  While we have learned much about Ultra-Diffuse Galaxies (UDGs) in groups and
clusters, relatively little is known about them in less-dense environments.
More isolated UDGs are important for our understanding of UDG formation
scenarios because they form via secular mechanisms, allowing us to determine
the relative importance of environmentally-driven formation in groups and
clusters. We have used the public Kilo-Degree Survey (KiDS) together with the
Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) to constrain the abundance
and properties of UDGs in the field, targeting sources with low surface
brightness (24.0$\leq$\bar{\mu}_{e,r}}$\leq$\26.5) and large apparent sizes
(3.0\arcsec$\leq$\bar{r}_{e,r}}$\leq$8.0\arcsec). Accounting for several
sources of interlopers in our selection based on canonical scaling relations,
and using an empirical UDG model based on measurements from the literature, we
show that a scenario in which cluster-like red sequence UDGs occupy a
significant number of field galaxies is unlikely, with most field UDGs being
significantly bluer and showing signs of localised star formation. An immediate
conclusion is that UDGs are much more efficiently quenched in high-density
environments. We estimate an upper-limit on the total field abundance of UDGs
of 8$\pm$3$\times10^{-3}$cMpc$^{-3}$ within our selection range. We also
compare the total field abundance of UDGs to a measurement of the abundance of
HI-rich UDGs from the literature, suggesting that they occupy at least
one-fifth of the overall UDG population. The mass formation efficiency of UDGs
implied by this upper-limit is similar to what is measured in groups and
clusters.
