Bee foraging activity is of considerable interest to almond growers. It provides a good indication of the colonies’ pollination efficacy placed in the orchards as it will ultimately impact the crop yield. Currently, bee foraging activity is inferred using bee flight hours, which are calculated purely based on meteorological data. A bee flight hour is defined as one hour where: there is no rain; the temperature is warmer than 55 ℉ (12℃), and wind speeds are less than 15 mph (24 km/h). While bees respond to ambient conditions with an almost clockwork-like precision, these calculations’ accuracy still hinges on small but significant variations in the microclimate of any given site and some intra-colony factors, such as colony size, which is known to affect the overall flight activity.
While the concept of using bee flight hours as a predictor of nut set seems logical, in reality, the situation is much more complicated. Variations in the microclimate at a hive will significantly influence bee foraging behavior, particularly temperature and solar radiation(1). Thus, even the positioning of a hive in the shade or full sun, or on a NW slope instead of a SE slope can impact colony activity. The reaction time between changes in microclimate and changes in colony activity is less than 1 minute. Bees continuously monitor their external environment to make decisions about whether to embark on foraging journeys.
Some in-colony factors will also impact foraging performance, such as colony size, queen, and brood status, the presence of disease or parasites(2). A minority of very active bees (about 20% of foragers) perform 50% of the colony’s total foraging trips(3)(4) . Foraging performance depends on bees’ individual experience (amount of foraging trips completed). Individual workers will continuously adjust their activity level to ensure that the colony's nutritional needs are adequately met. Environmental stressors that alter the activity or reduce the lifespan of foragers may prevent bees ever achieving maximal performance, thereby seriously compromising the effectiveness of the colony foraging force.
For these reasons, the direct measurement of colony flight activity would provide a much more accurate metric of the foraging efficacy of honey bee colonies in agricultural settings. Visual observations are laborious, inaccurate, and not practical for large scale monitoring. Hence there have been numerous attempts to produce sensors that can automatically count exiting and incoming bees. Measuring the direct measurement of colony flight activity presents a significant technical challenge. However, BeeHero has successfully trialed an optoelectronic sensor (Figure 1) that provides accurate real-time monitoring of a colony’s foraging rate: the actual number of foraging trips a colony makes during a day. The bee counter was trialed on beehives in almond orchards in the North region of the Central Valley during the past two Almond pollination seasons, 2019 and 2020, and produced some fascinating data.
Figure 1: Optoelectronic sensor used in Beehero trials during almond pollination
Figure 2 below shows bee activity on two consecutive days in relation to ambient temperature. The level of activity is significantly influenced by ambient temperature, demonstrating that an hour defined as bee flight hour can differ drastically in the number of actual flights, and thus visits to flowers, for any given period. Furthermore, the precision with which the instrument measures bee flight uncovers a number of other patterns of daily bee flight behavior: the exact time of the start and end of foraging, the speed with which the colony responds to changes in ambient conditions, and even the average forager trip time was seen as the distance between the exiting (green line) and returning bees (purple line) which in this example is in the region of 20 minutes.
Figure 2: Bee activity (out = green; in = purple) profile over two days in relation to ambient temperature (red line)
Given the disparate nature of the last two seasons regarding whether we were interested in quantifying the bee flight activity differences for the two seasons using the data from our bee counters’ data. The daily flight activity profiles for 2019 and 2020 are depicted in Figure 3. What is immediately apparent is that the number of days with maximum activity is higher in 2020, and the length of the significant flying day was significantly shorter in 2019.
Figure 3: Bee flight profiles for 2019 and 2020 almond pollination season.
Looking at cumulative bee flight activity for the two seasons, Figure 4 below shows the mean number of foraging flights (exits from the hive) for all the monitored hives (eight in 2019 and five in 2020). What is overwhelmingly clear is that there was about a three-fold increase in flight activity in 2020. On the other hand, using a formula for calculating bee fight hours (Figure 5)(5) shows at least a four-fold difference in bee flight hours in two seasons. Bee colony’s flight activity is not regular throughout the day. There are times of higher and lower activity caused by a myriad of intra- and extra-colony stimuli that regulate flight activity. Therefore a direct measurement of activity using a bee counter is expected to be much more accurate than a prediction based on a binary decision: flight/no flight.
Moreover, we have some indication that the bee flight hours calculation underestimates the flight activity during periods of inclement weather, as confirmed by our analysis of the temperatures bees started flying in the morning. As mentioned above, 2019 was a year marked by cold temperatures and frequent rainfall, and for that year, our data show that the bees will fly at temperatures well below the 12-13℃ threshold used for bee flight hours. During the second week of pollination, we found that the bees’ flight commenced at temperatures between 9-10℃. We hypothesize and plan to test the hypothesis in the coming seasons that this energy investment is paid off by the high-value reward when pollen is available. When the pollen is scarcely available, as is after petal drop, bees flight commenced at an average of 15℃.
Figure 4: Bee flight activity during the 2019 and 2020 seasons expressed as the mean number of foraging trips. Error bars represent the SEM
Figure 5: Bee flight hours in February 2019 and 2020 (Courtesy of Semios)
Finally, how does this data translate to yield? We know that this year’s yields are one of the highest on record, but that is partly due to the massive increase in total bearing acreage. What is more indicative is the yield per acre, and probably even more insightful measure is the actual nut set. Yield per acre is up by about 10%, and the nut set is up by an average of 21% compared to last year (Figure 6). Our data was taken in the North region, and it is interesting to see that the nut set per tree is almost double this year compared to last year, thus corresponding well with the increase in bee flight activity. However, care must be taken when interpreting the bee flight data. It is clear that in the South, the nut set remained unchanged from last year, thus indicating that other factors, which appear to be region-specific and not related to pollination, are at play. These could range from post-harvest water stress from the previous year to the disease. For the coming season, we aim to monitor multiple sites across all Central Valley regions as part of an effort to collect large amounts of bee data related to almond pollination. We’ll keep you posted!
Figure 6: 2020 California Almond Objective Measurement Report (July 7, 2020) USDA, National Agricultural Statistics Service, Pacific Region
1. Clarke, D., Robert, D. Predictive modelling of honeybee foraging activity using local weather conditions. Apidologie 49, 386-396 (2018) 2. Abou-Shaara, H.F; Veterinarni Medicina, 59, 2014 (1): 1–10; The foraging behaviour of honey bees, Apis mellifera: a review 3. Klein et al Honey bees increase their foraging performance and frequency of pollen trips through experience Scientific Reports volume 9, article 6778 (2019) 4. Tenczar, P., et al Automated monitoring reveals extreme interindividual variation and plasticity in honeybee foraging activity levels. Anim. Behav. 95, 41–48