The oak tree represents one of North America's most impressive native trees. Few trees exhibit such size, strength and longevity. Despite their infallible appearance, oak trees can fall victim to a variety of leaf-eating pests. Although some animals--including deer, beaver and rabbits--will eat oak leaves, the damage animals cause cannot compare to that done by insects. Insect pests can defoliate a tree in a very short period of time.
Forest Tent Caterpillar
Forest tent caterpillars are North American natives and feed on the leaves of hardwood trees, including oaks. The caterpillar is approximately two inches long, hairy and brownish-grey in colour. A fine blue line runs along each side of the body and characteristic white footprint-shaped markings run down the centre of its back. Damage done by the pest appears as holes in the leaves, similar to those that would be caused by a gunshot. Leaves give a tattered appearance when the tree is heavily infested. Oaks trees can withstand a year or two of defoliation. After that, stunted growth and some branch death may occur.
Gypsy Moth Caterpillar
Unlike the forest tent caterpillar, gypsy moth caterpillars are not native to North America. They are hairy and may grow to be three inches long. The pattern on the back of this caterpillar is what distinguishes it from other pests. Behind its black and yellow head are five pairs of blue spots followed by six pairs of red spots. Young caterpillars produce shot hole damage. Late-stage caterpillars consume entire leaves and are capable of totally defoliating a tree. Gypsy moth caterpillars prefer oak leaves to all others. Oak trees can usually withstand one year of defoliation. Two or more consecutive years of damage place undue stress on the tree, increasing the likelihood of death.
Oak Leaftier Caterpillar
Oak leaftier caterpillars are quite small, measuring only half an inch in length. They are greenish-yellow in colour and have a dark brown head. These are the caterpillars (in addition to leafroller caterpillars) frequently seen suspended from oak trees by silk threads. Affected leaves will be deformed, sparse and full of holes. Oak trees can usually survive two to three successive years of defoliation by this pest before branch death begins to appear.
Oak Leafroller Caterpillar
The oak leafroller caterpillar is a native pest that periodically will defoliate oak trees. The pest is approximately 1.2 inches long and is commonly known as the inchworm. It has a black head and a body that can be various shades of green. The caterpillars roll individual leaves to form enclosures which they use when not feeding. Leafroller caterpillars infrequently cause major defoliation and are more of a nuisance than anything else.
Scarlet Oak Sawfly
The oak sawfly is a type of wasp. It is the larvae of this wasp that feed on oak leaves. Scarlet oak sawfly larvae are dark green to black in colour and measure half an inch in length. They have a slimy coating that helps them adhere to the oak leaves. The larvae feed from the underside of the leaves, eating everything except the veins and a thin layer of upper leaf tissue. The extent of defoliation ranges from spotty to complete and is usually not fatal unless a tree is young or severely stressed.