Do ticks from cooler climates seek hosts at lower temperatures?

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Climate warming is changing the distributions and seasonal timing of many organisms and may also affect vectors of disease-causing pathogens. In Europe, the tick Ixodes ricinus is the primary vector of medically important pathogens, including Borrelia burgdorferi sensu lato, which causes Lyme borreliosis.

The researchers asked how climate change might affect I. ricinus host-seeking behavior, known as questing. They hypothesized that, to maximize survival, ticks had adapted their questing responses to temperature according to local climates. They predicted that ticks from cooler climates would quest at lower temperatures than those from warmer climates. This would suggest that I. ricinus can adapt and may therefore be resilient to climate change.

The team collected I. ricinus along a latitudinal gradient spanning a range of climates: northeast Scotland, North Wales, South England, and central France. In the laboratory, the ticks experienced temperature increases of 1°C per day, from 6 to 15°C. The proportion questing was recorded five times at each temperature, or each day. The researchers then estimated each population’s theoretical potential to quest throughout the year under future climate change projections.

As predicted, a greater proportion of ticks from cooler climates quested at lower temperatures than ticks from warmer climates. The proportion questing was strongly associated with key climate parameters at each location. Projections based on temperature alone suggested that populations could begin their activity season a month earlier under climate change, with implications for how long hosts are exposed to tick-borne pathogens.

The findings suggest that I. ricinus has adapted its behavior in response to climate, implying some potential to adapt to climate change. The authors say that accounting for differences between populations would improve continental-scale models predicting I. ricinus dynamics and disease risk.