A pH-responsive fluorescent nanopesticide for selective delivery and visualization in pine wood nematode control

The pine wood nematode Bursaphelenchus xylophilus is the causal agent of pine wilt disease, whose devastating epidemic has caused severe damage to pine trees. We developed a pH-responsive fluorescent nanopesticide for selective delivery and visualization in pine wood nematode control using thiamethoxam (THI), mesoporous silica nanoparticles (MSNs), and polyamidoamine (PAMAM) dendrimers. THI was encapsulated in MSNs and PAMAM dendrimers to form a THI@PAMAM@MSN nanopesticide with a THI loading rate of 21.8%. The THI released from THI@PAMAM@MSN was acid-sensitive owing to the strong protonation ability of PAMAM, with a cumulative 14-day release of 53.37% and 45.07% under acidic conditions at pH 4.5 and 5.5, respectively, which was greatly enhanced through the synthesis and accumulation of benzoic acid in nematode-infested pines. Owing to the fluorescence properties of THI@PAMAM@MSNs, we could visually demonstrate the transport of THI in pines and its effect on nematodes and vector insects. THI@PAMAM@MSNs exhibited excellent formulation stability, good wettability, and adhesion. In addition, compared with the commercially available preparation of THI@WG, THI@PAMAM@MSNs exhibited a higher transport capacity and control effect on pine wood nematodes (LC50 = 1.15 μg/mL). The control efficacy of THI@PAMAM@MSNs on pine wood nematodes in the field was 95.50%, 81.22%, and 57.87% at tree heights of 7, 6, and 5 m, respectively, and was significantly higher than that of THI@WG. The design of pH-sensitive nanocarriers increases the release of THI in infested pines, and fluorescence intensity facilitates the characterization of nanoparticles. Therefore, THI@PAMAM@MSNs can be used as a nanoformulation for controlling pine wood nematodes.