15. Assuming 5 per wheel and applying the proverb of "three men make a tiger"*, they just have to be the new cars of three famous people who are likely to get in the news for a wheel falling off.
That might work for once-off vaccinations of precisely targeted animals in densely populated areas but not delivering periodic high doses of medication to treat parasites to animals across 100s of millions of acres.
You have two types of screwworms, the common, and the south American. Are you sure ivermectin is as efficient against the species they're talking about?