As if ash trees have not been terrified enough this year with the threat of emerald ash borer on them, worried homeowners are seeing yet another injury to their ash trees. Luckily this one is not as potentially threatening as EAB – although it is much uglier!
Leafcurl ash aphid has struck trees in the Denver Metro Area. Symptoms are twisted, thick, gnarled leaves at the ends of branches. These clumps are often covered with the sugary exudate, honeydew, that is excreted by the insects which in turn collect pollen and other debris passing by in the air and can cause the clumps to appear webby and really messy – a scary thing to behold. The kind that sends one to the closet for, what else? something to spray on it!
But resist. First of all, it won’t help, and second, the colonies will begin to decline as new growth ceases being produced on your trees, which is about now. And third, by now the natural enemies have amassed to curtail the outbreaks.
Natural enemies of the leafcurl ash aphids - the beneficial insects that feed on aphids - are lady beetles and their larvae, lacewing larvae, flower fly larvae and parasitic wasps.
Exposed aphids can be taken care of with a strong jet of water, insecticidal soap and then if all else fails an insecticide. But leafcurl aphids have created a protected environment by curling the leaves tightly, thus preventing any of these remedies from being successful. More importantly, the use of insecticides will harm the natural enemies of aphids. So, it makes sense to resist using them and move on to what actually does work.
So what is the best management technique for the leafcurl ash aphid? First, this insect pest creates more of an aesthetic issue for your tree than a health threatening one. So carefully cutting the tips of the branches off and not letting them smash to the ground, but catching them in a disposable plastic bag and sealing it up carefully and taking it off to the trash – never the compost pile. Sanitation (the clean up of all falling leaves and the gnarled branch tips) is crucial for preventing infestations in future years as leafcurl ash aphids overwinter in the soil on the roots of the ash. Winged stages work their way through cracks in the soil to disperse to the foliage shortly after bud break in the spring. It is the young aphids that develop on the tender, emerging leaves that induce the thickening of the leaves and the tight curling. As new generations emerge and feed, the leaves continue to distort and the unsightly clumps form on the branch tips. Horticultural oils might be sprayed during the dormant stage just before bud break or a systemic applied to the soil as the soil warms and the insects emerge.
For additional information see CSU fact sheet 5.511 Aphids on Shade Trees and Ornamentals at http://www.ext.colostate.edu/pubs/insect/05511.pdf.