Ixodes rubicundus nymphs are short-day diapause-induced ticks with thermolabile sensitivity and desiccation resistance

The Karoo Paralysis tick, Ixodes rubicundus Neumann (Acari: Ixodidae), is a semi-voltine ixodid that survives stressful environmental conditions using morphogenetic diapause (eggs and engorged nymphs) and desiccation resistan ce. Both photoperiod and temperature influence diapause induction in the en gorged nymph. Ixodes rubicundus nymphs are typical long-day photoperiodic o rganisms. The critical photoperiod is similar to 13.5 h light, 10.5 h dark, and they display a thermolabile response. The period between detachment an d apolysis in engorged nymphs is modified by photoperiod; however, apolysis to ecdysis is not affected by photoperiod. Thus, initiation of development , but not the actual process is controlled by photoperiod. Most engorged ny mphs delayed metamorphosis when exposed to short-day regimen (LD 12 : 12 h) after feeding. Nymphs exposed to pre- and post-feeding long-day regimen (L D 14 : 10 h) developed. Times for 50% of nymphs to apolyse when exposed. to photoperiods of LD 14 : 10 h, 13.5 : 10.5 h, 13 : 11 h and 12 : 12 h were 28, 36, 40 and 58 days, respectively. Times for 50% of engorged nymphs to e cdyse ranged from 38 to 40 days after apolysis. Nymphs were sensitive to ph otoperiodic exposures before, during and after feeding. Nymphs exposed to l ong day (LD 14 :10 h) before and during feeding, moulted at 20 degreesC; ho wever, most exposed to 10 degreesC followed by 20 degreesC (post-feeding) w ent into diapause. Both short- (10 : 14 h) and long- (14 : 10 h) day expose d engorged nymphs survived 45 days at 0% r.h. (n = 73), but diapause-destin ed ticks kept at 13 degreesC lost the least mass (29.5 +/- 9.5%, SD), while nondiapause ticks at 23 degreesC lost the most (48.7 +/- 8.2%, SD). Termin ation of diapause and transition to development probably coincides with a d efinite increase of water vapour uptake by engorged nymphs. Comparatively, I. rubicundus engorged nymphs are more desiccation tolerant than a North-Am erican counterpart, Amblyomma cajennense (Fabricius) (Acari: Ixodidae), whi ch is also semi-arid- to xeric-adapted. Diapause conveys important survival attributes that enable engorged I. rubicundus nymphs to inhabit a semi-ari d environment with great temperature extremes, and to synchronize their act ivity periods with seasons and host utilization patterns.