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Dermacentor spp. -- Acarina, Ixodidae




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Two species of ticks, the American dog tick Dermacentor variabilis (Say) and the Rocky Mountain Wood tick Dermacentor andersoni Stiles, have been subject for biological control.  McMurtry (1978) reported that the dog tick was widely distributed in the United States east of the Rocky Mountains and is also found in California, Mexico and Canada.  It causes irritation to dogs and sometimes to humans, horses and cattle.  Of greatest concern is that it is also a vector of Rocky Mountain spotted fever in the central and eastern portions of North America, and is also known as an occasional vector of tularemia.


American Dog Tick.--The life cycle may be completed in 1-3 years, and there seems to be little activity during winter of in the hot summer.  Adults are most active in the spring and may live >2 years without food.  This is the only stage known to infest humans, dogs and domestic animals.  Small mammals, especially mice and rabbits, are considered to be the principal hosts.  Mating occurs on the host.  After engorging, they drop from the host and the females deposit their eggs in protected places in masses of 4,000-6,500 eggs, after which they die. The eggs hatch into six-legged larvae, which attach to a passing host.  After feeding for several days, they become engorged, drop tot he ground, and molt to the nymphal stage.  When the nymph is ready to feed, it likewise seeks a host on which to attach.  When the nymph has become engorged, it also drops to the ground where it molts to the adult stage.  Both larvae and nymphs were observed to live >one year in the absence of food (Smith et al. 1946).


The encyrtid parasitoid, Hunterellus hookeri Howard (= Ixodiphagus caucurtei du Buysson) was introduced from France into the United States where it was propagated and released on Naushon Island, Massachusetts, in an attempt to control the American dog tick (Larrouse et al 1928).  Small numbers of nymphs of D. variabilis parasitized by the French strain of H. hookeri were released on Capers Island, South Carolina in 1931 (Bishopp 1934).  A larger effort was made on Martha's Vineyard Island, Mass., where ca. 90,000 females of H. hookeri were released in two locations on the island during 1937-39.  The strain of parasitoid used originated in Texas (Smith & Cole 1943).


Larrouse et al. (1928) reported that in the season following the releases of H. hookeri on Naushon Island immature parasitoids were found in a single nymph of the dog tick and a single nymph of another tick species.  Subsequent surveys were conducted in 1940 by Cobb (1942) and in 1941 by Smith & Cole (1943).  In both cases a few parasitoids were found, but none were recovered from the American dog tick.  Both this species and Ixodes scapularis Say were still observed in abundance; therefore, there was no evidence that any success was achieved.  Bishopp (1934) reported recovery of the parasitoid from a single nymph of D. variabilis on Caper Island two years after release.  In an assessment of results of release of the parasitoid in Martha's Vineyard in 1937-39, Smith & Cole (1943) recovered no parasitoids from ticks in the release areas and observed no reduction in tick abundance that could be attributed to the parasitoid.  A later report by Smith et al. (1946) also indicates that the attempt was unsuccessful.


The biology of H. hookeri was reported in Wood (1911) Cooley (1928, Cooley & Kohls (1933) and Smith & Cole (1943), Cole (1965) and McMurtry (1978). 


Rocky Mountain Wood Tick.--The Rocky Mountain wood tick is a vector of Rocky Mountain spotted fever, a rickettsial pathogen caused disease that may be fatal to humans, but is primarily harmful to wild animals.  It can also carry tularemia, another disease primarily of wild animals, but also infectious to humans.  This tick is also responsible for tick paralysis, which affects the motor nerves starting in the legs and gradually spreading to the rest of the body.  This seems to result only if the tick feeds at the back of the neck or the base of the skull, and removal of the tick usually results in recovery.  Dermacentor andersoni occurs in the western United States, primarily in the Rocky Mountains and also in Canada.  Spotted fever occurs in other areas, but its chief vector there is the American dog tick, D. variabilis (see McMurtry 1978).


The eggs of D. andersoni are laid on the ground.  They hatch in spring or early summer into six-legged larvae and climb onto grass blades or other vegetation and attach to passing animals, usually small rodents.  When fully fed in a few days, the larvae drop to the ground to molt to the nymphal stage, which usually does not feed until the following spring, when they attach to small animals, become engorged, and drop to the ground to transform to the adult stage (McMurtry 1978).  Though some adults may attach to hosts the same season, they apparently pass the rest of the summer and winter in hiding and find a host the following spring.  Mating takes place on the host, and when fully fed the female drops to the ground to deposit her eggs.  Only the adult stage is known to attack humans and large animals (Cooley 1932).


The encyrtid parasitoid H. hookeri was brought to Montana for colonization (Cooley 1928, Cooley & Kohls 1933).  Over 4 million parasitoids were liberated during 1927-32, mostly in Montana but also in Colorado, Oregon and Idaho.  Various methods were used, including release of adult parasitoids, scattering parasitized nymphs in grass and low vegetation, and liberating squirrels, which had been infested with parasitized nymphs.  The methods for mass production are described by Morton (1928).


Recovery of parasitoids was made in only one instance in 1929, when a few which had emerged from D. andersoni nymphs taken from squirrels captured in the Bitter Root Valley of Montana (Cooley & Kohls 1933).  Cole (1965) reported that no reduction in the tick population was observed and no evidence has been obtained that the parasitoids were established in nature.


Other Ixodidae.--McMurtry (1978) reported on other species of Ixodidae that were subjects of biological control.  Alfeev (1940) reported on an experiment in which it was attempted to control Ixodes ricinus (L.) and I. persulcatus Schulze in a 250-acre pasture in the province of Leningrad, U.S.S.R.  Hunterellus hookeri was obtained from Montana in 1935 and propagated, and 2,600 adult parasitoids and 38,000 parasitized ticks were liberated.  But no recoveries were reported.



REFERENCES:     [Additional references may be found at:   MELVYL Library ]



Alfeev, N. I.  1940.  The utilization of Hunterellus hookeri How. for the control of the ticks Ixodes ricinus L. and Ixodes persulcatus Sch. with reference to the peculiarities of their metamorphosis under the conditions of the Province of Leningrad.  In:  Pavlovsky, 2nd Conf. Parasitol. Problems, Nov.  1940.  Akad. Nauk. S.S.S.R. Isv.  23-5.


Bishopp, F. C.  1934.  Records of hymenopterous parasites of ticks in the United States.  Wash. Ent. Soc. Proc. 36:  87-8.


Cobb, S.  1942.  Tick parasites on Cape Cod.  Science 95:  503.


Cole, M. M.  1965.  Biological control of ticks by the use of hymenopterous parasites.  A review.  World Health Organ., EBL 43.65.  11 p.


Cooley, R. A.  1928.  Tick parasites.  Montana State Bd. Ent. 7th Bien. Rept.:  10-16.


Cooley, R. A.  1932.  The Rocky Mountain wood tick.  Montana Agric. Expt. Sta. Bull. 268.  58 p.


Cooley, R. A. & G. M. Kohls.  1933.  A summary on tick parasites.  5th Pacific Sci. Cong. Proc. 5:  3375-81.


McMurtry, J. A.  1978.  Acarina, Ixodidae.  In:  C. P. Clausen (ed.), Introduced Parasites and Predators of Arthropod Pests and Weeds:  A world Review.  U. S. Dept. Agriculture, Agric. Handbk. No. 480.  545 p.


Morton, F. A.  1928.  Quantity production of tick parasites.  Montana State Bd. Ent. 7th Bien. Rept.:  32-5


Smith, C. N. & M. M. Cole 1943.  Studies of parasites of the American dog tick.  J. Econ. Ent. 36:  569-72.


Smith, C. N., M. M. Cole & H. K. Gouck.  1946.  Biology and control of the American dog tick.  U. S. Dept. Agric. Tech. Bull. 905.  74 p.


Wood, H. P.  1911.  Notes on the life history of the tick parasite, Hunterrellus hookeri How.  J. Econ. Ent. 4:  425-31.