KEY WORDS: Hypoderma lineatum. Hypoderma bovis. heel fly. warble. gadding
PERSPECTIVES AND OVERVIEWTwo species of cattle grubs (Diptera: Oestridae) affect cattle in the Northern Hemisphere: Hypoderma iineatum, the common cattle grub or lesser cattle warble fly, and Hypoderma bovis, the northern cattle grub or larger cattle warble fly. The word warble is Anglo-Saxon for boil.Adults of the cattle grub are commonly known as heel flies, warble flies, bomb flies, or gad flies. This parasite has been observed and recorded for centuries (159), and Bracy Clark (50) cited and discussed references from Biblical times to Shakespeare. Scientific observations on the biology were first recorded in the 1700s by Vallisnieri (cited in 224) in Italy, followed by an extensive review and taxonomic description by Brauer (34) in Austria in the mid 19th century. Major contributions followed in England by Ormerod (157) and Imms (93); in the United States by Riley (179), Osborn (158), Bishopp et al (24), and Mote (132); and in Canada by Hadwen (85). Other reviews completed before the widespread use of systemic insecticides for cattle-grub control include work by James (94) and Scharff (185) in the United States, Gansser in Austria (69), Gebauer in Germany (70), Grunin in Russia (81), Natvig in Norway (141), MacDougall in Scotland (124), and Bevan & Edwards (22) in England. The quality of these reviews makes it unnecessary to repeat detailed information about the stage descriptions and life cycles for these two important species. Rather, I attempt to summarize these contribu tions, emphasizing additions to our knowledge since the 1950s.I The U.S. Government has the right to retain a nonexclusive royalty-free license in and to any copyright covering this paper.
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BIOLOGY
Life Cycle and DamageFemale Hypoderma spp., which are bee-like in appearance (85), use a specialized ovipositor (75, 133) to attach their eggs to host animal hairs by means of an attachment organ (51), which is an outgrowth of the egg chorion. The most often selected site of attachment are hairs on the legs and lower body regions of cattle (83). The body temperature of the host animal aids in incubation ofthe eggs, which is normally 3-7 days (24). Newly hatched larvae (-1 mm in length) (42), crawl to the base of the hair shaft and penetrate the host (43) by means of proteolytic enzymes secreted from their blind midgut (1l5, 142). Occasionally, edema and inflammation occur at the penetration site, most often with older, previously infested animals (73). Once inside the host, the first-stage larvae migrate for 4 to 6 months, normally through the fascial planes between muscles, along connective tissue (86), or along nerve pathways (35), again aided by prote o lytic enzymes secreted from the anterior part of the migrating larvae (26). Digestion by the larvae is transcuticular (48); enzyme action aids in the transport of nutrients into the hemolymph. Hypoderma lineatum larvae travel forward to the resting site (12...
