In the wild, quaking aspen provide important breeding, nesting and cover habitat, and forage for a variety of birds and mammals. Quaking aspen is host to a variety of insects, some of which are important foods for other animals including woodpeckers. Wildlife species consume nearly every part of aspen at one time of the year ranging from the roots, to shoots to bark and catkins. Because of this, in the wild, it is truly a keystone species in many habitats.
Quaking aspen are native to the Missoula area, though they are probably at the edge of their range. This location can cause stress and make them susceptible to a variety of pests.
Though a beautiful tree, quaking aspen are maligned by many urban foresters because of their tendency to sucker, and for their predisposition to borer beetle infestations. However, borers are just one of the many insects that use the aspen.
Here is how it often goes…In our yard, it might begin with the bald-faced hornet looking for nesting material. The hornets chew the aspen bark, and mix it with their saliva to create their papery nests. The holes they create in the aspen bark allow the aspen borer beetle (see photo above), to easily open up the cambium to deposit the eggs (more on this later…). Bald-faced hornets are big and mean looking, and often mistaken for yellow jackets. Bald-faced hornets are truly beneficial in your garden, and they even pollinate flowers as summer goes on. One of their main prey is yellow jackets and they even lie in wait for them near food sources. You have probably noticed the two species near a food source, but, watch closely, and you may see the bald-faced hornets ambush and kill yellow jackets. Frankly, I am not a huge fan of yellow jackets, so this is pretty neat and gratifying to see.
Back to the borers. Once the adult borer lays its eggs in the bark of the aspen (see photo above, and notice all the pock marks on the aspen bark from bald-face hornet gnawing), the cool stuff really starts to happen. The aspen borers are a type of long-horned beetle (named for the very long antennae in adults- not for horns, unfortunately), and the one we have in our yard is Saperda calcarata, (also called Poplar borer). Some of these insects may reach two inches long, and are very well camouflaged mimicking the texture and colors of aspen bark and fungi (see photo below). The entire life cycle (from egg to adult) takes about three years in our area.The borer's eggs typically hatch within two weeks, and the newly emerged larvae chew through the bark, where they spend the first year. As the larvae grow, they eat and tunnel through the wood deeper into the heartwood and sapwood for two years. This tunneling produces the bulges and lumps you see in the trunk. All the while, they force coarse shavings, like sawdust out the holes. In the summer, you can literally watch this happen – just look for fresh sawdust at the base of an aspen at follow it up and you will see saw dust emerging from holes in the trunk. If you look closely enough, you may even see the larvae. The chewing and tunneling weakens the tree and allows the invasion of canker and fungi.
The accumulation of saw dust and borings beneath the aspen is home to the chrysalis of several moth species in our yard including the five-lined sphinx moth. Whereas butterfly caterpillars often pupate in a chrysalis that is suspended beneath a leaf or twig, sphinx moth caterpillars burrow into loose, dry duff to pupate. The loose, dry wood shavings the borer larvae produce is perfect for sphinx moth caterpillars to excavated and pupate within. The chrysalises are dark brown to almost black and look nearly dead. If you do find one when you are rooting around, if you watch carefully you might be able to see it move. Just place it back into the ground.
The five-lined sphinx moth, or commonly called a hummingbird moth, caterpillars are specific hosts of the Oenothera flava (yellow evening primrose) in our yard. The giant caterpillars have marking on them that mimic the shape of the narrow, serrated leaves- fantastically camouflaging the 4” caterpillars (see photo below).As a result of the larvae boring though the tree, the holes “bleed” or produce sap you can see running down the sides of the aspen. This sap is a great food source for a variety of insects, including butterflies. In our yard, the mourning cloak (Nymphalis antiopa, the Montanan state butterfly) is especially fond of it, and sap is its primary food. The mourning cloak only lays its eggs on twigs or in the shallow cavities of the quaking aspen, or related species, where the adults and caterpillars feed. In addition to butterflies, the sap attracts a variety of other insects from ants to yellow jackets and ultimately bald-faced hornets, which prey on the insects, and the cycle continues.
The presence of the aspen borer in the tree further weakens the aspen making it susceptible to more invasions of aspen beetles. These stresses on the tree also cause it to sucker- which is good because at this point the aspen is on its way to dying (though it make take 6-20 years for the initial trunk to truly die and by then a sucker is there to replace it). Quaking aspen seldom occur individually, and having a cluster of them is not only more natural and more beautiful looking, but it is also a way to provide for its longevity and succession.
In the fall, after about two years of growth, the larvae will move shallower- just beneath the bark, where they pupate during the winter. In February, in our yard, downy woodpeckers will drill into the bark looking for these pupae. In doing so, the woodpeckers drill holes that will create small cavities for insects to hide, lay eggs and even for adult aspen borers to deposit the eggs of a new generation. Once fully developed, the adult aspen borer beetles emerge through a hole in the bark, and spends its adult life mating, and dispersing is eggs to host quaking aspen.
The cavities and holes left by the borers create wonderful places for a variety of insects and spiders to raise their young. In turn, insect eating birds, like warblers, chickadees and others scour the aspens gleaning insects, spider eggs and others from the trunk, crevices and cavities of the aspen.
All this can happen around a quaking aspen in your yard, or, you could use a few types of insecticide and kill the borers.