Some Insect Pests of Wildlife

J. P. Linduska, Arthur W. Lindquist.

Combining such contradictory qualities as "adaptability" and "specialization," insects hold a key position in the economy of most living things. There is scarcely a plant that does not contribute to the needs of one or more kinds of "bugs," and a considerable segment of the insect world has become adapted to making a living off the larger forms of animal life. Wildlife supports these pests in great variety. And, despite a remarkable diversity of habits among birds and mammals, few, if any, escape attack by insects or their near relatives.

Moles and other subterranean mammals, which seldom appear above ground, are hosts to a complement of these arthropods. Bats, alternating periods of sustained flight with retirement in the deep recesses of a cave, are not without some parasites the little brown bat frequently carries two species of mites, one species of flea, and the bat bug, a form related to the common bed bug of humans. A semi-aquatic existence has not spared the beaver this "boring from without." Examination of 140 animals trapped in Minnesota produced one species of tick, one of mites, two types of beetles, and larvae of the screw-worm fly. Neither has the pelagic life of the northern fur seal entirely discouraged such parasitism. Young seals before their first oceanic crossing are host to two species of lice; two species of nasal mites affect young and old alike.

The distaste shown for some small mammals by their larger mammalian predators is not always shared by the lesser insect life. Short-tailed shrews, for instance, which may be commonly killed but seldom eaten by dogs, foxes, and cats, are entirely acceptable fare to several kinds of fleas, numerous species of mites, and occasionally ticks.

Among birds this relationship also appears to be practically universal. A survey of the ectoparasites of birds of the eastern United States yielded 198 species of lice, flies, ticks, and mites from 255 species and races of these animals. Such common songsters as the robin and song sparrow and also one of our commonest game species, the bobwhite quail, were found to harbor 15 species of external. parasites. The rabbit tick, which was recorded from 46 of the 255 types of birds examined, belied its common name by appearing as the most widely distributed of the 198 types of parasites found. The ubiquity of such pests of birds is apparent from studies in many areas, and 33 species of biting lice (Mallophaga) alone have been recognized from 29 species of wild birds just in the Province of Alberta.

Indeed, it seems unlikely that any bird or mammal can claim complete freedom from insects or related air-breathing arthropods, such as ticks and mites. Whales and other cetaceans are possible exceptions, but even in this instance gill-breathing arthropods of several types appear in substitution. The large whale lice (Cyamus) aside from bearing a superficial resemblance to some of the true lice, functionally present the same problem to whales as do their smaller insect counterparts to land animals.

Obviously the endless kinds of animals falling within the definition of wildlife, in combination with unlimited types of known insect pests, prevent anything like a detailed consideration of interrelationships of the two. Accordingly, this discussion must be confined largely to a generalized account of the subject in which occasional examples, illustrative of certain points, will be given in more detail.

INSECTS AND THEIR CLOSE RELATIVES differ markedly in their relationships to wildlife. Some depend on the host during their entire span of life. And, as is the case with many lice and mites, some have reached a level of specialization that restricts them to a' single species of animal. Others, including many ticks, experience intervals away from the host, and in the course of development a given individual may infest several species. Still others, such as adults of many of the biting flies and mosquitoes are even less discriminating in the selection of hosts and are essentially free-living except for periodic visits for the purpose of obtaining a blood meal.

Additional differences are to be found in the way in which these pests attack wildlife and in the importance of such attacks to the welfare of the individual. While these innumerable relationships between insects and wildlife defy any logical classification, a few considerations of the characteristics and habits of the more important parasite groups will provide an insight into the nature of the problem. A number of insect-borne diseases of wildlife are capable of transmission by species of widely different parasite groups, and an account of this phase of the problem will be reserved for later discussion.

TICKS, which are closely related to insects, are distributed widely, and practically all mammals are attacked by one or more kinds. The habits of most wild birds are such that opportunities for infestation are not great. However, even among this group, many that nest or feed on the ground become parasitized, and one group of ticks, the soft-bodied ticks (Argasidae), are common pests of birds. Cliff swallows are known to be attacked by a species of this group and heavy populations of the so-called blue-bug frequently occur in the nests of barn swallows. Another species has been taken from several of the raptorial birds (hawks and owls) and others of the soft-bodied ticks are known to feed on a number of kinds of mammals. Ticks of this group are responsible for transmission of relapsing fever, a spirochete disease important to humans and fatal to laboratory animals in some strains, but of unknown consequence to the hosts in nature.

Ticks hold a special significance for wildlife in their capacity for disease transmission a relationship we deal with later. Exclusive of this role, however, the mechanical effects of their feeding can be an important factor in predisposing animals to other pests and diseases. Through sheer numbers they frequently reduce the vitality of the host even to the point of occasionally being the direct cause of death.

The engorgement of female Rocky Mountain wood ticks has been recognized by William Jellison and Glen Kohls as being the cause of a noninfectious disease which they called tick-host anemia. After repeatedly producing this secondary anemia by heavy tick infestations on laboratory rabbits, these workers recognized the condition as being comparable to that previously observed in wild moose, jack rabbits, and foxes. They believed the disease occurred with some frequency in nature and that it could be the direct cause of death. Tick-induced anemia has been offered in explanation of a pronounced mortality among an island population of cottontail rabbits. This die-off, investigated by R. H. Smith and E. L. Cheatum, occurred on Fishers Island, N. Y., where a few dozen cottontails introduced in 1925 had reached near-plague proportions 13 years later. Ticks of the species Ixodes dentatus and the rabbit tick were abundant, and infestations on rabbits found dead were more than three times as heavy as those on live rabbits. Among dead animals suitable for autopsy the pathological conditions most consistently revealed were pale and watery-appearing blood and anemia, which, while generalized, was most apparent in the lungs and kidneys.

Feeding by ticks appears also to be an important underlying cause of subcutaneous abscesses reported in cottontail rabbits and wild hares. The condition, which has been termed lymphadenitis, or pyogenic disease, results from infection by one or more species of bacteria belonging to the genus Staphylococcus. The incidence or occurrence of the disease is not known, but among 84 cottontails examined from one area in New York State a rate of 17 percent was found. Likewise little is known of the disease as a mortality factor. Extensive involvement of the lymph nodes appears to be characteristic, however, and in some animals metastatic infections of the heart, lungs, kidney, and spleen have been noted. The rabbit tick and the species Ixodes dentatus have been associated with the disease in cottontails.

Tick paralysis, a disease of some consequence to humans in the Northwestern States and British Columbia, is known also to occur among a number of the wild mammals in that area. Engorging female Rocky Mountain wood ticks are associated with the disease, which is mainly of unknown causes. Most evidence favors the belief that a nerve poison, produced in the salivary glands and secreted by the tick during feeding, is responsible for the paralysis. Although removal of the offending tick is usually followed by rapid and complete recovery, it is likely that complete paralysis and death is more commonly the outcome in nature.

Ticks are believed to be of importance as a predisposing agent for such conditions as foot rot in deer and screw-worm common in many forms of wildlife. With other biting pests, they may serve to spread mechanically the disease, frequently fatal for rabbits, which is accompanied by so-called rabbit horns. This disease (rabbit papillomatosis) is caused by a filtrable virus. In the same way they probably carry the infectious agent responsible for fleshy "warts" on the feet and legs of rabbits.

Various degrees of blindness in birds are known to result from attachment and feeding by this pest in the region of the eyes, and occasional mortalities are reported wherein unusual tick infestations appeared to be the direct cause. F. C. Bishopp and Helen Trembley suspected that gross infestations of the lone star tick were responsible for mortality among wild turkeys on Bull's Island in South Carolina. Rabbits and other mammals occasionally are subject to massive subcutaneous infestations of ticks, and the appearance of these pests beneath the skin probably results from an allergic reaction by the host.

While comparatively little is known of the population relationships of ticks and wildlife, the widespread distribution, abundance of species, and immensity of numbers of ticks has led to some suspicions that population levels of one group may be regulated by the other. R. G. Green and his coworkers in Minnesota learned that cycles in numbers of the rabbit tick followed the population trend of the snowshoe rabbit and were related to it. In the fall of 1933, when snowshoes were at a cyclic peak, the estimated population of feeding ticks was 2,800,000 per square mile. With the hare population at a "low" in the fall of 1938, the numbers of ticks were at a corresponding low of 150,000 per square mile. The rabbit die-off was demonstrated to be independent of any immediate effects of the ticks, and due to "shock disease" in which hypoglycemia was the only definite finding.