Brown Granular Stuff Under Birch Tree

Q: There is brown granular stuff dropping underneath my birch tree. It turns to brownish red mud if it gets wet. 

A: They are probably droppings from a big population of sawflies. They very much resemble caterpillars but instead are the children of the dusky birch sawfly, a wasp-like insect. Since they are not caterpillars, products containing Bacillus thuringiensis won’t kill them. Imidacloprid is a systemic insecticide that can be applied to the soil around the tree in early spring before feeding activity is noticed.

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