Notes from the Lab

Bees forage more in apple orchards when they aren’t in bloom compared to when they are.

- November 1, 2022 - Scott McArt - (excerpt)

In the July 2022 issue of Notes from the Lab [162(7):795-797] I summarized a study from Michigan showing about three quarters of pesticide risk to honey bees conducting blueberry pollination comes from pesticides that aren’t registered for use on blueberries. Honey bees were simultaneously exposed to an average of 35 pesticides in pollen they collected during bloom, and the vast majority of exposures came from pesticides not used on focal farms or not used in blueberry at all.

We see this same non-intuitive pattern of pesticide exposure during New York apple pollination. For example, across 30 farms, more than two thirds of pesticide risk to honey bees conducting apple pollination comes from pesticides that aren’t sprayed on those farms during bloom (McArt et al. 2017).

How can these results be explained? Well, clearly many foraging bees must be flying outside of orchards. But where exactly are they foraging during bloom of a pollination-dependent crop like apple? How far from their colony are they foraging? And what about after bloom when growers of most pollination-dependent crops start spraying insecticides? How much foraging occurs in orchards after bloom, potentially exposing bees to high-risk insecticides? These are the topics for the fifty-ninth Notes from the Lab, where I summarize “Apple orchards feed honey bees during, but even more so after, bloom,” written by Taylor Steele and colleagues and published in Ecosphere [2022].

For their study, Steele and colleagues set up three observation hives at the Virginia Tech agricultural research and extension center (AREC), which is devoted to fruit crop research, mostly apples (see Photos 1 & 2). The landscape within a 2-km radius of the colonies was comprised of apple orchards (16%), forests (25%), and other landcover (59%), including grass/pasture, non-alfalfa hay, corn, soybean, developed/open space, and other crops (Photo 3). This is a typical landscape surrounding small- to medium-sized apple orchards in the Eastern U.S. Each colony was managed to decrease the likelihood of swarming and fed supplemental sucrose when needed.

In total, six colonies were monitored in 2018 and 2019. The authors filmed foraging bees communicating via the waggle dance on the ”dance floor” (the bottom frame of each observation hive) 3-4 times per week for the entire growing season (April-October). Because the waggle dance is a classic honey bee behavior for communicating the distance and location of good forage, a brief explanation is worthwhile.

The waggle dance is comprised of two parts: the waggle run, which is the information-rich portion of the dance, and the return phase. During the waggle run, a forager oscillates her abdomen at a specific angle relative to vertical for a particular duration of time, which indicates direction and distance, respectively. Then she stops, loops around to the left or right (return phase) and repeats the waggle run + return phase combination a variable number of times, with greater repetition indicative of greater resource quality.

By the end of the study, the authors had recorded 625  hours of “dance floor” videos, which were meticulously viewed, dancing foragers identified, and 3,710 individual waggle dances decoded. In other words, the authors had a very comprehensive dataset to reveal where honey bees were foraging over the course of two successive growing seasons.

So, what did they find? How far did honey bees forage from their colony throughout the season? As seen in Figure 1, some bees foraged over 11 km from their colony. But long-distance foraging was rare. The median foraging distance across both years was only 0.78 km and the upper quartile (i.e., the max foraging distance for 75% of bees) was just over 1 km. In other words, the majority of bees foraged within 1 km of their colony.

There was seasonal variation in how far from their colony workers foraged, but the only consistent pattern across years was a trend for greater foraging distance in May compared to other months. Notably, early May corresponded with peak apple bloom each year.

Did honey bees forage primarily in apple orchards during apple bloom? No. As seen in Figure 2, only 20% of foraging occurred in apple orchards during bloom (19% in 2018 and 21% in 2019). This is slightly greater than the 16% of surrounding land comprised of apple orchard (i.e., the horizontal red dashed line in Figure 2), but it certainly doesn’t indicate a strong preference for foraging in apple during bloom.

Instead, a greater proportion of foraging occurred in forests during apple bloom. In 2018, 37% of foraging occurred within forests and in 2019, 33% of foraging occurred within forests. Both of these percentages are quite a bit higher than the 25% of surrounding land comprised of forest (i.e., the horizontal black dashed line in Figure 2), indicating a preference for foraging in forests during apple bloom.

Beyond apple orchards and forests, about 45% of foraging occurred in other landscape types during apple bloom, including grass/pasture, corn, soybean, developed/open space, and other crops.

What about after bloom? Where did bees forage post-bloom? Perhaps most surprisingly, more honey bees foraged in apple orchards after bloom compared to during bloom! This can be seen in Figure 2, where 29% of bees foraged in orchards after bloom compared to 20% during bloom (red points and lines) averaged across both years. Conversely, a smaller percentage of bees foraged in forests after bloom compared to during bloom (black points and lines).

Well that’s interesting. Why would honey bees forage so much in apple orchards after bloom? Clearly, attractive food resources became available in orchards after apple bloom. At the same time, previously attractive food resources in places such as forests potentially became less available. To look into this, the authors surveyed the   ….