Trees can become afflicted with problems that can change their appearance and overall health. Iowa State University Extension and Outreach horticulturists offer tips on fighting these issues. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
Magnolia scale (Neolecanium cornuparvum) is the largest scale insect in Iowa. Adult magnolia scale females are pinkish orange to brown, elliptical and up to one-half inch in diameter. Females give birth to their young (known as nymphs or crawlers) in late summer.
Sap feeding by the scales causes stress to heavily infested plants and can result in stunted growth, yellowish foliage, branch dieback, or death of the plant. Magnolia scales produce large quantities of honeydew (sugary excretion) that accumulate on the tree’s leaves and twigs. Sooty mold, a black fungus that grows on honeydew, turns the honeydew-covered leaves and twigs black.
Magnolia scale is difficult to control. Two or more control strategies may need to be employed to effectively control magnolia scale. Remove and destroy heavily infested branches. Spray infested trees with a contact insecticide (horticultural oil, insecticidal soap or synthetic insecticide) in late August to early September to control the crawler stage of the insect. Another control option is a soil drench application of a systemic insecticide, such as imidacloprid, in July. Dormant horticultural oil also can be used in late fall or early spring before the buds begin to break.
The browning of the hackberry (Celtis occidentalis) leaves is probably due to lace bugs. Lace bugs are sap feeding insects commonly found on the leaves of shade and ornamental trees in Iowa. Trees most commonly affected are hackberry, sycamore and oak. Adult lace bugs have attractive wings that are beautifully sculptured with an intricate pattern of veins resembling lace, hence the common name.
Lace bugs feed on the underside of leaves. They pierce the leaf epidermis with their sucking mouthparts and cause the characteristic pale yellow, scorched or “bleached” discoloration on the upper leaf surfaces. The underside of heavily infested leaves will be speckled with small, black, shiny “varnish spots” (excrement). While lace bugs are present throughout the summer, damage symptoms usually don’t develop until August or September.
Lace bug damage varies greatly from year to year, mainly in response to variations in natural controls and weather conditions. Severe feeding may cause premature leaf drop, but healthy, well-established trees are not seriously harmed. Spraying infested trees with an insecticide in late summer is of little or no benefit to the trees. Further, spraying when it is too late for effective control may cause more harm than good by killing the insect’s natural enemies.
The “worms” are likely fall webworms. Fall webworms are hairy, tan to yellow caterpillars. As they feed, fall webworms construct tents or webs at the ends of branches. Tents are initially small, but the caterpillars enlarge the tents as they grow and consume the leaves within the tents. By the end of summer, tents may be two to three feet long and enclose entire ends of branches. Fall webworms feed on more than 200 species of deciduous trees. However, walnuts are their favorite host. In Iowa, the first sightings of fall webworms usually occur in early to mid-August.
Fall webworms do not cause serious damage to healthy, well-established trees. As a result, controls are not necessary. Damage to trees can be minimized by undertaking control measures as soon as the tents are discovered. Tents on branches that can be safely reached from the ground or with a ladder can be pruned out and the caterpillars destroyed. Insecticides also can be used for control, but must be applied with sufficient pressure to penetrate the tent and reach the caterpillars inside. Insecticide applications after mid-September are of no benefit and should be avoided in order to preserve predators, parasites and other biological control organisms.