John B. Welch

Characterization of Acaricide Resistance in Rhipicephalus sanguineus (Latreille) (Acari: Ixodidae) Collected from the Corozal Army Veterinary Quarantine Center, Panama

Abstract Rhipicephalus sanguineus (Latreille) were collected from the Corozal Army Veterinary Quarantine Center in Panama and characterized for resistance to five classes of acaricides. These ticks were highly resistant to permethrin, DDT, and coumaphos; moderately resistant to amitraz; and not resistant to fipronil when compared with susceptible strains. Resistance to both permethrin and DDT may result from a mutation of the sodium channel. However, synergist studies indicate that enzyme activity is involved. The LC50 estimate for permethrin was lowered further in the Panamanian strain then in susceptible strains with the addition of triphenylphosphate (TPP), but not with the addition of piperonyl butoxide (PBO). This suggests that esterases and not oxidases are responsible for at least some pyrethroid resistance. Elevated esterase activity and its inhibition by TPP were confirmed by native gel electrophoresis. The LC50 estimate obtained for coumaphos in the Panamanian strain was not lowered further than what was observed for susceptible strains by the addition of TPP or PBO. This indicates that enzyme activity might not be involved in coumaphos resistance. Resistance to amitraz was measured through a modification of the Food and Agriculture Organization Larval Packet Test. All tick strains were found to be susceptible to fipronil.

Responses of fertile and sterile screwworm (Diptera: Calliphoridae) flies to bovine blood inoculated with bacteria originating from screwworm-infested animal wounds.

Abstract A simple bioassay system was developed to study locomotory and ovipositional responses of screwworm, Cochliomyia hominivorax (Coquerel), flies to bovine blood inoculated with eight species of coliform bacteria that were isolated from screwworm-infested animal wounds. When exposed to odors from bacteria-inoculated blood which was incubated for 72 h at 37°C, ≈50% of 7- and 10-d-old gravid females landed on the blood by the end of 15 min test exposure. Only 17% of 7-d-old reproductively sterile females (from irradiated pupae) with previtellogenic ovaries and 2% of 4-d-old vitellogenic females responded to the same treatment. Females generally reacted in greatest numbers to bacteria-inoculated blood incubated for 72 h, followed by 48 h, then 24 and 96 h. Males of all ages tested were unresponsive. Although oviposition occurred in tests with gravid females lasting for 1 h, with both inoculated blood and an uninoculated control, the inoculated sample was significantly better than the control at 48, 72, and 96 h incubation duration. Our results are consistent with the conclusion that the inoculated blood, when incubated for 48–72 h, gives off volatile chemicals which attract gravid females and contains an oviposition stimulant that acts following contact and feeding. The volatiles, once isolated and identified, may be useful for sampling gravid females in the field as well as improving the oviposition system in the mass-production facility of the screwworm eradication program.

Seasonal and Spatial Distributions of Adult Screwworms (Diptera: Calliphoridae) in the Panama Canal Area, Republic of Panama

Abstract The distribution of screwworms, Cochliomyia hominivorax, (Coquerel) was studied in a seasonally moist lowland tropical forest in the Republic of Panama using a combination of field collections and satellite imagery. We found that different forest types could be distinguished and mapped using remotely sensed data. To determine the temporal and spatial distribution of flies, we collected flies coming to rotted liver at 82 sites in ten vegetation types (open areas, edge forest, dry scrub forest, forest successional stage 1, forest successional stage 2, forest successional stage 3, forest successional stage 4, forest successional stage 5, mature forests, palm swamp forest, and forest along streams) over three seasons (dry, transitional, wet). Nine of the vegetation types (excluding dry scrub forest) were identified and mapped using SPOT XS and Landsat 5 TM satellite data. Screwworm flies were most abundant during the transition from wet to dry season. Fly numbers were consistently higher in forest habitats, particularly those with trees 20–30 m in height and a fairly open canopy composed of many deciduous species that shed their leaves during the dry season. Screwworm numbers were also high in palm swamp forest, edge forest, and mature growth forest. Traps sampled in open areas had fewer flies and were unrelated to proximity to cattle. Females accounted for 88% of the total fly counts. This study further substantiates the importance of forests in the ecology and behavior of screwworm flies and demonstrates that remotely sensed data can be used to construct the spatial distribution of these flies in a tropical landscape. We discuss implications of this information to the screwworm eradication program.

Estimating Population Density Per Unit Area from Mark, Release, Recapture Data

In this paper, we describe a method for estimating animal population density per unit area from mark, release, recapture (MRR) data. Standard procedures are available for estimating population size from MRR data, but not density per unit area, which is often of more practical value. If data on dispersal movements are available in addition to cap- ture/recapture records, they can be used to estimate the area over which the study population ranges. These area estimates can then be combined with MRR abundances to yield estimates of population density. As a concrete example of how these methods can be applied, we calculated population density estimates for seven MRR data sets for the screwworm, Coch- liomyia hominivorax (Diptera: Calliphoridae), from Costa Rica. As a standard of compar- ison, we also used a different method to calculate estimates of screwworm density for five sets of USDA APHIS Screwworm Eradication Program data from Mexico. Mean densities for both sets of analyses were similar, ranging from 10 to 120 adult flies/km2. General agreement between the Mexican and Costa Rican estimates lends support to the dispersal approach we propose here. Although we have applied these methods to the screwworm as a specific example, they are entirely general and can be applied to virtually any organism for which both MRR records and dispersal data are available.

Screwworms, Cochliomyia hominivorax, Reared for Mass Release Do Not Carry and Spread Foot-and-Mouth Disease Virus and Classical Swine Fever Virus

Abstract Experiments were done to determine if transporting live screwworms Cochliomyia hominivorax Coquerel (Diptera: Calliphoridae) for developing new strains from countries where foot-and-mouth disease and classical swine fever are endemic, to the mass rearing facilities in Mexico and Panama, may introduce these exotic diseases into these countries. Are screwworms capable of harboring and spreading foot-and-mouth disease virus (FMDV) and classical swine fever virus (CSFV) when they are grown in virus-inoculated larval rearing medium? In one experiment, screwworm larvae were reared in a FMDV-inoculated artificial medium containing either 0.1 % formaldehyde or antibiotics as an antimicrobial agent. In another experiment, larvae were similarly reared in a CSFV-inoculated artificial medium containing 0.1% formaldehyde. In each experiment, samples of larvae and the rearing media were collected daily until pupation occurred. The presence of FMDV was assayed by observing cytopathic effects on cell cultures and a conventional reverse transcription-polymerase chain reaction (RT-PCR); CSFV was assayed using an avidin-biotin complex assay and a conventional RT-PCR. For media containing antibiotics, FMDV was detected in a larval sample collected on day 1 and in media samples on days 1, 2 and 3. No FMDV was detected from larval and media samples collected on all other days. For media containing formaldehyde, FMDV and CSFV were not detectable in larval or media samples collected on all sampling days. These results indicate that FMDV and CSFV cannot survive in rearing medium containing formaldehyde as an antimicrobial agent. Therefore, insects collected in endemic regions and reared using formaldehyde-containing medium for at least one generation at the collection site should be free of FMDV and CSFV and can be transported safely to a strain development/mass rearing facility.

Screwworm (Diptera: Calliphoridae) in the United States: Response to and Elimination of the 2016–2017 Outbreak in Florida

Abstract Eradicating screwworm, Cochliomyia hominivorax (Coquerel), from continental North American via the sterile insect technique has provided huge economic benefit to livestock producers by eliminating screwworm myiasis. After confirmatory identification of fly samples from infested deer by the USDA National Veterinary Services Laboratory on September 30, 2016, an alert was issued that screwworm myiasis was discovered in the Florida Keys. Personnel from USDA Animal and Plant Health Inspection Service, Agricultural Research Service, the State of Florida, U.S. Fish and Wildlife Service and local officials responded to the outbreak focus on Big Pine Key. After witnessing infested Key deer (Odocoileus virginianus clavium Barboyr & Allen), screwworm adult sampling was initiated at 0930 h on October 5, 2016 using nets to collect flies arriving at putrid liver, with the first female collected within 1 h. Larval samples were collected from infested animals for DNA analyses and to develop a “Florida outbreak” colony to test mating compatibility with the mass-produced strain used for sterile fly releases. Ground release chambers for sterile screwworm releases were placed in favorable habitats based on satellite image analyses. Sterile pupae were first placed in the chambers on October 11, 2016. Further liver trapping showed that 13 Keys were infested. One case, presumably through animal movement, occurred near Homestead on the Florida mainland. Ultimately there were 35 sterile fly release stations, including 4 located around Homestead, but no further cases were identified. About 188 million sterile flies were released until successful eradication was declared on March 23, 2017. Containing the outbreak prevented economic losses to livestock producers and other wildlife on the mainland and kept eradication costs to a minimum.