Sticky Sap, Holes, or Sawdust on the Trunk? Boring Insects Explained

What's Actually Living Under That Bark

Most of what people lump together as "boring insects" fall into two rough camps: bark beetles and wood borers, and they behave differently enough that the distinction matters.



Bark beetles are small, roughly the size of a grain of rice or smaller, and they tunnel through the thin layer just beneath the bark — the phloem — where the tree moves sugar and nutrients between roots and canopy. Pine engraver beetles, the species most active in this region's red pine stands, overwinter in the leaf litter at the base of trees and emerge in April or May to attack pole-sized and mature pines. They can push through two or three generations in a single season, which is part of why an outbreak in a stressed stand can look like it exploded overnight.

Wood borers are a different animal entirely, and their larvae are noticeably bigger — often an inch or longer, compared to a bark beetle larva that tops out around three-sixteenths of an inch. Most wood borers target wood that's already dead or dying rather than threatening a living, healthy tree. That's a useful rule of thumb, though it isn't universal, and a few species break it in ways worth knowing about.

Sticky Sap and Pitch Tubes: What They're Telling You

On a pine or spruce, a tree's first line of defense against a boring beetle is sap. When a bark beetle bores in, the tree pumps resin into the wound, trying to flush the insect out or trap it entirely. That resin dries into what arborists call a pitch tube — a small, popcorn-like blob of hardened sap sitting right at the entry point, usually somewhere on the lower or mid trunk.



Pitch tubes are a useful sign, but they're not foolproof. A healthy, well-watered pine can often out-produce the beetles trying to get in, which is exactly why pitch tubes show up so much more during drought years. A tree under enough water stress simply can't manufacture resin fast enough to keep up, and that's when a beetle population gets a foothold. If you're seeing pitch tubes on a pine that's already looked a little off — needles going yellow, then a reddish-brown by late summer — that combination is worth acting on rather than watching.

Not every drop of sap on a trunk means a beetle got through, though. Sap can weep from a mechanical wound, a pruning cut, or a crack from winter cold with no insect involved at all. The difference is usually in the shape: a clustered, popcorn-textured pitch tube at a distinct entry point reads very differently from sap simply running down bark from a wound higher up.

Sawdust and Frass: Reading the Pile at the Base

Sawdust-like material — technically called frass, a mix of chewed wood and insect waste — is one of the more reliable signs something is actively feeding, because it has to come from somewhere specific: a larva chewing its way through wood and pushing the debris back out through its tunnel.



Where you find it matters almost as much as the fact that it's there. Frass caught in bark crevices or piled at the base of the trunk usually means an active gallery somewhere above it. Fine, powdery frass tends to come from smaller borers and bark beetles; coarser, more visible shavings often point to a larger wood borer working through bigger wood. On birches and oaks specifically, bronze birch borer and twolined chestnut borer larvae pack their winding, S-shaped galleries with frass as they feed, and that debris is frequently the first thing a homeowner actually notices — well before the raised bark ridges or exit holes become obvious.

Timing helps narrow things down, too. Adult bronze birch borer and twolined chestnut borer beetles typically start emerging in early June in this part of the country, right around when black locust trees come into bloom, and they keep emerging into July. If you're seeing fresh frass appear in late spring or early summer on a birch or oak that's been struggling, that timing lines up.

Holes Tell Their Own Story

Exit holes — the openings adult insects leave when they chew their way out of the tree — vary enough in shape and size that they're one of the more useful clues you have, once you know what you're looking at.

A tight cluster of tiny, round, BB-sized holes scattered across a section of bark, sometimes described as looking like a shotgun blast, points toward bark beetles rather than a single larger borer. A D-shaped hole about an eighth of an inch across is the signature of metallic wood-boring beetles — bronze birch borer on a birch, twolined chestnut borer on an oak, and a few other species in that same family. These holes are small enough that you'll usually need to be standing close and looking carefully to catch them.

Round holes read differently depending on size, and even more on whether the tree is alive or dead. Most native wood borers that leave round exit holes are working in wood that's already dying, so finding one on a tree that still looks healthy is less alarming than it sounds. The exception is worth knowing by heart: a perfectly round hole about the size of a dime, on a tree that still has most of its leaves and looks otherwise fine, is the calling card of Asian longhorned beetle — a regulated invasive pest that attacks maple, boxelder, willow, elm, horse chestnut, and birch even when those trees are perfectly healthy.



Asian longhorned beetle hasn't turned up in Minnesota as of this writing, but it's established in parts of several other states, and new infestations almost always trace back to someone moving infested firewood or wood packing material into a new area. Beyond the dime-sized round exit holes, females also chew oval egg-laying pits into the bark that are roughly the same size, and because this beetle attacks living trees, you may see sap weeping from the bark right around those egg sites — a different picture than sap oozing from an old wound.

When It Isn't an Insect at All

Not every drip of sap or foul-smelling ooze on a trunk means you've got a borer problem. Bacterial wetwood, sometimes called slime flux, is exactly what it sounds like — a bacterial infection inside the wood that builds up gas pressure and forces sap out through cracks, old wounds, or pruning cuts. It tends to have a sour, fermented smell that plain sap doesn't, and it seeps out as a wet, sometimes frothy discharge rather than the hardened, clustered pitch tubes a bark beetle triggers.



The two can look similar at a glance, which is exactly why one detail matters: slime flux shows up without frass, without exit holes, and without the tunneling pattern a borer leaves behind. If you're seeing sap or ooze with no sawdust anywhere nearby and no small holes on the bark around it, you're probably looking at a wound issue rather than an active insect.

What a Diagnostic Walkthrough Actually Involves

Reading these signs correctly from a photo or a quick glance is hard, even for someone who's done it a while. A proper walkthrough starts at the base of the trunk and works upward — checking for frass, pitch tubes, exit hole shape and size, bark ridging, and any sap that doesn't match a normal wound pattern. It also looks at what's happening with the tree overall: canopy density, needle or leaf color, recent construction or grading nearby, and how long the symptoms have likely been building based on how far they've progressed.



That context is what separates a confident answer from a guess. The same D-shaped hole means something different on a birch that's been stressed by three dry summers than it does on one that's been perfectly healthy. Getting that context right is what determines whether the next step is watering and mulching, targeted treatment, or accepting that a tree is past the point where treatment makes sense.

Frequently Asked Questions

Reading What the Bark Is Already Telling You

A trunk covered in pitch, sawdust, or small holes isn't a mystery so much as a message, one the tree has usually been sending for weeks before anyone notices. Sap points to a defense response, frass means something is actively feeding, and hole shape and size narrow down who's responsible and how long the tree has likely been struggling. None of these signs adds up to a full diagnosis alone, but together they're usually enough to tell routine wear from something that needs attention now.

Getting these signs right matters most while a tree still has options. JT Tree Service, based in Grantsburg, Wisconsin, brings 25 years of hands-on experience and a MN DNR Certified Tree Inspector credential to properties across the St. Croix Valley, with the background to tell a bark beetle pitch tube apart from bacterial wetwood, or a routine borer from a pest that warrants a report to the state. That kind of read only comes from having looked at enough trunks to know the difference.