  We present statistically significant detections at 850um of the Lyman Break
Galaxy (LBG) population at z=3, 4, and 5 using data from the Submillimetre
Common User Bolometer Array 2 (SCUBA-2) Cosmology Legacy Survey (S2CLS) in the
United Kingdom Infrared Deep Sky Survey Ultra Deep Survey (UKIDSS-UDS) field.
We employ a stacking technique to probe beneath the survey limit to measure the
average 850um flux density of LBGs at z=3, 4, and 5 with typical ultraviolet
luminosities of L(1700A)~10^29 erg/s/Hz. We measure 850um flux densities of
(0.25 +/- 0.03, (0.41 +/- 0.06), and (0.88 +/- 0.23) mJy respectively, and find
that they contribute at most 20 per cent to the cosmic far-infrared background
at 850um. Fitting an appropriate range of spectral energy distributions to the
z=3, 4, and 5 LBG stacked 24-850um fluxes, we derive infrared (IR) luminosities
of L(8-1000um)~3.2, 5.5, and 11.0x10^11 Lsun (corresponding to star formation
rates of ~50-200 Msun/yr) respectively. We find that the evolution in the IR
luminosity density of LBGs is broadly consistent with model predictions for the
expected contribution of luminous IR galaxy (LIRG) to ultraluminous IR galaxy
(ULIRG) type systems at these epochs. We also see a strong positive correlation
between stellar mass and IR luminosity. Our data are consistent with the main
sequence of star formation showing little or no evolution from z=3 to 5. We
have also confirmed that, for a fixed mass, the reddest LBGs (UV slope Beta ->
0) are indeed redder due to dust extinction, with SFR(IR)/SFR(UV) increasing by
approximately an order of magnitude over -2<Beta<0 such that SFR(IR)/SFR(UV)~20
for the reddest LBGs. Furthermore, the most massive LBGs also tend to have
higher obscured-to-unobscured ratio, hinting at a variation in the obscuration
properties across the mass range.
