic acacias, which used in situ experiments to show that volatiles derived from pollen can specifically and transiently deter ants during dehis- cence, the effects being stronger in ant-guarded species and more effective on resident ants, both in African and Neotropical species. In these plants, repellence involves at least some volatiles that are known components of ant alarm pheromones, but are not repellent to beneficial bee visitors. 4. We also present new evidence of ant repellence by VOCs in temperate flowers, which is usually pollen-based and active on common European ants. We use these data to indicate that across a wide range of plants there is an apparent trade-off in ant-controlling filter strategies between the use of defensive floral volatiles and the alternatives of decoying EFNs or physical barriers. Key-words: ant guards, E,E-a-farnesene, evolutionary filters, extrafloral nectar, floral repel- lence, morphological floral barriers, pollen volatiles First, they have a limited potential as pollinators (Janzen Introduction: costs and benefits of ants on 1977; Ho¨ lldobler & Wilson 1990; Peakall, Handel & Beattie flowers 1991) because they are typically of small size, making them a Many plant species are able to attract ants for defence poor physical fit for the sexual parts of most flowers; their against herbivores. Some plants possess extrafloral nectaries smooth, hairless cuticles are poorly suited for pollen adhe- (EFNs) on their foliage, which attract nectar-gathering sion; and their low mobility due to winglessness makes them ants, while a subset of these species (myrmecophytic plants) less likely to effect cross-pollination. Moreover, most ants offer the ants specialized structures as shelter (domatia) and possess metapleural glands that produce anti-microbial sometimes protein bodies as food (Bentley 1977; Heil & agents, necessary for nest hygiene (Ferna´ndez-Marin et al. McKey 2003). Often, ants protect the plant from herbivores 2006), but detrimental to pollen longevity and fertility (e.g. that graze the photosynthetic tissues of leaves, but theoreti- Beattie et al. 1984; Galen & Butchart 2003). cally ants could also protect a plant’s flowers from flori- Second, ants may interfere with the plant’s effective pollin- vores. However, ants are usually unwelcome as flower ators. Aggressive ants may deter some incoming flower visi- visitors for several reasons. tors, including legitimate pollinators (e.g. Altshuler 1999; Galen 1999; Tsuji, Hasyim & Nakamura 2004; Gaume, Zach- *Correspondence author. E-mail: [email protected] arias & Borges 2005; Ness 2006; Junker, Chung & Blu¨ thgen Ó 2009 The Authors. Journal compilation Ó 2009 British Ecological Society Floral volatiles controlling ant behaviour 889 2007); notably, the invasive ant Linepithema humile is known Federle & Rheindt 2005). Dense leathery calyces (e.g. Dian- to significantly reduce the diversity of floral visitors to a wide thus) and inflated calyces (e.g. Silene) may also restrict ant range of plants (Lach 2008). Such effects will usually be detri- access to flowers, while thin and ⁄ or pendant flower stalks that mental, but can enhance outcrossing if ants make pollinators bend easily may also deter most larger ant species. move more frequently between plants (see Maloof & Inouye Other barrier defences include physically damaging struc- 2000). In addition, ants may act as nectar ‘thieves’, thereby tures such as small thorns, and penetrating or secretory tric- reducing the attractiveness of flowers to effective pollinators homes. Some Dalechampia species have moveable bracts that by removing nectar rewards, which may reduce pollinator close around the flowers at night, preventing 90% of visit frequency or duration (Galen & Geib 2007), and thus nocturnal florivory (Armbruster et al. 1997). Other floral reduce seed set. movements may have similar effects, and many diurnal or Third, ants can act as florivores themselves by harvesting post-visitation changes (flower shape, orientation, scent and certain floral structures or constituents. For example, ants colour) could be interpreted in this light, making a flower have been observed cutting the styles in Polemonium (Galen inconspicuous to ants and other enemies after pollination. 1983), partly destroying flower buds in the semi-myrmeco- In addition to barriers on stems, sepals and calyces, the cor- phyte Humboldtia (Gaume, Zacharias & Borges 2005), or olla itself may be defended internally with rings of fine hairs, destroying Cordia flowers (Edwards & Yu 2008). Izzo & Va- an extremely narrow tube or a specific constricted zone, or sconcelos (2002) demonstrated that Hirtella myrmecophila with nectary ‘lids’ so that only the tongue of a pollinator can plants produce flowers only on branches that lack the struc- reach the nectar. tures (leaf-pouch domatia) that the plant otherwise uses to For myrmecophytic plants, another architectural option is attract ants for protection against herbivory. to locate domatia well away from the flowers (Izzo & Va- Overall, ants are potentially deleterious to flowers, and sconcelos 2002; Raine, Willmer & Stone 2002), a physical fea- reducing these harmful effects on reproduction can exert sub- ture that is unusual in being inducible since domatia stantial selective pressures that favour adaptive responses by production can be triggered by the presence of ants on foliage plants. Perhaps unsurprisingly, therefore, many plant species (Blu¨ thgen & Wesenberg 2001). have adaptations to protect their valuable floral structures, which can be seen as ‘filters’ for ants (cf. Kautz et al. 2009). DECOYS AND BRIBES We propose that there are three main types: architectural bar- riers; decoys and bribes (e.g. food or lodging located some dis- Extrafloral nectaries (EFNs) occur in more than 90 angio- tance from the flowers); and chemical deterrents, often using sperm families and in some ferns (e.g. bracken, Tempel 1983), floral volatile organic compounds (VOCs). and can attract ants (and other predators) onto plants where In common with other plant defences, such filters can have they act as guards, with the EFN nectar regarded as fuel for variable properties: they may be permanent features, or tran- generalist plant protectors (Koptur 2005). Some recruited sient to coincide with flowering; they may be constitutive ants are so effective that they can provide biological control (varying mainly with plant age), or inducible and only of herbivores (e.g. Oecophylla weaver ants, Tsuji, Hasyim & appearing in response to ant presence or herbivore damage Nakamura 2004). (e.g. Heil 2002, 2004); and they may be broad-spectrum filters Herbivory (and hence potential selection for guards) is against any ants, or highly specifically targeted (especially in much older than insect pollination, so EFNs may well pre- myrmecophytes). Currently, the roles and interactions of date floral nectaries. In this light, we hypothesize that EFNs plant defensive filters in floral protection are relatively unex- may have evolved an important secondary role as attractive plored. Here, we review the current state of knowledge about decoys to keep ants away from the floral nectar and out of the filters that may protect flowers against ants. Initially, we flowers in both myrmecophytes and non-myrmecophytes briefly survey the ecology of architectural barriers and decoys (Wagner & Kay 2002). To tailor the bribe to its target, ant- and identify their limitations as filters, before turning to our defended plants sometimes offer EFN nectar with reduced main theme, which is the role of VOCs. sucrose (resulting from high invertase levels) and ⁄ or higher amino acid levels relative to floral nectars, both features that are often preferred by ants (Wagner & Kay 2002; Heil, ARCHITECTURAL