Mining larvae perish when their leaf wilts or is absconded prermaturely. It has been found in several studies that mined leaves are earlier to fall than unmined ones, but is does not seem plausible that this is a defense mechanism of the plant (Faeth, Connor & Simberloff, 1981a; James & Pritchard, 1988a; Preszler & Price, 1993a).
The behaviour of the weevil Caenorhinus pauxillus is rather unusual; the female gnaws the petiole of a leaf on which she has oviposited, causing the leaf to wilt and drop. The larva mines and develops successfully in the fallen leaf (Blommers & Vaal, 2002a).
Older parts of a mine may be filled with a sugary or granular substance. The micrsocope shows that it consists of inflated, more or less globular cells. This is a tissue that is made by the plant as a reaction to wounds: callus (Hering, 1951c; Jeanneau, 1972a).
Scaptomyza flava part of a corridor, filled with callus
Opened mine with frass grains, surrounded by beginning callus tissue. This foto shows that callus formation is stimulated by the presence of frass.
Also Lepidoptera mines may contain developing callus.
Stigmella alnetella on Alder: oldest part of the mine
callus as a threat
In some cases the growth of the callus can be so fast that it threatens the larva (Hering, 1962a). In a plant as Holly, with its leathery epidermis larvae sometimes are squeezed to death between the epidermis and the growing callus (Ellis, 2000a). But sometimes also failed mines in soft leaves seem to testify of a larva that fled unsuccessfully for the developing callus in its corridor.
Chrysoesthia sexguttella on Fat Hen