Amblyomma variegatum Reintroduced 
in the U.S. Virgin Islands 
Amblyomma variegatum, the tropical bont tick, was dis- 
covered on St. Croix, U.S. Virgin Islands, during July 1987. 
This tick had been considered eradicated since 1970, when 
the previous eradication program consisting of quarantine, 
rigorous individual animal treatment with coumaphos, and 
premises treatment with sprayable carbaryl came to an end. 
It is of interest that a few isolated male ticks were dis- 
covered sporadically on St. Croix after 1972, but no 
evidence of infestation or further spread could be found. 
The current outbreak is limited to one area of about 31 acres 
of brush pasture on which cattle, horses, swine, dogs, and 
chickens roamed freely, as well as mongooses and a few 
head of deer. Several cattle had died of a condition 
resembling dermatophilosis. Clinical cases of dermatophilosis 
were subsequently observed. A 5-acre portion of the 
premise was found to be heavily infested and has been 
treated twice with sprayable chlorpyrifos. 
Cattle, horses, swine, and dogs have been under quarantine 
and regular biweekly treatment with coumaphos. Cattle, 
swine, and chickens have now been removed from the 
premise. The biweekly scratch and treatment of all adjacent 
animals, as well as the trapping of chickens and mongooses 
for surveillance and elimination, is continuing. The hunting 
of deer has, thus far, been unsuccessful. The origin of this 
infestation is not clear. 
Boophilus Research Highlights for 1987 from the 
Cattle Fever Tick Research Laboratory, USDA, 
Agricultural Research Service 
Kerrville and Mission, Texas 
Research began several years ago on organophosphate 
(OP)-resistant Boophilus microplus from Mexico and 
continued with investigations on the biochemical 
mechanisms accountable for the altered susceptibility of the 
ticks to coumaphos and related acaricides. One study 
determined that in the resistant strain, which originated near 
Tuxtla, Chiapas, Mexico, two types of the enzyme acetyl- 
cholinesterase (AChE) were present. One form of AChE was 
prone to inhibition by OP’s, but the activity of the second 
form of AChE was less affected by these acaricides. These 
experimental results suggest that an altered AChE with 
decreased sensitivity to inhibition is probably an important 
factor in the altered susceptibility of the B. microplus in parts 
of Mexico to coumaphos, chlorfenvinphos, and related 
acaricides. A parallel investigation determined that in 
resistant ticks there was also enhanced metabolic 
detoxification of coumaphos and toxic metabolites of this 
compound. 
As the age of tick larvae increases, the ticks undergo 
undetermined biochemical and physiological changes that 
result in increased susceptibility to pesticides and lowered 
survival rates. It was hypothesized that a reduced quantity of 
AChE in older tick larvae is related to their increased 
susceptibility to acaricides. But, experimental evidence from 
investigations of the AChE of five species of ticks, including 
B. microplus, determined that in these ticks amounts of 
AChE actually increased with age. 
Because of an apparent failure of amitraz to control B. 
microplus on cattle in the vicinity of Juncos, Puerto Rico, 
engorged females of this species were collected and shipped 
to the Cattle Fever Tick Research Laboratory for tests of 
their susceptibility. Groups of calves infested either with F; 
larval ticks from Puerto Rico or larvae of an amitraz- 
susceptible B. microplus strain from Texas were sprayed 
with 0.025 percent amitraz, prepared from the emulsifiable 
concentrate formulation. Initial comparisons of the degree of 
control of the Puerto Rico and Texas ticks indicated that the 
strain of ticks from Puerto Rico was as susceptible to 
amitraz as the ticks from Texas. This test was repeated with 
the second laboratory generation of ticks from Puerto Rico 
and a known amitraz susceptible strain of B. microplus from 
Texas. Once again, the test results indicated that the ticks 
from Puerto Rico were fully susceptible to amitraz. 
A 0.025 percent concentration of amitraz wettable powder 
(WP), applied by immersing cattle in a dipping vat, controlled 
99.8 percent of the B. annulatus on the animals. In addition 
to the evaluation of efficacy, a study was performed with the 
assistance of APHIS, VS personnel to determine the stability 
of the amitraz WP in a dipping vat under South Texas condi- 
tions. During a period of about 1 month, 4,056 head of cattle 
were dipped in an amitraz-charged vat. Test results 
demonstrated that the active ingredient settled very rapidly 
in the freshly charged vat, but that the amitraz remained in 
suspension longer as the amount of contamination in the vat 
increased. There was a tendency for the amitraz concentra- 
tion to increase slightly after each replenishment, but in 
general the evaluation demonstrated that in a ranch vat in 
which large numbers of cattle are being dipped, amitraz WP 
remains stable and the concentration of the active ingredient 
can be maintained at the targeted level. 
Because of a need by tick eradication officials for an 
acaricide to prevent the hatching of larvae from eggs 
deposited on tick-infested premises, such as a sales yard, 
the ovicidal activity of coumaphos, diazinon, dioxathion, 
chlorpyrifos, and crotoxyphos against B. microplus eggs of 
different ages was determined. Test results proved that of 
the acaricides tested, only crotoxyphos, applied topically at 
a rate of 0.5 percent active ingredient, would provide 
sufficient ovicidal activity (=90 percent reduction in hatch) 
against eggs ranging in ages from 1 to 20 days for the 
compound to have value in premise cleanup activities. 
Investigations to more precisely determine the suitability of 
white-tailed deer as alternate hosts for B. annulatus have 
