How can milder winters increase varroa mite infestations?

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Honeybee at work — colony collapse disorder A western honeybee (*Apis mellifera*) forages in Italy.

Milder winters contribute to increased Varroa destructor infestations by disrupting the natural environmental constraints that typically limit mite populations.

Under normal winter conditions, lower temperatures help suppress mite reproduction within honeybee colonies. However, warmer winters allow for two primary factors that exacerbate the threat:

Extended Foraging Seasons: Milder weather enables honeybees to remain active for longer periods. This extended activity increases the opportunities for "drifter bees" to move between hives, facilitating the spread of mites from colony to colony.1
Reduced Population Limitation: In many regions, winter temperatures no longer remain low enough for a sufficient duration to significantly curb mite growth. Consequently, mite populations remain higher throughout the winter months, placing an increased parasitic burden on overwintering bee colonies and leading to higher rates of colony loss.1

These factors, often linked to climate change, are significant concerns in the context of colony collapse disorder, as they weaken the overall health of the colony and increase its vulnerability to the viruses transmitted by the mites.1