The potential of visual and olfactory signals in gall defence

Avoiding attacks is clearly better than suffering, or even overcoming attacks. Here we discuss the signalling ways by which galls (i.e., gall-inducers) may defend themselves from damage by avoiding attacks. Many colourful galls that are simultaneously chemically and/or physically defended, and/or omit repelling odours fulfilling the general criteria to be tentatively considered as aposematic. It has been shown experimentally that chemically defended galls also emit volatiles that repel relevant herbivores. Thus, both visual and olfactory gall traits may serve as adaptive signals that have been usually overlooked. It is also highly probable that the conspicuous colours (red, yellow) of many galls may also serve physiological functions, such as defence from reactive-oxygen production, from UV, and from excess visible light, or serve other, unknown functions. The certain role of camouflage (especially by being green), in defence from enemies, thus potentially increasing a gall-inducers’ fitness, was not given the attention it deserves. Detailed comparative and especially experimental studies on the adaptive role of gall shape, colouration and odours can further shed light on these phenomena.