In Praise of Bugs: The White Pine Weevil

[Guest post by Sue Sweeney]

The white pine weevil (Pissodes strobe) seems as good a candidate as any to start a national native bug appreciation day movement.  Chances are you’ll be much more likely to see this critter’s work than the critter, which may be a good thing seeing that the bug is an ugly as an orc  (IMHO that is). You can tell all weevils by their long, downward curving snouts, good for poking holes in plant tissue.  The white pine weevils are small beetle-types with splotchy, hard shells and the characteristic long snouts. Their children are pasty-white, legless grubs. Yuk.

When we talk about preserving biodiversity by preserving our native plants, part of the reason is so that our native insects with specialized diets, such as the white pine weevil, will have something to eat.  This statement may puzzle the gardener who thinks that bites out of the roses are a bad thing.  After all, who needs bugs, spiders, worms, ants, and those other tiny creepy-crawlers?  Yeah, some of the “bugs” pollinate the flowers  so that we have fruits and vegetables, some till the soil, some make honey, some spin silk, fireflies glow,  butterflies are beautiful, and some critters break dead things down into their original components for re-use, but…well, who cares about the rest of them?  Particularly, one as obscure and unattractive as the white pine weevil?

Talk about specialized, the white pine weevil’s whole job is to go around topping out our native eastern white pines.   According to the Ohio State University Extension, these little guys “prefer” eastern white pine and various spruces, but, it is said, that in a pinch they’ll also attack about 20 other pines, including, on rare occasion, a Douglas fir or two. In other parts of the country, due to a difference in diet, what us Yankees call the “white pine weevil” is called “Sitka spruce weevil” and “Engelmann spruce weevil”.

What the white pine weevil does, as a larva and an adult, is feed on the soft leaders at the top of the tree.  They seldom kill a tree but can significantly change its shape.  When left to their own devices, eastern white pines grow tall and straight.  If they loose their top leader, due to white pine weevils, utility companies, severe weather or other cause, they develop the soft, wind-swept shapes with which the trees grace many a New England skyline.

If you look around the Northeast, here and there, you’ll see a white pine grown to adulthood tall and straight, but most have developed multiple leaders and wonderful individualized shapes. There’s no way to say for sure that a particular pine got its shape from a weevil rather than other cause but the weevils do a lot of the work.

Interesting, sources pretty much agree that the only white pines likely to be killed by weevils are the under 4-footers growing in full sun, particularly if a bit over-watered (that means in a tree farm, tree nursery or front yard, rather than in natural part-shade understory conditions).

Equally interesting, they say that if the weevil population is in balance, usually only one weevil per tree will lay her eggs in the holes she makes right under the white pine’s top bud cluster.  In this case, most of the grubby larva will get smothered by the white pine’s thick, plentiful resin, provided for exactly this purpose, and the tree’s leader is likely to survive even if gets a bit (artistically) bent.

If there are too many adult weevils, multiple females will compete for egg-laying space on the same tree.  If simultaneously attacked by several weevil broods, the tree is sure to loss its terminal (top) leader and possibly also the next rung down, which will change is its shape and stunt its growth, making it ugly for a year or two while it sheds the dead top and makes new leaders. (If this happened to your tree, you can prune out the dead parts, if you like.)

The weevil population is kept in balance naturally through the white pine’s own defenses, predators, weather conditions, and the like.  The weevil population will increase in monocultures (one-kind-of-plant places) of its chosen food plant, especially if any of the trees are unhealthy.  The weevil population will also increase when the predators have been killed off by chemicals, unusual weather conditions, destruction of habitat or other ecological disturbance.

Tree nurseries, of course, hate white pine weevils, because the nurseries are trying to sell “prefect” little trees, grown quickly, packed together in full sun, and the weevils are actually taking a bite out of the cash register’s contents.  The lumber folk don’t like the weevils either because a severely bent tree is worthless for lumber.

It is true that, post-weevil infection, white pines are likely to develop multiple leaders or a deformed leader, and that such trees probably have shorter life spans than trees with a single straight leader.  If you want, you can fix the tree (pretty much anyway) by selecting the longest top branch and training it up into a new straight leader.  However, there’s a lot to be said for letting Nature take her course.

Once white pines were dominant in northeast forests but they lost significant population due to lumbering over the past 200 years.  So to the extent that the white pine weevil prevents the trees from being cut down by their worse predator, lumbering humans, is this so bad?  Further, the multi-trunked trees have denser crowns and are, therefore, better habitat; plus, as the multi-trunked trees age, the premature shedding of scaffolding branches opens up the tree cavities that are Nature’s prime residential real estate.

While the weevils make life difficult for the (non-organic?) tree farmers (according to the growers’ way of thinking), for the rest of us, though, the white pine weevil is one of nature’s great pruners, toiling hard to beautify the roadsides and hilltops, while enhancing habitat. Who knew?

Saving the bugs saves the birds: But that’s not the end of the story. Each specialized bug also has its predators, which have their predators, and so on. In the U.S. Department of Agriculture Forest Service’s leaflet devoted solely to the white pine weevil, Forest Insect & Disease Leaflet 21, they note that the white pine weevil is food for certain other insects, and for birds such as nuthatches, woodpeckers, chickadees, grosbeaks, and warblers, and small rodents such as voles, field mice, and shrews.

So, if we were to somehow eradicate the white pine weevils, not only would our skylines be less interesting, but the bug’s predators and parasites could starve or at least be decrased in number and become more vulnerable to the threats that turn marginal species into extinct ones, lessening biodiversity.

What we don’t know: Lastly, keep in mind that we don’t know much about Nature and her works. A century ago, we didn’t know that most trees can not live without symbiotic root fungi.  The flora and fauna are an interdependent net evolved to balance. Touch any part, and all other parts have to shift until balance returns. Ignorance of the consequences doesn’t change the result. Further, with Nature, “balancing out” can quite dramatic — think of earthquakes and thunder storms. When it comes to the flora and fauna, it can mean that whole species will die, even regional climates can change.

So, honor your bugs. When you see a hole in the leaf of your prized whatever, think of the enitre chain of life that starts with the little bug that made the hole.

Credits: The original version of this article was published December 2005 in the now-defunct themondaygarden.org, a year before Eric Grissell’s wonderful Insects and Gardens: In Pursuit of a Garden Ecology and two years before the publication of Doug Tallamy’s seminal work, Bringing Nature Home.  My inspiration was the work of  M.J. Hatfield, a self-taught naturalist and nature photographer (like me, only much better) who has put her hand to helping re-construct the Iowa prairie, while championing the cause of our native bugs.  She was one of the first, I believe, to see the connection between  the prairie’s need for birds to do the seeding and the birds’ need for native  insects.   M.J. is a major contributor to bugguide.net.

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