Prolonged effects of in-hive monoterpenoids on the honey bee Apis mellifera

Honey bees are exposed in their environment to contaminants but also to biological stressors such as Varroa destructor that can weaken the colony. Preparations containing monoterpenoids that are essential oil components, can be introduced into hives to control Varroa. The long-term sublethal effects of monoterpenoids used as miticides have been poorly investigated. Analysis of behavior of free-moving bees in the laboratory is useful to evaluate the impact of chemical stressors on their cognitive functions such as vision function. Here, the walking behavior was quantified under a 200-lux light intensity. Weeks and months after introduction of the miticide (74 % thymol) into the hives, decreases of phototaxis was observed with both summer and winter bees. Curiously, in spring, bees collected in treated hives were less attracted by light in the morning than control bees. The survival of bees collected in spring was increased by treatment. After a 1-year period of observation, the colony losses were identical in treated and non-treated groups. Colony loss started earlier in the non-treated group. In public opinion, natural substances as essential oils are safer and more environmentally friendly. We demonstrated that a monoterpenoid-based treatment affects bee responses to light. The latter results have notable implications regarding the evaluation of miticides in beekeeping.