Sugary secretions of wasp galls : a want-to-be extrafloral nectar?
Fecha
2017Autor
Aranda Rickert, Adriana Marina
Rothen, Carolina Paola
Diez, Patricia Alejandra
González, Ana María
Marazzi, Brigitte
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Background and Aims The most widespread form of protective mutualisms is represented by plants bearing
extrafloral nectaries (EFNs) that attract ants and other arthropods for indirect defence. Another, but less common,
form of sugary secretion for indirect defence occurs in galls induced by cynipid wasps. Until now, such galls have
been reported only for cynipid wasps that infest oak trees in the northern hemisphere. This study provides the first
evidence of galls that exude sugary secretions in the southern hemisphere and asks whether they can be considered
as analogues of plants’ EFNs.
Methods The ecology and anatomy of galls and the chemical composition of the secretion were investigated in
north-western Argentina, in natural populations of the host trees Prosopis chilensis and P. flexuosa. To examine
whether ants protect the galls from natural enemies, ant exclusion experiments were conducted in the field.
Key Results The galls produce large amounts of sucrose-rich, nectar-like secretions. No typical nectary and sub-
nectary parenchymatic tissues or secretory trichomes can be observed; instead there is a dense vascularization with
phloem elements reaching the gall periphery. At least six species of ants, but also vespid wasps, Diptera and
Coleoptera, consumed the gall secretions. The ant exclusion experiment showed that when ants tended galls, no dif-
ferences were found in the rate of successful emergence of gall wasps or in the rate of parasitism and inquiline in-
festation compared with ant-excluded galls.
Conclusions The gall sugary secretion is not analogous to extrafloral nectar because no nectar-producing structure
is associated with it, but is functionally equivalent to arthropod honeydew because it provides indirect defence to
the plant parasite. As in other facultative mutualisms mediated by sugary secretions, the gall secretion triggers a
complex multispecies interaction, in which the outcome of individual pair-wise interactions depends on the ecologi-
cal context in which they take place.
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