Transmission of plant pathogens by insect vectors is a complex biological process involving interactions between the plant, insect, and pathogen. Pathogen-induced plant responses can include changes in volatile and nonvolatile secondary metabolites as well as major plant nutrients. Experiments were conducted to understand how a plant pathogenic bacterium, Candidatus Liberibacter asiaticus (Las), affects host preference behavior of its psyllid ( Diaphorina citri Kuwayama) vector. D. citri were attracted to volatiles from pathogen-infected plants more than to those from non-infected counterparts. Las-infected plants were more attractive to D. citri adults than non-infected plants initially; however after feeding, psyllids subsequently dispersed to non-infected rather than infected plants as their preferred settling point. Experiments with Las-infected and non-infected plants under complete darkness yielded similar results to those recorded under light. The behavior of psyllids in response to infected versus non-infected plants was not influenced by whether or not they were carriers of the pathogen. Quantification of volatile release from non-infected and infected plants supported the hypothesis that odorants mediate psyllid preference. Significantly more methyl salicylate, yet less methyl anthranilate and D-limonene, was released by infected than non-infected plants. Methyl salicylate was attractive to psyllids, while methyl anthranilate did not affect their behavior. Feeding on citrus by D. citri adults also induced release of methyl salicylate, suggesting that it may be a cue revealing location of conspecifics on host plants. Infected plants were characterized by lower levels of nitrogen, phosphorus, sulfur, zinc, and iron, as well as, higher levels of potassium and boron than non-infected plants. Collectively, our results suggest that host selection behavior of D. citri may be modified by bacterial infection of plants, which alters release of specific headspace volatiles and plant nutritional contents. Furthermore, we show in a laboratory setting that this apparent pathogen-mediated manipulation of vector behavior may facilitate pathogen spread.

The present results suggest that varying levels of insecticide susceptibility exist in ACP populations across the citrus-growing areas of Florida. Increased levels of detoxifying enzymes in these populations may partially explain these differences. The present results indicate that insecticide resistance may become an emerging problem for ACP control if effective resistance management is not practiced.

The Asian citrus psyllid, Diaphorina citri Kuwayama, vectors Candidatus Liberibacter asiaticus (Las) and Candidatus Liberibacter americanus (Lam), the presumed causal agents of huanglongbing. D. citri generally rely on olfaction and vision for detection of host cues. Plant volatiles from Allium spp. (Alliaceae) are known to repel several arthropod species. We examined the effect of garlic chive (A. tuberosum Rottl.) and wild onion (A. canadense L.) volatiles on D. citri behaviour in a two-port divided T-olfactometer. Citrus leaf volatiles attracted significantly more D. citri adults than clean air. Volatiles from crushed garlic chive leaves, garlic chive essential oil, garlic chive plants, wild onion plants and crushed wild onion leaves all repelled D. citri adults when compared with clean air, with the first two being significantly more repellent than the others. However, when tested with citrus volatiles, only crushed garlic chive leaves and garlic chive essential oil were repellent, and crushed wild onions leaves were not. Analysis of the headspace components of crushed garlic chive leaves and garlic chive essential oil by gas chromatography-mass spectrometry revealed that monosulfides, disulfides and trisulfides were the primary sulfur volatiles present. In general, trisulfides (dimethyl trisulfide) inhibited the response of D. citri to citrus volatiles more than disulfides (dimethyl disulfide, allyl methyl disulfide, allyl disulfide). Monosulfides did not affect the behaviour of D. citri adults. A blend of dimethyl trisulfide and dimethyl disulfide in 1:1 ratio showed an additive effect on inhibition of D. citri response to citrus volatiles. The plant volatiles from Allium spp. did not affect the behaviour of the D. citri ecto-parasitoid Tamarixia radiata (Waterston). Thus, Allium spp. or the tri- and di-sulphides could be integrated into management programmes for D. citri without affecting natural enemies.

The high level of RR(50) values to imidacloprid and thiamethoxam suggest an unstable decline in the susceptibility of B. tabaci to imidacloprid and thiamethoxam, with possible cross-resistance or predisposition for dual resistance selection.

Previously, we reported that Candidatus Liberibacter asiaticus (Las)-infected Diaphorina citri are characterized by lower levels of cytochrome P450 monooxygenases than uninfected counterparts. In the present study, we investigated expression levels of family 4 cytochrome P450 (CYP4) genes in Las-infected and uninfected D.citri adults. Five novel CYP4 genes (CYP4C67, CYP4DA1, CYP4C68, CYP4DB1 and CYP4G70) were identified. Four of the five CYP4 genes were expressed at significantly higher levels in uninfected than Las-infected males, whereas only one was expressed at significantly higher levels in uninfected than Las-infected females. These results suggest that levels of cytochrome P450 monooxygenases in D.citri may be linked to expression levels of these CYP4 genes. Expression of all five CYP4 genes was induced by exposure of D.citri to imidacloprid, suggesting their possible involvement in metabolism of this toxin. Higher expression of the five CYP4 genes was found in nymphs than adults, which is congruent with previous results indicating higher levels of cytochrome P450 monooxygenases in nymphs than adults. These five CYP4 genes may be promising candidates for RNA-interference to silence overexpression of genes associated with insecticide resistance in D.citri. These newly identified genes may also serve as DNA-based screening markers for cytochrome P450-mediated insecticide resistance in field populations of D.citri.

Invasive fungus-growing ambrosia beetles are an emerging threat to forest ecosystems and fruit industries, but management tools are lacking. Here we explored the potential of beetle symbionts-ambrosia fungi-as a source of attractants. Our focus was the redbay ambrosia beetle, Xyleborus glabratus, and its symbiotic fungus, Raffaelea lauricola, which are devastating lauraceous hosts in the southeastern United States. We also tested three additional co-occurring beetle species and their symbionts. Each beetle species was consistently attracted to the odors of its symbiotic fungal species, occasionally also to symbionts of other species, but never to non-symbiotic Trichoderma. We further confirmed attraction to ethanol (positive control) in some species. Thus, ambrosia fungi produce volatiles attractive to their vector beetles, which may have potential as novel lures for ambrosia beetle management.

Candidatus Liberibacter asiaticus (Las) has been reported to increase the susceptibility of the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), to selected insecticides. Reduced general esterase activity in Las-infected, compared with uninfected, D. citri has been proposed as a possible explanation for this difference in insecticide susceptibility. The current study was conducted to quantify glutathione transferase (GST) and cytochrome P450 (general oxidase) activities in Las-infected D. citri to further explain the possible mechanisms for altered susceptibility to insecticides due to Las infection. GST and cytochrome P450 activities (indirectly through general oxidase levels) were quantified in Las-infected and uninfected D. citri nymphs and adults. Mean (±SEM) GST activity was significantly lower in Las-infected (468.23 ± 26.87 /µmol/min/mg protein) than uninfected (757.63 ± 59.46 µmol/min/mg protein) D. citri adults. Likewise, mean cytochrome P450 activity was significantly lower in Las-infected (0.23 ± 0.02 equivalent units [EU] cytochrome P450/mg protein) than uninfected (0.49 ± 0.05 EU cytochrome P450/mg protein) D. citri adults. Immature stages (second and fifth instars) were characterized by significantly lower GST activity than adults for uninfected D. citri. However, cytochrome P450 activity was significantly higher in second instar nymphs than adults and fifth-instar nymphs for uninfected D. citri. Lower activities of GST and general oxidase in Las-infected D. citri indicate that infection with Las alters D. citri physiology in a manner that could increase insecticide susceptibility. The reduced activities of these detoxifying enzymes due to Las infection may be explained by examining expression levels of associated genes in Las-infected and uninfected D. citri.

No abstract