his year the Giant Bark Aphid (Longistigma caryae) has been much more noticeable than in ordinary years. Reports have come in from Dallas, Houston and College Station of very heavy infestations on the leaves and twigs of live oaks.
The insect is about 6 mm. long, gray-colored with rows of dark spots, but seems even larger because of the long legs. Eggs laid in the bark in autumn are yellow at first, then turn black. The life cycle is simple, and populations of insects may stay on their host plant for generations. Because these aphids live all summer long, it is a good idea to try to eradicate these early populations now, if you see any signs of them.
Dr. Mike Merchant, entomologist from the Dallas area, says that he does not know why the populations of aphids have increased drastically this year. They have usually not been that large of a problem in other years. Opinions are mixed on whether or not Giant Bark Aphids actually kill oaks - but they can completely destroy entire limbs, especially in smaller sized trees.
The aphids are also found on basswood, hickory, pecan, sycamore, walnuts, and wax myrtles. Often there are signs of sticky honeydew under the trees, and then dark sooty mold will appear. Cars parked under infested trees may be affected by the rain of honeydew.
Look for foliage discoloration and dieback, with the underside of leaves populated by clusters of dark, large aphids. Masses of the dark eggs may be seen stuck to twigs and bark.
Begin by doing everything you can to promote tree vigor and health. On smaller trees try spraying the aphids off with a strong stream of water, insecticidal soap solution or horticultural oil application. Dr. Merchant suggests that if this does not make a difference, you might use Orthene or an insecticide containing imidacloprid such as Bayer Advance Lawn and Garden Spray. Because aphids are tough to kill, wait a few days to see if dead insects are coming loose from the leaves. If not, turn to the stronger insecticides such as Malathion, Dursban or Diazinon. If you see that aphid predators are beginning to increase in response to the infestation, let them do their work without further spraying.
For further information and images of giant bark aphids, see the following website: