Development of resistance to Hypoderma lineatum (Diptera: Oestridae) within a cattle herd.

Three experimental infestations of a herd of 27 cattle with the common cattle grub, Hypoderma lineatum, are described during a 4-yr period. The mean percentage of survival of larvae during internal migration was 67.7% in the initial infestation. In the 2nd infestation 1 yr later, the mean percentage of survival of larvae decreased to 40.5% during internal migration. Although fewer larvae survived to reach the tissues in the back in the 2nd infestation, more larvae in the back tissues survived (27.2%) and 53 more mature larvae (potential adults) were produced during the 2nd than the 1st infestation. The 3rd infestation resulted in no further decrease in larval survival to the back tissues (43%), but significant larval mortality in the back (5.7% survival) reduced the number of mature larvae. After 2 infestations, larval survival to the back tissues had stabilized at approximately 40%, whereas the significant decrease in larval survival in the back tissues during the 3rd infestation indicated that resistance manifested at this stage of the parasite life cycle may be important for H. lineatum population control. We can conclude that development of herd resistance through H. lineatum exposure may require several infestation cycles.