Fig. 1

Two alternative (but not mutually exclusive) hypotheses for the fall peak are shown: the direct development hypothesis (top panel) and the developmental diapause hypothesis (bottom panel). For simplicity, we show a fast 3-year life cycle of I. ricinus, which includes adult ticks (year y − 1 in the left panel) that lay eggs, the larvae (year y in the middle panel), and the nymphs (year y + 1 in the right panel). Masting occurs in year y − 1 (represented by the acorn symbol), the rodent population increases in year y (represented by the mouse symbol), and the larval blood meal occurs in year y (represented by the blood drop symbol). The origin of the spring peak of nymphs (green peak in year y + 1) is the same for both hypotheses and is as follows. Larvae that obtain their blood meal in the summer (green fraction in year y) moult into unfed nymphs in the same year, enter behavioural diapause, overwinter, and quest as unfed nymphs the following spring (green peak in year y + 1). The two hypotheses differ with respect to the origin of the fall nymphal peak (small orange peak). In the direct development hypothesis, larvae that obtain their blood meal early in the summer (orange fraction in year y) moult into unfed nymphs in summer and quest as unfed nymphs that same fall (small orange peak in year y). In the developmental diapause hypothesis, larvae that obtain their blood meal late in the summer (orange fraction in year y) overwinter as engorged larvae, moult into unfed nymphs the following summer, and quest as unfed nymphs that fall (small orange peak in year y + 1). The time lag between a masting event (acorns in year y − 1) and the spring peak of nymphs (green peak in year y + 1) is 2 years. In contrast, the time lag between a masting event (acorns in year y − 1) and the fall peak of nymphs (small orange peak) is 1 year under the direct development hypothesis and 2 years under the developmental diapause hypothesis