In the present study we aimed to investigate whether UV-B radiation can exacerbate effects of pesticides fenoxycarb, pirimicarb, and tebufenpyrad on the survival, reproduction, and population growth rate of the standard test species Daphnia magna. We applied sublethal pesticides’ concentrations and UV doses and observed no effects on survival. However, we observed synergistic effects of UV and pesticides on both cumulative reproduction and population growth rate (21 days) for fenoxycarb (100 μg/L) and pirimicarb (10 μg/L), but a less-than-additive effect for tebufenpyrad (5–10 μg/L). In the series exposed to UV and fenoxycarb or pirimicarb, the population growth rate dropped down to 0.1, while in the control series it was around 0.3. The results indicate that concentrations of some toxicants that are nontoxic in standard tests can cause harmful population-level effects when combined with UV.