P.M. Pollinators: Study Shines Light on Nocturnal Insects’ Role in Apple Production
By Paige Embry
Insects are indispensable members of the world. They serve as breakfast, lunch, and midnight snack for a host of other animals. They chow down on everything from poop to wood, freeing up the nutrients for re-use. Some even prey on other insects, helping prevent pests from running amok. And, of course, insects also help pollinate 80 percent or more of the world’s flowering plants, including around three-quarters of agricultural crops that we humans value.
This last insect job has received a lot of attention and research dollars, but most of the focus has been on daytime pollinators. Pollinators that fly at night—and, yes, there are a lot of them—have gotten much less scrutiny. A study published in July in the Journal of Economic Entomology took a look at nighttime pollinators, in particular at their impact of both on apple tree pollination in Arkansas as compared with their daytime counterparts. The results were illuminating—both for how much pollination happened at night and who was responsible.
The study took place over two years, and the researchers used four treatments: closed (flowers bagged day and night), open (no bag), diurnal (bagged at night, to allow only daytime pollinator visits), and nocturnal (bagged during the day, to allow only nighttime visits). Stephen Robertson, Ph.D., just completed his doctorate at the University of Arkansas and is the lead author of the paper. Every day during bloom (except during thunderstorms) he went out to the orchard at sunrise and sunset to take bags off some branches and put them on others. After bloom was over, Robertson and his colleagues counted fruit set, prodding each baby fruit with a finger to see if it was truly set or would fall off. They removed the set fruits and counted the seeds. “Seed set,” the authors write, “is a direct proxy of the level of pollination.”
Last, they looked to see if at least one flower per cluster had set fruit. Growers would rather have one pollinated flower each in five different clusters instead of five flowers in one cluster because, the authors write, “Growers typically reduce the number of developing fruit to one per cluster to ensure tree resources are devoted to fewer apples, thus generating higher quality and more valuable fruit.”
Changes in research design between the two years (e.g., different trees used, changes in methods) means each year needs to be looked at separately. In 2017 the clusters that were bagged both day and night (closed) showed nearly 30 percent pollination which can be put down to the potential self-fertility of some of the trees or problems with the mesh bags used that year. Nocturnal pollination rates in 2017 were 57.5 percent, while both diurnal and un-bagged clusters showed pollination rates in the mid-eighties. In 2018, 3.4 percent of closed clusters were pollinated, while the nocturnal, diurnal, and open treatments came out nearly the same (11.3 percent, 12.8 percent, and 10.8 percent, respectively). The vastly different pollination rates between years could be because different trees were used, natural variation in pollinators, or a late freeze in 2018 that killed most of the nearby blooms that acted as pollinizers for the trees being tested.
However, in both years, similar seed sets show that nocturnally pollinated fruit had similar pollination levels to those pollinated during the day. Although the results were quite varied between the years, they nevertheless show that nocturnal pollinators have the power to contribute to pollination services in apple production—and potentially other crops as well. Robertson says part of why he conducted this study was because he’d learned that moths were visiting both blackberries and a peach tree in the area. Robertson says, “What happens at night—where we’ve been somewhat ignoring it, somewhat dismissing it—there’s actually something going on. It’s potentially highly valuable input to fruit production.”
This study also shows that one grower’s bane may be another’s gift. The dominant night-time visitors were moths in the family Noctuidae, and the two most common have larvae that are considered pests: armyworm (Mythimna unipuncta) and variegated cutworm (Peridroma saucia). In a time where insect declines are increasingly well documented, it may be useful to realize that “pest” insects are not all bad.
Robertson says, “Insects don’t fall into these categorizations [good, bad] so neatly. … There’s more to the story.” In other words: Pest, pollinator, food source, balance keepers—insects—even the same insect—perform a variety of jobs.
“Let’s not dismiss [an insect] based on previous classifications and just hang our hats on this is a bad guy,” Robertson says. “There’s duality associated with this. There’s mutuality associated with all these things.”
Journal of Economic Entomology
Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website: www.paigeembry.com.