The objective of this study is to determine how enhanced nutrition of citrus plants may affect Asian citrus psyllid (ACP) biology. We have initiated this study with complementary field and laboratory experiments. Regarding the field trial, we are still following the Keyplex’ program. The two latest spray applications occurred in July and August. We followed ACP populations in the replicate sprayed and non-sprayed control plots from March to September. As mentioned in the previous report, from March to May we consistently observed more ACP on HLB-infected trees compared to uninfected trees. However, since June, when the ACP population increased significantly, we did not observe this difference between populations on infected and uninfected trees. Regarding the effect of Keyplex’ on infected versus non-infected plants, we observed the reverse trend on psyllid behavior: from March to May we did not observe a difference in ACP population between trees with the foliar nutrient supplement applications versus the control trees. However since June, we observed consistently more ACP on the nutrient supplemented trees than on the untreated control trees. Samples from these trees are being analyzed to see if the difference observed are statistically significant, but these results corroborate what we observed in previous laboratory experiments, where psyllids were more attracted to infected plants that were treated with the nutritional program compared to untreated, but infected control plants. Currently, we are performing an experiment to investigate the effect of nutrient sprays on HLB acquisition by ACP. Earlier, we observed that fewer ACP acquired the HLB bacterium from trees that were infected and supplemented with nutrients than from infected trees that were not supplemented after 10 days of psyllid exposure to trees in laboratory experiments. However, the results from field experiments did not corroborate these laboratory data. We did not find a significant difference between HLB acquisitions by psyllids when they were exposed to HLB-infected trees during the nymph stage on supplemented versus non-supplemented trees. We are currently analyzing the results of Candidatus Liberibacter (HLB pathogen) acquisition by ACP from field experiments after 10 and 28 days of psyllid exposure to trees. In similar laboratory experiments, we are analyzing psyllids after 28 days of exposure to trees. These experiments have compared psyllid acquisition of the HLB pathogen from infected trees that were either supplemented or not supplemented with nutritional sprays.
We have completed our annual survey of insecticide resistance in populations of Asian citrus psyllid across various growing locations throughout Florida. Five sites were evaluated in Lake Alfred, Winter Garden, Ft. Pierce, and La Belle across which 18,000 psyllids were collected and used in resistance screening assays. Insecticides at the LD50 value evaluated were carbaryl, chlorpyrifos, fenpropathrin, flupyradifurone, imidacloprid, and thiamethoxam, representing a range of chemistries available for psyllid population management. A focus on neonicotinoids was conducted to make comparisons of currently used chemistries, imidacloprid and thiamethoxam, first and second-generation, respectively, against, flupyradifurone, a new neonicotinoid, to determine base-line susceptibilities and to evaluate potential cross-resistance. An overall decrease in resistance levels was observed during this year’s survey for all insecticides tested as compared to previous surveys in 2011 and 2012. This drop may be due to better rotational practices between insecticides with different modes of action by growers, movement of psyllids from non-treated areas, or collection of psyllids from different sites than from previous years. The bottom line is that in the spring/summer of 2013, we did not observe that resistance appeared to be a significant problem in psyllid populations throughout Florida. Resistance found to be significantly higher in the field as compared with the laboratory susceptible control culture occurred for imidacloprid (1.8 fold ratio) and fenpropathrin (2.2 fold ratio) at Ft. Pierce and Lake Alfred, respectively. No other levels of resistance of statistical significance were observed in the field as compared with the susceptible laboratory population. A concomitant resistance against flupyradifurone for psyllids tested from Ft. Pierce was not observed; however, there was a trend suggesting cross-resistance. It is possible that resistance levels to neonicotinoids were too low at this site for a conclusive determination of cross-resistance between imidacloprid and the fourth-generation neonicotinoid, flupyradifurone. Additional sites are being screened for higher levels of imidacloprid resistance to identify populations to better evaluate potential cross-resistance between these insecticides. In addition, we have been developing insecticide resistant psyllid colonies against imidacloprid and flupyradifurone (and other modes of action) to address the possibility of cross-resistance directly in the laboratory.
Florida growers have reported that enhanced nutritional programs (ENPs) maintain productivity of HLB-affected trees. However, efficacy and sustainability of the nutritional approach for HLB disease management remains uncertain. Complementary studies of multiple ENPs and their individual components compared to the standard nutritional program (SNP) on nursery and field trees were initiated in 2010. ENPs did slow the development of nutrient deficiency in HLB-affected trees, but it did not prevent the development of deficiencies. The slowed nutrient deficiency was not reflected in a slowed yield decline. In both field and greenhouse trees, treatment of HLB-affected trees with ENPs resulted in a reduced fibrous root density compared to trees under the SNP. Healthy trees treated with ENPs had a slight increase in fibrous root density compared to SNP. It appears that ENP treatment of HLB-affected trees may accelerate fibrous root loss, increasing the susceptibility of the trees to stress conditions such as drought. In nursery trial, ENPs reduced the intensity of visual foliar symptoms, but not the percent of canopy with symptoms. Only on HLB affected trees, ENPs caused a reduction in total number of leaves in the canopy, although they did not change the number of new leaves produced. This suggests that ENPs induce increased leaf drop. Based on symptom observations, it is likely that the leaf drop is of leaves that would have expressed strong symptoms. Combined, these results suggest that ENPs are masking HLB damage to the tree by decreasing the percentage of canopy with easily visible yellowing, but not improving the biomass or health of the tree. A manuscript describing the results of this project has been submitted to a peer-reviewed journal and is currently under review for publication.
The objectives of this project were to screen insecticidal peptides for efficacy against the Asian citrus psyllid (ACP) citrus leafminer, brown citrus aphid (BCA), and T. radiata and to determine the lethal and sublethal effects of antimicrobial peptides on ACP and their associated microorganisms, including Candidatus Liberibacter asiaticus (CLas). A citrus tristeza virus (CTV) vector system was used to express candidate insecticidal peptides in plants tissue. After screening a range of insecticidal peptides against ACP in artificial feeding bioassays, four insecticidal peptides (A-D) were selected and transformed into citrus tristeza virus (CTV) vectors for further evaluation against citrus pests. For each insect species, we evaluated life history parameters (fecundity, nymph development, and survival) and feeding behavior. The effect of CTV-expressed peptides on ACP host preference and CLas titer were also evaluated in laboratory bioassays. We have identified one peptide that, when expressed in Valencia in a citrus tristeza virus-based expression vector system, elicits negative responses from two phloem-feeding citrus pests. Additionally, we have demonstrated that all peptides evaluated disrupt various aspects of ACP biology and behavior, including feeding and settling. Reductions in feeding and ACP CLas titer in response to peptide presence suggests that use of the CTV-peptide expression system may provide an effective tool for management of CLas transmission as part of an integrated pest management program.
Reducing metallic copper for canker control: Control on grapefruit – Nordox at 33% of the full rate (0.66 lb/acre) was mixed with soluble Magna-Bon (100 ppm rate ) and compared with Nordox at the full rate (1.33 lb/acre) and at a reduced rate (1.0 lb/acre). Nordox at the full rate (1.33 lb/acre) was applied in the first 5 applications and Magna-Bon (100 ppm rate; Nordox 1.33/lbA in the frst 5 applications followed by 5 applications of MB was compared with 10 apps of Nordox the full rate (1.33 lb/acre). Fruit disease control for the mixture of Nordox with Magna-Bon or the program with Nordox applied early and Magna-Bon applied later in the season provided comparable control to the full rate of Nordox. Each program reduced the metallic Cu applied per season by 50% Control on Hamlin oranges – Kocide at 33% of the full rate (1.0 lb/acre) was mixed with soluble Magna-Bon (100 ppm rate ; 37 oz.) and compared with Kocide at the full rate (3.0 lb/acre ) and at a reduced rate (2.5 lb/acre). Reduction of canker-=induce fruit drop for the mixture of Kocide plus Magna-Bon was comparable to the full rate of Kocide at half the metallic rate. The Kocide plus Magna-Bon mixture achieved canker control with 50% of the applied metallic Cu. SAR for canker control – Currently, we are collaborating with Syngenta in an EPA-approved Experimental Use Program (EUP) in two east coast grapefruit groves to support the labeling of Actigard (ASM) integrated with copper sprays for control of canker on bearing grapefruit.
Objective 1: Assays of non-bearing trees indicate that soil drench of chelated copper formulations is a consistently effective application method for increasing copper status of young leaves well above the baseline concentration. Therefore, preventative and curative activity of chelated copper bactericides are being followed in pre- and early stage HLB infected trees. Tree health ratings and samples for PCR status of the trees in all trials will be collected and analyzed later this fall. In the meantime, grower trials of Magna-Bon (MB) and Copper Quik (CQ) are underway based on promising visual observations of improved tree health after 18 months of soil applications of rates varying from 1-2 gal product per application and 2-3 applications per season. One grower location will provide the opportunity to compare fruit drop due to HLB in matched 3 acre sub-blocks with and without soil applications of CQ for the last two seasons.
Abandoned citrus groves in US citrus-producing regions are potential sources for both Asian citrus psyllid (ACP), and Candidatus Liberibacter asiaticus (CLas), the bacterium that causes Huanglongbing (HLB). In Florida and Texas where grower-driven area-wide management programs are underway in commercial groves, no psyllid control is being implemented in abandoned citrus. Since ACP adults are highly mobile, they can disperse from abandoned to productive citrus groves. If not controlled, these psyllids will stymie the effectiveness of area-wide management programs aimed at containing the spread of HLB in commercial citrus. Insecticide-based strategies are not feasible for ACP in abandoned citrus, but biological control of ACP could come from a combination of native or introduced arthropods and pathogens. ACP is susceptible to a native entomopathogenic fungus, Isaria fumosorosea (Ifr). Two Ifr strains highly pathogenic against ACP and available as spore formulations in the US are the commercial strain PFR97 WDG’ (Certis Inc.,USA) and a south Texas isolate (Ifr 3581). We will develop a novel autodissemination system for inoculating ACP with Ifr and use these infected psyllids to instigate epizootics and rapidly reduce ACP populations in abandoned groves. Our ultimate goal is to provide optimized strategies for integration of Ifr autodissemination into current management practices. Suppression of ACP in abandoned citrus by Ifr autodissemination will significantly lower the risk of immigrating adults spreading HLB to commercial groves. Our proposed project objectives for the first and second quarters of Year 1 (April 2013 to September 2013) were to evaluate: (1) Impact of Ifr dispensers on ACP populations in abandoned and managed trees, and (2) Effect of time and exposure on Ifr dispensers. For Objective 1, we will mark adult psyllids in abandoned grove plots in South Texas and compare the numbers of marked psyllids that move from abandoned into managed grove plots in the absence of Ifr dispensers (control) and when Ifr 3581 dispensers are hung in trees on the edge rows of the abandoned groves. ACP movement, infestation, densities, and Ifr infection will be monitored weekly on edge row trees of the abandoned and managed plots. For Objective 2, we will expose dispensers to outside weathering on citrus trees from groves also used for Objective 1 and evaluate the efficacy of Ifr formulation recovered at different time intervals. To accomplish Objective 1 and 2, we have located five suitable pairs of managed groves and abandoned groves in the citrus producing regions of Hidalgo County and Cameron County in the Rio Grande Valley. These pairs of managed and abandoned groves consist of orange or grapefruit trees and the abandoned groves have not been managed for at least 2 years. The efficacy of our management tactic will largely depend on the ability of our Ifr dispensers to attract adult psyllids to the dispensers’ spore-coated surfaces. The use of effective attractants for our dispensers is essential for the completion of Objectives 1 and 2. During the spring and summer of 2013, we conducted field trials to evaluate the relative efficacy of six different blends of synthetic aromatic compounds mimicking the flush volatile profiles of orange jasmine and Valley lemon for luring adult psyllids onto yellow sticky traps. Based on the results of these trials, we have selected two proprietary blends for dispensers that will be evaluated in field trials.
We conducted 3 assays to compare the effectiveness of a commercially available entomopathogenic nematode (EPN) species Heterorhabditis bacteriophora to that of Steinernema riobrave which is documented to be the most effective EPN pathogen of Diaprepes abbreviatus, but is not currently available commercially. In the basic assay, weevil larvae are preconditioned at 10 C or 27 C for 48 h. Then weevil larvae are exposed to very low rates (2 nematodes/cm2 soil surface) of EPNs at 18 C or 27 C for 6 days, during which mortality is evaluated at 2 day intervals. Thus, temperature regimes of 10C+18C simulate a wintertime cold event and 27C+27C simulates summertime conditions. Results of these trials revealed that: 1. H. bacteriphora was 50% as effective as S. riobrave in summer. 2. S. riobrave was 10% more effective in winter than it was with nonstressed weevils during summer 3. H. bacteriophora was 60% more effective in winter than it was with nonstressed weevils during summer 4. H. bacteriophora was 80% as effective in winter as Sr was with nonstressed weevils during summer We subsequently obtained an isolate of H. bacteriophora from upstate New York. In a single trial, not yet repeated, the New York isolate was somewhat more effective than the commercial isolate of H. bacteriophora during simulated winter cold events. We most recently obtained a commercial product containing Steinernema krauseii a temperate species capable of infecting hosts at temperatures as low as 8 C. In a single trial, not yet repeated, S. krauseii was ineffective against weevils under simulated summertime conditions. It was much more effective in the cold temperature regime, but inferior to H. bacteriophora and S. riobrave in its ability to kill weevil larvae.
Entomopathogenic nematodes: Amending soils to increase biological control of insect pests Because we found increased numbers of Steinernema diaprepesi in low pH treatments (as measured by qPCR of nematodes from soil samples) at the Bartow experiment (May 2013 report), we buried caged weevil larvae in all plots and evaluated the mortality rate after 7 days in the soil. In contrast to our predictions, weevil mortality and infection of weevils by S. diaprepesi were only 15% (74% vs 64%, ns) and 32% (33% vs 25%, ns) higher, respectively, in sulfur amended than non-amended treatments. Heterorhabditis indica infection of weevils was 8-fold higher (18% vs 2%, P<0.05) in the non-amended plots which conforms to the relationship measured in the Schumann ACPS trial in which H. indica were significantly more abundant in the high pH ACPS plots than in the low pH conventional citriculture plots. We Initiated a second round of buried sentinel weevils to confirm the results, to be reported in September 2013. In June 2013 the average fibrous root weight and numbers free living nematodes from sulfur-amended trees at Bartow were 60% (ns) and 56% (P<0.05), respectively, of those from non-amended trees. The abundance of Tylenchulus semipenetrans was 5-fold greater in non-amended than in sulfur amended plots (P<0.05). These results continue trends noted previously and suggest that sulfur is reducing fibrous root density somewhat (perhaps because roots are more efficient at lower pH), but is also affecting the citrus nematode independently of its affect on root availabilty. This is consistent with previous reports of the effects of pH on the citrus nematode and demonstrates that the potential for managing this nematode by pH adjustment is greater than previously thought. Completed a second manuscript (Cultural practices to manage a bacterial disease alter entomopathogenic nematode communities and increase the severity of a pest-disease complex) describing effects of ACPS and fabric mulch on citrus tree growth and yield, Diaprepes abbreviatus, Phytophthora nicotianae, EPNs and natural enemies of EPNs. First manuscript in this series (Campos-Herrera, R., et al. 2013. New citriculture system suppresses native and augmented entomopathogenic nematodes. Biological Control 66: 183-194.) published in June. Plant parasitic Nematodes: Characterizing a new nematode pest and the prevalence of resistance breaking populations of the citrus nematode. Evaluated nematicidal efficacy in the east coast, dagger nematode nematicide trial in June. No chemical treatment (oxamyl, and three experimental nematicides) reduced dagger nematode populations significantly. Plots will receive additional treatments at the end of the rainy season in early autumn and treatment effects measured in November. Dagger nematode spatial pattern at site on the central ridge was characterized and a second trial will be initiated this autumn.
Our long term goal is to develop a system for inoculating Asian citrus psyllid (ACP) with a native pathogenic fungus, Isaria fumosorosea (Ifr), and use these infected psyllids to rapidly ‘autodisseminate’ the pathogen to ACP populations in residential citrus. Specific research objectives for 2012-2013 were as follows: (1) Initiate Ifr-epizootics among ACP populations on dooryard citrus trees, (2) Evaluate seasonal and environmental effects on infection levels, (3) Determine the impact of pathogen on arthropod biological control agents. For objectives 1 and 2, we secured the permission of trailer park residents in the Rio Grande Valley to use their citrus trees for our field studies. To minimize the risk of spreading HLB, we agreed to: (1) inoculate and release only ACP adults collected from the study sites, and (2) use only trees infested by wild ACP. Since our last report submitted in April of 2013, the following trials were completed: “Quantify infection of Ifr-inoculated adults under field conditions and the effects of incubation treatments prior to field release”. To better assess what release rates of inoculated psyllids would be required to initiate epizootics within target trees, we conducted trials to quantify infection of Ifr-inoculated adults under field conditions and the effects of incubation treatments prior to field release. The rationale behind the incubation treatments were to develop a means of maximizing infection of inoculated psyllids after release on target trees. During June to September of 2013, we conducted field studies using orange trees at the ‘Victoria Palms’ trailer park in Donna, TX. Adult psyllids collected from orange trees at ‘Victoria Palms’ were inoculated with Ifr formulation and held (incubated) in humid vials with orange leaf disks for: 0 h, 24 h, and 48 h . Another set of adults were not inoculated and set aside as controls. Each psyllid was released into a ‘clip cage’ attached to a leaf on an orange tree at the trailer park. Groups of four clip cages (one cage for a control psyllid and one cage for each of the three types of inoculated psyllids) were attached to four apical leaves on the same stem. Ten groups of clip cages were setup on the North-East canopy of trees (total = 40 clip cages per tree). For eight consecutive days, we recorded the status of the psyllid within each clip cage. By the eighth day, all inoculated and incubated psyllids were dead and infected with Ifr. In comparison, 50 % of psyllids inoculated but not incubated were dead and only 20 % of these psyllids were infected with Ifr. None of the control psyllids were killed or infected by Ifr. These results demonstrated that incubation of Ifr-inoculated adults for at least 24 h will ensure 100 % mortality and infection within 8 days of release in residential citrus trees under summer weather conditions in Texas. We are currently repeating these studies under cooler and wetter fall conditions that should enhance infection of ACP by Ifr. “Effects of Ifr spores on lacewing larvae”. During June of 2013, we compared predation by lacewing larvae on healthy or Ifr-inoculated adults of ACP. We collected lacewing larvae from residential orange trees infested with ACP and held them for 12 h on orange leaf disks in plastic vials. After 12 h, we offered four healthy ACP adults and then four Ifr-inoculated ACP adults (inoculated by dusting with Ifr formulation 24 h earlier) to lacewing larvae that were similar in size and development. We observed each lacewing larva and found they accepted and fed on all the healthy ACP adults and also all the Ifr-inoculated adults. None of the lacewing larvae showed visible signs of infection by Ifr and appeared healthy after being held for five days in leaf-disk vials and fed additional healthy ACP adults. This study demonstrated that lacewing larvae do not appear to be adversely affected by feeding on ACP adults inoculated with Ifr formulation.
Our long term goal is to develop a system for inoculating Asian citrus psyllid (ACP) with a native pathogenic fungus, Isaria fumosorosea (Ifr), and use these infected psyllids to rapidly ‘autodisseminate’ the pathogen to ACP populations in residential citrus. Specific research objectives for 2012-2013 were as follows: (1) Initiate Ifr-epizootics among ACP populations on dooryard citrus trees, (2) Evaluate seasonal and environmental effects on infection levels, (3) Determine the impact of pathogen on arthropod biological control agents. For objectives 1 and 2, we secured the permission of trailer park residents in the Rio Grande Valley to use their citrus trees for our field studies. To minimize the risk of spreading HLB, we agreed to: (1) inoculate and release only ACP adults collected from the study sites, and (2) use only trees infested by wild ACP. Since our last report submitted in April of 2013, the following trials were completed: “Quantify infection of Ifr-inoculated adults under field conditions and the effects of incubation treatments prior to field release”. To better assess what release rates of inoculated psyllids would be required to initiate epizootics within target trees, we conducted trials to quantify infection of Ifr-inoculated adults under field conditions and the effects of incubation treatments prior to field release. The rationale behind the incubation treatments were to develop a means of maximizing infection of inoculated psyllids after release on target trees. During June to September of 2013, we conducted field studies using orange trees at the ‘Victoria Palms’ trailer park in Donna, TX. Adult psyllids collected from orange trees at ‘Victoria Palms’ were inoculated with Ifr formulation and held (incubated) in humid vials with orange leaf disks for: 0 h, 24 h, and 48 h . Another set of adults were not inoculated and set aside as controls. Each psyllid was released into a ‘clip cage’ attached to a leaf on an orange tree at the trailer park. Groups of four clip cages (one cage for a control psyllid and one cage for each of the three types of inoculated psyllids) were attached to four apical leaves on the same stem. Ten groups of clip cages were setup on the North-East canopy of trees (total = 40 clip cages per tree). For eight consecutive days, we recorded the status of the psyllid within each clip cage. By the eighth day, all inoculated and incubated psyllids were dead and infected with Ifr. In comparison, 50 % of psyllids inoculated but not incubated were dead and only 20 % of these psyllids were infected with Ifr. None of the control psyllids were killed or infected by Ifr. These results demonstrated that incubation of Ifr-inoculated adults for at least 24 h will ensure 100 % mortality and infection within 8 days of release in residential citrus trees under summer weather conditions in Texas. We are currently repeating these studies under cooler and wetter fall conditions that should enhance infection of ACP by Ifr. “Effects of Ifr spores on lacewing larvae”. During June of 2013, we compared predation by lacewing larvae on healthy or Ifr-inoculated adults of ACP. We collected lacewing larvae from residential orange trees infested with ACP and held them for 12 h on orange leaf disks in plastic vials. After 12 h, we offered four healthy ACP adults and then four Ifr-inoculated ACP adults (inoculated by dusting with Ifr formulation 24 h earlier) to lacewing larvae that were similar in size and development. We observed each lacewing larva and found they accepted and fed on all the healthy ACP adults and also all the Ifr-inoculated adults. None of the lacewing larvae showed visible signs of infection by Ifr and appeared healthy after being held for five days in leaf-disk vials and fed additional healthy ACP adults. This study demonstrated that lacewing larvae do not appear to be adversely affected by feeding on ACP adults inoculated with Ifr formulation.
Five studies were conducted to investigate factors influencing acquisition and inoculation of Candidatus Liberibacter asiaticus (Las) by Diaphorina. citri, e.g vector developmental stage, feeding periods, leaf age, symptom expression/bacterial titer in infected trees and insecticide treatments, in order to improved Huanglongbing management strategies. In study 1, we compared the ability to acquire Las by different developmental stadia and adults of D. citri on Las-infected plants over a range of acquisition access periods (AAPs). Mean acquisition efficiencies by adults and 1st, 2nd, 3rd, 4th and 5th instars were 42.9, 68.4, 60, 66.6, 84 and 65.3%, respectively. Third-instar nymphs and adults acquired Las in AAPs as short as 30 min, but efficient acquisition (>90%) required longer AAPs (1 and 4 days, respectively). Transmission efficiency reached a maximum of 9.4 and 4.8% (per individual) with a 12-h AAP by nymphs and adults, respectively. By comparing electrical penetration graphs (EPG) of nymphs and adults, we found no significant variations in probing behavior that could explain differences in transmission efficiency. In study 2, we observed that efficient acquisition depends on the availability of young leaves in Las-infected plants, apparently because phloem ingestion by D. citri adults is more frequent and last longer on younger leaves . In study 3, vector competence was assessed as the proportion of successful acquisition events by adult psyllids over a gradient in Las infection levels in citrus trees and over time after graft inoculation of this pathogen. We observed that plant infection levels (pathogen titer) increased rapidly over time, saturating at uniformly high levels (approximately 108 CN per g of plant tissue) near 200 days after inoculation ‘ the same time at which all infected trees first showed disease symptoms. Pathogen acquisition by vectors was positively associated with pathogen titer and time since inoculation, with acquisition occurring as early as the first measurement, at 60 days after inoculation. This result is problematic from a disease management perspective because it means that infected citrus trees may be sources of infectious vectors well before the disease symptoms become evident. Therefore, there is ample potential for psyllids to acquire the pathogen from trees during the asymptomatic (or latent) phase of infection, reducing the efficacy of diseased tree removal (rouging). In study 4, we conducted detailed transmission experiments coupled with EPG analysis of D. citri probing behavior in order to determine minimum time required for Las acquisition and inoculation, as well as the latent period. We observed Las transmission within an inoculation access period (IAP) as short as 45 min. EPG results showed that both acquisition and inoculation occur during the phloem phase, mainly during sustained sap ingestion (waveform E2). Nymphs and adults take an average of 63 min (16 – 241) and 75 min (16 – 241), respectively, to start E2. Thus, psyllids require ’15 min on citrus plants to reach citrus sieve elements and possibly acquire and inoculate Las. Once Las is acquired, D. citri is able to inoculate it in healthy plants after a latent period of at least 10 days at 25C. The mean latent period (LP50) is 16.5 and 18 days after acquisition by nymphs and adults, respectively. In Study 5, we evaluated the effects of insecticide mode of application (foliar vs. drench) on Las acquisition by D. citri nymphs and adults. Insecticides were not effective to avoid Las inoculation to healthy citrus trees, although they drastically affected bacterial acquisition. Therefore, insecticides should be more effective in preventing secondary spread, which requires bacterial acquisition by psyllids on infected plants, than primary spread by immigrating psyllids, which may be already infective when landing on healthy citrus trees in the orchard. The information on transmission biology of Las by D. citri generated in this project have direct implications on management of HLB and highlight the need of additional research to develop new control tools.
During this season, we are focusing our efforts on evaluating different formulations of imidacloprid. This research objective is in response to grower questions about the consistency of different imidacloprid formulations at a time when there are multiple generic products available. We are running two trials in commercial citrus groves – one trial on minneola tangelos and a second trial on grapefruits. We have completed 2 years of trials where we evaluated the uptake of clothianidin applied by soil drench and trunk spray. The results of the 2011 trials were very promising, whereas the results of the 2012 trials showed no residues in the trees. Due to the inconsistency of the results, we are conducting a third trial at Lindcove during the 2013 season. Soil drench and trunk spray applications at a single timing are currently being evaluated. In 2012, we evaluated the efficacy of thiamethoxam treatments on navel and Valencia oranges at several field sites in Redlands. At one of two sites where treatments were applied in October, thiamethoxam was only detected at 4 weeks. At a third site, where treatments were applied in mid-July, no thiamethoxam was detected up to 16 weeks after applications. At the latter site, monitoring was only begun at 8 weeks, so thiamethoxam residues may already have been established within the tree and then dissipated. Nevertheless, our data shows that thiamethoxam is not a persistent chemical in citrus trees. Our objective to conduct imidacloprid residue analysis on fruit from treated trees is now complete. Fruit from trees treated at 2 timings (Feb. and Mar.) with a 2X label rate of imidacloprid from 3 field sites in Bakersfield (Valencia oranges), Lindcove (navel oranges) and Hemet (grapefruit) have now been tested for residues. Samples were collected for up to 8 weeks after treatments at the Bakersfield and Lindcove sites and 16 weeks at the Hemet site. At the Bakersfield and Lindcove sites, residues were not detected (limit of detection of the assay was 0.01 ppm). At the Hemet site, residues were not detected in trees treated with a single application. However, in trees that were treated in 2 successive years with the 2X rate, residues of 0.01 ppm were detected, which is below the MRL established for citrus in the USA.
During this season, we are focusing our efforts on evaluating different formulations of imidacloprid. This research objective is in response to grower questions about the consistency of different imidacloprid formulations at a time when there are multiple generic products available. We are running two trials in commercial citrus groves – one trial on minneola tangelos and a second trial on grapefruits. We have completed 2 years of trials where we evaluated the uptake of clothianidin applied by soil drench and trunk spray. The results of the 2011 trials were very promising, whereas the results of the 2012 trials showed no residues in the trees. Due to the inconsistency of the results, we are conducting a third trial at Lindcove during the 2013 season. Soil drench and trunk spray applications at a single timing are currently being evaluated. In 2012, we evaluated the efficacy of thiamethoxam treatments on navel and Valencia oranges at several field sites in Redlands. At one of two sites where treatments were applied in October, thiamethoxam was only detected at 4 weeks. At a third site, where treatments were applied in mid-July, no thiamethoxam was detected up to 16 weeks after applications. At the latter site, monitoring was only begun at 8 weeks, so thiamethoxam residues may already have been established within the tree and then dissipated. Nevertheless, our data shows that thiamethoxam is not a persistent chemical in citrus trees. Our objective to conduct imidacloprid residue analysis on fruit from treated trees is now complete. Fruit from trees treated at 2 timings (Feb. and Mar.) with a 2X label rate of imidacloprid from 3 field sites in Bakersfield (Valencia oranges), Lindcove (navel oranges) and Hemet (grapefruit) have now been tested for residues. Samples were collected for up to 8 weeks after treatments at the Bakersfield and Lindcove sites and 16 weeks at the Hemet site. At the Bakersfield and Lindcove sites, residues were not detected (limit of detection of the assay was 0.01 ppm). At the Hemet site, residues were not detected in trees treated with a single application. However, in trees that were treated in 2 successive years with the 2X rate, residues of 0.01 ppm were detected, which is below the MRL established for citrus in the USA.
We are currently pursuing our first objective to evaluate the effects of nursery practices (watering, citrus variety, potting media) on imidacloprid uptake and retention. Two citrus varieties were selected for this trial – Parent Washington navel and Limoneira 8A lemon, both on Carrizo rootstock. The trees were budded in June 2012 and grown in 5′ pots at LREC. Trees were transferred to UCR Agricultural Operations on June 13 2013 and transplanted on June 17 to no. 5 pots. Two potting media were chosen for the study that differed principally in the proportions of sand (10-30%) and redwood (40-60%). The evapotranspiration rate was determined for the two soil mixes and this data was used to select three watering levels for the study. The three levels were defined as ‘replacement watering’ (100-120% ET), ‘overwatering (150-200% ET) and ‘severe overwatering’ (300-400% ET). For the trial, trees were placed in a 12 x 25 grid pattern and randomized according to variety, soil mix and watering level. When the trees had adapted to their new potting media environment, they were treated with 0.33 mls Admire Pro per cu ft potting media. Monitoring for ACP on all trees is being conducted at monthly intervals for the duration of the trial, and imidacloprid residues are being determined for all trees by ELISA at 2, 4, 8, 16 and 32 weeks post-treatment.
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