Diaphorina Citri Research Articles

Salivary effector DcE1 suppresses plant defense and facilitates the successful feeding of Asian citrus psyllid, Diaphorina citri.

Piercing-sucking insects secrete diverse repertoires of effectors into their hosts to weaken host defenses and promote infestation. The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most destructive insect pest in citrus orchards because of its role as a vector for the huanglongbing pathogen, Candidatus Liberibacter asiaticus (CLas). However, specific effector proteins and their functions in D. citri remain unclear. We demonstrate that DcE1, a salivary protein gene from D. citri, is predominantly expressed in the salivary gland tissues and is delivered into host plants during feeding. Transient expression in tobacco leaves revealed that DcE1 was subcellularly localized in the cytoplasm and plasma membrane, where it inhibited BAX- and INF1-induced cell death, suppressed callose deposition, and activated the salicylic acid pathway by upregulating the expression of endo-β-1,3-glucanase NtBGL2 and regulatory protein NtNPR1. Further, DcE1 knockdown by double-stranded RNA (dsRNA) injection decreased the survival rates of D. citri and interrupted D. citri phloem-feeding on host plants. These results indicate that DcE1 is a novel effector that promotes plant susceptibility and enables D. citri feeding. These findings enhance our understanding of D. citri-plant interactions and offer a potential new target gene for the development of citrus protection strategies. © 2024 Society of Chemical Industry.

Evaluation of sampling methods of Diaphorina citri Kuwayama (Hemiptera: Liviidae) in two citrus growing areas

Efficient, easy to implement and low-cost monitoring methodologies are necessary to obtain information on arthropod pest populations and to implement the most convenient and timely phytosanitary control practices. To optimize the sampling and monitoring of the Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), an insect associated with the transmission of Candidatus Liberibacter asiaticus and Ca. L. americanus, which cause Huanglongbing (HLB), a disease with a great impact on citrus orchards, three methods of adult sampling (yellow sticky traps, sweep net, and stem tap) were evaluated. For immature sampling, vegetative shoots were checked. The results demonstrate that the population density at the time of sampling affects the effectiveness and sensitivity of the sampling methods. Yellow sticky traps capture more adults and are the only effective method at low psyllid densities. Stem tap and sweep net are less expensive methods; however, they do not detect adults nor correlate with the number of nymphs and eggs in vegetative shoots when adult density is low. For adults, an optimal sample size was determined for each method. For yellow sticky traps, 3 to 5 traps for a 2-hectare plot with weekly frequency are recommended. For immatures, it is recommended to estimate the percentage of infestation by inspecting 45 to 55 vegetative shoots well-distributed within a 2-hectare plot, as a practical measure for farmers and extensionists to monitor D. citri.

Virus-Induced Gene Silencing Simultaneously Exploits ‘Attract and Kill’ Traits in Plants and Insects to Manage Huanglongbing

Abstract The vector-borne disease Huanglongbing (HLB) causes severe economic losses to citrus production worldwide with no available cure. Herein, we applied virus-induced gene silencing technology to engineer citrus that preferentially attracted and specifically killed Diaphorina citri, the vector associated with HLB. We engineered the infectious citrus tristeza virus (CTV-T36) clone to carry three truncated genes. The triple construct (CTV-tAwd-tWnt-tPDS) produces small interfering RNAs (siRNAs) against phytoene desaturase, PDS, to yield a phenotype with visual, olfactory, and gustatory cues that preferentially attracted D. citri. In addition, siRNAs targeted two genes related to flight in D. citri, abnormal wing disc (DcAwd) and wingless (DcWnt), that caused wing malformations and decreased survival in psyllids that fed on plants inoculated with the engineered virus. During two successive generations, D. citri reared on CTV-tAwd-tWnt-tPDS-inoculated plants exhibited higher mortality across life stages as well as reduced fecundity and fertility as compared with those reared on non-infected plants or CTV-wt-inoculated plants. Furthermore, CTV-tAwd-tWnt-tPDS-inoculated plants shortened the lifespan of D. citri by more than 20 days. Morphological abnormalities were noted in those adults that did successfully emerge on plants inoculated with CTV-tAwd-tWnt-tPDS, including cocked wings with a bowl-shaped depression and/or a convex shape. Phloem sap from CTV-tAwd-tWnt-tPDS-inoculated plants decreased the survival of D. citri adults, confirming that siRNAs were present in the sap of these plants. Collectively, we provide proof of concept for a novel variant of the attract-and-kill method where the cultivated crop is potentially transformed into a hyper-attractive population and transmission sink for a phytopathogen vector.

Transcriptomic profiling of 'Candidatus Liberibacter asiaticus' in different citrus tissues reveals novel insights into Huanglongbing pathogenesis.

'Candidatus Liberibacter asiaticus' (Las) is a gram-negative bacterial pathogen associated with citrus huanglongbing (HLB) or greening disease. Las is transmitted by the Asian citrus psyllid (ACP) where it colonizes the phloem tissue, resulting in substantial economic losses to citrus industry worldwide. Despite extensive efforts, effective management strategies against HLB remain elusive, necessitating a deeper understanding of the pathogen's biology. Las undergoes cell-to-cell movement through phloem flow and colonizes different tissues in which Las may have varying interactions with the host. Here, we investigate the transcriptomic landscape of Las in citrus seed coat vasculatures, enabling a complete gene expression profiling of Las genome and revealing unique transcriptomic patterns compared to previous studies using midrib tissues. Comparative transcriptomics between seed coat, midrib and ACP identified specific responses and metabolic states of Las in different host tissue. Two Las virulence factors that exhibit higher expression in seed coat can suppress callose deposition. Therefore, they may contribute to unclogging sieve plate pores during Las colonization in seed coat vasculature. Furthermore, analysis of regulatory elements uncovers a potential role of LuxR-type transcription factors in regulating Liberibacter effector gene expression during plant colonization. Together, this work provides novel insights into the pathogenesis of the devastating citrus HLB.

Reliable Augmentation and Precise Identification of EPG Waveforms Based on Multi-Criteria DCGAN

The electrical penetration graph (EPG) technique is of great significance in elucidating the mechanisms of virus transmission by piercing-sucking insects and crop resistance to these insects. The traditional method of manually processing EPG signals encounters the drawbacks of inefficiency and subjectivity. This study investigated the data augmentation and automatic identification of various EPG signals, including A, B, C, PD, E1, E2, and G, which correspond to distinct behaviors exhibited by the Asian citrus psyllid. Specifically, a data augmentation method based on an improved deep convolutional generative adversarial network (DCGAN) was proposed to address the challenge of insufficient E1 waveforms. A multi-criteria evaluation framework was constructed, leveraging maximum mean discrepancy (MMD) to evaluate the similarity between the generated and real data, and singular value decomposition (SVD) was incorporated to optimize the training iterations of DCGAN and ensure data diversity. Four models, convolutional neural network (CNN), K-nearest neighbors (KNN), decision tree (DT), and support vector machine (SVM), were established based on DCGAN to classify the EPG waveforms. The results showed that the parameter-optimized DCGAN strategy significantly improved the model accuracies by 5.8%, 6.9%, 7.1%, and 7.9% for DT, SVM, KNN, and CNN, respectively. Notably, DCGAN-CNN effectively addressed the skewed distribution of EPG waveforms, achieving an optimal classification accuracy of 94.13%. The multi-criteria optimized DCGAN-CNN model proposed in this study enables reliable augmentation and precise automatic identification of EPG waveforms, holding significant practical implications for understanding psyllid behavior and controlling citrus huanglongbing.

'Candidatus Liberibacter asiaticus' infection alters the reflectance profile in asymptomatic citrus plants.

Huanglongbing (HLB) is the primary and most destructive disease affecting citrus, caused by a pathogen transmitted by an insect vector, Diaphorina citri. There are no curative methods for the disease, and rapid and accurate methods are needed for early detection in the field, even before symptoms appear. These will facilitate the faster removal of infected trees, preventing the spread of the bacteria through commercial citrus orchards. It was possible to determine ranges of hyperspectral bands that demonstrated significant differences in relative reflectance between treatments consisting of healthy and infected plants from the first days of evaluation, when plants infected with 'Candidatus Liberibacter asiaticus' (CLas) were still in the asymptomatic stage of the disease. From the Week 2 of evaluation [58 days after infection (DAI) of plants] until the last week, spectral differences were detected in the red edge region (660-750 nm). From the Week 6 onwards (86 DAI), spectral differences between healthy and symptomatic plants were observed in bands close to the visible region (520-680 nm). Spectral differences were detected in the leaves of C. sinensis infected by CLas before the appearance of symptoms, making it feasible to use the hyperspectral sensor to monitor the disease. Our results indicate the need for future studies to validate the use of hyperspectral sensors for managing and detecting HLB in commercial citrus orchards, contributing to the integrated management of the disease. © 2024 Society of Chemical Industry.

Confirmation of Candidatus Liberibacter asiaticus in Asian Citrus Psyllids and Detection of Asian Citrus Psyllids in Commercial Citrus in Georgia (U.S.A.).

The Asian citrus psyllid (ACP) is the vector of Candidatus Liberibacter asiaticus (CLas), the causal agent of citrus greening or Huanglongbing (HLB), one of the most devastating citrus diseases worldwide. The citrus industry in Georgia (U.S.A.) is in the process of a rapid expansion, and based on experiences with HLB in Florida, there is great concern about the potential impacts of HLB on this emerging industry. Prior to 2023, ACP had been identified in residential citrus trees in isolated Georgia counties but little to no testing of psyllids for CLas had occurred. However, in 2023, one individual psyllid collected from Chatham County was confirmed positive for CLas by PCR and sequencing. Furthermore, during 2023, ACP adults and nymphs were identified for the first time in a Georgia commercial citrus grove. The finding of ACP in a commercial planting represents a significant risk for CLas dissemination, and thereby has the potential to stall the rapid expansion of Georgia's citrus industry. In the coming years, surveillance and testing of ACP from commercial groves will be essential for the early detection and management of HLB and its vector to reduce HLB spread within Georgia's commercial groves.

Grove-Level Analysis of the Titer and Prevalence of ‘Candidatus Liberibacter asiaticus’ and Wolbachia in Diaphorina citri, Vector of Citrus Huanglongbing

Huanglongbing (HLB, or citrus greening disease) affects all citrus varieties worldwide. In the United States, Asia, and South America, the causal agent is ‘ Candidatus Liberibacter asiaticus’ ( CLas), a phloem-limited, uncultured alphaproteobacterium. The hemipteran insect vector Diaphorina citri (Asian citrus psyllid) acquires and transmits CLas in a circulative, propagative manner. In addition to CLas, D. citri hosts multiple symbiotic bacterial species including Wolbachia (wDi). In D. citri, wDi has been sequenced and studied, but specific roles in D. citri biology are unknown. Using well-established quantitative PCR methods, we measured CLas titer in D. citri collected from four groves in central Florida with distinct HLB management strategies and tested whether CLas and wDi titer were correlated in a subset of these insects. Grove site had the largest effect on CLas titer. Sex had no effect on CLas titer, whereas a higher wDi titer was correlated with noninfected insects. Our results suggest that more directed follow-up research is necessary and important to clarify whether field management tactics influence CLas titer in D. citri and to better understand gene-by-environment interactions among D. citri, wDi, and CLas. Now that millions of trees in Florida have been treated with injectable formulations of oxytetracycline, which is likely to decrease bacterial populations in D. citri, this study may represent the last biologically meaningful snapshot of grove-level vector-pathogen ecology in the state during the HLB epidemic.

Control of Diaphorina citri (Hemiptera: Liviidae) using environmentally friendly chemical alternatives

Diaphorina citri poses a serious threat to citrus. Here, the nymphicidal activity of mineral and soybean oils, organosilicon, and citrus-essential-oil-derived adjuvants was evaluated. Additionally, mineral and soybean oils were studied for antifeedant and repellent effects on adults. Residual effect on the parasitoid Tamarixia radiata was also assessed. Nymphicidal activity ranged between 65–83%, with Silwet enhancing mineral and soybean oil efficacy. Evaluated soybean and mineral oil exhibited comparable antifeedant and repellent effects on adults. All products caused <20% mortality in T. radiata, even 24 h after spraying. Overall, tested adjuvants and oils offer promising eco-friendly alternatives for sustainable D. citri management.

The Asian Citrus Psyllid (&lt;i&gt;Diaphorina citri&lt;/i&gt;) in Africa: using MaxEnt to predict current and future climatic suitability, with a focus on potential invasion routes

The Asian Citrus Psyllid (ACP) (Diaphorina citri Kuwayama, 1908) (Hemiptera: Psyllidae) is a major citrus pest. The species has been introduced to West and East Africa, but has not yet spread to southern Africa, where it could have a devastating impact on citrus farming and livelihoods. A proactive response is key to mitigating the species’ impacts, particularly the ongoing monitoring of potential invasion routes and entry points into South Africa. Species distribution models (SDMs) were developed under current and future climates for ACP in Africa, and these models were used to (1) determine where the species likely poses a threat, (2) identify potential invasion routes into South Africa, and (3) assess how these factors will be affected under climate change. The SDMs indicated that there is an almost contiguous band of suitable climate along the east coast of Africa that joins the species’ current range in East Africa to South Africa, and under aggressive climate change a potential route of invasion through Namibia and Botswana. Much of South Africa is climatically suitable for the species, but under climate change, climatically suitable areas are likely to shift further inland. The spread of ACP into South Africa is unlikely to be prevented, but the outputs of the present models will inform monitoring activities and assist with preparations to respond to this predicted biological invasion.

The Asian Citrus Psyllid (&lt;i&gt;Diaphorina citri&lt;/i&gt;) in Africa: using MaxEnt to predict current and future climatic suitability, with a focus on potential invasion routes

The Asian Citrus Psyllid (ACP) (Diaphorina citri Kuwayama, 1908) (Hemiptera: Psyllidae) is a major citrus pest. The species has been introduced to West and East Africa, but has not yet spread to southern Africa, where it could have a devastating impact on citrus farming and livelihoods. A proactive response is key to mitigating the species’ impacts, particularly the ongoing monitoring of potential invasion routes and entry points into South Africa. Species distribution models (SDMs) were developed under current and future climates for ACP in Africa, and these models were used to (1) determine where the species likely poses a threat, (2) identify potential invasion routes into South Africa, and (3) assess how these factors will be affected under climate change. The SDMs indicated that there is an almost contiguous band of suitable climate along the east coast of Africa that joins the species’ current range in East Africa to South Africa, and under aggressive climate change a potential route of invasion through Namibia and Botswana. Much of South Africa is climatically suitable for the species, but under climate change, climatically suitable areas are likely to shift further inland. The spread of ACP into South Africa is unlikely to be prevented, but the outputs of the present models will inform monitoring activities and assist with preparations to respond to this predicted biological invasion.

The conserved role of miR-2 and novel miR-109 in the increase in fecundity of Diaphorina citri induced by symbiotic bacteria and pathogenic fungi.

Infection with pathogens can increase the fecundity and other fitness-related traits of insect vectors for their own advantage. Our previous research has reported that DcKr-h1 plays a critical role in the increase in fecundity of Diaphorina citri induced by the bacterium, "Candidatus Liberibacter asiaticus" (CLas) and the fungus, Cordyceps fumosorosea (Cf). However, the posttranscriptional regulation of this process remains poorly understood. Given the significance of miRNAs in gene regulation, we delved into their roles in shaping phenotypes and their underlying molecular mechanisms. Our results indicated that two miRNAs, miR-2 and novel-miR-109, jointly inhibited DcKr-h1 expression by binding to its 3' untranslated region (UTR). In both D. citri-CLas and D. citri-Cf interactions, the increased juvenile hormone (JH) titer and reduced abundance of miR-2 and novel-miR-109 ensure high levels of DcKr-h1 expression, consequently stimulating ovarian development and enhancing fecundity. These observations provide evidence that miR-2 and miR-109 are crucial players in the JH-dependent increase in fecundity in psyllids induced by infection with different pathogens.

Two new polyketone derivatives from the mangrove endophytic fungus Aspergillus sp. GXNU TH4b

Two new polyketones, exserone B (1) and cytosporone F (2), along with three known metabolites, were isolated from the mangrove endophytic fungus Aspergillus TH4b. The structures of 1 and 2 were determined by detailed NMR, and MS spectroscopic data. The absolute configurations of 1 and 2 were determined by single crystal X-ray diffraction analysis and the combination of experimental ECD and computational ECD, respectively. Compounds 1–2 have strong inhibitory activity against citrus Psyllid (Diaphorina citri) with 95.4% and 93.7% lethal at 1000 mg/kg.

Multiplex qPCR for the detection of bacteria associated with huanglongbing 'Candidatus Liberibacter asiaticus', 'Ca. L. americanus' and 16Sr IX group phytoplasma.

The occurrence of 'Candidatus Liberibacter' spp. and 'Ca. Phytoplasma' spp. associated with blotchy mottle symptoms poses challenges to huanglongbing (HLB) diagnosis using molecular techniques. The ability to detect multiple targets simultaneously and specifically is a key aspect met by qPCR. A set of primers and hydrolysis probes useful either in single or multiplex reactions for the detection and quantification of HLB-associated bacteria were developed. Sequences from conserved genes of the ribosomal proteins for Liberibacter and phytoplasma circumvent the lack of specificity and cross-reactivity problems related to 16S rDNA gene amplification, allowing precise and specific detection of HLB-associated bacteria in citrus and in the Liberibacter vector, Diaphorina citri. The triplex reaction exhibited high quality and precision as a robust tool for quantifying 'Ca. L. asiaticus' (CLas), 'Ca. L. americanus' (CLam) and 16SrIX phytoplasma. Triplex qPCR showed consistent results and comparable sensitivity to the RNR test, though Cq values were higher when compared to 16S rDNA qPCR. Detection tests using field samples indicate that the qPCR triplex can identify HLB-associated bacteria in samples with varying levels of symptoms, ranging from typical to asymptomatic. Assessment of field samples from growers indicated more than 78.6% had Cq lower than 35.0, below the cut-off established for qPCR reactions used in this work. qPCR triplex is a safe, specific, and sufficiently sensitive technique for detecting CLas, CLam and 16Sr IX phytoplasma simultaneously, in both citrus and D. citri samples. Its application is of importance in assisting growers in making decisions for HLB management.

Potent and selective ‘genetic zipper’ method for DNA-programmable plant protection: innovative oligonucleotide insecticides against Trioza alacris Flor

Chemical insecticides increased the chemical burden on natural ecosystems posing environmental health risk factor. The urgent need for a more sustainable and ecological approach has produced many innovative ideas, including eco-friendly ‘genetic zipper’ method (or CUAD platform) based on contact oligonucleotide insecticides. Oligonucleotide insecticides have enjoyed success recently on many sternorrhynchans showing highly adaptable structure for distinct insect pest species and selective mode of action. In this article, we describe the efficiency of the oligonucleotide insecticides (briefly, olinscides or DNA insecticides) Alacris-11 and Laura-11, as well as their combined use in mixture (1:1), designed for control of bay sucker (Trioza alacris Flor), an important psyllid pest of noble laurel (Laurus nobilis L.). These olinscides are based on short unmodified antisense DNA oligonucleotides that target ITS2 between 5.8S rRNA and 28S rRNA in pre-rRNA (Laura-11) and 28S rRNA region in mature 28S rRNA and pre-rRNA (Alacris-11). The maximum pest mortality, observed on the 14th day of the experiment, comprised 95.01 ± 4.42% for Alacris-11, 97.16 ± 2.48% for Laura-11, and 98.72 ± 1.14% for their mixture (1:1). The control oligonucleotide CTGA-11 did not cause any significant mortality (9.38 ± 0.57%), emphasizing selectivity in the action of oligonucleotide insecticides. The results show potent and specific nature of oligonucleotide insecticides for pest control and open up new frontiers in control of economically important psyllids in agriculture and forestry, including Asian citrus psyllid (Diaphorina citri Kuwayama) and many others. Scientists can easily adopt ‘genetic zipper’ method for plethora of insect pests because DNA is a programmable molecule and provides game-changing characteristics for plant protection.Graphical

Agroecological Alternatives for Pest and Disease Management in Mexican Lime [Citrus aurantifolia (Christm.) Swingle] Cultivation

agroecological alternatives for the integrated management of recurrent pests in Mexican lime (Citrus aurantifolia); as well as to establish possible synergies with the commercial bioinsecticide. Design/methodology/approach: This experiment was conducted in two phases: 1) visual monitoring and pre-identification of pests and fungal diseases in the Mexican lime crop, and 2) application and evaluation of the effect of application of commercial bioinsecticide (Biocanela) and mineral broths (bordeaux broth and sulfocalcium broth) alone and mixtures. Results: The pests with highest incidence were thrips (Pezothrips kellynus), Asian citrus psyllid (Diaphorina citri), whitefly (Bemisia tabaci), fruit fly (Drosophila melanogaster), and red spider mite (Tetranychus urticae), while the main fungal diseases were sooty mold (Capnodium citri), red algae (Cephaleuros virescens), citrus greasy spot (Mycosphaerella citri), and anthracnose (Colletotrichum acutatum). The mixture of mineral broths with the bioinsecticide Biocanela showed the highest fungicide and insecticide activity, and repellency on various pests and diseases of the Mexican lemon crop. Limitations on study/implications: Generate scientific knowledge regarding to the best agroecological alternatives for the integrated management of citrus pests and diseases that are economically and environmentally profitable. Findings/conclusions: Application of the mixture of sulfocalcium broth and Biocanela showed repellency effectiveness against pests such as thrips (Pezothrips kellyanus), Asian citrus psyllid (Diaphorina citri), whitefly (Bemisia tabaci), and fruit fly (Drosophila melanogaster). Likewise, it was effective against sooty mold (Capnodium citri), red algae (Cephaleuros virescens), and citrus greasy spot (Mycosphaerella citri), while the effect was lighter against anthracnose (Colletotrichum acutatum). Additionally, it induces new, healthy shoots in Mexican lemon trees.

The sandwich feeding assay for use with first instar nymphs of the Asian citrus psyllid, Diaphorina citri confirms the high susceptibility of this life stage to bacterial pesticidal proteins

Citrus greening or huanglongbing is the most important disease of citrus and threatens citrus production worldwide. As nymphs of Diaphorina citri play a crucial role in the acquisition and transmission of the citrus greening bacterium, suppression of this life stage is particularly important. However, the lack of a tractable feeding assay for use with first instar D. citri nymphs has impeded assessment of the toxicity of bioactives. Of several bacterial pesticidal proteins (BPP) that are toxic to D. citri adults, Mpp51Aa1 and Cry1Ba1, which have LC50 values of 110 and 120 µg/mL respectively in adults, were fed to 1st instar nymphs in a newly developed assay. For this new sandwich feeding assay, parafilm layers containing feeding solution were placed on top of two 35 mm Petri dishes, with a concave surface created on each. Fifty nymphs were transferred to the membrane on one Petri dish, and the second Petri dish placed on the top to create a “sandwich” with the 1st instar nymphs in the middle. Nymphs were fed for four days and the LC50 values for Mpp51Aa1 and Cry1Ba1 were calculated at 6.7 and 41.6 µg/mL respectively. Bioassays with bioengineered plants expressing Cry1Ba1 confirmed that the majority of D. citri mortality occurs during the 1st instar nymph stage, while egg laying adults are much less susceptible. Taken together, these results confirm that 1st instar D. citri nymphs are more susceptible to BPP than adults and demonstrate the utility of the sandwich feeding assay for effective screening of BPPs prior to investment into production of transgenic plants.

The feeding behavior of Diaphorina citri monitored by using an electrical penetration graph (DC-EPG) on citrus plants treated with Bacillus cereus and Bacillus velezensis

Diaphorina citri, an important pest and insect vector that can transmit the pathogenic bacteria Candidatus Liberibacter asiaticus, causing Huanglongbing disease, is one of many challenges in citrus agriculture. Integrated pest management by utilizing microorganisms is a wise and efficient alternative without damaging the environment. Utilization of Plant Growth Promoting Rhizobacteria (PGPR), such as Bacillus cereus and B. velezensis, is a potential strategy for the biological control of plant diseases or insect vectors. By inducing systemic resistance in plants, PGPR can enhance plant defense against diseases and insect pests while activating molecular and physiological changes in plants. This research aimed to determine the effect of B. cereus and B. velezensis on the plant growth and feeding behavior of D. citri. The height and volume of the plant canopy were observed periodically for 6 months, while the feeding behavior of D. citri was monitored using the Electrical Penetration Graph (DC-EPG). The results showed increased height and volume of the citrus plant canopy treated with B. cereus, indicating that B. cereus could act as a PGPR. The application of B. cereus and B. velezensis to citrus seedlings affected the feeding behavior of D. citri. D citri showed difficulty in penetrating the phloem tissue of citrus plants.

Climate covariate selection influences MaxEnt model predictions and predictive accuracy under current and future climates

The performance and transferability of species distribution models (SDMs) depends on a number of ecological, biological, and methodological factors. There is a growing body of literature that explores how the choice of climate covariate combinations and model parameters can affect predictive performance, but relatively few that delve into covariate reduction methods and the optimisation of model parameters, and the resulting spatial and temporal transferability of those models. The present work used the citrus pest, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), to illustrate how MaxEnt models trained on the insect’s native range in Asia varied in their predictions of climatic suitability across the introduced range when eight different covariate reduction methods were applied during model building. Additionally, it showed how model sensitivity varied across these different covariate combinations using three sets of independently validated occurrence points in the invaded range of the psyllid. Climatically suitable areas for D. citri differed by as much as two-fold between the best and worst-performing models in selected areas. Great care should be taken in the selection of the highest-performing predictor combinations and model parameter settings for SDMs, particularly in the case of invasive species where the assumption of environmental equilibrium is likely violated in the introduced range. Understanding how the predictive ability of SDMs can be influenced by the methodological choices made during the model building phase is vital to ensuring that ecological and invasion management programmes do not over- or underestimate climatic suitability and subsequent invasion risk.

Topping sweet orange trees as Diaphorina citri bait on the farm edge for huanglongbing management: Opportunities and limitations

Flush shoots of citrus have an important role in huanglongbing (HLB) epidemiology, as the insect vector of HLB, the Asian citrus psyllid (ACP), prefers them for feeding and reproduction. Additionally, ACP's acquisition and transmission of HLB bacteria mainly occurs in these flushes. As pruning stimulates sprouting, we hypothesized that periodically topping sweet orange trees on the orchard's edge (from 0 to 100 m) would attract ACPs from external areas to these flush shoots as bait, instead of ACPs dispersing throughout the orchard. Hence, it would direct HLB management on this edge width. For that, topped trees (TT) were compared to non-topped trees (NTT) located on the orchard's edge, with only NTT after edges (from 100 to 200 m). Vegetative growth, flushing frequency, ACP population, spray coverage, HLB incidence, fruit yield, and quality of Hamlin and Valencia Americana sweet orange trees were evaluated in commercial orchards over three growing seasons. Topping was performed frequently (every 30–45 d) on alternate rows to stimulate the year-round presence of shoot flushes within the orchard's edge. Insecticides were sprayed monthly to control ACP. Topping reduced tree height and canopy volume by 24 % and 20 % on average. Topped trees presented, on average, 3-fold more flush shoots than NTT over time. On Hamlin blocks, ACP numbers and cumulative HLB incidence were 70 % and 90 % higher, respectively, in TT compared to NTT. However, no consistent differences were observed for the Valencia Americana orchard. Moreover, ACP abundance and HLB incidence were similar after the edge regardless of topping. In the edge areas, although tree yield was reduced (∼65 % on average) on TT compared to NTT, topping significantly enhanced external fruit quality. This treatment increased fruit size and weight and promoted intense yellow coloration. Topping increased spray coverage at the top of Hamlin trees by 1.5-fold compared to control. Frequent topping as bait for ACP poses significant risks if the ACP population is not rigorously controlled, potentially leading to further reductions in yield from edge trees.