Crovect can be used off licence once the lambs are dry but technically you should speak to your vet about any off label medicine use. Ticks can kill baby lambs from tick pyaemia. Some carry a virus that hammers their immune system, tick bourne fever, as well as injecting bacteria into the blood stream. Most hill farms have to treat lambs near birth to avoid large losses with septicaemia or crippled lambs. Treating the ewes before lambing helps as the lambs get some chemical from the wool when they're sucking.