Ticks Hunt Mostly at Night, Year-Long Study of European Tick Behaviour Finds

The castor bean tick, Ixodes ricinus, is the most important vector of tick-borne pathogens in Europe, transmitting the agents of Lyme borreliosis, tick-borne encephalitis and a range of animal diseases. Yet one of the most basic questions about this arachnid has remained surprisingly unresolved: when, over the course of a day and a year, does it actually go looking for a host? A new study published in the journal Parasites & Vectors has now provided the most detailed field answer to date, and the results carry a warning for anyone who assumes tick exposure is mainly a daytime hazard.

Researchers at the University of Liverpool, working with Richard Wall of the University of Bristol, carried out an extraordinarily intensive sampling campaign at Lyme Park in northern England. Every month from January to December 2022, the team dragged a fabric blanket across fixed transects at four-hour intervals, completing full 24-hour sampling cycles. This design allowed them, for the first time, to separate seasonal variation in tick activity from daily, or diel, variation, two dimensions of behaviour that previous studies had almost always examined in isolation. Because the sampling rounds captured ticks actively questing, that is, climbing vegetation with outstretched forelegs to ambush passing hosts, the counts represent genuine host-seeking behaviour rather than simple abundance.

The headline finding is that questing activity in I. ricinus is predominantly nocturnal. Densities of host-seeking ticks were highest during the dark hours of the sampling cycle, and statistical modelling confirmed that darkness itself was a positive predictor of the number of questing nymphs. This is a striking departure from the popular image of ticks as creatures that latch onto walkers in broad daylight, and it has immediate implications for how people, pets and wildlife encounter infected ticks in the field.

The second major moderator of activity was atmospheric moisture, captured by a single, elegant variable known as the saturation deficit. Saturation deficit combines air temperature and relative humidity into a measure of how strongly the air draws water from living tissue, and it is widely used by entomologists to describe desiccation stress in small terrestrial arthropods. Because ticks lack efficient water-conservation mechanisms and must periodically rehydrate within the humid microclimate of the soil and leaf litter, high saturation deficit effectively forces them off the vegetation and into protective refuges. In the new study, the density of questing nymphs was negatively associated with saturation deficit, meaning that hot, dry conditions suppressed host-seeking even when the season was otherwise favourable.

Crucially, the researchers showed that the strength of the nocturnal activity pattern itself varied with the seasonal moisture environment. Night-time questing was most evident in nymphs during months with high saturation deficit, when daytime conditions were too punishing for ticks to remain exposed. In cooler or wetter periods, when desiccation stress was lower, activity was distributed more evenly across the day. In other words, darkness does not simply switch ticks on; rather, harsh daytime climates push their activity into the night, and benign climates allow it to spread out. This interaction between the daily light cycle and seasonal moisture availability had not previously been demonstrated under natural field conditions for this species.

To untangle these effects, the team analysed their 24-hour, year-round dataset using Generalised Linear Mixed Modelling, a statistical framework well suited to count data with repeated measures. Mixed models allowed the investigators to account for the structure of the design, in which the same transects were sampled month after month at identical clock times, while testing the independent contributions of darkness, temperature, humidity and saturation deficit to tick density. The modelling confirmed that the negative effect of saturation deficit and the positive effect of darkness on questing nymph density were robust after controlling for the sampling structure, giving confidence that the observed rhythms reflect biology rather than artefacts of the schedule.

The seasonal dimension of the results was broadly reassuring for the field. The annual pattern of activity, with questing peaking in the warmer months and collapsing through winter, was largely consistent with previously published data on European tick phenology. Larvae, nymphs and adults each showed characteristic seasonal windows of host-seeking, reflecting the developmental timing that determines which life stages pose a threat at any given time of year. Nymphs, the stage most often implicated in transmitting Borrelia bacteria and tick-borne encephalitis virus to humans, dominated the datasets in the ways that surveillance specialists have come to expect.

What makes the study transformative is not the seasonal story but the daily one. Diel patterns in tick behaviour have been chronically understudied, largely because most tick surveillance is conducted during working hours, typically between mid-morning and late afternoon. If ticks quest mainly at night, or shift their activity into nocturnal windows during dry spells, then standard daytime dragging may systematically misestimate both true abundance and true biting risk. The authors point out that these commonly overlooked daily rhythms have important implications for estimating abundance in scientific research and surveillance programmes, and for interpreting how hazard varies across the day rather than across the year alone.

There are also ecological consequences for the transmission of pathogens among wildlife. Different host species are active at different times: birds, small mammals and deer partition the 24-hour cycle in ways that determine which hosts a nocturnal tick is most likely to encounter. If I. ricinus concentrates its host-seeking in the dark, the feeding opportunities available to it, and therefore the transmission circuits available to the pathogens it carries, may be structured by the clock as much as by the calendar. Reservoir hosts that forage at night could contribute disproportionately to infection cycles, while daytime-active animals, including humans, may benefit from the desiccation-driven suppression of tick activity during hot, dry daytime conditions. Understanding these synchronies could sharpen models of disease emergence and inform the timing of interventions, from acaricide application to public health messaging about when and where exposure risk peaks.

For the public, the practical message is nuanced rather than alarmist. Tick bites remain possible at any hour, and risk is shaped locally by habitat, vegetation and recent weather. But the study suggests that people walking in tick habitat during cool, humid evenings and overnight periods may face greater exposure than conventional advice implies, while hot, dry middays may offer comparatively lower risk, not because ticks are absent but because they are sheltering from the desiccating air. Dog walkers and other pet owners, whose animals often roam at dawn and dusk, should take particular note. As climate change alters temperature and humidity regimes across Europe, the seasonal windows and daily rhythms of tick activity are likely to shift in ways that current, daylight-biased surveillance is poorly equipped to detect. This year-round, round-the-clock field study offers a methodological template for closing that blind spot, and a timely reminder that the ecology of one of Europe’s most consequential disease vectors unfolds largely after dark.

Subject of Research: Seasonal and diel environmental drivers of host-seeking (questing) behaviour in the European tick vector Ixodes ricinus.

Article Title: Seasonal and diel environment effects on host-seeking behaviour in the European tick vector, Ixodes ricinus

Article References: Noll, M., Wall, R., Makepeace, B. L., & Rose Vineer, H. (2026). Seasonal and diel environment effects on host-seeking behaviour in the European tick vector, Ixodes ricinus. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07679-5

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07679-5

Keywords: Ixodes ricinus, ticks, questing behaviour, host-seeking, saturation deficit, diel activity, seasonal activity, vector-borne disease, Lyme borreliosis, tick surveillance, Parasites & Vectors, field study