Aphids on trees
BROOKINGS – Aphids are appearing on many trees at this time of year, and many tree owners are calling John Ball, SDSU Extension Forestry Field Specialist to find out if their infested trees are OK.
Ball said that in most cases, although the sticky honeydew the aphids create can be annoying, aphids don’t cause much harm to trees.
“The aphid population is usually not high enough to do much harm,” he said. “The problem is just an annoyance to have to clear the honeydew from car windshield and patio furniture.”
How do I know if I have aphids in my trees?
“Aphid’s activity is generally marked by the abundance of honeydew they excrete on the lower leaves and any plants or objects beneath the tree,” Ball said. “I usually get calls from tree owners concerned about their weeping tree and what they are referring to is the sticky liquid that is covering anything beneath the tree.”
He added that walnuts seem to be particularly attractive to aphids and are one of the most common hosts he hears about.
There are two aphids, or their host injury, that are commonly seen this time of year: the wooly elm aphid and the poplar vagabond aphid. Ball shares what signs to look for and which treatment options will work – if any are necessary.
Wooly elm aphid
Another aphid Ball receives calls about this time of year is the woolly elm aphid.
“The feeding by the nymphs and adult aphids causes the young leaves to swell and curl around the colony,” he said. “The aphids usually do not harm the tree. The only problem is the honeydew that rains down on objects beneath the tree.”
Since the aphids are living in a curled leaf, Ball said most insecticides are ineffective as they work on contact and none of the pesticide contacts them. He said the best control is through the use of systemic insecticides such as imidacloprid that are applied as a soil drench and kill the insects as they feed on the sap.
These insecticides must be put on the tree before the problem is noticed as it can take up to 30 days before they begin working.
Poplar vagabond aphid
Ball recently received a picture of the gall formed by the poplar vagabond aphid.
“This aphid overwinters as eggs that hatch as the shoots are expanding,” he said. “The leaves at the tips are the feeding site for the nymphs and as they suck the sap from this foliage the leaves turn reddish and become deformed. This deformed leaves are hollow and form the gall.”
Ball explained that the adult aphids are leaving these galls now and flying off to their other host, grasses, where they feed on the roots.
“Another generation of aphids develops there and the new adults return to lay eggs on the poplars in the autumn,” he said.
Ball said that although the galls are common on cottonwoods and quaking aspens; they do not harm the tree.
“Once you see the galls it is too late for any control,” he said.
To learn more, visit iGrow.org.