In the young leaves of the Mustard crop, irregular streaks were seen which showed attack by leaf miner. Sample taken from the area showed soft, juicy sap with irregular swollen areas and empty cells In a view the pest in side view could be spotted.
On the attacked leaf the starting and end points of the mine can be clearly marked. Probably the mine started where the egg was laid and then on hatching, the larva tunnels into the leaf making serpentine mines that are clearly visible on one side of the leaf. The mines are the routes followed as the larvae feed on the internal cells of the leaf and moves ahead. The plant cells are ingested and the dark faeces excreted into the mine The mine gradually widens as can be seen in teased sections of the leaf. The flow of sap in the tunnelled mines was recorded. Another important feature seen was proliferation of cells within leaf tissue.
The literature says Mining insects are those whose larvae live in and feed on soft, parenchymatous plant tissue between the epidermal layers. While feeding, the mining insect forms a characteristic, externally visible tunnel called as mine, which appears as a white track on the leaf. Workers report these canals can assume a variety of shapes depending on the species involved and the tissue mostly consumed is the palisade parenchyma of mesophyll. Mining insects can be Lepidopterans, Hymenopterans, Coleopterans, and Dipterans (Dempewolf 2005).
Literature reports origin of gall-like structures in some leaf-mines. As per the reference article cited certain mining insects lead to formation of highly differentiated tissue (called nutritive tissue) for enhanced nutritive quality Mine inflation is due to the proliferation of this callus mass and the authors have used the word Mine gallers for such insects. (https://journals.plos.org/plosone/article id=10.1371/journal.pone.0209485 )