Rainfall affects detection of invasive ant environmental DNA in water

Yellow crazy ants (Anoplolepis gracilipes) are a widespread and environmentally disruptive invasive species that, as with any ant species, can be difficult to detect when it occurs in small, incipient colonies. Targeted environmental DNA (eDNA) has a history of use for the detection of aquatic organisms, but its application for the detection of terrestrial organisms is much less explored. Following development of an assay and field validation, we conducted field and laboratory tests to determine the effect of rainfall on eDNA deposited by live and dead yellow crazy ants. Simulated rainfall experiments over soil that had been exposed to live yellow crazy ants revealed that yellow crazy ant eDNA concentration initially increased with increasing flushes, likely as water caused eDNA to unbind from the soil, before decreasing with subsequent flushes. Once in the water, yellow crazy ant eDNA degraded rapidly. Simulated rainfall over soil exposed to dead yellow crazy ants for 28 days yielded consistent positive detections even after 20 flushes, representing 4 mm rainfall events each, over two days. Sampling from creeks and rivers downstream of known infestations revealed a strong effect of recent rainfall on detection. Targeted environmental DNA technology shows promise as a tool for detecting terrestrial insects and more testing will elucidate the contexts in which it is most sensitive.