Coverage of the 2011 Florida Citrus Show
Florida Citrus Show Extended Content Coverage: On A Mission
Activity abounds in the pursuit of improved integrated pest management strategies for the Asian citrus psyllid. ►Watch the video now.
By David Hall June 2011
Photo courtesy of USDA Areawide management of the Asian citrus psyllid (ACP) is a hot topic among many citrus growers and researchers. USDA-ARS has been working with Indian River citrus growers to implement an areawide management program for the psyllid in Martin, St. Lucie, and Indian River counties. Of interest is the influence of psyllid host plants grown in urban areas on populations of the psyllid in commercial citrus. Orange jasmine is a favored host plant of the psyllid and is widely grown as an ornamental landscape plant. No information was available on biological control agents attacking the psyllid in urban plantings of jasmine.
USDA-ARS is particularly interested in biological control by the parasitoid Tamarixia radiata and has been surveying different plantings from Ft. Pierce to West Palm Beach for psyllid infestations and Tamarixia. Results of the project show the psyllid is common in urban plantings of orange jasmine and that Tamarixia commonly attacks the psyllid in these urban plantings. The parasitoid persists in orange jasmine under regular management practices and contributes to area-wide suppression of the psyllid.
Going The Distance Recent USDA-ARS research on seasonal flight activity by the psyllid supports areawide management strategies. Captures of the psyllid on sticky traps deployed at various distances from citrus were used to make inferences on psyllid flight activity. Research results show flight activity by both male and female psyllids away from citrus can occur at any time of the year with consistent dispersal activity during the spring. Flights 500 feet from citrus were documented. Citrus is continually subject to infestations by immigrating adults and there is no time during the year a citrus grower could be assured immigration would not occur. Preventing flights between commercial citrus groves and from urban areas to commercial citrus should be an important IPM component for the psyllid. Hard To Resist Interest in host plant resistance prompted a USDA-ARS project to search for and identify plant resistance to the psyllid in citrus and related germplasm. A planting of 87 genotypes of citrus and citrus relatives was established, and these genotypes were surveyed during 2010 for infestation levels of psyllid eggs, nymphs, and adults. Among the 87 genotypes, the following were found to be most susceptible to colonization by the psyllid: Tien Chieh mandarin, curry tree, orange jasmine, Mato Buntan pummel, Diamante citron, alemow, Swingle citrumelo, and Nansho Daidai sour orange. In free choice situations, the following genotypes were colonized least by the psyllid: Japanese prickly-ash, white sapote, Simmons trifoliate, orangeberry/gin berry, Clausena harmandiana, Chinese box orange (brachytic form), and Australian finger lime var. sanguinea. No–choice experiments are being conducted with these apparently resistant genotypes, and plant volatiles associated with susceptible and resistant genotypes are being investigated in search of attractants and repellents for the psyllid.
Research on acquisition and transmission of the greening pathogen by psyllids gives clues on how fast the disease can be spread, whether all psyllid individuals are capable of transmitting the disease, whether there are seasonal trends in transmission, and other important information. Body Of Evidence While the greening pathogen is known to be persistent in infected psyllids, questions remain regarding if the pathogen actually replicates in the psyllid. One USDA-ARS project focuses on the transmission mechanism and distribution of the pathogen in the body of an infected psyllid. In some insect vectors of other plant diseases, internal body barriers to the movement of a pathogen have been found that reduce the ability of the insect to transmit the pathogen. These transmission barriers include the insect midgut and salivary glands. Results of research with the psyllid suggest just because a psyllid is infected doesn’t mean it can transmit the pathogen. This is because the pathogen, once ingested through feeding on diseased plants, moves into the psyllid’s alimentary canal but is not always able to pass through the gut wall into the body cavity and ultimately into the salivary glands. In order for the pathogen to be transmitted by an infected psyllid to a healthy plant, the pathogen must reach the salivary glands so it can be injected with the saliva during feeding on a host plant. Among psyllids with the pathogen in the salivary glands, the research has shown the pathogen may replicate and/or accumulate in the salivary glands. Of interest is why some psyllids have the pathogen in their gut and other tissues but not in their salivary glands. USDA-ARS is assessing the mechanism of this transmission barrier in hopes of finding a way to block transmission by the psyllids.
A Shot In The Arm Another interesting research project being conducted by USDA-ARS concerns finding antibiotics or antimicrobial compounds that could be used to control the greening pathogen in infected trees. Should effective chemicals be found, a way to treat infected trees with these chemicals would be needed. USDA-ARS is therefore evaluating methods of infusing chemicals into the phloem tissues of trees. Research is being directed toward finding a phloem infusion method that allows low-volume treatment of citrus. It is possible chemicals that control psyllids or deter their feeding might also be infused into citrus. This research project also is studying the infusion of natural plant products as possible antimicrobial agents or psyllid antifeedants. The ultimate goal of this research is to develop a cost-effective and sustainable grove treatment strategy that will prevent HLB symptom development and reduce the amount of psyllid feeding on treated plants. On The Molecular Level USDA-ARS is studying RNA interference (RNAi) as a psyllid management strategy [see more about this on page 12]. Double-stranded RNA (dsRNA) is used to silence complementary messenger RNA sequences (mRNA), which interferes with the production of proteins and ultimately interrupts an organisms’ metabolic processes reducing fitness and survival of the targeted organism. The research is concentrating on RNAi strategies to disrupt and thereby suppress the biological needs of psyllids. In lab experiments, RNAi treatments have increased ACP mortality within five days of initial ingestion. Research at ARS, Ft. Pierce, has demonstrated citrus trees can absorb dsRNA through roots or by injections, and that dsRNA is ingested by psyllids when they feed on plants treated with dsRNA.
USDA-ARS recently completed the sequencing the ACP genome, and are in the process of assembly, annotation, and release into the public domain. Projected benefits from sequencing the genome include identifying enzymatic and metabolic pathways to identify critical interdiction points; identifying gene targets for RNA interference; and expanding our understanding of the interaction between the bacterium and psyllid physiologies. All this new information will enable researchers to forge ahead in the development of improved psyllid population management strategies. About the author: David Hall is a research leader at the USDA-ARS lab in Ft. Pierce. Other research contributors include: Steve Lapointe, Bob Shatters, Wayne Hunter, Desouky Ammar, Kent Morgan, Paul Robbins, and Abby Walter.