Illinois. Although some larvae survive, winter cold may 
take, directly or indirectly, a heavy toll of unted larvae. 
Apparently, some larvae do not survive much more 
thana few months in anunfed state (Hooker et al. 1912). 
Because the larvae in Lee County are probably at least 
2 months old by late fall, many of those that have not 
found a host before the beginning of winter may be sup- 
posed to perish from starvation. The nonlethal starva- 
tion period possible for most larvae may terminate at 
about the time temperatures drop in winter. At this 
time larvae, numbed by cold and unable to seek a 
host, may die from lack of food rather than from cold 
directly. The extent to which cold alone or combined 
with other factors may adversely affect unfed larvae 
is an unsolved problem that is worthy of future in- 
vestigations. 
Low temperatures may influence adults as well as 
larvae. Adults of the continental rabbit tick, in both 
the north and south, go into hibernation or into a period 
ofrest even before the advent of cold weather. Possibly 
they require a period of rest or subjection to low temper- 
atures before they are able to feed, mate, and lay eggs. 
Such seems to be the case with adults of the eastern 
rabbit tick, [Ixodes dentatus, which, according to Smith 
(1945), must undergo hibernation in the unengorged state 
for one winter before they will feed. Similar behavior 
may be inherent in the continental rabbit tick. If so, 
the termination of this rest period may come about during 
the winter. Because of a longer period of low tempera- 
tures in the north, adults there would necessarily wait 
longer before seeking a new host than would adults in 
the south. We made no observations on whether the 
photoperiod affected the activity of the adults. 
It is presumed that in Lee County most rabbits 
become hosts to rabbit ticks at some time, if not many 
times. For example, between August 1] and August 30, 
1952, one rabbit, designated as Peter II, was infested 
with at least 60 ticks in the following sequence: August 
ll, 6 ticks; August 18, 13 ticks; August 21, 11 ticks; 
August 22, 0 ticks; August 25, 10 ticks; August 27, 
14 ticks; and August 30, 6 ticks. Other rabbits may 
encounter continental rabbit ticks as frequently as did 
Peter II. 
The proportion of rabbits in Lee County infested 
with the continental rabbit tick varied greatly with the 
season. During the period of greatest tick abundance, 
August and September, nearly 90 per cent of the rabbits 
examined had one or more ticks. By contrast, from late 
fall through the coldest part of winter, almost no rab- 
bits harbored ticks. In late spring nearly 75 per cent of 
the rabbits examined had ticks. In June, the number of 
infested rabbits dropped to 50 per cent or less. The 
number dropped again in late October and dwindled to 
almost: zero by mid-November. 
A larger number of larvae than nymphs and a 
larger number of nymphs than adults were found on rab- 
bits in Lee County. Each tick must find a host three 
times during its life, and the decline in numbers of ticks 
between stages may be directly correlated with the 
chances involved in procuring hosts. 
Larvae and nymphs of the continental rabbit tick 
often select ground-inhabiting birds as alternate, 
possibly equal, hosts. Adults of this tick rarely occur 
on birds; instead, they prefer rabbit hosts almost ex- 
clusively (Bishopp & Trembley 1945). Our records for 
for larvae and nymphs include such bird hosts as brown 
thrasher, towhee, prairie chicken, olive-backed thrush, 
tufted titmouse, quail, and swamp sparrow. Birds have 
been found infested as early in the year as April in 
central Illinois and as late as December in southern 
Illinois (INHS collection). Undoubtedly, migrating birds 
are responsible for dispersing some of these rabbit 
ticks northward in spring and southward in fall. 
Dermacentor variabilis (Say); variable wood tick, 
wood tick, American dog tick; fig. 5. — Insofar as num- 
bers are concerned, the variable wood tick is a minor 
ectoparasite of rabbits. In Illinois it is the principal 
vector of Rocky Mountain spotted fever in man. The 
relationship of this tick to rabbits, to the disease 
organism that produces spotted fever, and to man is 
undoubtedly an important one. Price (1954) recently 
stated that the strains of Rickettsia virus that cause 
Rocky Mountain spotted fever in man are maintained in 
nature primarily in ticks of the genus Dermacentor rather 
Fig. 5.—Dermacentor variabilis, variable wood tick, un- 
engorged adult female. This tick is a vector of Rocky Moun- 
tain spotted fever. [t was rarely found as an adult on rabbits 
in Lee County, Illinois, 1952-1954. In the immature stages, ~ 
which were occasionally found on rabbits in Lee County, it 
can be distinguished from other Illinois rabbit ticks by the 
presence of eyes. 
