Sawflies and Conifers

By Bruce Appeldoorn
Red-headed sawfly larvae feasting on the needles of Pinus mugo
Red-headed sawfly larvae feasting on the needles of Pinus mugo

Are you plagued by unwelcome, annual visitors?

Text and Photography Bruce Appeldorn

No, I’m not talking about an undesirable inlaw (they usually come for the holidays) or the pack of eight-year-olds with BB guns that come each summer when school’s out. I’m referring to those nasty red-headed pine sawfly larvae (Neodiprion lecontei) that strike each year at this time, chewing their way through your prized mugo pines (Pinus mugo). And they’ll be back again next year, guaranteed, right on time, exactly on schedule.

These caterpillar-like creatures hunt in a pack, decimating any two-, two/three-, or three-needled pines they can find. They’re creatures of the forest, native vermin that feast on pines in the natural landscape but also find garden specimens indescribably delectable. In their adult form, they are barely noticeable as an innocuous fly (Tenthredo mesoneda), but, as larvae, they are insatiable, ravenous beasts that are well-camouflaged and often are noticed only by the damage they leave behind. In the forest, they are a minor pest that prunes a bit of annual growth, but, for choice, smaller, ornamental garden conifers, sawfly larvae can be disfiguring and destructive. Fortunately, they feed only on pines.

Red-headed pine sawfly larvae appear each year from late June through August, but their precise arrival date will vary by elevation, USDA Zone, and weather. Higher elevations may have larvae appearing up to two weeks earlier. Each individual is a small, one-inch caterpillar that is straw colored or yellowish, with tiny black spots arranged in lateral lines on the body and a distinct, red head.

A group of these larvae will attack a single shoot of foliage, usually at the top of the plant, strip it clean of needles, and then move on to the next shoot. The damage can be remarkable, particularly on young or dwarf plants. Some reference books state that their damage is confined to second-year needles, but I find that they may devour any needle younger than two years old, probably only because still older needles are tougher to chew.

While the classic garden species of choice is mugo pine, I have seen other species included in their diet.* Loblolly pine dwarfs (Pinus taeda) are obviously high on their list of favorites, and they seem to go after some plants year after year. Pinus sylvestris (Scots pine) is apparently also delicious. As long as we’re making a list, we’d best include specimens of Pinus uncinata (Swiss mountain pine), P. heldrechii (Bosnian pine), P. nigra (Austrian pine), P. thunbergii (Japanese black pine), P. banksiana (jack pine), and a few others of minor importance.

The best defense is timely and persistent scouting—actively searching for these critters before and during their expected hatch-out date. Due to their camouflage, they can be hard to see initially, as they do look a bit like a growing pine bud—their red head being the bud itself. They will almost always be present in a group. A giveaway behavioral trait is that they will rear up on their hind legs and raise their heads when approached, trying to be their fiercest self in response to your nearby finger. Don’t worry, they don’t have stingers or biting mouthparts, only tiny mandibles just right for chewing pine needles. Like many insects, these guys are specialists.

In a few weeks, the gorged caterpillars will metamorphose into adult flies. These will breed, and females will lay eggs in slits in pine needles, where they will await the timely hatch-out in the following year. These slits will be visible under a magnifying glass for those so inclined to look. However, as this is a forest pest, new larvae will appear each year, almost guaranteed and on schedule. It is normal that some years will have heavier infestations than others.

Pine sawfly larvae can be controlled by physical or chemical means. “Physical” means getting down and dirty with them — a simple sideways motion between thumb and forefingers will crush the blighters, and you can get many of them in a single motion once you get the hang of it. There is something about this action that satisfies the need for garden revenge, and, after a few minutes, the problem is solved, and one feels much better. A gloved hand is recommended for the more squeamish among us.

Chemicals do not provide the same level of immediate gratification but are effective. Sevin™ dust will work but is not ornamental, unless you think of it as powdered sugar; Orthene™ (Acephate) is a systemic that can be applied as a spray before the caterpillars arrive and will kill the creatures as they feed (great for absentee or lazier gardeners). Neem oil or pyrethrins will satisfy those of us who insist on more “natural” methods. But for me, nothing is more natural than the search-and-destroy physical squish.

Of course, none of these control methods will work with the in-law problem. For that control, you’re on your own! Should vocal means fail, try to remember the old time-tested rule: “Always first try physical, then try chemical!”

*Editor’s Note. At my home in Adrian, MI (Zone 6), sawfly larvae enjoy most my Pinus resinosa ‘Morel’ (Morel red pine). What pests do you battle in your garden? I’d like to know.

Bruce Appeldoorn’s article first appeared in the September 2020 Southeastern Conifer Quarterly, pp. 4–5.

Comments

Eric Smith

I found that sawlflies like Cedrus deodara as well. They've come back 3-4 years in a row now.