Insect and mite pests on trees are an occasional problem in Iowa. Two pests in particular are causing noticeable, widely scattered discoloration of tree foliage this summer.
Hackberry Lace Bug. Although lace bugs of different species can occur on different species of trees, it is the hackberry trees that seem to suffer the worst. Sap feeding by the lace bug nymphs during June and July results in "bronzing" symptoms in August. Leaves have a characteristic bleached appearance from tiny whitish dots or chlorotic specks on the upper leaf surfaces. Plants heavily damaged by lace bugs usually have the underside heavily speckled with small, black, shiny "varnish spots" (excrement).
The wings and thorax of lace bug adults are beautifully sculptured with an intricate pattern of veins that resemble lace. The top of the body is flat and appears white or translucent. The wings extend out over the sides of the black body. Adults are approximately 3/8-inch long.
There is no benefit for the tree from lace bug control this late in the season. Most damage is done by the nymphs earlier in the season and spraying will not return green color to the already-damaged leaves. Some of the more heavily damaged trees will defoliate early. Lace bugs are an annoyance to people passing or sitting under the tree. Though the lace bugs are harmless to people, pets, structures, and landscape plants, there may be a desire to treat to alleviate the annoyance. Insecticides for lace bugs include insecticidal soap, Sevin, Orthene, malathion and Isotox.
Honeylocust Spider Mite. Honeylocust trees that already have yellow or brown leaves are probably infested with honeylocust spider mites. This pest problem is particularly noticeable on young honeylocust trees (10-20 feet-tall) in parking lots, sidewalk planters and other stressful sites. The mite infestation can be confirmed by examining the underside of the leaflets. Look along the midvein for mites and accumulations of cast skins.
Heavy infestations of honeylocust spider mites cause leaves to turn yellow and drop prematurely in August and September. Although this defoliation is annoying and looks terrible, the trees are mostly unaffected. New foliage appears as normal the following spring and trees grow as we would expect in the sites where planted. Spraying is not necessary, especially after the symptoms appear in August. Where tree appearance is very important preventive treatments in July would be recommended.
This article originally appeared in the August 21, 1998 issue, p. 115.