Aphids

• Look for soft-bodied insects on the undersides of river birch. Two types of aphids can infest birch trees, producing a sticky substance called honeydew as they suck juices from leaves. Honeydew in turn attracts sooty mold, a fungus that won't kill the river birch, but may make it unsightly. Common birch aphids are large and pale green, while the European birch aphid is tiny and yellow, but both have the same effect. Dislodge aphids with a hard stream of water from a garden hose, or spray with malathion or dimethoate insecticide as a last resort.
  
  

Leaf Miners

• Watch for pale pustules developing on river birch leaves. Leaf miners are a tiny wasp that bores into the leaf tissue and lays its eggs. Each swollen blotch on a leaf contains a larva, which feeds on the leaf in spring, by July an entire tree can be affected, so prompt attention must be paid to these bugs because they produce three to four generations in a single summer. Though river birch is rarely affected by leaf miner, it happens occasionally and can be controlled with insecticide spray applied in May before an infestation takes hold.
  
  

Leaf Spot Fungi

• Leaf spot diseases can occasionally attack river birch during unusually cool, wet spring weather. Symptoms include spots that range from yellow to brown, and can appear singly or cover the leaf, causing it to drop early. These fungi will not kill the river birch, unless the tree is very young and most of its leaves are affected. No treatment is necessary for older trees, but spray young trees with copper or sulfur fungicide in spring, while young leaves are expanding.
  
  

Dusky Birch Sawfly

• Look for small, caterpillar-shaped insects, feeding on the edges of river birch leaves. The dusky birch sawfly larva are about an inch long and yellowish green. A large infestation of these insects can defoliate a small tree. Their cocoons can overwinter in topsoil near the tree, and these bugs are capable of producing more than one generation each year. For small infestations, pick them off by hand and drown them in a bucket of water. For larger infestations, spray with an insecticide recommended for use on birches.