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A field trial was carried out during summer 1996 in a commercial beef cattle farm located in the central area of Argentina (30 degrees 16'S, 60 degrees 30'W) to evaluate the efficacy of a new insect growth regulator material (dicyclanil) in the prevention of myiosis caused by screwworm (Cochliomyia hominivorax) larvae. Forty bull calves (Bos indicus x Bos taurus), 5 to 6 months old were allocated by ranked paris to a control (G.1) and a treated group (G.2) of the same number of animals. On day 0, all experimental animals were surgically castrated and calves of group 2 were treated topically with 20 ml as a single dose of a ready-to-use formulation containing 5% w/v dicyclanil. Inspections for screwworm larva infestation were carried out on days 4, 8, 12, 16, 19, 23 and 25 after castration. The incidence of egg masses on the scrotal wounds evidenced a similar challenge for all groups (P > 0.05 chi(2)). However, 16 animals of G.1 (80%) and only one of G.2 (5%) developed active myiosis, respectively. This difference was significant (P=0.003 X(2)).

The prophylactic efficacy of ivermectin against navel or scrotal myiasis in calves was evaluated in eight trials in Argentina and Brazil. In two trials, calves were injected subcutaneously with ivermectin at a dosage of at least 200 microg kg(-1) within 24 h of birth. In the other six trials, two with two-month-old calves and four with four-month-old or older calves, all calves were treated with ivermectin at a dosage of at least 200 microg kg(-1) immediately after castration. In all trials, calves were maintained together on pasture and naturally exposed to Cochliomyia hominivorax. Navel and scrotal wounds were examined for myiasis daily for at least 14 days. Incidence of navel and scrotal myiasis was significantly lower (P < 0.01) in treated calves than in control calves.