What’s happening to my tree?

This is a question that we often receive at HGIC through our Ask Extension program. Many clients who write in for help think the holes may be from wood-boring insects, but SURPRISE – they are actually from a type of woodpecker! Who knew they could be so neat and precise? Boring insect holes will appear deeper and more random on the trunk or branches of a tree.

These holes are feeding wells of Yellow-bellied Sapsuckers, a native North American bird in the Picidae woodpecker family. This small forest woodpecker frequents mixed-evergreen and hardwood forests (and home yards) in Canada and the eastern U.S. Most of Maryland is within their wintering range. In spring, they migrate northward (and sometimes to parts of western Maryland) for the summer months.

What are Yellow-bellied Sapsuckers?

Yellow-bellied Sapsuckers are about the size of an American Robin. Despite their name, they don’t really have a yellow underside; it’s more white to creamy yellow-ish. Males have a more prominent red cap and a red throat, while females have a red crown with a white throat. Both sexes have vertical white wing patches along their folded wings, which contrast boldly against black plumage.

The short beaks of these birds help to drill fairly shallow wells to feed on the sweet sap, which in early spring begins to flow into tree canopies. Sugary sap, along with some protein from the occasional insects that get trapped in the oozing sap, helps to give the birds the energy they need for surviving the winter and making it to their summer nesting grounds. According to Cornell Ornithology, later in the season, they may also consume some of the cambium bark of trees. But why the perfect rows? It’s simply more efficient. They move from one shallow hole to the next to keep the sap flowing without damaging the inner structure of the tree.

Welcome to the Sap Diner!

You may have noticed two different types of patterns on trees, and this has to do with the birds feeding at different times in the season. Some bark damage looks more like the perfect grid of waffles waiting for the sweet syrup to fill the wells, while other damage is the straight holes of a Connect Four game grid. In early spring, the same time we tap Sugar Maples to collect sap for maple syrup making, sapsuckers drill deep holes to reach the sweet sap that is starting to flow in the inner sapwood of the tree. Later in the spring, after the tree produces leaves, the sugar is in the outer part of the trunk, bringing nutrients down to the tree’s roots, allowing the sapsuckers to drill shallower holes to access sap.
Rows of holes line the trunk of this tree resembling a Connect Four game grid, just waiting for some Sapsuckers to come and play! Photo: Home and Garden Information Center

Yellow-bellied Sapsuckers are considered a keystone species, meaning without their existence the ecosystem would be vastly different. Those sap taps don’t only feed the sapsuckers, but dozens of other species!

Birds:

• Hummingbirds: The sap wells are critical sources of food in the early spring for migrating Ruby-throated hummers since many nectar-producing flowers aren’t in bloom yet.
• Warblers: At least 7 species of these migratory songbirds frequent the sap wells.
• Woodpeckers: Other woodpecker species use the sap wells and the insects they attract as a food source.
• Many other songbirds: They visit these backyard feeding stations for a sugary treat as well.

Mammals:

• Any of the tree-dwelling mammals: Squirrels, chipmunks, mice, etc., lap up yummy sap.
• Porcupines: They are known to strip bark off trees, and they also benefit from the ready-made sap source.
• Bats: Some bat species stop by for the sugar and insects.

Insects:

• Butterflies and Moths: The wells are a great food source of nutrient-rich sap.
• Stinging insects: Bees, wasps, and hornets are often seen buzzing around the holes.
• Ants, beetles, and flies: They will crawl into the holes and usually become a snack for other visiting birds.

Wait, what about my tree?

So now that you know what those holes are from, should you be worried about your tree? Yes and no. Many studies have shown that yellow-bellied sapsuckers tend to drill on already-stressed trees. This is why we may notice it more in our backyards and parks rather than on a hike in the woods. Even if you diligently care for your holly or maple (or other trees or shrubs), these plants may not be in their natural wooded setting and could be stressed from any number of factors:

• drought
• soil compaction from regular lawn mowing or foot traffic
• lower levels of nutrients in urban/suburban soil (due to fallen leaves and other plant debris usually not being left in place to decompose and return nutrients to the ground)
• newly-planted specimens and those still getting established
• increasing heat (especially when temperatures remain warm overnight)
• insect or disease damage

However, severely-stressed trees also have poor sap flow, so you will likely also see drilling on fairly healthy trees. Sapsuckers prefer species that tend to be higher in sugar (maple, birch, apple) or trees with smooth outer bark (holly, beech, cherry). Younger trees also have less furrowed bark, so those may be more attractive. This doesn’t mean you shouldn’t plant these trees in your yard. They all add wonderful shade, fruit, or winter interest.

The bottom line is that heavy feeding from Yellow-bellied Sapsuckers could harm your young or stressed trees, but they don’t kill all trees. On a healthy tree, the damage may only be cosmetic, and the holes will seal over, unless the tree is entirely girdled. Many holes on a young or vulnerable tree could cause branch dieback, leaf wilting or browning, or further weaken potentially already-stressed trees.

So, what can you do? If the tree is young, you can protect the bark by loosely wrapping it with burlap or hardware cloth. This isn’t practical on large mature trees, though. In that case, its important to keep additional stress to the tree to a minimum.

• Monitor for watering needs: Even mature trees need about 1 inch of water a week during the growing season when there isn’t sufficient rain. Maryland often goes through periods of high heat and drought, so it’s important to protect our mature trees by irrigating when we can. Don’t forget about the fall, even if the weather is cooling off. This is a critical time to support the roots before the ground freezes.
• Add mulch: If there isn’t a mulch disc around the root zone, consider applying 2 to 3 inches of a biodegradable mulch, so its nutrients add to the soil as it decomposes. You can use shredded or chipped bark mulch, arborist wood chips, fallen leaves, pine needles, or another organic material. The organic matter that mulch adds discourages soil compaction,  improves soil moisture-holding capacity and moderates soil temperatures, and protects the tree’s trunk from mowing or weed-whacking injuries. Just keep the mulch pulled back a few inches from the tree’s bark and avoid “mulch volcanoes” around the root flare!
• Add more native plants to your yard or help advocate for native plants in a local park: There may not be a solution to prevent sapsucker feeding, but making sure we have an abundance of food sources for woodpeckers and other wildlife to choose from helps our urban/suburban landscapes recover if and when we do lose a tree. Explore the Maryland Native Plants Program resources to help select plants best-suited to your conditions.

Keep in mind that Yellow-bellied Sapsuckers are protected under the Federal Migratory Bird Treaty Act of 1918. This makes it illegal to kill, capture, or harm them, and as a keystone species, why would we, especially now that we know all they do for our ecosystems?

If you notice neat holes or grid patterns on your trees or shrubs, consider all the wildlife you are helping through that one plant! I’m sure they are all thankful for it.

Sources and additional resources to explore:

UME HGIC: Woodpecker and Sapsucker Damage on Trees

Cornell Lab All About Birds: Yellow-bellied Sapsuckers – This link allows you to listen to the sapsuckers’ various calls.

Georgia Dept. of Natural Resources Wildlife Division: Out My Backdoor: How Sapsuckers Help Hummers

Ohio State University: Buckeye Yard and Garden Online: Rows of Holes on Tree Bark

By Emily Clark Waterson, Certified Professional Horticulturist & Coordinator, Chesapeake Bay Landscape Professional, University of Maryland Extension Home and Garden Information Center