I have received several calls this week about oak trees leaving sap in people’s yards, decks, and cars. The oak trees aren’t actually dripping sap from their leaves, it is the honeydew left from aphids chewing on the leaves. The aphid essentially sucks the living juice out of the leaf, and the honeydew is the aphid’s excrement from their eating. Following is an explanation of the 0ak leaf aphid from the U.S. Forest Service.
Importance – Oak Leaf Aphids infest the undersides of leaves, leaf stalks and tender twigs of trees in the red and white oak groups throughout the East. Heavy infestations distort the foliage and weaken the plants. Distorted foliage will take on a curled appearance. Honeydew and sooty molds further mar the beauty of ornamentals.
Identifying the insect – The aphids are .04 to .06 inch (1 to 1.5 mm) long, soft-bodied, pearshaped, with a pair of cornicles (look like horns) at the bottom of the abdomen. They may be yellow, green, pink, or brown, with darker-pigmented blotches on the abdomen and dusky bands on wings. Winged and wingless forms occur.
Identifying the injury — Clusters of aphids feed largely on the underside of the leaves. Feeding injury curls and folds the leaves. Every leaf on a tree may be curled and distorted during heavy attacks. Leaf surfaces become sticky with honeydew followed by growth of black, sooty fungus.
Biology — Overwintering occurs as eggs deposited in bark crevices of host plants. The eggs hatch in the spring, and nymphs begin feeding on the leaves. There are several generations per year, but the highest populations have been observed during the spring.
Control — Natural enemies usually keep infestations in check. Insecticides are sometimes needed on ornamentals and other high-value trees. Insecticidal soap, neem oil and narrow-range oil (e.g., supreme or superior parafinic-type oil) provide temporary control if applied to thoroughly cover infested foliage. To get thorough coverage, spray these materials with a high volume of water and target the underside of leaves as well as the top. Soaps, neem oil and narrow range oil only kill aphids present on the day they are sprayed, so applications may need to be repeated. And, as always, make sure you read and follow the label directions.
Elysia Rodgers is the agriculture and natural resources educator for DeKalb County with the Purdue University Cooperative Extension Service.