Dusky birch sawflies
I have a 20-year old triple trunk river birch (Betula nigra 'Heritage') on my front lawn. For the past three years, dusky birch sawflies have been feeding on the leaves for the entire month of July and sometimes into August. The tree tolerates the feeding and does not seem to be suffering any adverse effects but I do not like what I think are their droppings (green/brown pellets the size of a pea with texture resembling a corn cob). They are covering my front porch, sidewalk and driveway, sticking to the bottom of shoes and staining wherever they fall. My questions are (1) why has this started only for the past three years; (2) should I have had the tree sprayed to kill/discourage the sawflies and (3) how much longer will this chewing/pooping continue?
Populations of this insect can fluctuate from year to year. Other than keeping the tree well watered during dry periods there are no preventative measures. No control is necessary on mature healthy trees. Hose off the debris.
In general, there are two generations a year. Larvae appear in April and May and again from July through early fall. However, this is difficult to predict.
Here is our publication on birch http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG58_IPM_Birch.pdf