Embryonic diapause and cold hardiness of Ixodes ricinus eggs (Acari: Ixodidae)

Abstract

Ixodes ricinus is medically the most important tick species in temperate Europe. The present study was undertaken to assess the effect of photoperiod and low-temperature exposure on termination of embryonic diapause and on cold hardiness in I. ricinus eggs. Engorged I. ricinus females (n = 9) were kept at 90% r.h., 15 °C longday (L17:D7), or 15 °C shortday (L14:D10), or at an outdoor site providing natural temperatures and daylength. Eggs of defined ages were continuously kept under these conditions or subjected to changes in photoperiod and temperature, and the effect on diapause incidence was observed. At 15 °C, non-diapause and diapause eggs hatched after 70-120 and 140-240 days, respectively. Interestingly, most egg masses did not show an all or nothing reaction but a certain percentage of diapause eggs. Cold exposure of eggs to 4 °C for 6 weeks completely terminated diapause, whereas the effect of photoperiod was negligible. Also exposure to temperatures between -10 and -20 °C for 24 h terminated diapause. At the outdoor site, eggs were laid in winter, from mid October to early April. All eggs hatched between mid June and early July. The supercooling point of these eggs was around -28 °C between November and January and rose to -27 °C in April. At constant 15 °C, the supercooling point rose from -30 °C in young eggs to -24 °C in physiologically older eggs. The lower lethal temperature in diapause and non-diapause eggs was -21.6 and -18.0 °C, respectively, but the difference was not significant.