Warbles or grubs are larvae of large flies belonging to the genus Cuterebra.  Warbles are usually found in rodents or lagomorphs but have been reported from deer, mink, fox and a variety of domestic animals.  At present it is thought that warble flies belonging to the genus Cuterebra are host specific.

Newly hatched cuterebrid larvae are grayish-white and 2-4 millimetres long.  As they develop they enlarge and become reddish-brown.  Just prior to pupation the prepupal stage is brown and may be 20-42 millimetres long.  In infected animals the larvae are commonly located in the region of the groin but may also occur beneath the skin on other parts of the body.  In Ontario parasitism by these larvae occurs only during the late summer and early autumn.

In the life cycle, female flies deposit eggs at entrances to burrows and other areas frequented by the host.  Eggs hatch in response to increased environmental temperature and possibly to carbon dioxide and moisture from the host.  The newly hatched larvae gain entrance to the host through the mouth, nasal openings or cuts in the skin.  The time required for larval development is dependent on many factors.  Bennett (1955) estimated that complete larval development of Cuterebra emasculator in chipmunks of Algonquin Park required 18-19 days under natural conditions.  Once mature, the larva or prepupal stage forces its way out of the host through a breathing pore in the skin.  Pupation occurs in detritus and loose soil.  The fly over-winters in the pupal stage.  The adults emerge in summer and will mate and lay eggs within a few days.

   Bennett (1955) found that 30% of 465 chipmunks which he examined from 1948 to 1953 in Algonquin Park were infected with larvae of C. emasculator.  Approximately 44% of the chipmunks examined between early August and early October were infected.  Multiple infection of chipmunks with C. emasculator is not uncommon. In a sample of 113 infected chipmunks Bennett (1955) found that 67% harboured one larva; 20% had two; 7% had three and 5% had four to eight larvae.

   The effects of cuterebrid larvae on their hosts have not been extensively studied.  However, it is common to have secondary bacterial infection of open wounds after the prepupal stages of C. emasculator leave chipmunks.

   The meat of rabbits or other animals infected with cuterebrid larvae is considered safe for human consumption following removal of any affected tissue.

Selected Reference:

   Bennett, G.F. 1955.  Studies on Cuterebra emasculator Fitch 1856 (Dipter: Cuterebridae) and a discussion of the status of the genus Cephenemyia Ltr. 1818. Can. J. Zool. 33:75-98.