Early Detection of the Invasive Alien Aquatic Plant Alternanthera philoxeroides (Alligator Weed) in Agricultural Catchments Using Loop‐Mediated Isothermal Amplification

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ABSTRACT Early detection of invasive alien species is critical for preventing ecological disturbance and economic loss. Alternanthera philoxeroides , an invasive aquatic plant native to South America, has become widespread in Japan, posing major threats to paddy‐field agriculture. Conventional monitoring depends largely on visual surveys, which are inefficient for identifying low‐density populations or fragmented plant debris. This study aimed to develop simple, field‐deployable methods for A. philoxeroides detection using (1) environmental DNA (eDNA) analysis based on the suspended glass fiber (SGF) method combined with loop‐mediated isothermal amplification (LAMP), and (2) LAMP‐based detection using pressed sap from weed debris collected at drainage pumping stations. The SGF + LAMP approach reliably detected A. philoxeroides eDNA at most sites within its known distribution range, whereas pressed sap–based analysis detected A. philoxeroides even in severely decomposed debris and at weight ratios as low as 0.1%. Both methods were validated in a catchment with no prior A. philoxeroides records, confirming its establishment along riverbanks and subsequent invasion in paddy fields. Collectively, these approaches enable rapid, sensitive, on‐site detection without requiring specialized equipment or taxonomic expertise, providing practical tools for the early monitoring and management of A. philoxeroides and other invasive plants in agricultural water networks.

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