Urban Tree Pests

Trees are a vital component of urban and suburban landscapes. Depending on the species and location, they may face an assortment of challenges. One important group is biotic organisms. Biotic tree pests are composed of pathogens (fungi and bacteria predominately) that cause disease, insects and mites, and weeds. If a biotic pest warrants a pesticide treatment, the person responsible for the tree must have a pesticide applicator license.

For our discussion on biotic pests, let us focus on insects. Even excluding disease and weed pests, this can be an intimidating number of potential pests. It is usually not too difficult to find an insect on any given tree. However, this does not mean that any insect found on a tree is a threat. Many insects that feed on trees cause cosmetic/aesthetic damage only, meaning the tree's health is not negatively impacted. Learning the key insects then becomes critical. A key pest is one that is routinely a problem (causing cosmetic damage that is unacceptable to a client), or it can be one that has the potential if left unchecked to cause significant damage or death.

Insects feed on trees using chewing mouthparts or piercing and sucking mouthparts. Those with chewing mouthparts feed on foliage, wood, or tree roots. Insects with piercing and sucking mouthparts extract fluid from plant tissue. An individual tree can host numerous species of both types. Let us look at a few representatives from each group.

Chewing Mouthparts

Insect borers can be a serious threat to tree health. The native species of borers, such as ash/lilac borer (Podosesia syringae) and roundheaded apple tree borer (Saperda candida), target trees that are under stress and are weak. This can be a common occurrence for urban trees since they are growing in a harsh environment. Borers that are not native, commonly called exotic or invasive species, attack and kill healthy trees. Asian longhorned beetle (Anoplophora glabripennis) and emerald ash borer (Agrilus planipennis) are two introduced species.

Asian longhorned beetle (ALB) was discovered in New York City in 1996. Since then, it has been found in five other states. Many of the populations have been eradicated. ALB larvae feed on over 20 species of deciduous trees. Maples are a particular host preference for larval development, especially Norway maples that were used as street trees. Emerald ash borer (EAB) was found in Michigan in 2002. Twenty years later, it can be found in 36 states. Green and white ash are the primary hosts. ALB larvae start feeding in the cambium and then move deeper into the heartwood. EAB larvae feed in the cambium. The larval feeding of each species eventually kills the tree.

Even though these beetles attack healthy trees, the first tactic for management is prevention. This starts with proper tree selection for the location, use of proper planting techniques, follow-up maintenance (especially water during the first years after transplant), and pruning. Healthy trees have a greater chance of warding off borers. However, since ALB and EAB attack healthy trees, insecticides are required to protect trees from larval feeding. The active ingredients that have efficacy include dicrotophos (Bidrin), dinotefuran (Safari, Transtect, Zylam), emamectin benzoate (TreeAge), and imidacloprid (Merit and many others).

Japanese beetles (Popillia japonica) have been in the United States since 1916. Unlike the borers, Japanese beetle adults feed externally on the foliage. The feeding damage is called skeletonization. The larvae of Japanese beetles feed on turfgrass roots, overwintering to emerge in June of the following year. Trees that are near turf areas are going to have the most damage. The level of skeletonization depends on the population and the host tree. Lindens are a particular favorite. The damage can be viewed in two ways. It can be tolerated with nothing done to intervene, or an insecticide can be applied to deter and/or kill the adults. Some of the insecticides work as contact materials so are applied to the foliage. These include the active ingredients bifenthrin, carbaryl, and permethrin. They are non-selective meaning they will kill other non-target insects, including beneficial insects (parasites, pollinators, and predators). Chlorantraniliprole (Acelepryn) is a systemic that can be applied to the foliage also.

Bagworms (Thyridopteryx ephemeraeformis) are caterpillars that encase themselves in the foliage of the plant they are feeding on. They feed on numerous species of trees. The damage that they do to needled evergreens (arborvitae, juniper, spruce) is the most serious. Early instar caterpillars clip bits of foliage and tie them together to make a portable case (or bag) to insert their bodies. They walk around the host plant housed in this protective cover. Significant damage can be done before the insect is noticed, and this can severely impact the appearance and long-term health of a needled evergreen. The active ingredients spinosad (Conserve) and Bacillus thuringiensis (BT, many products) are very effective and do not negatively impact most beneficial insects.

Piercing-sucking Mouthparts

There are many species of aphids that can show up in urban landscapes. Two that are often encountered are linden aphid (Eucallipterus tiliae) and tuliptree aphid (Illinoia liriodendra). They become a nuisance problem in urban settings because their sticky excrement (called honeydew) drops from the canopy onto surfaces below – sidewalks, parking lots, chairs/benches. Honeydew also attracts stinging insects. Though there is a complex of beneficial insects that may find an aphid population, they often do not reduce the population level to an acceptable level for the situation. This forces the tree care professional to use an insecticide. Horticultural oil and insecticidal soap are efficacious against aphids. Once they have dried, there is no residual so that beneficial insects are not impacted. Pymetrozine (Endeavor) also has minimal impact on beneficials. Other insecticide options include acetamiprid (TriStar), cyantraniliprole (Mainspring), and imidacloprid (Merit and others).

Scale insects are unusual as far as insects go. Adult females do not look like insects. Instead, they look like weird growths on the host plant. Many of the scales encountered on ornamental plants are placed into two categories – armored scales and soft scales. A designation is based on the body cover and where the scale feeds inside of the plant. Armored scales have a hard cover called a Test that is not fused to the body. Soft scales have a soft cover that fuses to the female body. Armored scales insert their mouthparts into individual plant cells. Soft scales insert theirs into the phloem. Feeding in the phloem causes the ingestion of more carbohydrates than can be digested, so they are excreted as a sticky fluid called honeydew. For example, Japanese maple scale (Lopholeucaspis japonica) is an armored scale and tuliptree scale (Toumeyella liriodendri) is a soft one. Both are key urban tree pests.

The key to effective scale management is proper identification so that it can be determined when the nymphs (immatures) hatch. These newly hatched scales are called crawlers because they have legs and move on the host plant. Crawlers are susceptible to horticultural oil, insecticidal soap, and pyriproxyfen (Distance, Defiance, Fulcrum, Proxite). The other reason scale identification is important is the hard scale group is not killed by neonicitinoid insecticides, which include imidacloprid. The soft scales are killed by acetamiprid, dinotefuran, and imidacloprid.

Spotted lanternfly (SLF, Lycorma delicatula) is a new invasive species. It presents two challenges in an urban/suburban environment. The first is the nuisance factor. In general, people are not fond of a rather sizeable insect flying and walking around outdoor areas. The other nuisance issue is SLF is a copious honeydew producer. Any outdoor structures (sidewalk, deck, patio, bench, furniture) that are under a tree where SLF's are feeding will become covered in sticky excrement. The other issue is whether the feeding of SLF on desirable trees leads to long term tree health problems. Currently, there is a lack of data on the effects of prolonged feeding on trees. Red and silver maple, river birch, black walnut, and willow are preferred hosts of adults in late summer. It must be mentioned that tree of heaven is the favorite host, but it is considered an invasive species and is not typically managed.

SLF immatures, of which there are four developmental stages, feed primarily on herbaceous plants and thin-barked woody plants. They have a contrasting color pattern (black and white for the first three immature stages; red, black, and white for the fourth and final immature stage) that makes them easy to see. They are excellent jumpers. The adults are winged with the second pair of wings being bright red. Adults can be managed with dinotefuran or imidacloprid, which are systemic, or with a contact insecticide (bifenthrin, carbaryl, permethrin).

Urban trees must survive in a less than ideal environment. They must deal with problematic environmental/manmade conditions that suburban and woodland trees do not. These can be the primary health issue. However, key biological agents (diseases and insects) can cause damage or kill trees. It is important to recognize them and know the respective management options.