This research is the work of BeeHero's Head of Scientific Strategy, George Clouston, and Head of Bio Research, Dr. Doreet Avni. It provides detailed information and insight into the impact of California's recent adverse weather conditions on this year's almond pollination.

Cold weather in California came at the worst possible time for almond growers this year, with the winter storm sweeping across California, and snow falling in areas that rarely see it — including some orchards in the Sacramento Valley. It has been very cold during the bloom (about 20 degrees lower than expected this time of the year), with snow, rain, and wind.

Weather is the one factor that growers can not control during pollination season, and temperature, rainfall, snow, and wind are affecting both the almond bloom and the honey bees as their pollinators. The trees require a combination of specific temperature, chill hours, and weather conditions to bloom and produce a successful crop; while honey bee colonies' spring development and flight activity are determined by those same weather elements. Therefore, adverse weather conditions may lead to reduced numbers of almond flowers, and lowered pollination efficiency.

Typically, almond trees require temperatures between 60°F and 70°F  during the day and between 40°F and 50°F at night to initiate bud break and bloom. Warm weather can lead to early bloom, while cold weather can delay bloom. Heavy rainfall during the almond bloom period can be detrimental to pollination, as it can wash away pollen, and in some cases, can even damage the flowers. Flowers can also be damaged by strong winds, snow, or frost. Snowy conditions can cover the almond trees, preventing sunlight from reaching the buds and flowers, delaying the bloom and reducing the time window for pollination, and subsequently, the potential for reduced yield.

Optimal pollination is a result of grower management practices, good weather conditions, and healthy, active honey bee colonies. An active honey bee colony is one that has adequate bee and brood frames, plenty of stored nectar and pollen, and a good laying queen. Such a colony, in the right weather conditions, would be highly active and a good pollinator. Bees will typically stay inside their hives during rainy weather, they won't leave their colony if the temperature is below fifty degrees, and wind will minimize their flight time.

Data from BeeHero's Pollination Research Stations clearly shows the impact of this year's extreme weather on the foraging activity of honey bees. Using specialist bee counters, we are able to monitor the actual number of bee trips undertaken by the colonies throughout pollination.

Figure 1 compares daily temperature with the number of daily bee trips made by 4 monitored colonies located in the Central Valley, at one of our 15 Pollination Research Stations. The graph shows bee activity averaged nearly 60,000 trips per day from the 17th to the 21 February, then declined as the temperature started to fall, reaching a low of just a few hundred bees on 24 February. Foraging commenced again in the days afterward, though activity was about half what it was in warmer weather. By March 2nd, when temperatures recovered to the 60’sºF, activity was back to around 60,000 trips per day.

Figure 1. Shows bee activity averaged nearly 60,000 trips per day from the 17th to the 21st of February, then declined as temperatures started to fall.

bee activity relative to temperature during almond pollination

A closer look at the data on one of the cooler days reveals some fascinating insights into bee foraging behavior. Figure 2 shows the foraging pattern of a hive during a day of changeable weather conditions. Following morning rainfall, there is a surge in bees leaving the hive within minutes of the rain stopping,  even though the temperature was still only 50-52ºF. Foraging increases rapidly, with over 200 bees per minute leaving the hive by the middle of the day. Activity then decreases as the temperature drops and rain returns at about 1.30 pm.   Indeed, in this case, the bees start to reduce the number of trips before the rain arrives, and are all safely in the hive before it starts to rain!

Foraging activity, however, is not just influenced by the weather but also the abundance of pollen and nectar-producing flowers available to the bees. At this time, the bloom was still strong, with high-value pollen and nectar resources available. This makes it worthwhile for the bees to invest energy in foraging, resulting in a massive surge in bee trips as foragers try to maximize returns to the hive during the weather window. These data demonstrate how bees continuously monitor their external environment to decide whether to embark on foraging journeys or return to the hive, balancing the colony's needs against the external food resources available and prevailing weather conditions and assessing the risk and reward of leaving the hive.Despite the cold and wet weather, we can see how bees fly in suboptimal conditions and rapidly adjust flying behavior to optimize foraging opportunities and pollination, maybe even changing activity based on their own weather forecasts! Time will tell the ultimate impact on nut set and yield. At the conclusion of this year's bloom, we will present a complete analysis of the 2023 pollination.We appreciate you as a BeeHero grower, and we hope this information gives you better insight into the impact of weather on your pollination. For more detailed information and visibility into the bloom status statewide, visit our Pollination Research Stations online.