Plant essential oils are commonly used to manage insects; they are widely available and some are inexpensive.  In this research we have selected five botanical oils costing less than $100 US per kilogram, to evaluate for repellency to Asian Citrus Psyllid (ACP), the insect vector of the causal pathogens of huanglongbing.  In olfactometer assays, fir oil was repellent; clove and camphor oils were attractive; and litsea and citronella oils elicited no response from ACP females.  In no-choice settling experiments, neither the low nor high fir oil treatment deterred ACP from settling.  Subsequently, ACP were presented with a choice test between control plants and fir oil plants with a single dose of fir oil contained in a polyethylene vial.  In this case, ACP disproportionately settled on control plants, avoiding fir oil baited trees completely.  Finally, we conducted a field trial using yellow sticky traps baited with a high or low dose of clove or camphor oil deployed from seven mL polyethylene vials.  We expected that the botanical oil baited yellow traps would catch more ACP than unbaited controls.  There was no significant increase in trap capture over the course of our experiment in male, female, or total ACP capture.  We hypothesize that this result may have been caused by sub-optimal release rates or the overriding visual cue elicited by yellow sticky traps. Our ongoing experiments are designed to improve the behavioral activity of release devices for these essential oils, which may have practical utility for ACP management.

Candidatus Liberibacter asiaticus (Las) is a fastidious, phloem-inhabiting, gram-negative bacterium transmitted by Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae). The bacterium is the presumed causal agent of huanglongbing (HLB), one of the most destructive and economically important diseases of citrus. In this study, we investigated whether Las is transmitted between infected and uninfected D. citri adults during courtship. Our results demonstrate that Las is sexually transmitted from Las-infected male D. citri to uninfected females at a low rate (4%) during mating. Sexual transmission was not observed following mating of infected females and uninfected males or among adult pairs of the same sex. Las was detected in genitalia of both sexes and in eggs of infected females. A minimum latent period of 7 days was required to detect the bacterium in recipient females. Rod shaped and spherical structures resembling Las were observed in ovaries of Las-infected females with transmission electron microscopy, but were absent in ovaries from uninfected D. citri females. The size of the rod shaped structures varied from 0.39 to 0.67 mm in length and 0.19 to 0.39 mm in width. The spherical structures measured from 0.61 to 0.80 mm in diameter. This investigation provides convincing evidence that a plant pathogenic bacterium is sexually transmitted from male to female insects during courtship and established evidence that bacteria persist in reproductive organs. Moreover, these findings provide an alternative sexually horizontal mechanism for the spread of Las within populations of D. citri, even in the absence of infected host plants.

In July 2012, we captured Asian citrus psyllids (ACP), Diaphorina citri, at the Lake Kissimmee State Park (Polk county, FL). ACP were captured on yellow sticky traps deployed in a wet flatwood ecosystem. Specimens were sent to the Florida Department of Agriculture and Consumer Services (FDACS) and were all identified as D. citri.

From the 12 July through 8 October 2012, we monitored the ACP population at this location. Capture of ACP on 19 July reached a maximum of 1.3 ACP per trap per week. ACP collected were submitted to qPCR and 20% of captured ACP in this forest were positive for Candidatus Liberibacter asiaticus (Las).

After exploration of the surrounding area, we found four non-cultivated tangerine plants on the border of Lake Rosalie, 1 km away from the original ACP collection site. These four plants were tested for Las and all were negative. Yellow sticky traps were also deployed on these citrus trees but no ACP were collected at this location.

Plants found in the original area of collection were identified, and to our knowledge, none are currently known as alternative hosts of ACP. We are performing bioassays and thus far, we found that ACP were able to feed and survive on gallberry (Ilex glabra L.). These results suggest that ACP may have a wider alternative host acceptance range and / or higher dispersal ability than previously thought and occur within a dense Florida forest in the absence of surrounding citrus groves within at least 3 km.

In this investigation, we experimentally demonstrated specific mechanisms through which a bacterial plant pathogen induces plant responses that modify behavior of its insect vector. Candidatus Liberibacter asiaticus, a fastidious, phloem-limited bacterium responsible for causing huanglongbing disease of citrus, induced release of a specific volatile chemical, methyl salicylate, which increased attractiveness of infected plants to its insect vector, Asian citrus psyllid (Diaphorina citri), and caused vectors to initially prefer infected plants. However, the insect vectors subsequently dispersed to non-infected plants as their preferred location of prolonged settling because of likely sub-optimal nutritional content of infected plants. The duration of initial feeding on infected plants was sufficiently long for the vectors to acquire the pathogen before they dispersed to non-infected plants, suggesting that the bacterial pathogen manipulates behavior of its insect vector to promote its own proliferation. 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 and was similar under both light and dark conditions. Feeding on citrus by D. citri adults also induced the release of methyl salicylate, suggesting that it may be a cue revealing location of conspecifics on host 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.