Hard ticks have three life cycle stages: larvae hatch from eggs and have six legs; they shed their skin (moult) to become nymphs, which have eight legs; nymphs moult to become adults, which are male or female.
Males have a hard covering (scutum) over their backs; in females, the scutum covers only a small portion of the tick’s back, allowing the rest of the body to expand when she takes a blood meal.
The three stages each spend a number of days on a host taking a blood meal before they moult to the next stage. Fed and unfed stages differ markedly in size. Newly hatched larvae are very small and are often called ‘seed ticks’ because they look no larger than some grass seeds. When engorged, they are approximately pin-head size. Engorged nymphs are approximately match-head size, whilst engorged adult females vary in size according to species. Engorged female ticks on Australian livestock can be 10–15mm long.
The hard ticks are further distinguished by their life cycles into 1-, 2- or 3-host ticks. In Australia 1-host and 3-host ticks affect cattle.
The cattle tick is a 1-host tick (Figure 1).
A larva finds a host and remains on it until it drops off as an engorged adult, approximately 21 days later. The moulting from larva to nymph and nymph to adult occurs on the one host. Each of the three stages lasts 5 to 7 days. Engorged female ticks drop off the host and lay their eggs on the ground (approximately 2,000 to 3,000 eggs in the case of the cattle tick) and these hatch to new larvae in approximately 3 weeks. The spent female tick dies. The cattle tick can therefore complete its life cycle, from newly laid egg to engorged adult, in approximately 6 weeks.
In a 3-host tick life cycle each stage engorges on blood from a different host for around 5 to 7 days then drops to the ground—the larvae and nymphs to moult, and the adult female ticks to lay their eggs and die. Each stage has to find a new host. This means that it can take as long as two years for a 3-host tick to complete its life cycle from egg to engorged adult (Figure 2).