Insects and Diseases

Calico Scale (Eulecanium cerasorum)

Calico ScaleCalico Scale can be a pest of all stone fruits, including plums, cherries, and almonds. It can also impact honey locust, walnut, elm, maple, dogwood, and crabapple trees. When an outbreak occurs, a large volume of sticky sap is excreted from the scale. This excretion can coat tree leaves, branches, and anything under the trees. The coating can result in increased growth of sooty mold. If the sooty mold fungus is extensive, this may weaken the tree by interfering with the trees ability to photosynthesize light. This could weaken the tree and allow for other pests or diseases to kill the tree.

Description: Calico Scales have distinctive white and black markings and are 6-8 mm in diameter. As they age Calico Scales lose their black and white markings and turn dark brown.

Symptoms: Large amounts of honeydew are produced in the spring by developing adult females. Sooty mold may grow on the honeydew and stunted leaves may occur where twigs are heavily infested. No symptoms may be present in light and medium infestations.

Management: In cases of light Calico Scale infestation, the pest may be removed by mechanical means such as brushing or with your fingernail. For medium to heavy infestations, application of horticultural oils in late winter/early spring to control eggs. Applications during early summer and fall will control crawlers.

Eastern Tent Caterpillars (Malacosoma Americanum)

Tent CaterpillarsThe Eastern Tent Caterpillar (ETC) is a native caterpillar that makes a very noticeable silken web, specifically in the crotch of branches. This web is used for thermal protection during cold periods in the spring when the caterpillars are active. During sunny and warmer days, ETC will venture out from the web and fed on foliage.

Primary host trees: This pest is very specific to apple and cherry species.

Description: Has a long white stripe down the length of its back. Caterpillars are active into late-May or early June.

Pupation: Generally occurs starting in late May and the adult moths appear a few weeks later.

Management: Monitor for new webs, which can be physically removed at night or on cool days when all of the caterpillars are inside. These webs can then be stepped on or squashed, buried or burned.

Memosa Webworm (Homadaula anisocentra)

WebwormThe Memosa Webworm is present throughout southern Michigan, especially the counties south of a line from Grand Rapids to Flint. The past few years of mild winters have allowed the Memosa Webworm to expand further north than usual.

Primary host trees: Often affects honeylocust trees.

Symptoms: The small caterpillars web together leaves, stripping the green tissue and leaving brown lace. Heavily infested trees may turn entirely brown, appearing as if they suddenly died from drought. However, this damage is not fatal and the trees should put out a full flush of new growth next spring.

Management: Trees rarely die from a single defoliation, especially late in the summer. However, 100 percent defoliation two years in a row can be crippling and sometimes deadly. The webworms go through two generations per year, with the caterpillars feeding and damaging honeylocust trees in June and July, then again in late August and early September. Insecticides application should begin with a Bacillus thuringiensis product when injury first begins in late June.

Asian Long-Horned Beetle (Anoplophora glabripennis)

Asian BeetleLocalized infestation of the Asian Long-Horned Beetle (ALB) includes Illinois, New York, and New Jersey. The beetle has been found in wooden crating and packing material associated with trade goods from Asia. To date, infestation has not been sited in Michigan.

Primary host trees: The ALB feed on several species of hardwood trees. The favorite target are norway maples, but other maple species, horse chestnut, elm, box elder, mulberry and poplar trees are often hosts. Pine, spruce and other conifers are not attacked.

 Egg laying sites: Female beetles chew round holes in the bark along the trunk and on the limbs where they lay the eggs. Beetle larvae and adults push out chewed wood, which resembles sawdust. Larvae spend the winter in the tree emerging as adults in late spring through 3/8-inch diameter tunnels that cut off the flow of water and nutrients in the tree.

First symptoms: Dark wet areas on branches and trunks are often seen on infested trees. The sweet sap may attract bees. Damage from the beetle and secondary pests will kill a tree within a few years.

Preventing ALB from attacking trees: When planting yard or ornamental trees, homeowners within regulated areas should select varieties that ALB does not prefer. Homeowners may also visually inspect tree health by keeping a close watch for signs of distress that may occur as the result of an infestation.

Pinewood Nematode (Pine-Wilt Disease) (Bursaphelenchus xylophilus)

Pine WiltThe Pinewood Nematode invades the stems and branches of pine regardless of the age or size of the tree. The Pinewood Nematode is a microscopic worm that clogs up the conduits within a tree carrying sap and other nutrients. A pine sawyer beetle is the vector of pinewood nematode. When the beetle feeds on the bark and twigs of susceptible live trees or when the female lays eggs in freshly cut timber or dying trees, transmission of the Pinewood Nematode occurs. In late spring, 2006 several Oakland communities have noted pine trees riddled with BB sized holes.

Host Trees: Pine Wilt disease affects are not limited to attacks on scotch pine and austrian pine. The nematode has also been isolated from larch, balsam fir, spruce and cedar.

Symptoms: Signs that become evident in late spring or summer include lack of resin exudates from bark wounds and foliage that becomes pale green then yellow and finally reddish brown when the tree succumbs to the disease.

Management: According to the U.S. Forest Service, there is no cure for Pine Wilt disease. Residents should be aware that removal and destruction of pine trees killed by the pinewood nematode will help prevent spread to adjacent, healthy pines. Maintaining the vigor of pine trees by fertilization and irrigation during dry periods may assist prevent the disease from developing.

Tar Spot (Rhytisma acerinum and R. punctatum)

Tar Spot is quite evident in many areas of Michigan over the past few years. According to Michigan State University research findings high levels of pathogen spores accompanied by conducive environmental conditions in the spring can initiate high levels of infections.

Tar Spot

Primary host trees: Spores are evident particularly on maple species

Symptoms: Beginning in the spring and early summer you may see tiny yellow spots on the leaves up to an inch or so in diameter. Over the course of the summer the spots will become darker with the production of fungal material. The fungus develops over winter on fallen leaves then infects the upper surfaces of leaves in spring during moist conditions. Occasionally some leaf withering and drop will occur.

Management: Tar Spot is primarily a cosmetic, harmless, nuisance disease that causes no harm to the tree, primarily because of its late appearance in the season. Tar Spot disease is of such little significance that the best approach is to do nothing.

Sirex woodwasp (Sirex noctilio)

WoodwaspThe Sirex woodwasp is most commonly detected in areas associated with wood packing materials. Recently, these insects have been discovered in a Michigan Department of Natural Resources (MDNR) trap in Macomb County. This raises concern because the Sirex woodwasp has the potential to cause significant mortality of many species of Pine trees. Adults usually emerge from July through September, with peak emergence during August.

Description: Woodwasps have a dark metallic blue or black body, reddish-yellow legs, and black antennae. Adults have a spear-shaped plate at the tail end. In females, a long ovipositor under this plate is used to inject the insect’s eggs into the sapwood of the tree, which ultimately kills the tree.

Symptoms: Infested trees may have resin beads at the egg-laying site. This egg-laying site is further distinguished by a round exit hole. During the three to six months following an attack infested trees wilt and needles will become yellow and then red.

Management: The Sirex woodwasp has been successfully managed using biological control agents, which include predators, parasites and pathogens. In some cases, a parasitic nematode is used to infect the larvae and sterilize the female Sirex woodwasps.

Updated: November 2007