Diaphorina citri (Hemiptera: Psylloidea) in China: Two Invasion Routes and Three Transmission Paths

The loss of Halloween gene function seriously affects the development and reproduction of Diaphorina citri (Hemiptera: Liviidae) and increases its susceptibility to pesticides

The citrus industry has suffered severe losses as a result of Huanglongbing spread by Diaphorina citri. Controlling the population of D. citri is the key to preventing and controlling the spread of Huanglongbing. Ecdysteroids are key hormones that regulate insect development and reproduction. Therefore, the Halloween gene family involved in the ecdysone synthesis of D. citri is an ideal target for controlling the population growth of this insect. In this study, we successfully cloned four Halloween genes expressed during D. citri development. Silencing of one of the four genes resulted in a significant decrease in 20E titers in nymphs and significant decreases in the developmental, survival and emergence rates. Inhibiting Halloween gene expression in adults impeded the growth of the female ovary, diminished yolk formation, lowered vitellogenin transcription levels, and hence impaired female fecundity. This showed that Halloween genes were required for D. citri development and reproduction. DcCYP315A1 and DcCYP314A1 were highly expressed when D. citri was exposed to thiamethoxam and cypermethrin, and silencing these two genes made D. citri more sensitive to these two pesticides. Inhibition of DcCYP315A1 and DcCYP314A1 expression not only significantly delayed the development and reproduction of D. citri but also increased its susceptibility to pesticides. Therefore, these two genes are more suitable as potential target genes for controlling D. citri.

Population Diversity of 'Candidatus Liberibacter asiaticus' and Diaphorina citri in Sichuan: A Case Study for Huanglongbing Monitoring and Interception

Citrus huanglongbing (HLB) is present in 10 provinces in China and is associated with 'Candidatus Liberibacter asiaticus' (CLas), which is transmitted by the Asian citrus psyllid (Diaphorina citri, ACP). To date, HLB and ACP have expanded to Yibin city of Sichuan Province, posing an imminent threat to the citrus belt of the upper and middle reaches of the Yangtze River, an important late-maturing citrus-producing area in China. To understand the epidemiological route of CLas and ACP in newly invaded regions of Sichuan and thereby better establish an HLB interception zone ranging from Leibo to Yibin, we evaluated the molecular variability of 19 CLas draft genomes from citrus or dodder (Cuscuta campestris). They include three type-specific prophage loci, three variable number tandem repeat loci, a miniature inverted-repeat transposable element, and population diversity of 44 ACP mitochondrial genomes. The results indicated that CLas isolates in the newly invaded area (Pingshan) were more diverse than those in the HLB endemic areas (Leibo and Ningnan). Phylogenetic analysis based on mitochondrial genomes demonstrated that ACPs in Leibo, Pingshan, and Xuzhou (rural areas) represent a new mitochondrial group (MG4), distinguished by the three unique single-nucleotide polymorphisms in cox1, nad4, and cytb. However, the ACPs sampled from the urban areas of Cuiping and Xuzhou belonged to the southeastern China group (MG2-1). Altogether, our study revealed multiple sources of ACP and CLas in the HLB interception zone and proposed their transmission route. This study contributes to the formulation of precise HLB prevention and control strategies in the HLB interception zone in Sichuan and could be useful for HLB management efforts in other regions.

Control efficacy and joint toxicity of thiamethoxam mixed with spirotetramat against the Asian citrus psyllid, Diaphorina citri Kuwayama

BACKGROUND

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, is one of the most devastating pests in citrus orchards, and has caused huge economic losses worldwide. Chemical control is the most effective way for psyllid control. Herein, the toxicity of nine insecticides to ACP adults and the joint action of thiamethoxam + spirotetramat were determined by a topical application method in the laboratory; field plot experiments were conducted to evaluate the control efficacy of one self-made thiamethoxam + spirotetramat 40% suspension concentrate (SC) comparing with thiamethoxam 21% SC, spirotetramat 22.4% SC, tolfenpyrad 15% SC and bifenthrin 100 g/L emulsifiable concentrate against ACP using foliar sprays in 2018-2019.

RESULTS

The highest toxicity to ACP adults was achieved by beta-cyfulthrin, bifenthrin, thiamethoxam and acetamiprid, with median lethal doses of 0.247 to 1.382 ng/adult at 24 h after treatment. High toxicity was observed by chlorpyrifos, spirotetramat and tolfenpyrad, but moderate toxicity by pyriproxyfen and buprofezin. For mixutres of thiamethoxam and spirotetramat, a 25:15 mass ratio showed the highest synergistic effect, with a co-toxicity coefficient (CTC) of 246.52; while a 10:30 mass ratio exhibited an additive effect, with a CTC of 109.84. Thiamethoxam + spirotetramat 40% SC at 60-80 mg/kg can effectively control ACP with a control efficacy of 72.92 to 99.29% during 3-30 days. Moreover, foliar sprays of all tested insecticides at the tested rates had no phytotoxic effects on citrus trees.

CONCLUSION

A one-time foliar spray of thiamethoxam + spirotetramat 40% SC at 80 mg/kg could be recommended to control ACP during its infestation period in citrus groves.

Knockdown of the Trehalose-6-Phosphate Synthase Gene Using RNA Interference Inhibits Synthesis of Trehalose and Increases Lethality Rate in Asian Citrus Psyllid, Diaphorina citri (Hemiptera: Psyllidae)

Simple Summary

In this study, we cloned and characterized a trehalose-6-phosphate synthase (TPS) gene from D. citri (DcTPS) for the first time. Meanwhile, we used RNA interference (RNAi) technology to efficiently disrupt DcTPS gene function in order to elucidate its role in the growth and development of D. citri. Our results suggest that dsRNA-mediated gene-specific silencing resulted in a strong reduction in relative expression of DcTPS and survival rate of nymphs, as well as an increase in malformation. This work was undertaken to establish a foundation for further research on the functions of D. citri trehalose-6-phosphate synthase. This will provide a new target for the control of D. citri in the field.

Abstract

Diaphorina citri Kuwayama is the vector of citrus “huanglongbing”, a citrus disease which poses a significant threat to the global citrus industry. Trehalose-6-phosphate synthase (TPS) plays an important role in the regulation of trehalose levels of insects, while its functions in D. citri are unclear. In this study, full-length cDNA sequences of the TPS gene from D. citri (DcTPS) were cloned and its expression patterns at various developmental stages were investigated. The results indicated that DcTPS mRNA was expressed at each developmental stage and the highest DcTPS expression was found in the fifth-instar nymphs of D. citri. Additionally, mortality and deformity of D. citri were observed after 24 and 48 h by feeding with three different dsRNA concentrations (20, 100 and 500 ng/μL). The results indicated that DcTPS expression was declined, and mortality and malformation in nymphs were increased via feeding with dsDcTPS. Moreover, the enzyme and trehalose content were decreased, while the content of glucose was significantly higher than that of untreated (control) individuals. This suggests that DcTPS might be vital for the growth and development of D. citri and further studies of the genes should be related to molting and metabolism for controlling D. citri.