By Kathy Salisbury
Director, Ambler Arboretum of Temple University

The Mugo Pine (Pinus mugo) in front of the Hilda Justice Building is nearly defoliated. For the second September in two years, this plant is looking quite dead. The needles have been devoured leaving just brown bumpy twigs.

If you look closely, not everything is brown. Clustered at the ends of the branches are black and green "caterpillars" with russet heads. 

These creatures are the larvae of the Redheaded Pine Sawfly. Sawfly, not moth or butterfly, this means these are not actually caterpillars at all, despite the striking resemblance. 

How do you know if you are looking at a caterpillar (the larva of a moth or butterfly) or a sawfly larva? Sawfly larva have three pairs of true legs on their abdomen and 6 pair of pro-legs, stubby little appendages. True caterpillars do not have more than 5 pairs of pro-legs.  Sawfly larvae also tend to congregate in large numbers while caterpillars are mostly solitary creatures. 

The larva of the redheaded pine sawfly makes it difficult to count their prolegs, gathering in clusters like they do.  

If you should happen to disturb the clusters of Redheaded Pine Sawfly larva watch closely. They will lift their heads and rears up in a defensive stance while regurgitating a chemical from the plant they are consuming, which they have sequestered from the needles just for this purpose. If you look closely in the photo above, you can see the defensive posture and tiny little clear bubbles at the mouths of the larvae, exhibiting this defense mechanism. 

In large enough populations these insects can kill their host plants, which include mostly hard pines like our native Virginia Pine, Pitch Pine and Red Pines just to name a few. 

As far as controls go, there are natural predators of sawflies, including birds and parasitoids. If you find them in your landscape, you can hand-pick them and squish them or place them in a jar of soapy water. Because they are not caterpillars, controls designed for caterpillars like the biological control BT (Bacillus thurningiensis) will not work.