Leaf mines as visual defensive signals to herbivores

Leaf‐mining insects produce conspicuous and distinct leaf mines on various types of plant leaves. The diversity of leaf‐mine morphology has typically been explained by several factors, such as selective feeding on plant tissues, improvement of microclimate, faecal disposal, reduction in the efficiency of parasitoid search behaviour and leafminer phylogeny. Although these factors are certainly associated with mining patterns, masking the mines, rather than making them conspicuous, appears to be more advantageous for deterring parasitoids and predators of leafminers. However, here, I propose that prominent leaf mines may serve to signal or cue herbivores to avoid feeding on the mined leaves. Because most leafminers are sessile and complete their development within a single leaf, herbivory of mined leaves is detrimental to leafminer survival. Other herbivores appear to avoid consuming mined leaves for a variety of reasons: leaf mines mimic leaf variegation or mottling; mined leaves induce chemical and physical defences against herbivores; and leaf mines mimic fungal infection, animal excrement, and necrosed plant tissues. Hence, natural selection may have favoured leafminers that produce conspicuous mines because of the increased survival and fecundity of thereby reducing herbivory on mined leaves.