Abstract
To simulate effects of pesticides on different honeybee (Apis mellifera L.) life stages, we used the BEEHAVE model to explore how increased mortalities of larvae, in-hive workers, and foragers, as well as reduced egg-laying rate, could impact colony dynamics over multiple years. Stresses were applied for 30 days, both as multiples of the modeled control mortality and as set percentage daily mortalities to assess the sensitivity of the modeled colony both to small fluctuations in mortality and periods of low to very high daily mortality. These stresses simulate stylized exposure of the different life stages to nectar and pollen contaminated with pesticide for 30 days. Increasing adult bee mortality had a much greater impact on colony survival than mortality of bee larvae or reduction in egg laying rate. Importantly, the seasonal timing of the imposed mortality affected the magnitude of the impact at colony level. In line with the LD50, we propose a new index of “lethal imposed stress”: the LIS50 which indicates the level of stress on individuals that results in 50% colony mortality. This (or any LISx) is a comparative index for exploring the effects of different stressors at colony level in model simulations. While colony failure is not an acceptable protection goal, this index could be used to inform the setting of future regulatory protection goals.
Highlights
A number of stressors have been implicated in honeybee losses in many parts of the world[1], including habitat loss;[2] viral diseases;[3] parasites such as Varroa destructor,[4,5] which can be a disease vector;[6−8] and use of pesticides.[9]
We present the LIS10, which predicts 10% colony failure from an imposed stress. We argue that these indices will be useful for comparing the impact of imposed stressors at colony level and could inform the setting of pesticide protection goals in the future, once the indices have been applied to a wider variety of stressors and their variability has been quantified
Colony dynamics were affected to varying degrees depending on the season and the reduction in the egg laying rate (Figure S1)
Summary
A number of stressors have been implicated in honeybee losses in many parts of the world[1], including habitat loss;[2] viral diseases;[3] parasites such as Varroa destructor,[4,5] which can be a disease vector;[6−8] and use of pesticides.[9]. An innate difficulty in studying the effect of pesticides on honeybee colonies is the level of replication needed to capture low-level effects at the field scale.[10] The European Food Safety Authority (EFSA) has described specific protection goals for honeybee colonies, stating, “The magnitude of effects on colonies should not exceed 7% reduction in colony size”.11. To assess whether this level of impact is occurring a minimum of 60 pairs (control and treatment) of colonies and fields are needed for each study.[11] If multiple stressors are to be studied even higher numbers would be needed. Ecological models can help in designing and targeting empirical studies, generating specific hypotheses that later may be tested experimentally, and can be used to assess the risk of environmental chemicals to honeybees.[11]
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