1a. Objectives (from AD-416)
1. Develop functional genomic resources and employ these resources to examine the genetic basis of biological traits that can potentially be used for biologically based control, including implications for the geographic origins of infestations. 2. Expand current biocontrol efforts by discovering and developing new parasites and pathogens; improving mass culture and field release systems; and defining host specificity of natural enemies. 3. Characterize semiochemicals through investigation of pheromone biosynthesis and release; use these findings to develop novel biologically-based control and surveillance methods, including the detection and treatment of incipient or low level populations. 4. Develop integrated pest management plans that utilize available control methods, perform comprehensive risk assessment, and that can be adapted to specific stakeholder needs, including local eradication.
1b. Approach (from AD-416)
1. Normalized cDNA libraries will be sequenced using 454-pyrosequencing technology. Resulting sequences assembled and automatically and manually annotated for key words, gene function, and Gene Ontology terms. The 454 data will be used to search for potential microbes infecting fire ants and determine their nature, distribution, relationship, and effects of these microbes in fire ants. SNPs within the data will be identified and applied to high-resolution identification of the source population of introduced S. invicta. Microarrays will be constructed and used to identify differentially expressed genes from parasite-infected and -uninfected fire ants and identify genes co-expressed with the social form-specific gene Gp-9 allelic variants. 2. Microsporidia: After approval, V. invictae will be released in the U.S. following the procedures for K. solenopsae introductions. Phorid Flies: Additional decapitating phorid flies will be released using procedures based on our previous successful releases in the U.S. The impact of phorid flies will be assessed by multiple methods, e.g. monitoring the establishment, expansion, distribution and parasitism rates of phorid fly species across the fire ant range. Viruses: Baculovirus expression methodology will be emulated in order to express Solenopsis invicta viruses (SINV-1, -2 or -3) for study and development as a microbial pesticide. With purified preparations of SINV, a number of basic studies will be conducted, including lethal dose evaluations against different stages of ants, transmission studies within and between colonies, field-testing, and formulation development. Other potential biocontrol agents will be investigated as time and importance dictate. 3. The function of the fire ant PBAN/pyrokinin family of neuropeptides will be investigated by: a) in vivo injection into female and male sexuals and immatures and observed for phenotypic changes; b) manipulation of hormone/receptor capabilities through the use of agonists, hyper-agonists, and antagonists of the neuropeptide hormones; and c) use of RNAi gene knockout methods – followed by monitoring for phenotypic and/or behavioral changes. Monitoring and surveillance methods will be developed for fire ants, using known attractants. A variety of existing fire ant bioassays will be adapted and applied to non-fire ant invasive pest ant species to generate the tools to create better baits and effective monitoring systems, e.g. attractants and repellents. 4. Potential geographic range expansion of tramp ant species will be modeled using CLIMEX a program that can be used to predict where a pest ant of interest can survive. Phagostimulants will be studied to improve the acceptability of baits for non-Solenopsis pest ants by adapting methods used for fire ants. Standard laboratory colony tests will be used to assess bait formulation effects on brood volume, adult populations, and queen survivorship. A combination of monitoring tools, baits, and biologically based-control methods will be applied to selected invasive ant species as these new tools become available.
3. Progress Report
Fire ant decapitating flies that develop in fire ants infected with the fire ant pathogen Kneallhazia (=Thelohania) solenopsae, can acquire the pathogen. We recently found that K. solenopsae was transmitted to fire ant colonies by feeding them infected parasitic flies. This is the first direct evidence that fire ant decapitating phorid flies can vector a fire ant pathogen. A new fly, Pseudacteon cultellatus, was released and recovered after 6 months, thus it survived at least one full generation in the field. This fly specializes small workers found in multiple-queen fire ant colonies. Overall decapitating fly parasitism rates (several species have been released) from fire ant colonies around Gainesville, FL varied between 1 and 5% or more. The first ant DNA virus, SiDNV, was discovered from the fire ant. Studies to characterize and evaluate its impacts on fire ant colonies are underway. Genome integration of S. invicta RNA virus 1 (SINV-1) into the fire ant host genome was found to not occur – important result supporting the potential of this virus as a microbial insecticide. These studies provide critical basic information necessary to exploit the viruses as microbial agents against fire ants. Unit scientists are the world authorities on viruses in ants. A patent application was filed on RNAi to silence genes critical to normal functioning of fire ant colonies. A Cooperative Research and Development Agreement was put in place that will facilitate the development of RNAi for control. An invention disclosure was submitted and approved for the use of RNAi silencing to silence a key fire ant receptor gene. Natural product derived extracts and compounds were evaluated as fire ant repellents, useful to exclude fire ants where they are not wanted. Alarm pheromone levels over time in workers and female sexuals/queens were determined. Queens have an unusual pattern over time suggesting unknown alarm pheromone functions. A draft genome of the fire ant was generated. Several unique aspects of the genome are likely linked to the complex social behavior of this species. These data provide a new resource for functional genomic studies. A population genetic study of introduced fire ants revealed that a number of queens mate with more than one male depending on the social form of her first male mate. A comprehensive population genetic study of fire ants in China revealed several patterns regarding the invasion history, routes of subsequent dispersal, and signatures of genetic bottlenecks of this pest ant species in China. Two commercially available ant baits (containing metabolic or nerve inhibitors) consistently killed colonies of the invasive crazy ant, Nylanderia pubens. However commercially available insect growth regulating (IGR) baits had minimal effects although they were predicted to have better potential against this ant. Collections of the invasive crazy ants, N. pubens, were made from four regions in Florida and their genome sequenced. This information will be used to survey crazy ant populations for pathogens, e.g. viruses, and may lead to new biological control agents for this invasive ant.
1. Fire ant decapitating flies can parasitize over 5% of colony workers. Decapitating flies clearly impact fire ant populations in South America because fire ants have evolved a suite of defenses against their attacks. However, the overall magnitude of decapitating fly impacts on fire ant colonies in the United States is poorly known. ARS scientists in Gainesville, FL, determined that the direct impacts of parasitism on fire ant colonies average about 1-2% of workers, but some colonies have parasitism rates in excess of 5%. These results indicate that, at certain sites and at certain times, parasitism rates may be enough to tilt the competitive balance in favor of native ants, which mostly do not sting humans, livestock, or wildlife.
2. Fire ant pathogen hitches ride with dispersing queens and parasitoids. Single-queen fire ant colonies in the field were infected with the fire ant pathogen Kneallhazia solenopsae. Winged female sexuals from these colonies take flight and mate with males 300 ft in the air, then return to the ground and attempt to found new colonies. ARS scientists at Gainesville, FL, found that female sexuals collected during mating flights from these colonies were infected. This is the first report of artificial inoculations resulting in infections of dispersing queens, which could prevent the establishment of new colonies. Another possible mode of pathogen transmission is via fire ant decapitating flies. When the flies develop in fire ants infected with the fire ant pathogen K. solenopsae, they can acquire the pathogen. Recently ARS scientists found that K. solenopsae was transmitted to fire ant colonies when pathogen infected parasitic flies were fed to the ants. This is the first direct evidence that the fire ant decapitating phorid flies can vector a fire ant pathogen.
3. Densovirus discovered in fire ants. Discovery of viral pathogens in fire ants has been very difficult until the tools of molecular biology have been brought to bear on the problem. ARS scientists at Gainesville, FL, used transcriptome sequencing to discover a new virus in the fire ant. This virus is the first DNA virus discovered in the fire ant and ants in general. Importantly, this class of virus is characterized as more virulent than other classes of viruses and may offer advantages in biocontrol of fire ants. ARS researchers have discovered all known virus species from ants worldwide and are considered the authorities on the subject. Studies to characterize and evaluate the impacts of this new virus on fire ant colony health are underway.
4. Major reductions found in fire ant populations. The major problem with fire ants in the United States is that they are 5-10 times more abundant than they are in their native range in South America. Population studies conducted by ARS scientists at Gainesville, FL, in the 1990s showed that sites with high densities of multiple-queen fire ant colonies were largely stable over a 6-year period. Resurvey of two large areas south of Gainesville this Spring showed a major and surprising switch from multiple-queen colonies to single-queen colonies along with a dramatic drop in fire ant population densities. The regional extent and cause of this change is currently unknown, but studies are underway to determine if biological control agents (pathogens and phorid fly parasitoids) released in the early 2000s are implicated.
5. Better baits for the invasive crazy ant, Nylanderia pubens. Many economically important invasive ant species are difficult to control, for example the crazy ant, Nylanderia pubens. ARS scientists at Gainesville, FL, determined that two commercially available ant baits (containing metabolic or nerve inhibitors) consistently killed small colonies of N. pubens. However, commercially available insect growth regulating (IGR) baits had minimal effects on this species, even though IGR baits were hypothesized to have better potential to suppress the huge populations of this invasive pest. Research to improve the efficacy of IGR baits is on going.
Choi, M.Y., Vander Meer, R.K., Shoemaker, D.D., Valles, S.M. 2011. PBAN gene architecture and expression in the fire ant, Solenopsis invicta. Journal of Insect Physiology. 57(1):161-165.