Resinous trees help honey bees fight disease

By Mike Wilson, PhD Student at the University of Minnesota

Honey bees are the most important pollinators of agricultural crops world-wide, and we count on honey bees to help us produce nutritious food in the U.S. You may know that honey bees are required to produce fruits and nuts like apples, peaches, almonds, and blueberries, but they are also required to produce vegetable seed (so we can eat things like onions and broccoli), to produce oil seeds (so we can use canola or sunflower oil), and to produce alfalfa (so we can feed it to cows and produce milk). Many plants that we use for food, fuel, or fiber require, or are enhanced by, bee pollination. Unfortunately honey bees, as well as wild bees, are facing serious decline in the U.S. due to complex interactions between poor habitat, pesticide exposure, pests, and diseases. 

The grocery store is a boring place without honey bees! Photo by Whole Foods.

Most people know that bees collect pollen and nectar from flowers near their hives, but honey bees also need to collect sticky resins from plants. If you've ever parked your car under a pine tree, then you’re probably familiar with resins from having to clean them off! Many plants, including pine, spruce, birch, and poplar, secrete antimicrobial resins from wounds, stems, and leaf buds. Resins prevent insects and mammals from eating plants, but also prevent infection from bacteria and fungi. Honey bees are known to collect both the deep red resins of balsam poplars and the bright yellow resins of eastern cottonwoods in Minnesota.

Resin droplets on a cottonwood bud. Bees seed out resinous plants, collect their resins, and take them back to their nests. Photo by Radical Botany.

Bees do not eat resin, but mix it with wax and use it as a building material. When honey bees nest in tree cavities, they coat the entire inside of their cavity with lots of resin and even use it to build part of their comb. We think that rough surfaces, like those found on the inside of natural tree cavities, stimulate bees to collect resins. In man-made hives built with smooth lumber, honey bees do not use resin like they do in natural cavities, but mostly to glue down movable frames and covers. You will hear beekeepers call these deposited resins “propolis”.  

Cross Section of a natural tree nest. You can see the layer of resin used to coat the entire cavity, and also resin used in comb building. Photo by Gary Reuter.

Research at the University of Minnesota bee lab and elsewhere has shown that the presence of resins in the nest is very important to honey bee health. Having resins in the nest decreases the mortality of bee eggs and larvae and prevents bacteria from colonizing adult bees. Resins in the nest also greatly reduce infections caused by chalkbrood fungus, a pathogen that kills bee larvae. Bees will actually self-medicate themselves by collecting more resin than usually if you experimentally treat colonies with this fungus! It is critical to have enough resinous plants, like cottonwoods, around to make sure that honey bees can take advantage of all these health benefits. 

Top-down view of a man-made hive. Resin is deposited on the tops of the movable frames. Notice the resin forager with red resin on her hind legs! Photo by Mike Simone-Finstrom.

Remember that we are not doomed to eating only gummy bears and saltine crackers! Improving and creating good bee habitat in both large and small spaces will greatly help all beneficial insects, thereby also helping us to produce food. If you want to read more about bees and what you can to help them out, please check out our website or connect with us on Facebook. Good luck out there!