Assessing the risk of containerized citrus contributing to Asian citrus psyllid (Diaphorina citri) spread in California: residence times and insecticide residues at retail nursery outlets.
For phytophagous insects and plant pathogens, the unregulated movement of plant material can inadvertently promote long-distance spread, facilitating biological invasions. Such human-assisted spread has contributed to the invasion of the Asian citrus psyllid (Diaphorina citri), a vector of the pathogens associated with huanglongbing. Following the detection of D. citri in California, regulations were instituted to limit movement of D. citri host plants, by mandating insecticide treatments of citrus nursery stock, and limiting the amount of time host plants can reside at retail sites. We used a set of surveys and a field experiment to evaluate how well these steps mitigate the threat of containerized citrus playing a role in D. citri spread. A qualitative analysis of data collected by state regulators throughout Southern California found that containerized citrus may reside at retail sites for extended durations, in extreme cases upwards of 2 years post treatment. More detailed surveys at nearly 30 retail sites in Southern California showed that the majority of citrus plants were present past the 90 day regulatory limit, 33% had been treated more than 1 year prior, and 90% had imidacloprid residues below those known to be effective against D. citri nymphs. A field experiment confirmed that imidacloprid residues in trees grown in containers were affected by citrus species, watering level, soil mix, and time since treatment. Overall, plants had D. citri-effective residues for approximately 12 weeks, suggesting that imidacloprid treatments should protect the majority of containerized citrus against D. citri for approximately the duration of the 90 day regulatory limit. To further protect trees from infestation, nurseries should be encouraged to adopt practices that maximize the effectiveness of insecticide treatments, including ways to reduce residence times of host plants at retail sites.