Phenology of Asian Citrus Psyllid (Hemiptera: Liviidae), With Special Reference to Biological Control by Tamarixia radiata , in the Residential Landscape of Southern California

Modeling climate change impacts on potential global distribution of Tamarixia radiata Waterston (Hymenoptera: Eulophidae)

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama (Hemiptera: Liviidae), is an efficient vector of "Candidatus Liberibacter" species, the causative agents implicated in citrus greening or huanglongbing (HLB). HLB is the most devastating citrus disease and has killed millions of citrus trees worldwide. Classical biological control using Tamarixia radiata Waterston (Hymenoptera: Eulophidae) against ACP has been successful in some regions. Climatic conditions are critical in determining suitable areas for the geographical distribution of T. radiata. However, paucity of information on climate change impacts on the global spread of T. radiata restricts international efforts to manage ACP with T. radiata. We investigated the potential global distribution of T. radiata using 317 native and non-native occurrence records and 20 environmental data sets (with correlation coefficients (|r| > 0.7)). Using the Maximum Entropy model, these data were analyzed for two shared socioeconomic pathways (SSPs) and two time periods (2030s and 2050s). We showed that habitat suitability for T. radiata occurred in all continents except Antarctica. However, the highly suitable areas for T. radiata were found in parts of the Americas, Asia, Africa and Oceania. The climate suitable areas would increase until the 2050s. The predictions showed that mean temperature of coldest quarter and precipitation of warmest quarter were the most important environmental variables that influenced the distribution of T. radiata. The model reliably predicted habitat suitability for T. radiata, which can be adapted in classical biological control programs to effectively manage ACP in an environmentally friendly manner.

Tamarixia radiata global distribution to current and future climate using the climate change experiment (CLIMEX) model

The phloem-limited bacteria, "Candidatus Liberibacter asiaticus" and "Ca. L. americanus", are the causal pathogens responsible for Huanglongbing (HLB). The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the principal vector of these "Ca. Liberibacter" species. Though Tamarixia radiata Waterston (Hymenoptera: Eulophidae) has been useful in biological control programmes against D. citri, information on its global distribution remains vague. Using the Climate Change Experiment (CLIMEX) model, the potential global distribution of T. radiata under the 2050s, 2070s, and 2090s for Special Report on Emissions Scenarios A1B and A2 was defined globally. The results showed that habitat suitability for T. radiata covered Africa, Asia, Europe, Oceania, and the Americas. The model predicted climate suitable areas for T. radiata beyond its presently known native and non-native areas. The new locations predicted to have habitat suitability for T. radiata included parts of Europe and Oceania. Under the different climate change scenarios, the model predicted contraction of high habitat suitability (EI > 30) for T. radiata from the 2050s to the 2090s. Nevertheless, the distribution maps created using the CLIMEX model may be helpful in the search for and release of T. radiata in new regions.

Source or Sink? The Role of Residential Host Plants in Asian Citrus Psyllid Infestation of Commercial Citrus Groves

Citriculture landscapes in the U.S. are typically habitat mosaics of commercial groves interspersed with residential areas supporting a variety of unmanaged citrus. Diaphorina citri the vector of Candidatus Liberibacter asiaticus, the causal agent of Huanglongbing feeds on citrus in both habitats. We postulated that residential citrus function as a 'source' of D. citri that infest groves, functioning as sinks. Here we report on an experimental mark-release-recapture study conducted at the interface of a residential neighborhood and groves. Adult D. citri marked with colored fluorescent powders were released in both habitats (n = 15,300) and their movement within and between milieus monitored. Although the recapture rate of marked psyllids was very low (0.23%), the results were instructive. Most of the recaptured psyllids in residential trees (84.6%) were released within that habitat. In contrast, approximately half of the marked psyllids recovered in groves were released in residential areas. Of all the recaptured psyllids, about 40% changed habitats, but the change was skewed toward movement from residential to grove habitat. These data strongly suggest that there is a constant exchange of D. citri adults between the two habitats, with residential citrus trees functioning as a source habitat of psyllids. The further the residential trees are located from groves, the less likely they will serve as sources of D. citri. Hence, to reduce the risks of citrus grove colonization by D. citri, new groves should be established away from residential habitats where possible, and psyllid management practices must also be implemented in residential habitats.

Compatibility of a Beta-cyfluthrin-Based 'Attract-and-Kill' Device with Tamarixia radiata (Hymenoptera: Eulophidae) for Suppression of Diaphorina citri (Hemiptera: Liviidae) on Residential Citrus

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is a serious threat to the U.S. citrus industry because it spreads huanglongbing (HLB), a bacterial and incurable citrus disease. The nymphal parasitoid, Tamarixia radiata Waterston (Hymenoptera: Eulophidae), has been extensively released for biological control of D. citri in South Texas since 2010 but provides insufficient control. An 'attract-and-kill' (AK) device was evaluated for its compatibility with T. radiata for suppression of D. citri on dooryard citrus. The AK device is visually attractive to D. citri adults because it is the same color as young citrus flush and kills individuals on contact with the toxicant beta-cyfluthrin. This study evaluated 1) lethality of AK devices to T. radiata adults under lab conditions; 2) efficacy of AK devices for year-round psyllid suppression on individual dooryard lemon trees; 3) discovery and parasitism of D. citri colonies by T. radiata on lemon trees with or without AK devices. Contact with AK devices for 5 s or more was lethal to adult parasitoids. Deployment of 20 AK devices per tree provided significant year-round suppression of D. citri on infested lemon trees and reduced mean attack intensity (cumulative psyllid-days) of adults by 66% and nymphs by 82%. Discovery and parasitism rates of D. citri colonies by T. radiata were similar on control trees and trees protected by AK devices. An AK device that targets adult psyllids could be used to effectively complement biological control of D. citri by T. radiata in residential landscapes.

Parasitoid vectors a plant pathogen, potentially diminishing the benefits it confers as a biological control agent

Huanglongbing (HLB) is a destructive disease of citrus primarily transmitted by the Asian citrus psyllid (ACP). Biocontrol of ACP is an environmentally sustainable alternative to chemicals. However, the risk of parasitoid rational application in ACP biocontrol has never been evaluated. Here we show, the dominant parasitoid of ACP, Tamarixia radiata, can acquire the HLB pathogen Candidatus Liberibacter asiaticus (CLas) and transmit it horizontally when probing ACP nymphs. If these ACP nymphs survive the probing, develop to adults and move to healthy plants, CLas can be transmitted to citrus leaves during feeding. We illustrate the formerly unrecognized risk that a parasitoid can potentially serve as a phoretic vector of the pathogen transmitted by its host, thus potentially diminishing some of the benefits it confers via biocontrol. Our findings present a significant caution to the strategy of using parasitoids in orchards with different infection status of insect-vectored pathogens.

Temperature-Dependent Biological Control Effectiveness of Tamarixia radiata (Hymenoptera: Eulophidea) Under Laboratory Conditions

The parasitism rate and host-feeding rate of Tamarixia radiata (Hymenoptera: Eulophidae), an ectoparasitoid of Diaphorina citri (Hemiptera: Liviidae), were evaluated at 20, 27.5, 30, and 35°C, at 70 ± 5% RH, and 14 h of photoperiod. The biological control efficacy of T. radiata was evaluated by linking the age-stage predation rate with the two-sex life table. The net host-feeding rate (C0) by T. radiata was 32.05, 54.40, 17.25, and 1.92 nymphs per female parasitoid at 20, 27.5, 30, and 35°C, respectively. The total net nymphs killing rate (Z0) was 103.02, 223.82, 72.95, and 6.60 nymphs per female parasitoid at 20, 27.5, 30, and 35°C, respectively. Noneffective parasitism rate was observed at 35°C because of high mortality at this temperature. Our results indicated that temperature had meaningful effects on parasitism and host-feeding rate parameters in the laboratory, and may affect biological control efficiency of the parasitoid in the field. The highest host-feeding rate and total biological control efficiency of T. radiata were recorded at 27.5°C. Most importantly, we found that host-feeding activity of the parasitoid is temperature-dependent, and changed across temperature regimes: the host-feeding rate increased as the temperature increased up to 30°C, started to decrease after this temperature and declined to its minimum level at 35°C. This information is valuable for developing biological control and integrated pest management techniques for Asian citrus psyllid management.

Effect of Horticultural Mineral Oil on Huanglongbing Transmission by Diaphorina citri Kuwayama (Hemiptera: Psyllidae) Population in a Commercial Citrus Orchard in Sarawak, Malaysia, Northern Borneo

Diaphorina citri Kuwayama transmits a destructive citrus disease caused by a fastidious bacterium 'Candidatus Liberibacter asiaticus' (CLas) designated as Huanglongbing (HLB) which posed a risk of detrimental threat to the Malaysian citrus industry. All D. citri life stages show a lumped habit on young flushes and its population fluctuations was closely related to accessibility of young flushes. The study aimed to investigate if the appearance of young flush shoots on citrus influences ACP population fluctuation and if horticultural mineral oil (HMO) could reduce spread of HLB transmission by ACP in a commercial healthy orchard. Field research was carried out from 1 April 2011 to 1 December 2014 in a 2-year-old 1 ha citrus farm that consisted of 200 PCR-certified disease-free grafted non-bearing honey tangerine (Citrus reticulata L.) in southwestern Sarawak, Malaysia. The experiment had two treatments namely control (unsprayed) and nC24 HMO with four replications arranged in a simple randomized block design. ACP eggs, nymphs, and adults per flush shoot was assessed and HLB incidence was monitored for visual inspection of the citrus trees for the current existence of usual signs of characteristic symptoms of HLB such as yellowing shoots, leaf mottling, and corky or enlarged veins on leaves. HLB-specific primer was employed in 16S rDNA polymerase chain reaction to detect the CLas gene in diseased trees. Increase in abundance of D. citri is mainly affected by the citrus flushing cycles and their life stages are completed on these flush shoots. Relative degree of aggregation index for D. citri adults increased during periods of cyclic production of new flush. HMO-treated plots produced a significantly lower percentage up to 11.43% of diseased trees against 42.20% in untreated control plots. HMO is effective against D. citri and recommended to be incorporated in the IPM program to prevent infection and reduce the spread of HLB.

Distribution, Phenology, and Overwintering Survival of Asian Citrus Psyllid (Hemiptera: Liviidae), in Urban and Grove Habitats in North Florida

Cold hardy citrus is an emerging industry in north Florida. However, it is under the threat of Candidatus Liberibacter asiaticus (CLas), the agent of the citrus disease huanglongbing. Distribution and phenology of the Asian citrus psyllid, Diaphorina citri (Kuwayama), the vector of CLas, was investigated over a 2-year sampling period in north Florida. Diaphorina citri was only found in backyard and ornamental citrus along the Gulf of Mexico, and was not observed in cultivated citrus groves during the 2 years (2017-2018) of the survey. Diaphorina citri population peaks occurred approximately 2 mo later than in central Florida with major population peaks occurring in July. The number of D. citri adults was significantly higher on CLas infected than uninfected citrus trees, whereas more nymphs were found on uninfected trees. Most D. citri were negative for CLas except in Franklin county where both infected trees and psyllids were found. We were able to find adult D. citri during all winter months, despite temperatures as low as -5.5°C. During two consecutive winters, we conducted experiments to determine D. citri cold hardiness by caging D. citri under ambient conditions in mid-November and assessing survivors in the following spring. In 2018, approximately 21%, of D. citri adults survived overwintering whereas 16% survived in 2019 despite lower temperature in 2018 than in 2019. As we are at the earliest stage of HLB infestation, management of D. citri and CLas in north Florida should focus on removal of CLas-infected trees to reduce the reservoir of pathogen.

Optimization of sampling and monitoring of vegetative flushing in citrus orchards

Citrus trees produce flushes throughout the year, but there are no criteria established for a precise shoot monitoring in orchards under tropical climate. Methods for quantification of flush dynamics would be useful for horticultural and pest management studies because different insect vectors feed and reproduce on flushes. We estimated the minimum number and distribution of trees for sampling and determined the flushing pattern over time in ‘Valencia Late’ orange trees grafted onto ‘Swingle’ citrumelo rootstock. Shoots within a square frame (0.25 m²) on two sides of the canopy were counted and classified by their phenological stage. The minimum number of samples was estimated using the mean number of shoots and area under the flush shoot dynamics (AUFSD). The temporal and spatial distribution analysis was performed by Taylor’s power law and by multiple correspondence analysis (MCA). Additionally, a shoot maturity index (SMI) based on visual qualitative assessment of flushes is proposed. Considering the mean number of shoots, it was necessary to sample two sides of 16 trees to reach a relative sampling error (E_r) of 25%, whereas by the AUFSD, only five trees were necessary to reach an E_r of 10%. Flushes were predominantly randomly distributed over time and space. Testing eight transects, sampled trees should be distributed throughout the block, avoiding sampling concentration in a certain area. MCA showed that the west side and the upper sampling positions of trees were more likely to be associated with younger shoots. AUFSD and the evaluation of both sides of the canopy yielded a smaller number of trees to be assessed. The SMI was a reliable metric to estimate the shoot phenology of orange trees, and correlated well (R² > 70%) with the mean number of shoots within the square frame. Therefore, SMI has the potential to make shoot monitoring in the field more practical.

Response surface methodology reveals proportionality effects of plant species in conservation plantings on occurrence of generalist predatory arthropods

Multivariate geometric designs for mixture experiments and response surface methodology (RSM) were tested as a means of optimizing plant mixtures to support generalist predatory arthropods. The mixture design included 14 treatment groups, each comprised of six planters and having a proportion of 0.00, 0.17, 0.33, 0.66, or 1.00 of each plant species. The response variable was the frequency of predators trapped on sticky card traps placed in each group and replaced 2 times per week. The following plant species were used: Spring 2017: Euphorbia milii, E. heterophylla, and Phaseolus lunatus; Summer 2017: E. milii, Fagopyrum esculentum, and Chamaecrista fasciculata; and, Summer 2018: E. milii, F. esculentum, and Portulaca umbraticola. Predator occurrence was influenced by: 1) Linear mixture effects, which indicated that predator occurrence was driven by the amount of a single plant species in the mixture; or, 2) Nonlinear blending effects, which indicated that the plant mixture itself had emergent properties that contributed to predator occurrence. Predator abundance was highest in the Spring 2017 experiment and both linear mixture effects and nonlinear blending effects were observed. Predator occurrence decreased in subsequent experiments, which were conducted in the warmer summer months. In both Summer experiments, only linear mixture effects were observed, indicating that predator occurrence was driven by the amount of a single plant species in the test mixtures: Euphorbia milii in 2017 and Portulaca umbraticola in 2018. The results showed that not only did the species composition of a plant mixture drive predator occurrence but that proportionality of species contributed to the outcome as well. This suggests that, when formulating a plant mixture to aid in conservation biological control consideration should be given to the proportion of each plant species included in the mixture. RSM can be an important tool for achieving the goal of optimizing mixtures of plants for conservation biological control.

The Effects of Constant and Fluctuating Temperatures on Development of Diaphorina citri (Hemiptera: Liviidae), the Asian Citrus Psyllid

The effects of six average daily temperatures, 15, 20, 25, 30, 32, and 35°C, that were either constant or fluctuating over 24 h on development times of California-sourced Diaphorina citri Kuwayama nymphs were examined. Thermal performance curves for immature stages of D. citri were characterized using one linear and six nonlinear models (i.e., Ratkowsky, Lobry-Rosso-Flandrois, Lactin-2, Brière-2, Beta, and Performance-2). Daily thermal fluctuations had significant effects on development times of D. citri nymphs, which differed across experimental temperatures. Diaphorina citri nymphs reared at constant temperatures completed development faster than those reared under fluctuating profiles with equivalent temperature means. Linear model estimates of degree-days required for completion of cumulative development of D. citri were 25% lower for constant temperatures when compared with fluctuating temperature regimens. Nonlinear model estimations of optimum developmental temperature and upper theoretical temperature bounds for development were similar for individuals reared under constant and fluctuating temperatures. Nevertheless, the estimated values of lower theoretical temperature limits above which development occurred were lower under fluctuating than constant temperatures. A meta-analysis of published D. citri temperature-dependent development literature, synthesizing datasets of five globally distributed populations (Brazil, California, China, Florida, and Japan) reared under different constant temperatures on six different host plants (i.e., Citrus limonia, C. sinensis cv Natal, C. sinensis cv. Pêra, C. reticulata, Fortunella margarita, and Murraya paniculata), together with the results of this study (C. volkameriana), revealed convergence in estimates of developmental parameters. These results have implications for predicting D. citri invasion and establishment risk and subsequent population performance across various climactic gradients and geographic regions.

The Enemy is Outside: Releasing the Parasitoid Tamarixia radiata (Hymenoptera: Eulophidae) in External Sources of HLB Inocula to Control the Asian Citrus Psyllid Diaphorina citri (Hemiptera: Liviidae)

Huanglongbing (HLB), the most destructive citrus disease worldwide, was first recorded in Brazil in 2004, and since then, more than 50 million trees identified with this disease have been eliminated. The disease is managed mainly by controlling the psyllid vector Diaphorina citri Kuwayama, 1908 (Hemiptera: Liviidae). Although the presence of the insect in commercial citrus groves is low, HLB infection rates increase in areas bordering the groves. The disease is transmitted by psyllids from host citrus plants in areas outside the managed groves, such as abandoned or organic groves and residential trees, and from orange jasmine plants in urban settings. In order to provide information to support HLB control, this study evaluated the biotic and abiotic variables that affect the dynamics of D. citri populations after releases of the parasitoid wasp Tamarixia radiata (Waterston, 1922) (Hymenoptera: Eulophidae) in external sources of HLB inocula. The study was divided into two parts. After releasing the parasitoids in non-commercial areas, we determined the following: (a) the variables that significantly affected the number of nymphs collected on shoots in the same non-commercial area; (b) the variables that significantly affected the number of adult psyllids collected in a neighboring commercial citrus area. Our results indicated that the number of nymphs in external areas was affected only by the host plant and rainfall. However, periodic parasitoid releases significantly reduced the number of adult psyllids collected in the commercial area. The results indicate that the release of parasitoids in external sources of inocula has the potential to maximize actions for D. citri control, contributing to the reduction of psyllid populations in commercial areas. Consequently, this strategy may help to manage the disease infection without an increase in insecticide use.

Development and Field Validation of a Beta-cyfluthrin-Based 'Attract-and-Kill' Device for Suppression of Asian Citrus Psyllid (Hemiptera: Liviidae) on Residential Citrus

An 'attract-and-kill' (AK) device was evaluated for suppression of adult Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), on residential citrus. The AK device, made from weather-resistant plasticized PVC, lured D. citri adults by simulating the color of citrus flush and killed them with beta-cyfluthrin. This study evaluated: 1) lethality of AK devices weathered up to 8 wk on residential citrus; 2) survival of psyllids caged with potted plants and AK devices; 3) psyllid suppression achieved by AK devices on individual dooryard trees. AK devices weathered for up to 8 wk remained lethal to psyllids. Greenhouse trials evaluated survival of adult psyllids caged for 4 d with orange jasmine plants that were: 1) treated with an (beta-cyfluthrin-infused) AK device; 2) treated with a blank (no insecticide) AK device; or 3) 'untreated' with no AK device. After 4 d, psyllid survival was on average 95% lower among adults exposed to plants with AK devices than adults exposed to untreated plants or plants with blank AK devices. Less than half of the adults exposed to plants with AK devices were alive after 1 d and nearly all were dead after 4 d. Deployment of 20 AK devices per tree provided significant psyllid suppression on infested lemon trees from winter to summer and reduced mean reproduction (cumulative eggs) by 91% and mean attack intensity (cumulative psyllid-days) of adults by 59% and nymphs by 53%. AK devices could be an effective control option for D. citri in urban areas.

The Influence of Temperature Variation on Life History Parameters and Thermal Performance Curves of Tamarixia radiata (Hymenoptera: Eulophidae), a Parasitoid of the Asian Citrus Psyllid (Hemiptera: Liviidae)

This study examined the effects of seven constant and fluctuating temperature profiles with corresponding averages of 12 to 38°C on the life history of the Punjab, Pakistan-sourced Tamarixia radiata (Waterston) released in California for biological control of Diaphorina citri Kuwayama. One linear and seven nonlinear regression functions were fit to egg-to-adult development rate data to characterize thermal performance curves. Temperature fluctuations significantly affected both development and longevity of T. radiata. Estimates of degree-days predicted by the linear model were 30% higher for the fluctuating regime than the constant regime. Nonlinear model estimations of theoretical minimum and maximum developmental thresholds were lower for the fluctuating regime when compared to the constant regime. These predictions align with experimental observations. Parasitoids reared under fluctuating profiles at low average temperatures developed faster (15°C) and survived longer (15-20°C) when compared to those reared under constant regimes with corresponding means. In contrast, high average fluctuating temperatures produced parasitoids with an extended developmental period (35°C) and reduced longevity (30-35°C). A meta-analysis of published T. radiata development datasets, together with the results of this study, indicated convergence in degree-days and theoretical minimum developmental thresholds among geographically distinct parasitoid populations. These findings demonstrate the significant effects of temperature on T. radiata life history and have important implications for optimization of mass-rearing and release efforts, improvement of predictions from climate modeling, and comparison of T. radiata population performance across climatic gradients and geographic regions.

Impact of the Temperature on the Phenology of Diaphorina citri (Hemiptera: Liviidae) and on the Establishment of Tamarixia radiata (Hymenoptera: Eulophidae) in Urban Areas in the Lower Colorado Desert in Arizona

The invasive pest Diaphorina citri Kuwayama was first detected in Arizona in 2009. Since late 2013, the parasitoid Tamarixia radiata (Waterson), the main biocontrol agent of D. citri, has been released as part of a biological control program on citrus grown in urban areas of two western Arizona counties in the lower Colorado desert environment. Here we report a 3-yr survey aimed at evaluation of T. radiata releases on D. citri populations and assess the impact of the climate conditions on the phenology of D. citri and on the establishment success of T. radiata. We also monitored the phenology of D. citri as part of this assessment on different citrus host species. We show that the high summer temperatures in the Arizona desert halt the development of D. citri for about 3 mo every year which appears to have limited the establishment and impact of T. radiata. At survey sites distant from release areas the parasitism rates over the season ranged from 0 to 75% and on average peaked around 50% in 2016 but it was low or absent in 2015 and 2017, respectively. We discuss the consequences of this phenology of D. citri in the desert areas for the prospects of long-term establishment of T. radiata and the management of this key citrus pest.

An assessment of interspecific competition between two introduced parasitoids of Diaphorina citri (Hemiptera: Liviidae) on caged citrus plants

Two parasitoids attacking nymphs of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) and Diaphorencyrtus aligarhensis (Shafee, Alam & Agarwal) (Hymenoptera: Encyrtidae) are being released in California, USA in a classical biological control program. To evaluate the effect of multiple parasitoid species on D. citri mortality, we conducted mesocosm experiments under controlled conditions using a complete block design with 6 treatments (D. citri nymphs exposed to: no parasitoids; D. aligarhensis or T. radiata alone; D. aligarhensis or T. radiata released first (by 48 h); and both species released simultaneously). Parasitism of D. citri nymphs by T. radiata exceeded 60% and was unchanged when D. aligarhensis were present. Parasitism by D. aligarhensis was greatest when T. radiata was absent (∼28%) and was reduced in all treatments with T. radiata present (<3%). D. citri mortality and parasitoid-related mortality of D. citri was consistent across parasitoid treatments. Laboratory results suggest that competition between D. aligarhensis and T. radiata is asymmetric and favors T. radiata. It may be difficult for D. aligarhensis to contribute significantly to D. citri biological control where T. radiata is present. However, results reported here suggest that competition between T. radiata and D. aligarhensis is not likely to reduce parasitism by T. radiata or reduce parasitoid-induced mortality of D. citri.

Hexaacetyl-chitohexaose, a chitin-derived oligosaccharide, transiently activates citrus defenses and alters the feeding behavior of Asian citrus psyllid

Plants have a perception system triggered by pathogen and pest signals to initiate defense. These signals include evolutionarily conserved molecules from microbes and insects termed pathogen/herbivore-associated molecular patterns (PAMPs/HAMPs). Here we showed that hexaacetyl-chitohexaose (HC), an oligosaccharide from chitin, a structural component in insect exoskeletons and fungi cell walls, upregulated defense-associated genes WRKY22, GST1, RAR1, EDS1, PAL1 and NPR2, and downregulated ICS1 at 1 h after HC treatment in Sun Chu Sha mandarin leaves. The effect was transient as defense gene transcriptional changes were not observed at 18 h after the treatment. Electrical penetration graph (EPG) recordings were used to study the feeding behavior of Asian citrus psyllid (ACP) following the HC treatment. ACP is the hemipteran vector of Candidatus Liberibacter asiaticus (CLas), the pathogen associated with huanglongbing (HLB). Adult ACP displayed reduced intercellular probing, reduced xylem feeding count and duration, and increased non-probing activity on HC-treated citrus compared to controls. During an 18-h recording, percentage for total duration of xylem ingestion, phloem ingestion, intercellular probing were lower, and the percentage of non-probing behavior was higher in HC-treated leaves than in controls. In host-selection behavior studies, HC treatment did not alter the attractiveness of citrus leaves under light or dark conditions. In addition, ACP feeding on HC-treated leaves did not show differences in mortality for up to 10 day of exposure. In summary, we report that HC induced a transient defense in citrus and an inhibitory effect on ACP feeding but did not affect host selection or the insect fitness under the tested conditions.

Field Efficacy of Autodissemination and Foliar Sprays of an Entomopathogenic Fungus, Isaria fumosorosea (Hypocreales: Cordycipitaceae), for Control of Asian Citrus Psyllid, Diaphorina citri (Hemiptera: Liviidae), on Residential Citrus

Autodissemination and foliar sprays of PFR-97 (Certis Inc., Columbia, MD) microbial insecticide, a blastospore formulation of Isaria fumosorosea Wize (Hypocreales: Cordycipitaceae), were evaluated for control of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), on residential citrus. Seasonal trials on dooryard trees in South Texas evaluated: 1) pathogenicity of I. fumosorosea (Ifr) spores on autodisseminators (dispensers) deployed up to 3 wk on grapefruit trees; 2) psyllid control on several citrus species by dispensers and sprays; 3) infection range of the dispenser. Decline in spore pathogenicity over time was similar among dispensers during fall, winter, or spring and decreased by 30% after 1 d, 59% after 7 d, 81% after 14 d, and 100% after 21 d. Dispensers or sprays were equally effective for psyllid control on heavily infested lime trees from fall to spring and reduced mean reproduction (cumulative eggs) by 90% and mean attack intensity (cumulative psyllid-days) of adults by 76% and nymphs by 82%. Dispensers or sprays were also equally effective for psyllid control on lightly infested lime trees from spring to mid-summer and on orange or grapefruit trees from fall to winter. Very light infestations on grapefruit trees from spring to mid-summer were not significantly reduced by dispensers or sprays. Psyllid control was not improved by combining dispensers and sprays. Adult psyllids infected by Ifr were recovered in trees located 3-4 m away from trees with dispensers but not at greater distances. PFR-97 dispensers could be a treatment option for D. citri in settings where chemical control is problematic.

Modeling the Phenology of Asian Citrus Psyllid (Hemiptera: Liviidae) in Urban Southern California: Effects of Environment, Habitat, and Natural Enemies

Modeling can be used to characterize the effects of environmental drivers and biotic factors on the phenology of arthropod pests. From a biological control perspective, population dynamics models may provide insights as to when the most vulnerable pest life stages are available for natural enemies to attack. Analyses presented here used temperature and habitat dependent, instar-specific, discrete models to investigate the population dynamics of Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). This pest is the target of a classical biological control program with the parasitoid Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae). The population trends of D. citri eggs, nymphs, and adults, citrus flush growth patterns, and T. radiata activity were monitored monthly on orange and lemon trees at 10 urban sites in southern California for a 2-yr period. Cumulative D. citri egg, nymph, and adult days recorded at each site, were regressed against accumulated degree-days (DDs) to model the population dynamics of each development stage in relation to temperature. Using a biofix point of 1 January, the model predicted that 10% and 90% of eggs were laid by 198 and 2,255 DD, respectively. Populations of small and large D. citri nymphs increased slowly with 90% of the population recorded by 2,389 and 2,436 DD, respectively. D. citri adults were present year round with 10 and 90% of the population recorded by 95 and 2,687 DD, respectively. The potential implications of using DD models for optimizing inoculative releases of natural enemies, such as T. radiata into citrus habitat infested with D. citri, are discussed.

Spatiotemporal dynamics of the Southern California Asian citrus psyllid (Diaphorina citri) invasion

Biological invasions are governed by spatial processes that tend to be distributed in non-random ways across landscapes. Characterizing the spatial and temporal heterogeneities of the introduction, establishment, and spread of non-native insect species is a key aspect of effectively managing their geographic expansion. The Asian citrus psyllid (Diaphorina citri), a vector of the bacterium associated with huanglongbing (HLB), poses a serious threat to commercial and residential citrus trees. In 2008, D. citri first began expanding northward from Mexico into parts of Southern California. Using georeferenced D. citri occurrence data from 2008-2014, we sought to better understand the extent of the geographic expansion of this invasive vector species. Our objectives were to: 1) describe the spatial and temporal distribution of D. citri in Southern California, 2) identify the locations of statistically significant D. citri hotspots, and 3) quantify the dynamics of anisotropic spread. We found clear evidence that the spatial and temporal distribution of D. citri in Southern California is non-random. Further, we identified the existence of statistically significant hotspots of D. citri occurrence and described the anisotropic dispersion across the Southern California landscape. For example, the dominant hotspot surrounding Los Angeles showed rapid and strongly asymmetric spread to the south and east. Our study demonstrates the feasibility of quantitative invasive insect risk assessment with the application of a spatial epidemiology framework.

Oviposition Behavior and Survival of Tamarixia radiata (Hymenoptera: Eulophidae), an Ectoparasitoid of the Asian Citrus Psyllid, Diaphorina citri (Hemiptera: Liviidae), on Hosts Exposed to an Entomopathogenic Fungus, Isaria fumosorosea (Hypocreales: Cordycipitaceae), Under Laboratory Conditions

Antagonistic interactions between the nymphal parasitoid, Tamarixia radiata Waterston (Hymenoptera: Eulophidae), and the ARSEF 3581 strain of the entomopathogenic fungus, Isaria fumosorosea Wize (Hypocreales: Cordycipitaceae), could disrupt biological control of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Three interactions were evaluated under laboratory conditions at 25 °C: 1) parasitoid survival if parasitized hosts were exposed to ARSEF 3581 blastospores before or after host mummification; 2) parasitoid survival if mummies containing larva or pupa were exposed to ARSEF 3581 hyphae; 3) parasitoid oviposition on infected hosts with visible or without visible hyphae. Topical application of blastospore formulation onto the dorsal surfaces of live nymphs parasitized with second-instar wasp larva (3 d after parasitism) reduced host mummification by 50% and parasitoid emergence by 85%. However, parasitoid emergence was not affected by topical application of blastospore formulation onto mummies that contained fourth-instar wasp larva (6 d after parasitism). Parasitoid emergence was reduced by 80% if mummies containing fourth-instar wasp larva were covered with blastospore formulation colonized by fungal hyphae. In comparison, parasitoid emergence was not affected if mummies containing wasp pupa (9 d after parasitism) were covered with formulation colonized by fungal hyphae. Female parasitoids oviposited on infected hosts without visible hyphae but not on infected hosts with visible hyphae. Our findings suggest that I. fumosorosea could detrimentally affect T. radiata, if both natural enemies are simultaneously deployed for biological control of D. citri However, temporal separation of the fungus and parasitoid could reduce antagonism and enhance control of D. citri.