Clip 'N Keep:
Three common beetles

By Jennifer Hobson, OMAFRA Nursery Crops Specialist, Horticultural Science Division, University of Guelph

Of the many insect pests we find plaguing our woody plants, beetles certainly do their fair share of damage. It is no surprise to find that beetles comprise the largest order of insects, making up about 40 per cent of the known species of insects (Bland and Jaques, 1978). Beetles have thickened, leathery front wings (elytra) which cover the thinner membranous hind wings that are used for flight. The mouthparts of adults and most larvae are adapted for chewing, and most of that chewing occurs on plants. Larvae (or grubs) look quite different from the adult stages with their elongate bodies and lack of wings. It is usually the larval stage that is most injurious to our horticultural plants. Here are a few beetle pests that are common in Ontario:

Viburnum Leaf Beetle (Pyrrhalta viburni)
Small yellowish larvae hatch as Acer platanoides finish bloom. Larvae feed from leaf undersides, leaving only the veins behind. Larvae feed for about six weeks then pupate in the soil and emerge as adults in late June. Small, brown adults mate on foliage (Photo #2). Females chew holes in stems, lay eggs inside and secrete chewed bark to form a protective cap. Eggs overwinter under cap, and swell in April as larvae get ready to hatch (Photo #4).

Hosts: Viburnum opulus, V. trilobum, V. dentatum, V. lantana and others

Injury or symptoms:
  1. Skeletonized leaves result as larvae chew interveinal tissue (Photo #1). Repeated years of defoliation may cause dieback and death of shrub.
  2. Egg-laying may also kill twigs (Photo #3).
  1. Prune out egg-laying sites found on one- and two-year old twigs, collect clippings and destroy.
  2. Newly-hatched larvae (one to14 days old) are susceptible to chemical control.

Bronze Birch Borer (Agrilus anxius)
Larvae develop over a two-year period inside the host. They feed by chewing large tunnels in the cambium, just under the bark which cause swollen ridges that are noticeable from outside the tree (Photo #6). This damage cuts off sap flow to the upper portion of the branch, causing leaf necrosis, defoliation and dieback. Pupation takes place inside the tunnel. Adult beetles chew a hole in the bark and emerge, from late May to early July. Adults lay eggs on bark (especially in crevices). Eggs hatch, and larvae begin tunnelling on the side of the tree to which they were attached. Therefore, chemical control must be applied before egg-laying.

Hosts: Betula papyrifera, B. pendula and B. populifolia.

Injury or symptoms:

  1. Dieback and death of leader and branches near top of tree in June/July (Photo #5). Tree mortality.
  2. Large, swollen ridges around circumference of bark indicating larval tunnels.
  3. D-shaped adult emergence holes (Photo #7).
  1. Since birch trees have shallow roots, deep watering and removal of competing vegetation around base will help overcome infestation.
  2. Prune out and destroy infested branches before May.
  3. Spray chlorpyrifos thoroughly over bark to control hatching larvae, when Spiraea vanhouttei is in full bloom.

Pine Shoot Beetle (Tomicus piniperda)
As larvae feed they form patternless tunnels (galleries) under the bark of stressed and dead trees (Photo #11). This feeding damage causes sap flow to be cut off from the rest of the tree, causing further dieback and death of the host. Larvae pupate under the bark and adults emerge in late June, approximately when the Catalpa speciosa are in late bloom. The adults move up to the foliage to bore into the base of a shoot. They cleanly chew out the pith, working toward the tip and exit out through another chewed hole (Photo #9). Shoot feeding weakens the tips, causing them to die and break easily. Adults overwinter just under the bark at the base of trees, and emerge in late winter when daytime temperatures reach 10-12° C for a few days ("spring flight"). Adult females lay eggs under bark in late winter.

Hosts: Pinus sylvestris, P. mugo, P. nigra, P. resinosa, P. strobus and others

Injury or symptoms:
  1. Shoot tips hanging from branches July-November (Photo #8).
  2. Non-patterned larval galleries are formed under the bark of stressed/dying trees.
  3. Adult beetles can often be found in broken shoot tips littered on the ground (Photo #10).
  1. These beetles have been associated with drought conditions. Irrigate stressed trees where possible.
  2. Set out trap logs to attract breeding adults by January. Remove trap logs about six weeks after "spring flight" to control populations in the immediate area.
  3. Remove and destroy stressed or dying trees by May. They are attractive to breeding and feeding adults.

Bland, R.G. and Jaques, H.E. 1978. How to Know the Insects. 3rd Ed. W.C. Brown Company Publishers, USA.
Johnson, W.T. and Lyon, H.H 1986. Insects That Feed on Trees and Shrubs. 2nd edition. Comstock Publishing Associates, Ithaca, New York.
Pirone, P.P. 1978. Diseases and Pests of Ornamental Plants. 5th edition. J. Wiley & Sons, New York.