Integrated Pest Management Strategies for Asian Citrus Psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae) and Huanglongbing in Citrus for Sarawak, East Malaysia, Borneo

Natural Infection of Murraya paniculata and Murraya sumatrana with 'Candidatus Liberibacter asiaticus' in Java

The phloem-limited bacterium 'Candidatus Liberibacter asiaticus' (CLas) is the putative causal pathogen of the severe Asiatic form of huanglongbing (citrus greening) and is most commonly transmitted by the Asiatic citrus psyllid Diaphorina citri. CLas severely affects many Citrus species and hybrids and has been recorded in the Citrus relative, orange jasmine, Murraya paniculata (L.) Jack (syn. M. exotica L.). In this study, 13 accessions of three Murraya species (M. paniculata, M. sumatrana Roxb., and M. lucida [G.Forst.] Mabb.) and the Papuan form of a putative hybrid (M. omphalocarpa Hayata) were identified morphologically and molecularly based on sequence identity of the matK-5'trnK region of the chloroplast genome, and infection on these plants under field conditions was determined by PCR and quantitative real-time PCR (qPCR) on two to four occasions over 14 months. CLas was repeatedly detected in leaflet midribs by PCR and qPCR on four and three accessions of M. paniculata and M. sumatrana, respectively. It was not detected in leaflet midribs of single accessions of M. lucida and M. omphalocarpa. The species identification of the CLas-positive accessions was further confirmed using all the molecular taxonomic markers consisting of the six fragments of the maternally inherited chloroplast genome and part of the nuclear-encoded internal transcribed spacer (ITS) region. The results indicated that natural infection of M. paniculata and M. sumatrana with CLas can occur in Java. To our knowledge, this is the first demonstration of the natural infection of M. sumatrana with CLas. Further studies are required to determine whether infections persist in the absence of D. citri.

Detection of Candidatus Liberibacter asiaticus and five viruses in individual Asian citrus psyllid in China

Introduction

Asian citrus psyllid (ACP, Diaphorina citri) is an important transmission vector of "Candidatus Liberibacter asiaticus" (CLas), the causal agent of Huanglongbing (HLB), the most destructive citrus disease in the world. As there are currently no HLB-resistant rootstocks or varieties, the control of ACP is an important way to prevent HLB. Some viruses of insect vectors can be used as genetically engineered materials to control insect vectors.

Methods

To gain knowledge on viruses in ACP in China, the prevalence of five RNA and DNA viruses was successfully determined by optimizing reverse transcription polymerase chain reaction (RT-PCR) in individual adult ACPs. The five ACP-associated viruses were identified as follows: diaphorina citri bunyavirus 2, which was newly identified by high-throughput sequencing in our lab, diaphorina citri reovirus (DcRV), diaphorina citri picorna-like virus (DcPLV), diaphorina citri bunyavirus (DcBV), and diaphorina citri densovirus-like virus (DcDV).

Results

DcPLV was the most prevalent and widespread ACP-associated virus, followed by DcBV, and it was detected in more than 50% of all samples tested. DcPLV was also demonstrated to propagate vertically and found more in salivary glands among different tissues. Approximately 60% of all adult insect samples were co-infected with more than one insect pathogen, including the five ACP-associated viruses and CLas.

Discussion

This is the first time these viruses, including the newly identified ACP-associated virus, have been detected in individual adult ACPs from natural populations in China's five major citrus-producing provinces. These results provide valuable information about the prevalence of ACP-associated viruses in China, some of which have the potential to be used as biocontrol agents. In addition, analysis of the change in prevalence of pathogens in a single insect vector is the basis for understanding the interactions between CLas, ACP, and insect viruses.

Effect of Systemic Insecticides Applied via Drench on the Mortality of Diaphorina citri on Curry Leaf

Huanglongbing (HLB), the most serious disease in citriculture, is caused by the bacteria Candidatus Liberibacter spp., which is transmitted by the Asian citrus psyllid (ACP) Diaphorina citri. HLB is mainly controlled with insecticides, necessitating the development of alternative methods, e.g., the use of trap plants such as curry leaf Bergera koenigii, which is highly attractive to the ACP. We evaluated the effects of the main systemic insecticides used by citrus growers, applied via drench to adults of D. citri on the curry leaf tree. We tested the persistence of three pesticides: thiamethoxam, thiamethoxam + chlorantraniliprole, and imidacloprid in protected cultivation and the field condition at 7, 14, 28, 42, 56, 70, 98, and 154 days after the application. Different concentrations of insecticides containing the active ingredient thiamethoxam were tested on adults to determine the LC₁₀ and LC₅₀. Finally, we assessed the sublethal effects on the oviposition and development of D. citri. The insecticides controlled the adults for long periods. However, in the field experiment, from 42 days after application there was a decrease in mortality caused by pesticides applied via drench, while in the protected cultivation, mortality did not decline until the last day of evaluation. The median lethal concentration (LC₅₀) for thiamethoxam was 0.031 g of active ingredient per plant, and for thiamethoxam in a mixture, the LC₅₀ was 0.028 g a.i. per plant. In the experiment with sublethal doses, D. citri did not oviposit on the treated plants. Our findings suggest that the attract-and-kill system using the curry leaf tree and systemic insecticides is effective for the control of D. citri and contributes to the integrated management of HLB.