Background: Honeybee Flight Activity Index

Honeybee (Apis mellifera) Flight Activity Index and Mason Bee (Osmia cornuta) Flight Threshold Status

The bee foraging estimates presented here are not designed to indicate a specific level of activity, but as a general guide to indicate hours when honeybees and mason bees are likely to be visiting flowering ground cover and crop plants. Actual bee activity will vary with site-specific and date-specific factors such as flower density, nectar flow, recent weather, local honeybee, and mason bee population density.

The chart serves to highlight hours to avoid spray application to prevent direct pesticide contact on honeybees and mason bees. Spray application of materials hazardous to an area during hours when honeybees, mason bees, and other pollinator insects are actively visiting flowering ground cover or crop plants exposes the bees to harm from direct spray contact and fresh spray residue.

Spraying outside of bee activity hours does not prevent all risk of harm. Minimizing pesticide application, and selection for materials with the lowest impact when the application is needed, are necessary first principles. Mowing to remove clover, dandelion and other attractive flowers of ground cover plants before spray application is another important tool.

The minimum temperature for honeybee flight is 54º F. The optimum temperature for flight activity is 72-77º F, but activity continues up to about 100º F before declining. Minimum flight temperature may increase by 4-7º F during late spring and summer, but this is not reflected in the activity index as it is based on field studies done in the early spring. Using a lower minimum threshold provides a conservative buffer that overestimates, and reduces the chance of underestimating, honeybee activity when temperatures are in the 55-61º F range.

There is an interaction between light level and temperature, such that the temperature to begin foraging in the morning is higher at low light levels than at higher light level. The threshold temperature to cease foraging in the afternoon as light declines are higher than the threshold temperature to begin foraging in the morning, but other factors such as nectar flow can also cause cessation of foraging. The formula to estimate flight activity does not attempt to estimate cessation of foraging. As a result, the formula may overestimate honeybee activity in the afternoon when the falling temperature or nectar flow brings an end to foraging. For the purpose of this chart, it is better to overestimate activity, and the need to avoid contacting honeybees with pesticide spray, than to underestimate the risk of bee exposure to pesticide spray.

Even with favorable temperature and sunlight, rain and wind speed above 15 mph can cause honeybees to stop foraging. Rain is not included in the flight activity estimates. Mason bees can continue to forage during rain, but honeybees are less likely to do so. The absence of rain is another potential source of a high bias in honeybee activity estimates. Of course, rain often makes conditions unsuitable for spray application, so overestimating honeybee activity during rain hours is less likely to affect grower selection of spray timing because growers are less likely to spray while it is actively raining.

ORANGE AREA = Honeybee foraging activity index for the hour ending at the marked time. This is a relative estimate based on flight response of the honeybee (Apis mellifera) to temperature and amount of sunlight, as estimated by a multiple regression formula based on data published in Burril and Dietz, 1981, and constrained by a minimum activity threshold derived from Vicens and Bosch, 2000.
YELLOW AREA = A presence/absence indicator for Osmia cornuta, a mason bee. If temperature and light conditions have met minimum requirements for this species to be foraging, the value is 1. If not, the value is 0.
- These estimates do not represent activity by other important pollinators, including bumblebees that may forage at temperatures as low as 40F. In addition, bumblebees may stay in the field overnight.
BLUE LINE along the bottom of the chart indicates hours during which foliage is wet. If a spray material that can be harmful to honeybees as undried surface residue is applied during hours of wet foliage, the residue may not dry until foliage dries. The risk to bees could be increased if foraging activity overlaps a period of the undried residue of such material. For application to dry foliage, one hour of good drying conditions is often sufficient.
PURPLE DASHED LINE = wind speed, which ends honeybee foraging at 15 mph, and is also included to help identify low wind periods for good coverage and drift control that overlaps with periods of low honeybee activity.
VERTICAL GREEN LINE = today’s date and the beginning of forecast values.

Model Logic Sources and Notes

Burrill, R, M, and A. Dietz. 1981, The Response Of Honey Bees To Variations In Solar Radiation And Temperature. Apidologie, Springer Verlag, 12(4), pp.319-328. <hal-00890551>

Source of data for regression formula to estimate honeybee activity index above minimum flight temperature and light thresholds.
“… honey bees will not fly if there is no light regardless of temperature.”
“Above threshold levels one factor, the temperature is only predictive when the other factor, radiation intensity, is considered and vice versa. Under the circumstances, activity responses to temperature appear to be approximately linear.”

Vicens, N. and J. Bosch. 2000. Weather-dependant pollinator activity in an apple orchard, with special reference to Osmia cornuta and Apis mellifera (Hymenoptera: Megachilidae and Apidae). Environ. Entomol 29:413–420.

Source of honeybee and mason bee minimum temperature thresholds based on light level.

“On all five days on which video recordings were taken, O. cornuta was fully active for longer periods (range: 3 hours, 20 minutes – 9 hours, 50 minutes; sum: 33 hours, 40 minutes) than A. mellifera (range: 0 minutes -5 hours, 30 minutes; sum: 15 hours, 10 minutes) Both activity initiation and termination occurred at lower temperature and solar radiation in O. cornuta than in A. mellifera.”
“Both activity initiation and termination occurred at lower temperature and solar radiation in O. cornuta … than in A. mellifera “
Bee counts on the trees and the activity counts at the O. cornuta and A. mellifera nesting sites in relation to ambient temperature and solar radiation “… establish the approximate temperature-radiation thresholds for the two species. The lowest temperature at which O. cornuta activity was registered was 9.8º C, with a solar radiation of 225 w/m2. At higher temperatures, this species was active at lower radiations (e.g., 69 w/m2 at 10.5º C or 34 w/m2 at 16.0º C). A. mellifera had a similar response but at higher thresholds (12.2º C – 329 w/m2, 13.2C 233 w/m2, 15.8º C – 151 w/m2).
Temperature and radiation thresholds for A. mellifera found in this study are similar to those reported elsewhere (Burrill and Dietz 1981, Kevan and Baker 1983, Winston 1987, Free 1993). A. mellifera is fully active at temperatures higher than 12-14º C and solar radiation higher than 300 w/m2, being particularly sensitive to drops in solar radiation below this limit. Moderate wind, even at favorable temperatures and light intensity may also cause activity to cease.

[*** Editor note: The honeybee activity formula used for this chart sets value to 0 if wind speed is > 15 mph]

“A. mellifera is an endothermic species, able to warm up through shivering of the flight muscles, and attain thoracic temperatures 15C above ambient temperature (Heinrich 1979). However, because it requires a high thoracic temperature to fly, it cannot make uninterrupted flights at ambient temperatures lower than 12-15º C (Heinrich 1979, 1993).

[*** Editor note: The honeybee activity formula sets the minimum flight temperature no higher than 15.5º C (60º F) as long as light level is >0. The data from Vicens and Bosch (2000) indicate that it would be more realistic to require temperatures above 60º F at very low light levels that can occur in the first hours after sunrise on cloudy days, but Burril and Dietz, using different methods, did observe honeybee departures from observation hives at very low light levels. In order to reduce the chance of underestimating honeybee foraging at low light levels, the temperature requirement was set to never exceed 60F (15.5º C).

The minimum temperature for flight is no lower than 12.2º C (54º F) once light level exceeds that which occurs after 8:00 a.m. in May – July except on very overcast days. During late spring and summer, the true flight temperature threshold in May through summer at this light level may be higher than 12.2º C, but is constrained to reduce the chance of underestimating honeybee activity.

Multiple sources concur with these limits. Because the formula constrains the minimum flight temperature requirements in the direction of overestimating honeybee activity, estimates may be unrealistically high, especially late in the day. But it is better to overestimate bee activity in order to over-protect honeybees, rather than the alternative. In addition, afternoon hours are often unacceptable for spray applications due to the wind, drift potential and other concerns, in which case an overestimate of honeybee activity would not result in a different grower decision about suitable spray timing.

For the mason bee Osmia cornuta, the minimum flight temperature with low light is constrained to be no higher than 60º F. With “high” light (i.e. after 8:00 a.m. during May-August except on very cloudy days) the forage temperature threshold is set at 49.6º F,]

Back to Vicens and Bosch excerpts:

(During fruit tree bloom)…”Even on days with good weather, the foraging activity of most insects is low or nonexistent in the early morning and late afternoon, limiting further the time available for pollination.”
“The estimated numbers of nesting O. cornuta females necessary to pollinate 1 hectare (=2.5 acres) is ” … ” 530 for apples (Vicens and Bosch 2000), in contrast to 2–7 colonies of A. mellifera, with thousands of foragers per colony (McGregor 1976, Free 1993). These differences between the two bee species in numbers of individuals required are the result of the higher pollinating efficacy of O. cornuta on fruit tree flowers (Bosch and Blas 1994, Monzon 1998, Vicens and Bosch 2000), its strong preference to visit fruit trees (Marquez et al. 1994), and its capacity to forage for longer periods, both daily and seasonally (this study). Because of its greater tolerance to cool, overcast weather, O. cornuta’s beneficial effects as an orchard pollinator will be comparatively greater on years with inclement conditions during flowering.”
“The abundance of Diptera [flies accounted for about 10-40% of flying insect found on apple flowers in this study] does not reflect their pollinating efficacy. Our observations are consistent with the long-recognized low pollinating contribution. The apparent greater activity of Muscoid ßies during the daylight hours with the lowest temperature and solar radiation is the result of their habit of spending the night on the flowers.”
“Of the two managed bee species, the activity of A. mellifera was more dependent on climatic factors than that of O. cornuta. O. cornuta tended to be distributed uniformly on the flowers throughout the day and were active on days when other pollinators were scarce or absent. The establishment of precise activity thresholds is difficult because responses depend on a combination of factors that include not only weather variables but also the physiological and behavioral state of the insect, as well as floral resource availability (Stone 1994). Thus, activity thresholds vary, not only among individuals but also throughout the day and the nesting season. “

[*** Editor note: Minimum temperature thresholds for honeybee flight appear to be lowest in spring. Both the Vicens and Bosch (2000), and Burril and Dietz (1981) studies were done during apple bloom. Minimum flight thresholds and estimate of flight activity based on data from these studies are more likely to set lower thresholds than what actually apply during late spring and summer. Thus, activity estimates during marginal conditions are biased high. As noted above, it is better to over- than under-estimate activity for purposes of this chart.]

“Both in O. cornuta and A. mellifera, as well as in Megachile rotundata (Szabo and Smith, 1972), there is an inverse correlation between temperature and solar radiation during daily activity initiation, so that females start foraging at lower temperatures on days with clear skies. When conditions are particularly unfavorable, however, bees will wait until both weather factors rise above “normal” levels.”
“… nectar availability might have affected the termination of foraging activity in this study, notwithstanding the significant influence of weather factors found for both species.”

[*** Editor note: The estimates in this chart essentially do not include forage activity termination thresholds, thus late afternoon and early evening values are likely to be biased high.]

“As a rule, under moderate wind and relative ambient humidity of <95%, O. cornuta is fully active from 10 to 28º C (see also Maddocks and Paulus, 1987, Bosch, 1994) and 200 w/m2. Some females even forage under light rain or strong wind. O. cornuta is a relatively large Osmia with abundant pilosity and dark coloration on the head and thorax (completely black in females), and these traits are commonly associated with bees able to fly at low temperatures (Heinrich 1993, Stone 1994).”

[*** Editor note: This study only included one Osmia species. The authors note that another Osmia species that has been studied in more detail, Osmia rufa, has more stringent environmental requirements for flight than O. cornuta, and that where they coexist O. cornuta begins flying at lower temperatures. They also comment that flight temperature requirements in bee species tend to be similar with other species in the same family. While by no means definitive, these observations lend credence to the idea that the temperature and light thresholds for Osmia cornuta may be useful as a protective estimate for other species in the same family, i.e. for other species of mason bees.]

Thorp, R.W. 1996. Bee management for pollination, p. 132–138. In: W.C. Micke (ed.). Almond production manual. University of California, Division of Agriculture and Natural Resources, Publication 3364.

“Bee flight activity is governed by the abiotic environment. Honeybees forage when temperatures are 55° F (12.8° C) and higher; they do not forage in rain or in wind stronger than 15 miles (24.1 km) per hour. Cloudiness can also reduce flight activity, especially when it occurs near threshold temperatures.”

Boyle-Makowski, R. M. D. 1987. The importance of native pollinators in cultivated orchards: their abundance and activities in relation to weather conditions. Proc. Entomol. Soc. Ont 118:125–141

“The importance, efficiency, and activity of native and domestic insect pollinators were studied for three consecutive years, during the blossom period in a semi-dwarf apple orchard of the Niagara Peninsula [Canada]. Insect numbers and activity were correlated with apple cultivar (Empire, MacIntosh, Golden Delicious, Red Delicious] located on the east and west slopes of the orchard, and with temperature, humidity, and light intensity fluctuations.”
“The physical factors, especially temperature, were mainly responsible for the variation in the numbers of honeybees.”
[Andrenids – solitary, ground-nesting “mining bees” ] ” … the range of activity is slightly narrower than that of the honeybees but their numbers do not fluctuate with changing weather conditions. The Diptera although numerically ranked second in importance in 1978 and 1979 were less effective than the Hymenoptera as they carried little pollen on their bodies.”
“… native pollinators become important in pollination during blossom seasons with adverse weather conditions, these insects compensated for the reduced activity of the honeybees, an observation that has been underestimated in orchard management.”

Kevan, P. G., and H. G. Baker. 1983. Insects as ßower visitors and pollinators. Annu. Rev. Entomol. 28: 407Ð 453.

“Temperature effects are often closely related to light effects, especially in basking insects.”
“Honey bees cease foraging when wind speeds exceed 24-34 kph” (15-21 mph)
“The lowest temperature at which flight actively commenced in honeybees is about 10°C, but in spring flight usually begins at 12-14° C, in May at 14–16° C, and in summer, the main foraging period, at 16-18° C. A decrease of flight activity in the evening starts at slightly higher temperatures. Foraging increases with temperature up to approximately 38° C, but this figure may be low, and under some circumstances foraging may continue up to temperatures of 42° C or 48° C.”

Wafa, A. K. 1955. Temperature as a factor controlling the commencement of honeybees flighting and honeybee flight activity distribution during the various months of the year. (Hymenoptera: Apidae). Bull. Soc. Ent. Egypte 39:315-334.

“High significant positive correlations were found between the minimum temperature of the environment and the temperature at which the bees flew. The bees usually ceased to fly after the outside temperature had reached its maximum for the day, and they occasionally remained in the field after the temperature had begun to fall. The extent to which this happened differed markedly at different times of the year and was dependent upon the influence exerted by other factors of the environment.”

Abrol, D. P. 2006. Diversity of pollinating insects visiting litchi flowers (Litchi chinensis Sonn.) and path analysis of environmental factors influencing foraging behavior of four honeybee species. Journal of Apicultural Research. 45( 4): 180-187.

“In general, 15.5-18.5 degrees C temperature, 600-1700 Ix light intensity, and 9-20 mW/cm(2) solar radiation appeared to be the minimum ecological conditions for the commencement of flight activity in Apis species. Cessation of activities in all the honeybee species was controlled mainly by the decline in values of light intensity and solar radiation irrespective of other factors.”

Corbet, S.A., N. M. Saville, M. Fussell, O. E. Prŷs-Jones and D. M. Unwin. 1995. The Competition Box: A Graphical Aid to Forecasting Pollinator Performance (JSTOR website). Journal of Applied Ecology 32(4), pp. 707-719.

“Thermal constraints on flight activity limit the pollinating effectiveness of bees. Each species of social bee has a microclimatic ‘window’ within which foraging fight can be sustained. To predict whether a given species of social bee is worth testing as a pollinator in a given climate, it is useful to know at least the lower limits of that microclimatic ‘window’. We consider how information from a series of bee counts through a day can be used to characterize a bee species in terms of activity/microclimate relations as a basis for predicting the diel pattern of foraging activity…”
“We use examples of field data to explore the relationship between microclimate and activity for the honeybee Apis mellifera and several species of bumblebee, Bombus. Regression analysis is used to relate activity to T-g [T-g = black globe temperature = temperature measured inside a black sphere to include the effect of solar radiation on a body] and to identify the lower temperature threshold for activity from field bee counts. In field analysed here, the bumblebees Bombus terrestris/lucorum, B. pascuorum and B. hortorum began foraging at lower temperatures than honey-bees or B. lapidarius.”

[*** Editor note: Lower black globe temperature threshold for foraging was stated as 11º C (52º F) for honeybees, and 6º C (43º F) for Bombus (bumblebee) species.]

Benedek, P. 1976. Effect Of Environmental Factors On The Foraging Rates Of Honey Bees In Red Clover Fields. Zeitschrift Fuer Angewandte Entomologie 1(1): 14-20.

“Foraging rates of honeybees were definitely influenced by several environmental factors. The impact of air temperature proved to be the most important of the weather conditions.”
“Sky cover had slight reversed effects while light wind (below 4.5 m/s) [Ed. = 10 mph] and flight distance within 600 m did not affect the foraging rates of honeybees on red clover inflorescences. Strong wind decreased the speed of flower visits or prevented it.”
“… the foraging rates of honeybees are always affected by a complex system of factors, individual components of which can partly interfere with or support each other .”

Ramírez, F., and T. L. Davenport. 2013. Apple pollination: A review. Scientia Horticulturae. 162: 188–203.

“Honeybees remain near hives during overcast and rainy days. Their flight speed is 22 Km/h, thus, higher wind velocities affect their flight (Mayer et al., 1985).” (22 km/h = 14 mph)

Kumar, N. and R, Singh. 2005. Relative abundance of Apis spp. on rabi season sunflower (Helianthus annus L.) Journal of Entomological Research (New Delhi) 29(1): 65-69.

“The standard regression coefficients revealed that temperature played main role in fluctuating the population of Apis spp.”

Szabo, T. I. 1980. Effect Of Weather Factors On Honey Bee Apis-Mellifera Flight Activity And Colony Weight Gain. Journal of Apicultural Research 19(3): 164-171.

“Multiple correlation coefficients of humidity, temperature, wind speed and light intensity with flight activity ranged from + 0.701 to + 0.978, all highly significant.”

Nunez, J. 1977. Circadian Variation Of Flight Activity In Colonies Of Apis-Mellifera-Ligustica. Journal Of Insect Physiology. 23(3): 397-392

“Daily flight activity is confined to the warm hours when the temperature is higher than a certain threshold. Flight activity shows two intensity maxima: one in the morning correlated with an increased nectar flow in the hive and another in the afternoon between 15 and 16 h (solar time) not correlated with nectar flow (correlated with photoperiod).”