An unrecognized fine-scale host-plant adaptation in a leaf miner: correct dorsoventral egg orientation is essential...

Larvae of leaf miners live within leaves and feed on internal tissues, making successful entry into the leaf essential for their endophagous lifestyle. In the leaf-mining moth Acrocercops transecta, females lay eggs on the surface of host-plant leaves. Immediately after hatching, larvae penetrate the leaf directly through the eggshell, representing the only opportunity to enter host leaves. Under laboratory conditions, we observed substantial mortality caused by failure of leaf entry. The nearly transparent eggshell of A. transecta enabled observation of embryonic development, revealing that correct dorsoventral orientation, ventral side facing the leaf epidermis, is essential for successful mining, whereas inverted orientation leads to failure. We developed a novel egg transplantation method and showed that correcting inverted embryos by flip transplantation restored mining success, while inverting normally oriented embryos prevented it. Notably, inverted embryos were also observed under natural conditions, causing mortality in first-instar larvae. Furthermore, under laboratory conditions, the mortality rates differed between the horticultural host Juglans regia and the natural host J. mandshurica. These results reveal a previously unrecognized source of mortality in leaf miners and demonstrate that precise dorsoventral orientation during oviposition is essential for offspring survival, highlighting a fine-scale egg-loading mechanism shaping successful host adaptation in insects.