The linked paper says this:

> we recommend here that ecological thresholds for neonicotinoid water concentrations need to be below 0.2 μg/L (shortterm acute) or 0.035 μg/L (long-term chronic) to avoid lasting effects on aquatic invertebrate communities

I assume the map refers to these values. μg/L is the same as parts-per-billion in case anyone was wondering.