Influence of Tree Size and Application Rate on Expression of Thiamethoxam in Citrus and Its Efficacy Against Diaphorina citri (Hemiptera: Liviidae)

Comparison of the effectiveness of thiamethoxam and its main metabolite clothianidin after foliar spraying and root irrigation to control Myzus persicae on peach

The green peach aphid, Myzus persicae, is one of the most economically important pests in peach-growing areas around the world. In many countries, the application of insecticides is the main method to control and reduce the population of M. persicae. In this study, we investigated the effects and persistence of thiamethoxam against M. persicae by foliar spraying and root irrigation. The residues of thiamethoxam and clothianidin in peach were determined to assess food safety. The results showed that thiamethoxam treatment significantly reduced the population of M. persicae through foliar spraying and root irrigation. And the persistence of root irrigation on M. persicae was significantly longer than that of spraying. Thiamethoxam and clothianidin were absorbed by the roots, transported to other parts of the plant, and concentrated in the leaves, especially new leaves. The final residues of thiamethoxam and clothianidin in peaches were below the maximum residue limit (MRLs). These results suggested that thiamethoxam is more effective in M. persicae control through root irrigation than foliar spraying. The persistence of root irrigation on M. persicae was significantly longer than that of spraying. These results shed some light upon the control of M. persicae by root irrigation of thiamethoxam.

Uptake of soil-applied thiamethoxam in orange and its effect against Asian citrus psyllid in different seasons

BACKGROUND

Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is an important pest of citrus worldwide because it transmits the bacteria causing huanglongbing (HLB). We investigated the effects and persistence of two soil application rates of thiamethoxam on ACP populations in two flushing seasons in the field. Thiamethoxam and clothianidin residues in the fruit were detected to evaluate food safety.

RESULTS

Soil application of 50% thiamethoxam water-dispersible granules at concentrations of 4 and 10 g tree^-1 significantly decreased ACP populations, and there was a positive correlation between control efficacy and the persistence of thiamethoxam and clothianidin in leaves, providing longer-term protection for up to 90 days in the fall compared with 60 days in the spring. Higher thiamethoxam and clothianidin amounts were observed in new leaves than in old leaves. Thiamethoxam and clothianidin residues at a high rate in fruit were 0.012 and 0.010 mg kg^-1 at harvest, respectively, and neither insecticides was detectable at low rates.

CONCLUSIONS

These results demonstrate that soil-applied thiamethoxam plays a role in defending ACP, and provides an extended period of control efficacy. This knowledge could provide a reference for the control of ACP by soil application of thiamethoxam to reduce HLB spread. © 2018 Society of Chemical Industry.

Evaluating the Effect of Imidacloprid Administered in Artificial Diet on Feeding Behavior of Diaphorina citri (Hemiptera: Liviidae) Using Electropenetrography

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the vector of Candidatus Liberibacter asiaticus (CLas), the presumed cause of Huanglongbing (HLB) in citrus. Management strategies were developed in Florida that used soil-applied neonicotinoids to protect young trees. Despite the implementation of intense management programs, infection spread among the most intensively managed groves. We used electopenetrography to test five imidacloprid doses (0.55, 5.5, 55, 550, and 5,500 ppm) administered in artificial diet to approximate the dosage required to reduce feeding activity and prevent salivation/ingestion activity. We failed to detect a significant effect of 0.55 ppm imidacloprid on probing behavior, pathway, or salivation/ingestion activity when compared with the untreated control. We observed a significant reduction in the number of probes and the number of pathway with both 5.5 and 55 ppm imidacloprid. We detected a significant reduction in the number of salivation/ingestion events at both 5.5 ppm and 55 ppm imidacloprid (57 and 54 percent, respectively) compared with the untreated control, and a reduction in number of sustained (>600 s) salivation/ingestion at 55 ppm. While reductions in feeding activity were apparent at dosages of at least 5.5 ppm, we were unable to prevent salivation/ingestion with dosages as high as 5,500 ppm, which is greater than what is known to occur following application in the field. While soil-applied imidacloprid may slow the spread of CLas, our findings suggest that prevention of CLas inoculation in the field is unlikely. Management strategies must be refined to prevent the spread of HLB in Florida.

Spatial and Temporal Distribution of Soil-Applied Neonicotinoids in Citrus Tree Foliage

Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the insect vector of Candidatus Liberibacter asiaticus (CLas), the presumed cause of huanglongbing (HLB) in citrus (Rutaceae). Soil-applied neonicotinoids are used to manage vector populations and thus reduce the spread of HLB in Florida citrus. Studies were conducted in the greenhouse and field to quantify the spatial and temporal distribution of three neonicotinoid insecticides within individually sampled leaves and throughout the tree canopy. Following field application, no difference in parent material titer was observed between leaf middles versus leaf margins following application of Platinum 75SG or Belay 2.13SC; however, imidacloprid titer was higher in leaf margins than leaf middle following application of Admire Pro. The bottom region of trees contained more imidacloprid than other regions, but was not different from the spherical center region. In the greenhouse, imidacloprid and clothianidin titers peaked 5 wk following application of Admire and Belay, respectively, and thiamethoxam titer peaked 3 wk after application of Platinum. There was no effect of leaf age on uptakes of any insecticides tested. Titers of soil-applied neonicotinoids quantified in the field failed to reach known levels required to kill D. citri. Exposure of D. citri to sublethal dosages of neonicotinoids is of concern for HLB management because of possible failure to protect treated plants from D. citri and selection pressure for development of neonicotinoid resistance. Our results suggest that current soil-based use patterns of neonicotinoids for D. citri management may be suboptimal and require reevaluation to maintain the utility of this chemical class in citrus.