The recent emergence of several plant diseases caused by psyllid-borne bacterial pathogens worldwide (Candidatus Liberibacter spp.) has created renewed interest on the interaction between psyllids and bacteria.In spite of these efforts to understand psyllid association with bacteria,many aspects of their interactions remain poorly understood.As more organisms are studied,subtleties on the molecular interactions as well as on the effects of the bacteria on the psyllid host are being uncovered.Additionally,psyllid-borne bacterial phytopathogens can also affect the host plant,which in turn can impact psyllid physiology and behavior.Here,we review the current literature on different aspects of the influence of bacteria on multitrophic interactions among plants,psyllids,and pathogens.We then highlight gaps that need to be addressed to advance this field,which can have significant implications for controlling these newly emergent and other plant diseases.

Insect herbivore outbreaks frequently occur and this may be due to factors that restrict top-down control by parasitoids,for example,host-parasitoid asynchrony,hyperparasitization,resource limitation and climate.Few studies have examined hostparasitoid density relationships during an insect herbivore outbreak in a natural ecosystem with diverse parasitoids.We studied parasitization patterns of Cardiaspina psyllids during an outbreak in a Eucalyptus woodland.First,we established the trophic roles of the parasitoids through a species-specific multiplex PCR approach on mummies from which parasitoids emerged.Then,we assessed host-parasitoid density relationships across three spatial scales (leaf,tree and site) over one year.We detected four endoparasitoid species of the family Encyrtidae (Hymenoptera);two primary parasitoid and one heteronomous hyperparasitoid Psyllaephagus species (the latter with female development as a primary parasitoid and male development as a hyperparasitoid),and the hyperparasitoid Coccidoctonuspsyllae.Parasitoid development was host-synchronized,although synchrony between sites appeared constrained during winter (due to temperature differences).Parasitization was predominantly driven by one primary parasitoid species and was mostly inversely host-density dependent across the spatial scales.Hyperparasitization by C.psyllae was psyllid-density dependent at the site scale,however,this only impacted the rarer primary parasitoid.High larval parasitoid mortality due to density-dependent nymphal psyllid mortality (a consequence of resource limitation) compounded by a summer heat wave was incorporated in the assessment and resulted in density independence of host-parasitoid relationships.As such,high larval parasitoid mortality during insect herbivore outbreaks may contribute to the absence of host density-dependent parasitization during outbreak events.

"Candidatus Liberibacter solanacearum" (Lso) are phloem-restricted and unculturable Gram-negative bacteria.Presently five haplotypes have been identified worldwide;but only haplotypes A and B are associated with the vector Bactericera cockerelli ((S)ulc.) in the Americas.Previous studies showed that Lso-infection reduces B.cockerelli reproductive output and that Lso haplotype B is more pathogenic than Lso haplotype A.To understand the interaction ofLso haplotype B and B.cockerelli,the fitness of Lso-free and Lso B-infected insects,and the expression of vitellogenin (Bc Vgl-like),a gene involved directly in the insect reproduction were analyzed.Statistical differences in the number of eggs oviposited,and the total number of progeny nymphs and adults were found among crosses of insects with or without Lso.Significant differences in sex proportions were found between Lso B-infected and Lso-free crosses:a higher proportion of F1 adult females were obtained from Lso B-infected mothers.A significant reduction of BcVgl-like was observed in crosses performed with Lso B-infected females compared to the Lso-free insects.In female cohorts of different age,a significant reduction of BcVgl-like expression was measured in 7-d-old Lso B-infected females (virgin and mated) compared with 7-d-old Lso-free females (virgin and mated),respectively.The reduction of Bc Vgl-like transcript was associated with a lower number of developing oocytes observed in female's reproductive systems.Overall,this study represents the first step to understand the interaction of Lso B with B.cockerelli,highlighting the effect of Lso B infection on egg production,Bc Vgl-like expression,and oocyte development.

The potato/tomato psyllid Bactericera cockerelli causes serious damage to several solanaceous crops by direct feeding and vectoring Candidatus Libedbacter solanacearum,a bacterial pathogen.Electron beam (eBeam) irradiation is an environmentally friendly,chemical-free alternative method that is increasing in use for disinfestation of insect pests.We hypothesize that this irradiation technology will have detrimental effects on potato psyllid and thus impede its disease vectoring.To this end,we explored the effects of eBeam treatment ranging from 50 to 500 Gy on survival,development and reproduction of this pest.Impact on psyllids was apparently dose-dependent.When irradiated at 350 Gy,eggs could not hatch,1st instar nymphs failed to emerge,and although a small portion of irradiated 5th instar nymphs survived,the emerged adults were mostly deformed.Abnormality in eclosed adults suggests harmful effects of eBeam on metamorphosis.Reproduction was seriously impaired when female psyllids were exposed to eBeam at the 5th instar nymphal or young adult stage,presumably due to inability to form oocytes.In addition,reciprocal crosses between irradiated and untreated psyllids indicated that female psyllids were more radiosensitive than males to eBeam.Taken together,these findings indicate that eBeam negatively impacted potato psyllid development and reproduction,which would inevitably compromise its disease transmission capacity.A dose of 350 Gy can be considered as a reference dose for effective control of potato psyllids.

Asian citrus psyllid,Diaphorina citri Kuwayama (Hemiptera:Liviidae),trans-mits the causal pathogen of huanglongbing and is a global pest of citrus.D.citri pop-ulations exhibit resistance to multiple insecticide modes of action in areas where these chemicals have been overused.We performed genome-wide transcriptional analysis for two field populations of D.citri (Wauchula and Lake Alfred,Florida,USA) that exhibit 1300-fold resistance to the neonicotinoid insecticide,thiamethoxam,and compared it to that of susceptible psyllids collected from the same area and without imposed selection.The Lake Alfred population responded to insecticide resistance by up-regulation of 240 genes and down-regulation of 148 others.The Wauchula population exhibited similar pat-terns to the Lake Alfred population with up-regulation of 253 genes and down-regulation of 115 others.Gene Ontology annotation associated with cellular processes,cell,and cat-alytic activity were assigned to differentially expressed genes (DEGs).The DEGs from Lake Alfred and Wauchula populations were mapped to Kyoto Encyclopedia of Gene and Genomes pathways and implicated enrichment of metabolic pathways,oxidative phos-phorylation,extracellular matrix-receptor interaction,terpenoid backbone biosynthesis,and insect hormone biosynthesis in the resistant populations.Up-regulation of 60s ribo-somal proteins,UDP-gluscoyltransferases,cytochrome c oxidases,and CYP and ABC transporters among thiamethoxam-resistant D.citri implicates a broad array of novel and conventionally understood resistance mechanisms.

The Asian citrus psyllid,Diaphorina citri,is the vector of the pathogenic bacteria that causes Huanglongbing,the most devasting disease affecting citrus worldwide.As cultivation of citrus tends to expand northward,the tolerance of psyllids to freezing temperatures needs to be investigated.While mortality of D.citri to cold temperature has been previously studied,much less is known regarding the acclimation potential of psyl-lids to cold temperatures.We first evaluated cold resistance of D.citri depending on color morph and sex.Subsequently,we compared mortality of D.citri at-4℃for 10 hours between unacclimated psyllids,psyllids that were gradually acclimated to cold over 1 or 2 weeks,and psyllids intermittently acclimated also for 1 or 2 weeks.We did not find major differences in cold resistance between color morphs and sex.We found that after 1 week of gradual acclimation,the survival of psyllids at-4℃increased to 86％,compared to 20％in the control group.Survival did not increase after the second week of gradual acclimation.Although intermittent acclimation improved survival compared to the control group,it was less effective than gradual acclimation with a survival at 30％and 70％after 1 and 2 weeks of acclimation,respectively,although this difference with gradual acclima-tion was not significant at week 2.These data show that gradual cold acclimation allows D.citri to survive brief periods below freezing.It may serve as a mechanism responsible for increasing northern establishment of this pest.