As far as I am able to judge from my own experience, mosquitoes are more or less equally numerous and ferocious in the Canadian, the Hudsonian, and the Arctic Zones of the Northwest. Naturally the season begins earlier in the more southerly localities. In two seasons (1914, 1920) at the western end of Lake Athabaska they began to be troublesome about the middle of June, whereas at Nueltin Lake this stage was reached about the first of July. In the Athabaska and Great Slave lakes region (Canadian and Hudsonian Zones) I have never had occasion to regard black flies as serious in respect to either numbers or ferocity; but there is universal agreement that conditions are vastly different and worse on the Barren Grounds. At Nueltin Lake the Simulium hordes become troublesome at approximately the same time as the mosquitoes. Toward the end of August there is a merciful diminution in the numbers of both mosquitoes and black flies on the Barrens, and after the first of September they may be practically disregarded, except on an occasional day of unseasonable warmth.
It may be remarked in passing that one of the insect terrors of the Athabaska region, the so-called “bulldog” (a species of Tabanidae), did not come to my attention as a pest at Nueltin Lake though I collected two species of Tabanus. Malloch (1919), in reporting on the Diptera of the Canadian Arctic Expedition of 1913-18, does not include a single species of Tabanidae. On the other hand, Twinn (1950: 18) states that 17 species have been found at Churchill; he refers to tabanids as “very abundant in forested regions of the North.” The “bulldog” may be presumed to contribute to the summer misery of the Woodland Caribou and the Moose as well as of man.
The season during which the adult warble flies and nostril flies harass the Caribou probably lasts only a few weeks in July and August. While the adults evidently cause no pain, they no doubt arouse an instinctive dread in the prospective hosts of their larvae. While I have no information as to whether they follow the hosts into the wooded country, it would appear quite likely that they do so if we are to credit statements by Franklin (1823: 242), Richardson (1829: 251), and B. R. Ross (1861: 438) that these pests infest the Woodland Caribou as well as the Barren Ground species. Furthermore, Cephenemyia has been reported in Pennsylvania as a parasite of the White-tailed Deer (Odocoileus) (Stewart, 1930?). The scarcity of available study material may be judged from the fact that the Canadian Arctic Expedition of 1913-18 secured only three adults of Oedemagena and none of Cephenemyia (Malloch, 1919: 55-56).
From the foregoing it may be seen that the wooded country represents a virtually fly-free haven for the Barren Ground Caribou during nearly ten months of the year. Is it any wonder, then, that they hasten throughout August toward or into the shelter of the woods, to gain freedom from the winged scourges of the Barrens? (See also [Retrograde autumnal movement].)
In the spring migration of 1947 the last of the Caribou passed the Windy River area on July 1, just before the mosquitoes and black flies had become seriously troublesome. It might be surmised that the animals keep marching northward in advance of the appearance of these flies, as long as that is feasible; and that when fairly overwhelmed by the winged hordes, they desist from further progress in that direction. The fawns are born at such a time (in late May or June) as to pass their first few tender weeks before becoming subject to serious insect attacks.
(See also [Retrograde autumnal movement]; [Relations to flies].)
References.—Richardson, “1825”: 328-329; Hoare, 1930: 33, 37-38; Jacobi, 1931: 193-195; Soper, 1936: 429; Hamilton, 1939: 247, 301; Clarke, 1940: 95-96; Porsild, 1943: 389; Banfield, 1951a: 27-29.
[ Effect of combined environmental factors on distribution]
The sum total of environmental factors apparently makes the Barren Grounds a distinctly more favorable summer habitat for the present species than the wooded country, since the latter region is virtually entirely deserted at that season. In general, the wooded country must be a more favorable winter habitat, since the bulk of the animals evidently resort to it at that time; yet its advantages are by no means clear-cut or overwhelming, since a very considerable proportion of the Caribou elect to spend the winter on the Barrens (Hanbury, 1904: 93, 120, 139; Hoare, 1930: 22; Clarke, 1940: 8-9, 11, fig. 4; Anonymous, 1952: 267).