Snags

While most wildlife get the things they need from living trees, there are still a large number of animals that rely heavily on dead trees. In nature there are always trees that come to the end of their lives, and they continue to provide food and shelter long after they have stopped living. Trees are fantastic in that way. In a living tree, most of the wood is unavailable for other organisms, while in dead trees it becomes a relatively good source of food for wildlife. Of course, it is true that a dead tree is more likely to topple than a living tree. If you think that a tree could come down on your house, you may still be able to save wildlife (and your money) by only cutting down part of the tree. Unfortunately, humans have gotten into the habit of just removing dead trees without any consideration given to the environmental impact. Removing dead trees lowers the density of bird populations.

 

Snags help wildlife in the following ways:

1) They are a source of food for many insects and other invertebrates. The tunnels made by beetle larvae provide nesting sites for many solitary bees.

2) The insects in these trees are a source of food for many birds. If you leave a tree to decay naturally in your garden you may well be rewarded with the appearance of a pileated woodpecker. These birds are one of the most impressive in North America.

3) Woodpeckers excavate holes in these trees for nesting. When they leave (and they only tend to use a hole once), the holes get used by other birds, racoons, flying squirrels and bats. The holes provide nesting sites and winter roosting sites to avoid inclement weather. The importance of these holes for the survival and propagation of bird populations should not be underestimated.

4) Several birds of prey use snags for perching as do many smaller birds.

5) As the snag continues to rot, it will be used by amphibians and smaller mammals.

I have a snag in my suburban garden. The tree had to come down due to serious injury in a storm but I left a stump that was 11 feet high. I considered the maximum height allowable by looking at the snag's position in the garden relative to the fences and the building structures. Within 2 years, unusual insects started showing up in the garden. Horntails are large ichneumonid wasps that do not sting. They lay eggs in the crevices and the larvae that hatch burrow inwards and feed on the wood. Older trees with thicker trunks have a lot more value to wildlife. Our preconceived notions of gardens that they should be clean and tidy often make us think that a snag would be ugly; however, you will love watching the wildlife that shows up in your garden to take advantage of your snag.

Below you will find photos of some of the incredible insects that may arrive in your garden if you have a dead tree:

	Horntails are fairly big and robust ichneumon wasps. They are completely harmless to humans and help speed up the decay process in dead trees.

Many insects have parasites or parasitoids. In this case, horntail larvae are fed upon by parasitoid wasps called Megarhyssa that have very long ovipositors used for depositing eggs. They locate the larvae of horntails using vibrations and olfaction. Then, they insert their long ovipositor into the wood. The ovipositor may be incredibly thin, but it has a drill mechanism that grinds away at the wood to create a hole through which the wasp can lay an egg on the horntail grub. The horntail grubs chew through wood quite slowly, so they cannot run away. Within one week, the egg of the Megarhyssa hatches and starts its development by feeding on the non-essential parts of the grub. This is typical of parasitoid wasps. The Megarhyssa larvae eventually pupate inside the dead grub and when they emerge as an adult, they have to chew their way out of the wood.

	Megarhyssa macrurus is a parasitoid of horntails. They only feed on horntail larvae and without snags, these stunning insects would not have a habitat. The exceptionally long tail seen here is an ovipositor capable of drilling into wood to lay an egg on a tunneling larva beneath the surface.

Female Megarhyssa atrata are among the longest insects in North America if you include the ovipositor. In the wasp shown below, the ovipositor was about 5 inches (127 mm) long which allows this wasp to reach deeper horntail grubs in the snag. There are quite a few species that feed on insects tunneling through dead trees. The length of the ovipositor is one biological factor that differentiates an ecological niche by reducing competition. Wasps with shorter ovipositors are restricted to finding grubs closer to the surface of the wood.

 

	Megarhyssa atrata, a parasitoid of horntails. is the longest insect in North America. The circular membrane is part of the abdomen and aids the insect in laying an egg.

Other insects that feed on wasp larvae may use existing holes to reach horntail grubs. The horntail therefore supports many other species of wasp.

 

	Ibalia anceps is also a parasitoid of horntails. However, they do not have the huge ovipositors that a Megarhyssa uses. Instead, they must find grubs that are more accessible through existing tunnels.

 

	Arotes decorus is another common wasp on trees that lays eggs on the larvae of some beetles. They are much smaller than Megarhyssa and they do not have the ability to drill into the wood. Instead they insert their ovipositors into holes made by the beetles themselves.

 

 

There are also beetle larvae that are tunnelling through the snag and you might be lucky enough to find the Eastern eyed click beetle that show up to lay eggs on these larvae. The striking fake eyes are a feature common to insects that want to appear bigger than they actually are as it might prevent predation.

	The two toothed tumbling flower beetle, Yakuhananomia bidentata, found on Norway Maple. The grubs of this beetle are the host of the parasitoid, Arotes decorus, shown above. Since both the beetle and the wasp have been found on this tree, it is reasonable to assume that Norway Maple is an acceptable ecological substitute.

 

 

	Alaus occulatus

The fungus that feed on rotting trees need to reproduce. Some do it by travelling with the insects that tunnel through the tree while others spread in a more typical way by producing a fruiting body that we call a mushroom, which releases spores into the air. Feeding on the mushrooms you may observe yet more insects that live on snags such as fungivorous beetles.

	Within a few years, mushrooms will appear on the trees and wherever they are, beetles will follow. In this case, a member of the genus Triplax, a pleasing fungus beetle, is found feeding on the gills of the mushroom.

You may also observe visits by yellowjackets, which are usually harmless when found away from their nests. They need a supply of wood to construct their paper nests.

	Snags often are visited by yellowjackets. They scrape the wood and then fly back to the nest.