By: Sandy Feather ©2010
Penn State Extension
Q. Last spring, I saw green worms eating my Mugo pines. By the time I noticed, they had devoured most of the needles at the ends of the branches. Last year's growth covered up the damage somewhat, but I'd hate to see my Mughos eaten again. Can I do something to prevent them from damaging these evergreen shrubs this spring?
A. The larvae (immature stage) of an insect called the European pine sawfly often feed on mugo pines (Pinus mugo), as well as other pine species. The larvae resemble caterpillars, with green bodies and black heads. On close inspection, European pine sawfly larvae have a light green stripe down the middle of their backs, bordered by a dark green stripe on either side, then a white stripe between the dark green stripe and the legs.
Depending on moisture and temperatures, they are active in our area sometime from April through May. It is typical for them to feed en masse, a characteristic known as gregarious feeding. Their early hatch means that they must feed on last years' needles, leaving the plants looking like clipped poodles. By the time new growth comes on susceptible plants, the European pine sawfly larvae are pupating in the soil under the plants. (Bob's note: Early May around Pittsburgh)
Sawfly larvae are well camouflaged, but the
telltale needle stubble gives them away
Successful control depends on you monitoring the plants for their activity. If you control them immediately after they hatch, you can avoid such severe damage. You could even check them now for the presence of eggs.
The light tan eggs are laid on the needles in a row, appearing as a straight, dotted line down the needle. If it would not leave the plants looking scalped, you could prune out those twigs with a large number of eggs to reduce the number of survivors.
Burn the twigs, or tie them securely in plastic bags and send them out with the trash. Or you can wait until they hatch, then spray them with insecticidal soap, BioNeem (azadirachtin) or Sevin (carbaryl), while the larvae are very small and before they cause serious damage.
There are other species of pine sawflies that are active in western Pennsylvania. The introduced pine sawfly feeds on many species of pine, and is active from June through September. They have dark green bodies marked with yellow and gray spots, and black heads. Another species is the redheaded pine sawfly. They are quite distinctive, with creamy white bodies marked with black spots and red heads. Redheaded pine sawfly larvae are typically active from July through early September, but I have seen them at work as late as November.
Although these sawfly larvae look like caterpillars, caterpillars are the larvae of moths or butterflies. These are the larvae of sawflies. The main reason for this distinction is that the biological insecticide Bacillus thuringiensis (Bt) is specific to the larvae of moths and butterflies; it is not at all effective against sawfly larvae.
One way to tell the difference is to count the abdominal prolegs, those found between the insect's rear end and its midsection or thorax. The larvae of moths and butterflies have five or fewer pairs of abdominal prolegs; the larvae of sawflies have six or more pairs.