Our experiment proved our initial hypothesis that Ixodes anatis would act comparably to most other nidicolous tick species in terms of conditions of preferred temperature and humidity, false. Temperature had a larger impact on development than RH on engorged larval and nymphal stages of I. anatis.
Under laboratory conditions, the requirements for larvae were narrower than for nymphs. Engorged larvae showed optimum development (moulting times and survival) at 10–20 °C when SDs were < 1–2 mmHg (RH > 94%). Engorged nymphs survived and moulted up to 25 °C but, like larvae, appeared to favour a range of 10–20 °C, although with the ability to survive a somewhat drier atmosphere, tolerating a SD range of 1–10 mmHg. Females laid eggs at all temperatures and the range of humidity tested, although the pre-oviposition period was from six to 14 days longer at SD of < 1 mmHg as compared to 2–3 mm of Hg. The prolongation of development at the lower temperature may have exposed the eggs to a greater decline in their water balance than would have occurred at higher temperatures. Also, breaking the eggs into smaller batches would have possibly increased surface area and subjected them to increased dehydration, however, this requires further investigation.
Under field conditions, the temperatures in the burrows varied slightly across the 3 months with a mean temperature of 11 °C (range: 10–13), mean RH of 67% (range: 65–69) and a calculated SD between 3–4 mm of Hg, which were at the lower end of the favourable range for both larvae and nymphs, but ambient humidity was a little drier than the larvae would seem capable of tolerating. Having said this, the RH was measured in the burrow air, not at the soil surface which may have been slightly more humid. From previous experiments conducted on burrows (D. Galvez, personal communication, 2018) we know that while external temperature fluctuates, the diurnal temperature within the burrow remains relatively constant. In addition, over the year, the microclimate in a burrow is not as extreme as in the external environment and remains within a range of ± 6 units for both temperature and humidity.
In the burrows, the developmental success rate for both larvae and nymphs was very high (99% and 91.8% respectively). In the laboratory however, larvae exposed to similar conditions (10 °C and 62.1%-83% RH; SD 1–4 mmHg), did not survived. It is possible that engorged larvae in burrows were in closer contact to available soil moisture, and able to absorb it in through the cuticle or experience reduced water loss. Larvae in laboratory chambers were surrounded by humid atmospheric air, but at a level perhaps less than that experienced by larvae in burrows. Ogden et al., (2004) reported that even small fluctuations or changes in temperature and humidity can affect the developmental times in ticks. It is also possible that these differences may have been caused by our routine checks as larvae are less tolerant to minor changes in temperature and RH (Chilton and Bull, 1993), and another study by Padgett and Lane (2001) found that when larvae were left undisturbed, they had a higher success of moulting that the ones that were disturbed more often.
In studies with kiwi-occupied burrows (Bansal et al., 2019; Swift et al., 2015) larvae were most prevalent from January to June (summer and autumn), and lowest in October (spring; usually a damper season). Nymphs, on the other hand, were less prevalent in January, with highest numbers from June to December. In the present study the artificial ' burrows' did not have any kiwi, which is very likely to have influenced temperature and humidity levels, both from physiological exhalations, body warmth (Calder, Parr and Karl, 1978) and deposited waste material.
In general, in many species of Ixodidae, immature stages of these ticks survive better at moderate to high RH (> 90%) and between 18 °C to 25 °C but die off rapidly at 75% RH at similar temperature conditions (Ginsberg et al., 2017; Needham and Teel, 1991; Padgett and Lane, 2001; Troughton and Levin, 2007). We found that the optimum temperature preferred by Ixodes anatis to complete development is between 10 °C to 15 °C which is lower than many other species of Ixodid ticks. Extended developmental times as a function of low temperature preference may be an adaptation for survival in burrows which are unoccupied for long periods as well as to the cold temperatures in New Zealand. However as in other species, the bioclimatic requirements of larvae are at the lower end of the range tolerated by the species overall. To a large extent this determines both seasonal patterns and habitat suitability for the species because, if larvae are disadvantaged, the life cycle can be disrupted. Nymphs, however, are generally more desiccation resistant and have a better tolerance of higher temperatures than do larvae, with engorged females capable of withstanding even greater bioclimatic extremes (Chilton and Bull, 1993; Heath, 1975, 1981; Needham and Teel, 1991).
The kiwi is a nocturnal animal and can range widely in search of food as well as use a multitude of burrows within its range (Dixon, 2015, Jamieson et al., 2016). The tick too is exclusively host-specific (aberrant hosts are very rare; see Heath 2010) and this suggests it would be an advantage for the tick to be sedentary and to be capable of sustained quiescence in the event of the spasmodic presence of hosts. There has been no success in finding questing I. anatis outside of kiwi burrows, reinforcing the inference of the tick's sedentary nature and thus its adaptation to stable, but relatively cool and damp conditions in the burrows and reflecting the findings in this study as well as the evolutionary consequences of its association with the kiwi.
The best survival strategy for the kiwi tick is to have a mix of stages in each burrow, ready to take advantage of the return of a host. A quicker development cycle for engorged larvae over the warmer time of year provides unfed nymphs that are able not only to withstand cooler times of the year but also the attendant added risks of dehydration. Unfed stages were not tested in these experiments and such a study would throw additional light on the biology of I. anatis in relation to its host.