Bonsai common pests - Vine weevils.

These evil pests are a double edged sword in multiple ways.

Both larvae and adults damage plants with the adults damaging visibly above ground not only the bark but also the leaves. The larvae do damage invisibly below ground at the roots.

The root damage is only detected when they become sufficiently compromised that the leaves wilt.

Adults are usually detected by leaf damage and are also more active at night, so the culprits may be spotted and identified then.

image source

Larvae in the form of curved creamy white legless grubs are detected by the vigilant repotter. It is always good practice to carefully examine the soil during repotting as this is an excellent time to identify and eradicate any generally invisible uninvited inhabitants.

By I, Opuntia, CC BY-SA 3.0, Link

This simplest treatment for these is to remove the offenders manually remembering that adults are most active after dark. In severe grub infestations the potting soil can be soaked with an appropriate insecticide.