Sawflies becoming the bane of spruce

We got a bit of the answer to this question a few years back when the spruce bark beetle infestations devastated parts of our local forests from Homer to Eagle River -- in fact, throughout Southcentral. It wasn't a pretty picture.

I bring this up because in recent weeks there has been a lively discussion of another insect that attacks spruce. This one is a naturally occurring member of the order of insects known as "Hymenoptera," which includes bees, wasps, ants and sawflies. It is known scientifically as Pilonema alaskersis Rohwer, but you and I will probably call it the yellow-headed spruce sawfly and references to it won't be kindly.

If you grow larches or gooseberries, you may be familiar with the habits of sawflies. The adults lay eggs that emerge as larvae sometime during the spring or summer. These caterpillar-like creatures have an unfathomable and unending appetite and can strip the leaves off of their host plants in a matter of days. This is almost OK for gooseberry bushes because they don't eat or otherwise impact the berries. It is less acceptable when they hit larches, but since these trees drop their needles in the fall, the impact of loosing them earlier is completely forgotten come winter.

But imagine your spruce trees with no needles all winter long. Imagine lots and lots of them all over the place with no needles. This could happen. The 1/8 inch, yellow-headed sawfly larvae feed for a month or so, starting on the new foliage (needles) and then moving to the mature ones. This is not a pretty thought, no matter how much for granted we take these trees. They are, appreciate them or not, the predominant tree in most of the forests of Alaska.

In mid-summer, the larvae fall out of the trees and go into the soil where they overwinter in cocoons. Come warm spring weather, they emerge as flies. Well, usually. For some, the conditions are not right and they stay as pupae for another year or even more.

The damage caused by the larvae is first observed in the tops of trees which start to get a brownish-red color. More and more of the tree foliage is damaged and entire trees can be defoliated. Then new branches and shoots stop growing. After a couple of years or so, there can be just top kill or tree death.

So what should we do to stop this invasion? Well, first of all, we need to note that these are naturally occurring insects in Alaska. That tells me that somewhere there are naturally occurring fungi, bacteria, viruses, spiders, insects and birds and other animals that normally keep them in check. We need to find out what these are and take measures to make sure they are around and can do their job.

As for cultural controls, it seems the best thing we can do is keep our trees healthy. That means making sure they get ample water during the dry months, have a good needle or brown mulch at least under their drip lines, and that they are properly fertilized (the mulch will take care of that) as well as grown in soils that are not compacted. This might be the final excuse homeowners need to go out and aerate lawns.

Neem-based products seem to work on the gooseberries that are infested. These should help when infestations are expected or observed in a tree. A good spray of cold water, especially if mixed with a dishwashing soap or Dr. Bonner's type liquid soap will also help keep populations down.

The point is, we all need to make sure our conditions do not foster this plague on spruces. We also need to spend some time this fall and winter researching and discussing how we can most effectively protect our spruce (and pines which they also will often infect) from Pilonema alaskersis Rohwer without having to resort to dangerous pesticides.

Jeff Lowenfels is a member of the Garden Writers Hall of Fame. You can reach him at teamingwithmicrobes.com or by calling 274-5297 during "The Garden Party" radio show from 10 a.m. to noon Saturdays on KBYR AM-700.