Common Insect Pests of Elm in North Carolina

Introduction

Elms (Ulmus spp.) are valued shade and street trees across North Carolina, but they are vulnerable to a range of insect pests. Some cause only minor, cosmetic damage, while others play a major role in spreading serious diseases such as Dutch elm disease. This factsheet highlights the most common insect pests of elm in North Carolina, their identification and biology, and management options to help homeowners, arborists, and land managers protect these important trees.

Elm Bark Beetles

Native Elm Bark Beetle

The native elm bark beetle (Hylurgopinus rufipes) is a small, dark brown insect that is about 1/8 inch long. The beetles bore into the bark of stressed, dead, and dying elms to breed and construct tunnels, called galleries, in which they lay eggs. An adult beetle bores int othe tree, creating an entrance; females construct egg tunnels, called "galleries", to the left and right of the entrance tunnel in between the bark and the wood. Elms are the only hosts for native elm bark beetles, which vector Dutch Elm Disease (DED).

Since they carry DED fungal spores in their mouthparts and on their bodies, native elm bark beetles are high-impact pests. The beetles acquire DED fungal spores when they emerge from infected elms and can introduce the fungus into healthy elms while feeding. Dutch elm disease fungus blocks an elm’s water-conducting tissue (xylem), which kills the tree. To reduce beetle populations and limit the spread of DED, eliminate beetle breeding sites. Remove and destroy dead and dying elms and elm firewood. Insecticide sprays (e.g., pyrethroid) may provide limited protection when applied in April and May, but are not a substitute for sanitation. Time sprays to local adult activity; sanitation (prompt removal/debarking of infested elm wood) is essential. More information about the native elm bark beetle is available from NC State Extension.

Smaller European Elm Bark Beetle

The smaller European elm bark beetle (Scolytus multistriatus) is a dark, shiny, reddish brown insect that is approximately 1/8 inch long. After feeding on healthy elm trees, the beetles bore into the bark of dead or dying elm trees and create egg galleries in a straight line parallel to the grain of the wood. The larvae, which are tiny, white grubs, feed and develop through the summer months, then overwinter in “brood” trees, which are elms infected with DED. Once they have fully developed, adult beetles emerge and fly to healthy elms to feed on the tree. While feeding, the beetles cut into the elm’s xylem vessels and introduce DED fungal spores. As a vector of DED, smaller European elm bark beetles are also considered a high-impact pest.

Of the two elm bark beetles, the smaller European elm bark beetle is more aggressive and more successful at colonizing weakened elms. Female beetles release an aggregating pheromone to signal a suitable breeding site, which initiates a mass attack on the tree. Rapid removal of DED-infected, beetle-infested elms may slow the spread of DED. Debark and destroy infected elms to prevent the successful emergence of smaller European elm bark beetles months later. As with the native elm bark beetle, insecticides may provide some protection, but should not be used in lieu of sanitation. More information about the smaller European elm bark beetle is available from NC State Extension.

Adult Native Elm Bark Beetle (Hylurgopinus rufipes).

Javier E. Mercado, Bark Beetle Genera of the U.S., USDA APHIS PPQ, Bugwood.org

Inner bark of a tree with many trails scribed throughout

Native Elm Bark beetle egg galleries.

Jim Baker, North Carolina State University, Bugwood.org

Adult smaller European Elm Bark beetle.

Gerald J. Lenhard, Louisiana State University, Bugwood.org

Inner bark with centipede-shaped etchings

Smaller European Elm Bark Beetle egg gallery

Joseph OBrien, USDA Forest Service, Bugwood.org

Elm Zigzag Sawfly

Detected in NC in 2022, the elm zigzag sawfly (Aproceros leucopoda) is a shiny, black, winged insect that is a little more than ¼ inch long. Females reproduce parthenogenetically (without a male) and lay up to 60 eggs singly on the tips of elm leaf serrations. Eight days later, grayish-white larvae emerge. Developing and mature elm zigzag sawfly larvae are green with a black band on their head and T-shaped black markings above their second and third pair of legs. Elm zigzag sawfly larvae spin one of two types of cocoons: net-like cocoons attached to leaves in the summer months and solid-walled cocoons in soil or leaf litter in the winter.

Young elm zigzag sawfly larvae defoliate elm leaves in a characteristic zigzag pattern while older larvae defoliate more broadly across leaf tissue, often obscuring the zigzag pattern. Generally, defoliated elms can recover from feeding damage. However, defoliation year after year can weaken an elm tree, leaving it susceptible to other pests, diseases, or branch dieback.

Since most trees recover from minor defoliation, control may not be warranted. Hand-remove elm zigzag sawfly larvae from small trees. Chemical protections are being investigated. To be most effective, apply an insecticide treatment in the spring as soon as larvae are detected on elm leaves. More information about elm zigzag sawfly is available from NC State Extension.

Adult elm zigzag sawfly

Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org

When young elm zigzag sawfly larvae feed on elm leaves, they create a characteristic “zigzag” pattern for which the species is named.

Tom Macy, Ohio DNR Division of Forestry, Bugwood.org

Elm-Grass Root Aphid

The elm-grass root aphid (Tetraneura ulmi), which is also referred to as the elm sack gall aphid, is a tiny insect that creates galls (abnormal growth of plant tissue) by feeding on elm leaves. The stalked, hollow sack galls are approximately ⅜ inch tall and green or yellow-green. The elm-grass root aphid has a complex life cycle that involves grasses and elm trees. In June and July, winged aphids fly from sack galls to colonize the roots of grasses and reproduce by giving birth to live nymphs. Then, in September, fully developed aphids return to an elm to produce nymphs, which grow into wingless males and females. Each fertilized female lays a single overwintering egg. In the spring, females hatch from the overwintering eggs. Aphid feeding induces a sack-gall where the females give birth to a second generation of wingless aphids.

Elm-grass root aphid galls are primarily an aesthetic concern and do not affect an elm tree’s health. Overwintering aphids are preyed upon by a variety of natural predators, including minute pirate bugs, predaceous caterpillars, and birds. If an elm is severely infested year after year, consider applying a systemic insecticide during the winter to prevent gall formation the following spring. More information about the elm-grass root aphid is available from NC State Extension.

Elm grass-root aphid nymphs create galls (deformed plant growths) by feeding on elm leaves.

Daniela Lupastean, University of Suceava, Bugwood.org

Elm Leafminer

Elm leafminer sawflies (Kaliofenusa ulmi) are tiny, dark, non-stinging wasps that emerge from new growths on elm trees. Female elm leafminer sawflies lay up to 25 eggs, scattering them across an elm leaf or laying them near the leaf’s central vein. Small, pale-white larvae with dark brown heads hatch 10 days later and begin to feed on elm leaves. The leafminer sawflies drop from the elm tree, dig into the topsoil, and spin tough, brown, paper-like cocoons where they spend the rest of the summer, fall, and winter before pupating in the early spring.

Elm leafminer sawflies attack American (U. americana), Camperdown (U. glabra ‘Camperdownii’), English (U. minor ‘Atinia’), Scotch (U. glabra), and Siberian (U. pumila) elm. They may attack the same elm tree year after year. Elm leafminer larvae mine inside the leaves rather than skeletonizing from the outside. Larvae feed between the upper and lower leaf surface, causing blotch mines, which resemble small tan patches on the elm leaf. In some cases, leaves are mined so heavily that the elm tree defoliates.

Consider hand-picking and removing mined leaves if the elm tree is small. Tiny, newer mines indicate that the application of a systemic insecticide (e.g., imidacloprid) may still be effective. Apply early in the season, before larvae finish mining and drop to the soil (often by early to mid-summer). More information about elm leafminer is available from NC State Extension.

Small black winged insect on a green leaf

Adult elm leafminer sawfly

Whitney Cranshaw, Colorado State University, Bugwood.org

Elm leafminer larvae feed between the upper and lower surfaces of elm leaves, creating tan patches called “blotch mines.”

Brian Kunkel, University of Delaware, Bugwood.org

Elm Leaf Beetle

The elm leaf beetle (Xanthogaleruca luteola) is a tiny insect. Immature larvae are small, black, and grub-like and feed on the leaves of elm trees. Adult elm leaf beetles, which are slightly less than ¼ inch long, are yellow to olive green in color and have a black stripe that runs along the outer edge of their front wings. Female elm leaf beetles lay 5 to 25 eggs in a group on the underside of elm foliage, and may lay several groups of eggs. Larvae hatch 3 weeks later. Once hatched, the larvae feed on the underside of elm leaves, avoiding leaf veins and the upper surface. Elm leaf beetle larvae migrate to pupate in the cracks of the trunk and lower limbs of an elm. Adults emerge 7 to 14 days later in the mid- to late summer.

Although all elm species are subject to attack, elm leaf beetles typically prefer Chinese elm (U. parvifolia). Larval feeding on the lower surface of leaves may result in partial or complete defoliation of elm trees. Severely colonized foliage turns dark brown and drops prematurely. Landscape trees are usually more heavily infested than forest trees. Feeding damage rarely kills an elm tree, but intense defoliation can stress or weaken an elm, making it susceptible to other pests or diseases, such as Dutch elm disease.

Several natural enemies, including predaceous stink bugs and plant bugs, attack elm leaf beetles. Consider applying a systemic insecticide (e.g., imidacloprid) to kill the foliage-feeding beetles. To target larvae and adult elm leaf beetles, choose to spray insecticides directly onto the leaves, rather than using systemic treatments. Wrap a foot-wide adhesive band with insecticides around an elm’s trunk to prevent freshly pupated adult beetles from crawling up the tree to lay eggs on the foliage. More information about elm leaf beetle is available from PennState Extension.

Small yellow and black beetle on a green leaf

Adult elm leaf beetle feeding on an elm leaf

Whitney Cranshaw, Colorado State University, Bugwood.org

European Elm Scale

The European elm scale (Gossyparia spuria) is a small, elongated, sap-sucking scale insect that resembles a mealybug in appearance. Female scales are wingless, about ⅛ inch long, and surrounded by a white, waxy fringe. They lay eggs 2 to 3 weeks after elm leaves fully form in the spring and continue to lay eggs into the summer. In June and July, small, yellow nymphs hatch from the eggs and feed on the primary leaf veins on the underside of elm leaves. During August and September, nymphs return to elm twigs to overwinter in bark crevices.

American elm is highly susceptible to European elm scale. Severe infestations cause an elm to yellow and shed leaves. Immature scales use piercing-sucking mouthparts to siphon sap from an elm’s vascular tissue (phloem). Nymph feeding can cause yellowing, premature leaf drop, and branch dieback. In addition, female scales secrete honeydew, which can cause black sooty mold to grow. While not harmful itself, sooty mold can be unsightly, and honeydew can attract stinging insects.

Many natural enemies control European elm scales, including mites, predatory plant bugs, and spiders. Chemical management should only be used against live European elm scales. Consider applying horticultural oils in the late fall and early spring to smother overwintering nymphs in bark crevices. More information about European elm scale is available from Colorado State University Extension.