Diurnal Patterns of Flight Activity and Effects of Light on Host Finding Behavior of the Asian Citrus Psyllid

The Asian citrus psyllid, Diaphorina citri (Hemiptera: Psyllidae), is an invasive pest of citrus in the United States. The psyllid feeds and reproduces primarily on new flush growth of citrus and other rutaceous plants. Because it vectors the bacterial causal agents of the deadly citrus greening disease, D. citri is potentially a pest of economic importance in all citrus growing areas where it occurs together with the disease. We investigated the diurnal patterns of its flight activity in the field and the effects of light on its host selection and egg laying behaviors. The numbers of adult psyllids caught on yellow sticky traps were 3 to 4-fold higher during daytime than nighttime. Daytime flight activity of D. citri adults also varied with time of the day with peak catches occurring at midday from 1200 to 1500 h. Illumination of the traps at night increased their attractiveness to adult psyllids by 5-fold. Similarly, light significantly increased plant colonization by adults and female egg deposition on potted plants in the laboratory. These results showed that the flight activity and host selection behavior of adult psyllids are regulated by light and circadian rhythms. Thus, adult psyllids utilize light as visual cues in their host-plant selection process.     

Factors associated with Diaphorina citri immigration into commercial citrus orchards in São Paulo State,Brazil

Seasonality often influences multiple aspects of vector-borne plant diseases. With respect to the management of plant disease vectors, an understanding of the factors driving seasonal changes in vector flight activity may improve management outcomes by facilitating more proactive application of control measures. One of the most challenging issues for citrus Huanglongbing management is to forecast when disease primary spread by bacteriliferous Diaphorina citri will occur. We monitored the temporal patterns of immigrating psyllids, using upwards of 1,200 traps spread among 9 citrus farms and checked on a weekly basis over 4 years. This dataset was analysed with a set of hierarchical models to estimate the effect of climatic variables on citrus foliage production (i.e. flush), and effects of climatic variables and citrus foliage dynamics on D. citri catches over the season. The results showed substantial seasonal variability in immigrating D. citri abundance, with the critical dispersal/migration period occurring between end of winter and spring. During this period, 65% of the total psyllids were collected on sticky traps. Seasonality of immigrating D. citri coincided with changes in certain climate variables, with negative effects of humidity and daily maximum temperatures, and a positive effect of rainfall amount in prior weeks. Maximum temperature and both daily minimum temperature and rainfall during prior weeks were also associated with new citrus flush production, which itself was positively related to immigrating D. citri abundance. Based on these results, citrus growers should be aware and intensify the frequency of psyllid control tactics (i.e. insecticide and/or kaolin sprays) during this period in order to prevent Ca. L. asiaticus transmission. These results are an important step towards developing the predictive framework needed to refine D. citri and huanglongbing management.     

Visual Responses of Adult Asian Citrus Psyllid (Hemiptera: Liviidae) to Colored Sticky Traps on Citrus Trees

The effects of five differently-colored sticky traps in capturing adult Diaphorina citri were evaluated in citrus orchards. Trap catches of D. citri were monitored fortnightly on blue, green, red, white and yellow sticky cards placed on three citrus varieties during D. citri active flight period from April to July in south Texas. Evaluation of mean trap catches of each color by repeated measures analysis of variance produced three separate groups: yellow traps caught significantly more D. citri adults than the other four traps; red and green traps caught significantly more D. citri than blue and white traps, which were not significantly different. Although the number of adult psyllid captured on all trap types significantly increased with time during the trapping period, the performance of traps did not change with time. Trap catches were also significantly influenced by the citrus species; traps placed on lemon trees captured more D. citri than those placed on sweet orange and grapefruit, suggesting that plant preference exhibited by D. citri may influence the performance of traps. The ratio of trap reflectance between the 680 to 700 nm and the 450 nm was significantly correlated with total trap catches in all host species studied. Thus, this index was a good indicator of the attractiveness of adult D. citri to colored traps. Additionally, we compared the reflectance values of young versus mature flush shoots of the three host plants used in this study as related to densities of D. citri recorded in colored traps. We discussed the importance of visual cues in the host finding behavior of adult D. citri.     

Secondary hosts of the Asian citrus psyllid,Diaphorina citri Kuwayama: Survivorship and preference

The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most economically important and difficult to manage citrus pest in Florida due to its role as vector of huanglongbing or citrus greening disease. Spread of the disease is a function of dispersal patterns and thus influenced by availability of suitable hosts. Young leaves of citrus or related plants are required for reproduction, but in their absence, secondary hosts may provide needed resources to enhance survival. Therefore, survivorship on and preference for three abundant weed species was investigated. The suitability of potential secondary host plants Bidens alba (L.), DC, Ludwigia octovalvis (Jacqu.) P. H. Raven, and Eupatorium capillifolium (Lam.) Small was compared to a reproductive host, Murraya paniculata (L.) Jack, in no‐choice tests by assessing survivorship of D. citri adults confined to these plants in cages. Preference was evaluated by choice tests where D. citri adults were released into cages containing all three secondary hosts alone or with M. paniculata. Both B. alba and E. capillifolium increased D. citri survivorship by twofold compared to starvation conditions with only water available. Choice trials revealed no difference in initial selection between true and secondary hosts; however, the true host was favoured over time. This result suggested that hosts were selected initially by sight, and only later by taste and/or smell. While secondary hosts are unable to support reproduction or long‐term survival, these findings establish the ability of D. citri to use secondary hosts that are ubiquitous in Florida citrus groves as temporary reservoirs for food and moisture when ideal host conditions are scarce or absent.     

Incidence of “Candidatus Liberibacter asiaticus” in a Florida population of Asian citrus psyllid

The incidence of a bacterium “Candidatus Liberibacter asiaticus” was assessed in a Florida population of Asian citrus psyllid, Diaphorina citri. The bacterium is the presumed causal agent of Asiatic huanglongbing, a serious citrus disease. Adult D. citri were periodically collected between May 2010 and September 2012 in a block of diseased trees located in east‐central Florida. The psyllids were individually subjected to molecular analyses (quantitative polymerase chain reaction assays using HLBaspr primers) to determine whether the bacterium was present and, if so, the population level of the pathogen based on qPCR cycle threshold (CT) values. Significantly greater percentages of females tested positive for the pathogen than males, but there were no significant differences between females and males with respect to population levels of the pathogen within the psyllids. No significant differences were found among the three D. citri colour morphs with respect to percentages of adults testing positive for the pathogen. Among 47 sample dates, a mean of 17.5% of adults per sample date tested positive (CT < 36) for the pathogen with a mean CT value of 31.1. The incidence of the pathogen was generally higher during late fall or early winter and often lower during mid‐ to late summer. There was a significant negative correlation between percentages of D. citri testing positive and air temperature. Increases in the incidence of the pathogen may not necessarily correspond to increases in transmission, as a number of factors both internal and external to D. citri can influence transmission. Transmission rates may be highest during periods when D. citri infestation levels are large, a high percentage of adults carry a high population of the pathogen in their salivary glands, and citrus flush is abundant.     

Patterns of habitat use by the Asian citrus psyllid,Diaphorina citri,as influenced by abiotic and biotic growing conditions

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Effect of plant barriers and citrus leaf age on dispersal of Diaphorina citri (Hemiptera: Liviidae)

Studies designed to measure dispersal capacity of Diaphorina citri Kuwayama (Hemiptera: Liviidae) are needed to provide the epidemiological knowledge necessary to improve management of citrus huanglongbing. In this study, a mark–release–recapture technique was used to investigate whether 1) host or non‐host plants of D. citri can act as barriers for dispersing insects and 2) presence or absence of young citrus leaves influence movement of D. citri towards citrus plants. The experimental field consisted of four circular and adjacent areas containing citrus trees, Citrus sinensis (L.) Osbeck cv. ‘Hamlin’, planted in concentric circles at 18, 24 and 30 m from the release centre. Insect activity was monitored by recapturing at each distance using yellow stick traps. Dense plantings of tall non‐host plants of D. citri such as corn had no effect on insect dispersal towards citrus plants when compared to a shorter cover crop such as grass. In contrast, suitable host plants acted as traps decreasing movement of D. citri. Diaphorina citri dispersed at greater speeds in the absence of young leaves reaching 140 m within 6 hours after release, whereas in the presence of young leaves, individuals reached at most 60 m at 1 day after release. Results suggest that D. citri control measures may be more efficient during periods of highest vegetative activity when insects are less active. Moreover, the use of suitable host plants for D. citri as trap plants may be a potential tactic to prevent movement of insects into the crop.     

Monitoring methods for Diaphorina citri Kuwayama (Hemiptera: Liviidae) on citrus groves with different insecticide application programmes

The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae), is a major citrus pest that transmits the bacteria Candidatus Liberibacter asiaticus and Ca. L. americanus associated with huanglongbing (HLB) disease. Diaphorina citri population densities can affect the effectiveness of its monitoring and sampling methods. Thus, we compared different methods for adult D. citri monitoring in groves with and without insecticide application programmes. Four short‐term experiments were carried out, each one lasting four consecutive weeks. In these experiments, sticky cards with different colours (yellow, light green, green and dark green), sweep net, two suction device models, visual inspection and stem tap sampling were assessed. Two long‐term experiments were conducted for 4.5 and 5 years, in which only yellow sticky card and visual inspection for D. citri monitoring were assessed. For the short‐term experiments, psyllids were detected by all monitoring methods during all sampling periods in areas without chemical control. However, in areas with psyllid control via fortnightly and monthly applications of insecticides, only sticky cards, regardless of their colour, were able to detect the presence of D. citri. Similarly, for the long‐term experiments, yellow sticky cards were more effective than visual inspection for detecting and quantifying D. citri in all areas with or without insecticide application. Therefore, in areas where HLB is present and chemical control of psyllid is required, sticky cards are the most reliable option for monitoring D. citri.     

Leaf age affects the efficacy of insecticides to control Asian citrus psyllid,Diaphorina citri (Hemiptera: Liviidae)

Huanglongbing (HLB), the most destructive citrus disease worldwide, is associated with the bacteria transmitted by the psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae). One of the main tactics used to manage this disease is the chemical control of D. citri. In this study, the efficacy of four insecticides commonly used by the citrus growers against the D. citri adults was assessed in two vegetative growth stages of flush: young shoots (~ 10 cm in length) and mature leaves. The bioassays were carried out under the greenhouse and field conditions. The insecticides bifenthrin, dimethoate, imidacloprid and thiamethoxam were tested at the recommended rates of 15, 450, 40 and 25 mg/L of water, respectively. The insecticides were sprayed onto the leaves of each stage until runoff. Psyllid mortality was assessed at different periods after insecticide application (0, 1 and 2 weeks). In both greenhouse and field trees, the period of control was shorter in the young shoots than in the mature leaves. In addition, the efficacy of insecticides rapidly decreased in the young shoots over time, mainly for imidacloprid and bifenthrin (less than 1 week). Thus, these results suggest that the number of spray applications to control D. citri should be more frequent during the flushing period.     

Chemical and behavioral analysis of the cuticular hydrocarbons from Asian citrus psyllid,Diaphorina citri

Huanglongbing (HLB) is the most destructive disease of citrus worldwide. The Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Psyllidae), is the vector of the phloem‐inhabiting bacterium, Candidatus Liberibacter asiaticus, which is presumed to cause HLB in Florida citrus. Laboratory and field studies were conducted to examine the behavioral responses of male and female D. citri to their cuticular extracts. In olfactometer assays, more male D. citri were attracted to one, five, or 10 female cuticular extract equivalent units than blank controls. The results were confirmed in field studies in which clear or yellow traps baited with 10 female cuticular extract equivalent units attracted proportionately more males than clear traps baited with male cuticular extract or unbaited traps. Analyses of cuticular constituents of male and female D. citri revealed differences between the sexes in chemical composition of their cuticular extracts. Laboratory bioassays with synthetic chemicals identified from cuticular extracts indicated that dodecanoic acid attracted more males than clean air. Traps baited with dodecanoic acid did not increase total catch of D. citri as compared with blank traps at the dosages tested; however, the sex ratio of psyllid catch was male biased on traps baited with the highest lure loading dosage tested (10.0 mg).