Oak trees have been noted as major centers of insect activity, supporting hundreds of native insect species that feed on everything from leaves to twigs and acorns. They are the keystone species of our forest ecosystems making them an ideal tree to plant in support of our native biodiversity. But, what happens when all this insect feeding gets out of hand? Can there ever be too much oak and insect interaction?
Oak lace bugs (Corythucha arcuate) are one insect participating in the oak feeding bonanza these days. In the last few weeks, I observed several populations around central Illinois on our state tree, the white oak (Quercus alba), which is their preferred host species.
Lace bugs get their name from their lacey appearance, with clear, subdivided wings as adults that look a lot like lace. They are actually really interesting looking insects if you are able to see them under slight magnification. In Illinois, multiple species of lace bug occur feeding on a variety of trees, as well as some herbaceous plants. Although some species have been introduced, oak lace bugs are native.
These tiny insects (¼ inch) feed on oak leaves with piercing and sucking mouth parts they use to access the sugars with leaves. Their feeding results in foliar symptoms we commonly see later in summer, including stippling (tiny spots), discolored blotches on leaves and, in extreme cases, early leaf drop. As populations build in late summer, the combined effect of feeding across the season can create a canopy of damaged-looking leaves, causing alarm.
In addition, lace bugs also leave some telltale signs, such as black frass (excrement) and eggs on the underside of leaves. Most folks first notice the stippling and blotching on upper leave surfaces and don’t bother to take a look at the underside. However, the undersides of leaves tell the complete tale, with nymphs and adults often present and visible as tiny specks that move when touched. Minute black eggs can be seen in carefully laid clusters, although they look very similar in size and color to the frass, which is randomly distributed across the leaf surface and often intermixed with eggs.
Lace bugs overwinter as adults in buds, bark and other nooks and crannies they can find on their host plant. Egg laying begins in early spring as oak leaves emerge, with several generations occurring across summer. It takes around 30 days from hatching to reaching adulthood and over the course of summer, their population builds and the damage incurred on leaves becomes more and more apparent.
It’s often not until this time of year that folks call our office with a “sick” oak since populations have built up and damage is more apparent. However, it’s rare that I recommend any treatment other than observation; damage to the tree is largely aesthetic.
The cumulative effect of this leaf damage doesn’t amount to much for a large healthy tree since leaves are still able to function, producing sugars for energy via photosynthesis, and much of the tree’s annual sugar production already occurred earlier in the year.
In rare cases, I do recommend some control on newly planted trees with heavy infestations. A steady stream of water will knock off the majority of these insects and effectively reduce their population. Be sure to direct the spray at the underside of leaves where these insects feed. I also recommend watching heavily infected plants the following year and repeating the treatment periodically throughout the season to limit population buildup over summer.
Mature, healthy trees should bounce back just fine from any late summer damage. I do recommend monitoring trees with heavy infection this year for the next few years. Repeated infestation and heavy feeding can reduce tree vigor over subsequent years. In addition, it may be a sign of other stress the tree is under, making it more susceptible.
Lace bugs are native insects and despite their damaging feeding, they are part of our native ecosystems. Keep in mind that they have lots of native predators and insect populations are known to swing from year to year. Hopefully the predators that feasted on lace bugs this year will have a large presence next year and things will return to balance.
Ryan Pankau is horticulture extension educator with University of Illinois Extension serving Champaign, Ford, Iroquois and Vermilion counties.
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