Veterinary, Medical and Urban Entomology
Agriculture depends on people, animals, and plants. National Program 104 solves problems that affect people and animals, protecting them from the insects and ticks that cause direct injury or disease. The program is particularly well suited to the mission of the Agricultural Research Service because research in this area that addresses livestock problems often also solves human problems and vice versa. For example, the invasive imported fire ant causes discomfort, injury, or sometimes severe illness in almost half of the population of 11 states, but it also affects livestock through direct injury and the destruction of pasture land. Another example is the threat of the introduction of Rift Valley fever virus into the U.S. Mosquitoes would transmit this virus to ruminant livestock, disrupting the industry in large regions of the country. Humans could suffer severe illness from infections caused by mosquito bites or by exposure during meat production. One way to think of NP 104 is that it develops the information necessary to protect creatures with two or four legs from creatures with six or eight legs.
ARS has a long history of achievement in this field. In fact, the first discovery of a disease-causing pathogen transmitted by an arthropod was performed by the microbiologist, Theobald Smith, in 1889-1893 while working for the U.S. Department of Agriculture. He found that Texas cattle fever was caused by a protozoan and that the protozoan was transmitted between cattle by a particular kind of tick. Smith’s discovery was well-documented and often discussed at the time, leading to the astoundingly significant association of malaria, yellow fever, and dengue with mosquitoes. His discovery also led to an eradication campaign started in the early 1900s that continues to this day, almost eliminating the threat of Texas cattle fever in our country. The importance of mosquitoes as vectors of important diseases caused a renewed interest in accurate taxonomy and, again, USDA was at the center of activity. L.O. Howard, H.G. Dyar, and F. Knab produced the definitive scholarly work on mosquitoes of the Americas, with Knab completing the manuscript in 1918 as he was dying from sand fly-borne leishmaniasis. In 1942 the War Department called on the USDA to develop new insecticides and repellents to protect military personnel from the ravages of scrub typhus and malaria, particularly in the Pacific Theater. Scientists at the Orlando laboratory eventually screened over 20,000 compounds and produced a series of compounds that could be applied to the skin or clothing. Before the introduction of chloramphenicol in 1949, scrub typhus was almost a death sentence and the repellents dimethyl phthalate, ethyl hexanediol, and benzyl benzoate were all that stood between soldiers and this infection. Thanks to USDA scientists Carroll Smith and Harry Gouck, the screening effort in the 1950s found that DEET was a highly effective insect repellent. DEET remains the standard repellent active ingredient today. ARS also developed the current repellent treatment for military uniforms during the 1980s. The Department of Defense has again asked ARS to produce better products to protect the U.S. military and the agency has responded with a major effort.
Finally, no discussion of USDA achievements would be complete without mentioning the research that led to the defeat of the screwworm fly. This fly actively seeks even the smallest wounds in cattle, lays its eggs, and then causes what is often mortal injury to the animals. In 1950, E.F. Knipling began an effort to release sterile males of the fly over huge areas, preventing the females from laying fertile eggs. This technique has been refined greatly over the years, but it continues and has successfully eradicated the fly as far south as Panama. The benefits from this program are almost incalculable, considering the economic benefit to the American beef industry, the relief from animal suffering, and the benefit to small landholders in Mexico and Central America.
In 2005, National Program 104 continued an active agenda of research based on its five-year plan started in 2003. The program includes 20 core-funded projects performed at 10 locations in Florida, Maryland, Nebraska, Mississippi, Louisiana, Texas, Wyoming, Washington, and Panama. Fifty-one permanent scientists have contributed to 117 peer-reviewed scientific publications. In the course of this work they have collaborated with 24 universities, 13 industrial partners, and 27 governmental or non-governmental organizations.
Some of the achievements of 2005 are briefly described below within the general functional categories of insect pest management (IPM). Risk assessment answers the questions of whether or not there is a problem and where that problem occurs, usually from existing sources of information. Surveillance is the process of on-the-ground measurements to provide targeting information for control efforts. Arthropod control includes a wide variety of techniques which, at their best, are applied in an integrated fashion to achieve the most efficiency with the least possibility of environmental or occupational hazard. Finally, monitoring is the continuous process of assessing the success of control measures in order to validate sustained effort and to detect any changes that might require additional action.
Much of the basic research on pathogens and vector arthropods is relevant to the risk assessment portion of IPM. The improved understanding that comes from study of the relationship of the vector to the pathogen leads to a more realistic appraisal of risk in particular scenarios. Descriptive studies of the spatial and temporal distribution of pathogens and vectors are also very useful for risk assessment. Since all interventions are local in nature, the most relevant information must be made available on a very fine scale that can be used by farmers, ranchers, mosquito abatement authorities or other action sources.
Some species of these small flies, also called no-see-ums or sand flies, are important vectors of viruses to livestock. Vesicular stomatitis virus received a great deal of attention during the last year. The growth of the virus in the biting midge was described in several ways, including the influence of infection on gene expression in specific tissues, the time course of viral RNA expression in the vector, and detection of the virus in ovaries of one species of biting midge. The presence of the virus in the ovaries suggests that the pathogen can be passed from mother to daughter midge, complicating practical efforts to stop the chain of transmission. ARS also worked on spatial risk models for these vectors, taking advantage of remote sensing data and new, powerful software for spatial analysis.
Imported Fire Ants
Established colonies of fire ants will accept newly mated queens. This behavior was unexpected, since colonies are generally very aggressive toward non-members. Presumably, the acceptance of newly mated queens improves the flexibility and genetic diversity of the colony, making fire ants that much more difficult to control. Surveys of native ant species continued in the southern states, including presentation of identification keys on the Internet. The ants of Mississippi have been completed, recording 154 species, 59 of which were new state records. Work is currently being conducted on the ants of Alabama, with plans to complete the ants of Louisiana, Arkansas, and Tennessee. These studies are essential to be able to determine the ecological damage created by imported fire ants and, more important, to be able to make sound decisions about measures to defend the country against importation of other invasive ant species.
The movement of stable flies was studied by marking 50,000 of them, releasing them, and then trapping to determine how many had gone how far. Recapturing 3,000 flies, researchers showed that most flies moved just 1 km.
An extensive survey of mosquitoes in Wyoming showed that both Culex pipiens (the house mosquito) and Culex tarsalis (the western encephalitis mosquito) were infected with West Nile virus and that they were probably the important vectors in the state. Culex tarsalis was probably responsible for transmitting the virus to the sage grouse, further reducing the population of this rare bird. Feeding by this mosquito on the grouse was demonstrated by comparison of the DNA in the blood eaten by mosquitoes and the genome of the grouse. The major larval source of these mosquitoes is apparently the water that gathers on the surface in the process of coal bed methane generation. The mosquitoes transmitting West Nile virus were also studied in Connecticut, testing over 800,000 specimens. ARS scientists also developed a computer model for risk assessment of West Nile virus based on temperature and the time required for the virus to develop to infectious levels in the mosquito.
Showed that screwworm flies are unlikely vectors of Foot & Mouth Disease virus and of African swine fever virus and this is important because screwworm flies are endemic in regions with Foot & Mouth Disease.
Researchers found facultatively anaerobic bacteria in the gut of the Formosan subterranean termite. They also found Klebsiella pneumoniae, a bacteria capable of significant nitrogen fixation. The size of Formosan subterranean termite workers was found to be correlated to the tunnel size. New methods to analyze colony structure were developed using genetic tests. Researchers examined the basic biology of flight and sound production, showed that juvenile hormone III is important in the regulation of caste numbers and gut fauna and developed a tissue culture cell line for the Formosan subterranean termite.
Surprisingly, it was found that the major American vector of Lyme disease, Ixodes scapularis, transmits the bacteria to cattle. A weather-based model for distribution of this species was developed. Researchers showed that an invasive forest weed makes no difference in the abundance of this tick in the Northeast.
Great progress was made in sequencing the genome of Boophilus microplus, the vector of Texas cattle fever and a target for a major APHIS control program. This genome will be a resource for discovery in many aspects of tick biology. Progress included the creation of 45,000 expressed sequence tags veering 35-60% of the genes. cDNA libraries were made from RNA expressed in salivary glands early and late in feeding. This work led to construction of dsRNA with inhibitory capabilities. Scientists developed DNA markers for chromosomes and methods for finding chromosomes, documented a very large quantity of repetitive DNA in the tick genome and wrote a white paper as a first step to assemble the funding and collaboration to complete the sequencing of the genome.
Surveillance provides the detailed information necessary to determine exactly where and when to apply control measures. Sometimes the result is that no control is required at all. In addition to determining the presence of the arthropod, it is also often useful to determine whether or not it is infected with a pathogen, what population the insects or ticks came from, and the physiological status of the arthropod.
Imported Fire Ants
The alarm pheromones of the red imported fire ant were found to be species specific, possibly providing a tool for surveillance or control. Studies of the sensillae of both larval and adult fire ants may also lead to development of new ways to repel and attract ants. The thermal signature of fire ant mounds was studied in order to provide the means to detect them from aerial or satellite photos. Genetic studies developed methods for detecting pathogens and distinguishing between colonies with a single queen (monogyne) or multiple queens (polygyne).
Researchers developed a new PCR technique that reduces the time required for detection, the cost of the test, and the difficulty created by background signal. This test was developed for detection of Bluetongue virus, but it could be applied to any RNA-based virus. Scientists completed the sequence of the vesicular stomatitis virus, including the expression of five genes in a bacterial system. This accomplishment will improve the possibilities for understanding viral transmission and development of new detection techniques.
Great progress was made at both a practical and theoretical level in the development of attractants. A lure for the major vector of West Nile virus in the southern U.S. was created that specifically attracts the gravid female mosquitoes. The older, gravid mosquitoes are more likely to be infected than mixed populations that include newly emerged females that have not had time to bite and develop a viral infection. At a more basic level, researchers discovered a suite of chemicals that explains much of the reason why some mosquitoes prefer to bite birds and others prefer to bite mammals. Some of the chemicals are repellent to particular mosquito species and others are attractant to particular species. ARS scientists also developed a method for marking larval mosquitoes that remained in the emerging adult, providing a technique for marking large numbers of mosquitoes before they take flight.
Scientists found that commercially available attractants, as well as a newly discovered substance, can modify the direction of tunnel construction. Measured the distance traveled by winged termites, documenting their flight 900 m across the Mississippi River.
Researchers applied a variety of detection systems for Formosan subterranean termite, each useful for different situations. These included remote sensing from aerial photographs, acoustical and video detection in living trees, infrared detection, and motion-activated detection.
ARS scientists discovered a series of chemicals that tend to cause ticks to stop walking (arrestants). These chemicals were located in dog hair, deer urine, feather rubbings, and white-tail deer tarsal gland extracts.
It is in the nature of integrated pest management that many different kinds of control techniques are necessary to make an efficient program. Not only can one kind of control be applied when another fails, but some control methods are more appropriate for one situation than another. Although the program stresses non-chemical control methods, the use of rational chemical control is a feature of most successful integrated pest management programs.
Researchers conducted studies on a strategy to stop bluetongue virus transmission by modifying the biting midge that transmits it. Results in the laboratory were promising that it would be possible to block virus transmission by introducing a small piece of double-stranded RNA into the vector. At the opposite end of the spectrum, ARS scientists designed an integrated program for midge control on white-tailed deer farms in order to reduce the risk of epizootic hemorrhagic disease virus.
Imported Fire Ants
ARS was very active in making improvements in fire ant control during 2005. Integrated control programs were developed to protect the nursery industry in Tennessee and visitors to the Natchez Trace, a segment of the National Trail System. Integrated control that included the use of chemicals, pathogens, and parasitoids eliminated 95% of the ants, in contrast to chemical control alone that eliminated only 50-80% of them. A bait formulation was developed that resists moisture from early morning dew, making it possible to distribute the bait during a larger part of the day. Progress continued in the use of parasitoid phorid flies that develop within fire ant workers, eventually killing them. The first species introduced, Pseudacteon tricuspis, was shown to spread about 12 miles per year from a point of introduction. Two other species, P. curvatus and P. litoralis, have been recently introduced and may be more effective. A new species of predatory ant, Solenopsis daguerri, was discovered in Argentina. At least five new pathogens of fire ants were discovered, including bacterial pathogens discovered by fatty acid analysis, the fungus Myrmicinosporidium durum, the microsporidian Vairimorpha invictae, the neogregarine Mattesia sp., and the virus SINV-1. The discovery of a viral pathogen of fire ants was particularly dramatic, as it was the first virus ever discovered in this ant. Physiological studies relevant to the development of new control techniques included the genetic description of the detoxifying enzyme glutathione S-transferase, description of the amino acid sequence of the pheromone biosynthesis activating neuropeptide, and detection of the upregulation of the transferring gene during Beauveria bassiana infection.
The research program supported an integrated approach to fly control by developing chemical and biological methods. An alternative class of insecticides (neonicotinoids) was evaluated against the horn fly and a number of new control products were tested against house flies. Combining attractants and chemical control, the ARS is working on an entirely new house-fly control strategy based on attracting and killing the pests. Progress was made in biological control of house flies in the discovery of chemicals emitted by larvae that attract a parasitoid wasp. Using two other parasitoid wasps, 50% control of house fly was achieved in a poultry house.
Two new mosquito-infecting viruses were discovered that may eventually contribute toward biological control. Also, progress was made on description of the genome of CrCPV, a virus of Culex restuans. The use of permethrin on cattle for mosquito control was examined in order to develop tools to stop the potential spread of exotic, mosquito-borne viruses like Rift Valley fever virus. Working at a more basic level, potentially new insecticides were developed by exploring which chemicals inhibit or overstimulate neuropeptide reception in the insect brain. The physiologically essential ecdysis triggering hormone was modeled in the computer and used to develop an analog chemical that blocks reception. ARS scientists addressed the problem of removing all insects from a passenger airliner (disinsection) by using an air curtain at the door and then heating the cabin after the passengers had disembarked. Using a very new technique, workers injected a short segment of specifically designed genetic material (dsRNA) to eliminate the ability of a mosquito to transmit virus. Researchers also tested the popular strategy of trapping mosquitoes in order to get some relief from biting pests. They found that the technique worked well if the sources of mosquitoes were limited (as on an island) and the number of species was limited.
In the area of personal protection, a major effort was made to screen large chemical libraries for repellent chemicals, including those that could work spatially. Another chemical screening effort took advantage of the long history of repellent testing at ARS to begin computer modeling of repellent properties. A separate effort found 17 chemicals secreted by humans that tend to reduce biting by mosquitoes. New techniques were also developed, including a slow-release formulation for volatile repellent compounds, a fluorometric technique for detection of important detoxifying enzymes in mosquitoes, and a screening method for potential repellent products utilizing a column with odorant binding protein.
Demonstrating the ability to respond quickly and thoroughly, ARS scientists eradicated the screwworm fly from the isand of Aruba. They also used Jamaica as a test bed for the application of new methods. The process of rearing sterile male flies may be improved by using sex pheromone to separate the males according to relative libidity. Other new potential improvements in rearing included development of a gelling agent for the maggot medium that is cheaper and biodegradable and progress toward development of a method for storing frozen, genetically altered embryos.
ARS scientists performed a long series of experiments relevant to control. First, they found that two common kinds of hardboard made in the tropics (bagasse and hemp) were not resistant to termite attack, but that two species of tropical wood and three species of American wood were highly resistant to Formosan subterranean termite. Other work involved use of a biocontrol fungus, orange oil, azadirachtin (neem), and cecropin. Two very practical results were that commonly-used fipronil baits eliminated termites from a 5 m radius up to 5 years; and, granular boric acid added to chipboard could protect the material from termites.
A large demonstration project in Formosan subterranean termite control resulted in 50% reduction of the pests in the French Quarter of New Orleans. Education was a big part of this campaign, with production of web sites directed toward the general public.
A major, seven-location study of the effectiveness of the “4-poster” device was completed. This device lures deer to a position where they rub their heads and necks against rollers with an acaricide. The study showed that the 4-posters reduced the abundance of the Lyme disease vector tick by 69-80% and reduced the abundance of the lone-star tick (vector of Rocky Mountain spotted fever and other pathogens) by 95-99%.
ARS scientists demonstrated a series of repellent compounds on ticks, including benzoquinones from millipedes and the ARS-developed compound SS-220.
Researchers discovered two new nematode pathogens of ticks and demonstrated the effectiveness of two others for control.
Many of the same techniques used in surveillance are also used in monitoring. In other words, the data on where the target arthropods are located before control also provides information on how well the control methods functioned. Veterinary, medical, and urban entomology would probably benefit from more attention to this important function. Not only would better monitoring reverse program failures earlier, it would also tend to sustain the support for successful programs.
Researchers combined remote sensing with a diffusion model to quantiate screwworm risk, quantitating the width of necessary barriers.
Scientists used alates (winged termites) to evaluate the success of the control program against the Formosan subterranean termite, monitored the spread of the Formosan subterranean termite across Texas, including population genetics characterization and developed PCR genetic detection techniques for important pathogens of termites.