Predicting Honeybee Colony Failure: Using the BEEHAVE Model to Simulate Colony Responses to Pesticides

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.

Characteristics of specific life stages used in the BEEHAVE model Table S2 Percent survival of simulated colonies with mortalities imposed as defined percentages Figure S1 Average numbers of bees alive in the colony at the end of three years when egglaying rate is reduced Changes to the BEEHAVE model used in the simulation, including those made to the Netlogo Interface and to the model code.

Table S1
Characteristics of specific life stages used in the BEEHAVE model relating to life stage duration, food requirement and major source of mortality. Values are parameters set in the model (Becher et al. 2014) * In-hive bees will become foragers after between 7 and 50 days depending on colony conditions † Nectar consumption increases with level of brood care

Table S2
Percentage of the 30 replicate colonies surviving 3 years of an imposed stress of a set percent mortality on one life stage for one month of the year. A colony that has more than 4000 bees at the end of three years is assumed to survive the winter.

Figure S1 Reducing egg-laying rate
The mean number of bees alive in a colony (± standard error) at the end of three year simulations (n = 30) when the colony is subject to a reduction in the egg-Laying rate (ELR) for one month in the year.

Figure S2 Increasing mortality as a multiple of control
The number of bees alive in the colony (± standard error) at the end of three year simulations (n = 30) when the colony is subject increased mortality, (multiple of the model control mortality) of certain life stages for one month in the year. The standard error of the mean is shown.
A: Larval daily mortality B: In-Hive worker mortality C: Forager daily mortality D: Forager per trip mortality.

Figure S3 Increasing mortality by a set percentage
The number of bees alive in the colony (± standard error) at the end of three year simulations (n = 30) when the colony is subject increased mortality (set percentage mortality) , of certain life stages for one month in the year. The standard error of the mean is shown.
A: Larval daily mortality B: In-Hive worker daily mortality C: Forager daily mortality D: Forager per trip mortality

Figure S4 Increasing mortalities of three life stages by multiple of control and by a set percentage
The number of bees alive in the colony (± standard error) at the end of three year simulations (n = 30) when the colony is subject increased mortality (set percentage mortality), of larvae, inhive workers and foragers for one month in the year. The standard error of the mean is shown.

Figure S5
Detailed effects of imposing 25% daily larval mortality in June. Black solid line represents the control scenario and the red dashed line represents the treatment scenario. The treatment period is between the vertical lines. Data was collected from BEEHAVE simulations set up exactly as described in the methods, taking values at the end of each day in the model. Detailed effects of imposing 5% daily in-hive worker mortality in June. Black solid line represents the control scenario and the red dashed line represents the treatment scenario. The treatment period is between the vertical lines. Data was collected from BEEHAVE simulations set up exactly as described in the methods, taking values at the end of each day in the model.
A: Number of workers in colony; B: Number of larvae in colony; C: Number of larval deaths from lack of food or brood care; D: Age when workers first become foragers (days); E: Honey store of the colony (J); F: Pollen stores in the colony (g)

Figure S7
The increase in in-hive worker deaths in the colony after the addition of an extra 5% daily in-hive worker mortality in a month compared to the control. The points show the increase in number of in-hive worker deaths each day during the treatment month compared to the same month in a control simulation. The values below each month give the total difference in in-hive worker deaths for the month in question.

Changes to BEEHAVE model that were used in all simulations in this study
The BEEHAVE model can be downloaded at www.beehave-model.net. Included in the download are a manual and ODD (Overview, Design concepts and Details) document describing the model and how to operate it in more detail.

Changes to the interface tab
Added "Inputs" for new global variables: Added "Switches" to turn stress effects on and off ReducedEggLaying -Daily ELRt will be reduced during treatment period LarvalMortality -Daily larval mortality will be increased during treatment period InHiveMortality -Daily in-hive mortality will be increased during treatment period ForagerMortalityPerDay -Daily forager mortality will be increased during treatment period ForagerMortalityPerTrip -Per Trip forager mortality will be increased during treatment period S12 Supporting Information; Rumkee et al ES&T Added "Chooser" to select how mortality effect will be applied PesticideMortalityType Options: "Multiple"mortality is applied as a multiple of the default mortality "Set Percent"mortality is applied as a defined percent.

Changes and Additions to the code tab
Reducing egg-laying rate starting on the treatment day lasting for the treatment period In 'NewEggsProc' Altering larval mortality, either by multiplying the control mortality or by setting the mortality to a defined percent.
In 'WorkerLarvaeDevProc' Replaced line: Altering the mortality applied to foragers at the flower patch, either by multiplying the control mortality or by setting the mortality to a defined percent.