The Hawaiian Beet Webworm (Spoladea recurvalis) is found worldwide, but mainly in the tropics.
It is a small moth with a wingspan of 22–24 mm.
They feed on the underside of the leaves.
I found quite a number of individuals in the weeds next to the lake at Atashika, where they were mostly sitting on the underside of the leaves (3rd photo) .

Atashika, Mie Prefecture, Japan, 28 September 2018