The Occurrence of Oestrus ovis L. (Diptera: Oestridae) in the Bighorn Sheep from Wyoming and Montana

Abstract

Three previous and five new records of the domestic sheep bot, Oestrus ovis, from the bighorn sheep are given. The life history and descriptions of adult and larval forms are presented. The significance of the occurrence of the parasite in the abnormal host is discussed. The infrequent occurrence of an aberrant parasite in an abnormal host is ordinarily of particular interest only to specialists acquainted with these groups. Repeated occurrences, however, may have a special biological significance particularly when they are found in related hosts occupying similar habitats. This could well be the case of the domestic sheep botfly, Oestrus ovis L., as it is found in bighorn sheep, Ovis canadensis Shaw, in North America. The parasite normally occurs in domestic sheep, Ovis aries, and goats, Capra hircus, in almost every sheep-raising district in the world. It is found generally throughout the United States and Canada and is considered one of the most important pests of domestic stock. While in flight the female fly deposits a small droplet containing two or three minute larvae in the nostrils of the selected host. These first-instar larvae migrate into the nasal passages and remain 1 to 9 months, feeding on the mucous exudate caused by their constant irritation of the mucosa. Cobbett and Mitchell (1941) report that the second-instar larvae then migrate into the frontal sinus, where development is accelerated. When the third stage is completed the larvae migrate out through the nostrils and drop to the ground to seek a suitable medium for pupation. The pupal period varies considerably according to the climatic conditions and season, but averages about 6 weeks. The adult flies mate shortly after emergence and the gravid female seeks out the hosts to deposit her larvae. Mature, gravid females will produce an average of 500 larvae within their life span of 2 or 3 weeks. The adult flies frequently cause considerable annoyance when attacking the host animals. Received for publication 1 November 1965. Domestic sheep become terrified when approached by the loud-buzzing insect and will often bunch together with their heads inward and held close to the ground. Others may run frantically, shake their heads, or suddenly push their muzzles against the ground or between other sheep. The migration of the larvae over the delicate membranes causes the animal to rub its nose on the ground and to strike sharply with its forefeet. Infected sheep usually have a pronounced nasal discharge, sneeze frequently, and grate their teeth. Loss of appetite is common, emaciation follows, and death is not unusual in severe cases. According to Cobbett and Mitchell (1941) inflammation of the mucosa is always associated with the presence of dead larvae, and secondary bacterial invasion is probably the cause of death. DESCRIPTION OF ADULT The adult female fly measures approximately 11 mm in length. The overall color is dark gray with the exception of the head and legs, which are dull yellow. The parafrontal areas are a series of deep, pock-like depressions containing numerous thin hairs. The mouthparts are vestigial and are reported to be nonfunctional although it is possible that the adult fly may take in moisture during its brief mating period. The thorax is a frosted gray with four black, parallel stripes composed of numerous minute tubercles, each bearing a hair. The legs are yellow with moderately strong bristles on each tarsal segment. The wings are hyaline and membranous, approximately 8.0 mm in length with minute spinules the entire length of the costal vein. The abdomen is usually black with an irregular pattern of gray pollinose spots. DESCRIPTION OF LARVAE The first instar (Fig. 1) is tapered at both ends and measures about 1.0 mm in length. The oral hooks are heavily sclerotized, black, and strongly recurved. The hooklets located on the caudal segment (Fig. 2) are diagnostic for the species. Two terminal protuberances each support 10 or 11 of these hooklets, which are best viewed from the