ISA Rocky Mountain Chapter

Tree Pests

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Emerald Ash Borer (EAB) was found in Boulder, CO, in September 2013. As a non-native insect, EAB lacks predators to keep it in check. EAB only attacks ashtrees in the genus Fraxinus (so mountain ash are not susceptible). Approximately 15% of the trees that make up Colorado's urban forest are ash.



Scale Insects protect themselves with a covering of wax. Each species has a distinctive pattern on this waxy "shell", which can vary greatly in form and color. Most soft scales (Coccidae) tend to be rounded; armored scales (Diaspidadae) tend to be shaped like small nipples or elongate funnels. Cochineal scales, found on cactus pads, are covered with loosely piled white wax. Newly hatched scales, the crawlers are pin-head size and typically pale yellow or pinkish in color. Scales are found throughout the state where trees, shrubs, and other perennial plant hosts are present. The most common scales are found on shade trees; the insects are more easily spotted on the twigs than the leaves.

Japanese beetle has an oval form is about 7/16-inch in length. It is generally metallic green with coppery-brown wing covers, which do not quite cover the tip of the abdomen. Along the sides are five patches of whitish hairs. The antennae are clubbed at the end and may spread to a fan-like form. Adults feed on leaves, buds and flowers of many common garden and landscape plants. On leaves feeding is usually restricted to the softer tissues between the larger leaf veins, which results in a characteristic feeding pattern known and described as ‘skeletonizing’. More generalized ragged feeding occurs on softer tissues, notably flower petals; rose flowers are particularly susceptible to Japanese beetle injury. Damage on individual plants may be patchy, concentrated where aggregations of feeding beetles occur.

Aphids are the most common insects found on trees, shrubs, and garden ornamental plants. Over 350 different aphid species occur in the state but most can feed on only a few species of plants. However, with so many kinds of aphids, few plants grown in Colorado do not support at least one aphid. Most species rarely injure plants or even attract attention, but a few aphid species do cause problems. Aphids feed by sucking sap from plants. When the number of aphids on a plant are very high for an extended period, their feeding can cause wilting and sometimes even dieback of shoots and buds. Some aphids can cause leaf curling when the insect infests emerging leaves. 


Ips beetles, sometimes known as “engraver beetles,” are bark beetles that damage pine and spruce trees.Mature ips beetles enter trees and tunnel, producing a yellowish- or reddish-brown boring dust. The dust accumulates in bark crevices or around the base of the tree, and the affected parts of the tree discolor and die. Small round holes in the bark of infested trees indicate the beetles have completed development in that part of the tree and have exited. The presence of woodpeckers, a common predator of the ips beetle, may indicate infestation. These symptoms are similar to mountain pine beetle, so be sure to properly identify the beetles you find associated with your tree.

Dutch elm disease flagging in elm tree

Dutch elm disease is an aggressive fungal disease of elms native to America. The fungus is spread  from tree to tree via the European elm bark beetle. As the beetles tunnel in to lay eggs, the fungus enters the plants' water-conducting system. Once inside the tree, the fungus begins to plug the vascular system. As a result, leaves wilt and the affected tree dies within a few months to a year.



Mountain pine beetle (Dendroctonus ponderosae) activity subsided and remained low with a total of 5,000 acres of active mountain pine beetle infestation detected in the state in 2015. The epidemic has ended in many areas of Colorado as mature pine trees have been depleted following the outbreak that impacted more than 3.4 million-acres of Colorado forestland from 1996-2013. Many of the pine forests impacted by the outbreak, especially in portions of Middle Park and North Park, now have a gray cast due to the large numbers of dead trees.



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