I've long admired silver linden (Tilia tomentosa) with its dark green leaves accentuated by silver undersides. This tough tree is able to handle many of the urban slings and arrows that send less hardy trees to wood chippers. It's also less susceptible to the depredations of Japanese beetles (Popillia japonica); unlike its littleleaf cousin, T. cordata.
However, this eastern European native seems to be having a growing challenge with our native basswood lace bug (Gargaphia tiliae). I first encountered basswood lace bugs on silver linden in a northern Kentucky landscape in 2004. Although the lace bugs had turned the silver lindens into golden-brown lindens by August, the bugs remained rare throughout much of Greater Cincinnati for many years.
This has changed in recent years. In fact, from my perspective, basswood lace bugs on silver linden are gradually replacing hawthorn lace bugs (Corythucha cydoniae) on their namesake host as harbingers of the lace bug season.
Lace bugs (order Hemiptera; family Tingidae) are so-named because of the lace-like pattern of veins and membranes in their wings. Most lace bug species found in Ohio live on the lower leaf surface and linden lace bugs are no exception.
All lace bugs are tiny insects with the adults measuring no more than 3/16" long. The nymphs are even tinier and appear to be covered in small spikes.
Both the adults and nymphs use their piercing-sucking mouthparts to suck juices from the leaves. Although feeding is done on the lower leaf surface, the damage appears on the upper leaf surface as tiny chlorotic spots (= stippling). High winds and heavy rains can drive lace bugs to lower leaves, so the stippling damage may be heaviest in the lower canopy.
The stippling may at first appear as distinct 1/4 - 1/2" diameter spots on the upper leaf surface created by 1st instar nymphs feeding near the cluster of eggs from which they hatched. This symptom is particularly evident with basswood lace bugs on silver linden owing the dark green upper leaf surface.
Eventually, the stippling will coalesce to produce large white patches and heavily stippled leaves look "bleached-out." As the damage progresses, portions of the leaf, or entire leaves, will turn yellow to copper-brown. Another tell-tale feature of lace bug feeding is the appearance of black tar-like fecal deposits that add to the unsightly appearance of heavily infested leaves.
High lace bug populations can produce enough leaf damage to cause early leaf drop, branch dieback, and even the death of small trees and shrubs. Many lace bugs in Ohio have 2 to 3 generations per season with the leaf damage ramping up with each successive generation. This means it's critical to target the current first generation with control measures to halt further damage this season.
Lace bugs can also be a serious nuisance pest. They have a penchant for dropping from heavily infested trees onto unsuspecting hikers, picnickers, and patrons of outdoor bars and cafes. They don't feed on people, but they can use their piercing-sucking mouthparts to deliver a pinprick bite particularly if they fall between a person's neck and shirt color. Adding insult to injury, floating lace bugs can ruin a good Mai Tai.
A Rogues Gallery of Lace Bugs
Despite its common name, hawthorn lace bug has one of the most cosmopolitan palates of any lace bug found in Ohio. It will feast on a wide variety of rosaceous plants as well as a few plants outside of the rose family such as common buttonbush (Cephalanthus occidentalis). However, they are most commonly found on hawthorn (Crataegus spp.), serviceberry (Amelanchier spp.), Cotoneaster spp. and Firethorn, (Pyracantha coccinea).
Other lace bugs that will soon be joining the lace bug scene in Ohio include buckeye lace bug (C. aesculi); oak lace bug (C. arcuata), sycamore lace bug (C. incurvata); walnut lace bug (C. juglandis); and chrysanthemum lace bugs (C. marmorata).
Buckeye, oak, and walnut lace bugs confine their feeding to their namesake hosts. Sycamore lace bugs may be found on American sycamore and to a lesser extent on London planetree.
Chrysanthemum lace bugs (C. marmorata) are unusual in two ways. They are found on both the lower and upper leaf surfaces and they may feed on a wide range of herbaceous perennials in the Asteraceae family including asters, black-eyed Susans, goldenrods, and sunflowers. These lace bugs may occur in greenhouses as well as landscapes. Indeed, landscape infestations may originate in greenhouses.
Suppression includes simply applying a heavy jet of water to blast away the lace bugs. Of course, adults have wings, so the "water park ride" approach may be overcome by adults flying back to the leaves. Insecticidal soaps and horticultural oils are effective; however, direct contact is required. So, make sure to target the undersides of leaves. The same is true for pyrethroid insecticides; they are stomach poisons but lace bugs are sucking insects.
Some systemic insecticides such as acephate (e.g. Orthene, Lepitect) and the neonicotinoids dinotefuran (e.g. Safari, Transtect) and imidacloprid (e.g. Merit, Xytect) are effective against lace bugs. However, imidacloprid must be applied with enough lead-time to allow the active ingredient to migrate to the leaves in a high enough concentration to kill the bugs; it may be too late this season.
Of course, before using any insecticide, you must read and follow label directions paying close attention to plant safety as well as avoiding impacts on pollinators. This includes making certain sprays do not drift or runoff onto non-target flowering plants.