Common Insects of Minnesota
Pine Sawfly
Description: Damage is caused by the larval form which look like caterpillars. Severe feeding can heavily defoliate a pine tree.
Trees Affected: Mugo pine, Austrian pine, Scotch pine
What to look for: Actively feeding sawflies during May,
Pine trees with “tufted” appearance, consumed needles
Treatment Options: one insecticide application is effective.
Remove by hand for lighter infestations.
Keep trees healthy with proper watering and mulching.
Scale Insects
Description: Non-mobile sucking insects feeding on needles, leaves, and stems of a wide range of trees and shrubs
Trees Affected: Most tree and shrub varieties
What to look for: Yellowing, stippling, or wilting leaves or needles, physical presence of scales
Treatment Options: Dormant oil spray applications,
systemic insecticide applications, spray during crawler stage. Do not fertilize.
Spruce Gall Adelgids
Description: Small pineapple-shaped gall growing on spruce branches causing distortion or side-branching.
Trees Affected: Spruce
What to look for: Pineapple-shaped galls, distorted growth patterns.
Treatment Options: Prune out light infestations, systemic insecticide applications,
insecticide spray application in early spring during crawler stage
Japanese Beetles
Description: Flying insect that feeds on a wide variety of trees during summer.
Heavy infestations can leave a tree completely defoliated.
Trees Affected: Linden, elm, crabapple, birch, serviceberry, rose, chokeberry, cotoneaster, and others
What to look for: Beetles feeding on leaves in July, skeletonized leaves, heavy defoliation in July
Treatment Options: Systemic soil applications, insecticide spray during periods of leaf feeding.
Perform a lawn application of insecticide to kill the larval stage of beetle in the
ground prior to hatching in late June / July.
*Please note many chemicals are prohibited on linden trees to protect pollinators.
Tent Caterpillars
Description: Caterpillars that feed on a wide variety of trees and build “tents” or “webs” on tree branches.
Trees Affected: Commonly prefer crabapple, cherry, birch, linden, and ash.
What to look for: Physical presence of webs or tents throughout the tree crown,
skeletonized leaves, defoliated portions of trees, or feeding caterpillars.
Treatment Options: Hand pick out infestations on smaller trees or
prune out infestations. Webs can protect against insecticide applications.
Aphids and Mites
Description: Small insects that feed on a wide variety of trees and shrubs by sucking out sap from needles, leaves or stems.
Trees Affected: Most tree and shrub species
What to look for: Stunted, deformed, or discolored leaves. Look for honeydew, black sooty mold or high numbers of ants (for aphids)
Treatment Options: Usually only need treatment when population numbers are high. Insecticide sprays or systemic insecticide soil applications. A Hint from The BIG Oak = Do NOT fertilize infested trees or shrubs. This will not restore health but will explode aphid and mite population numbers making it worse.
Boring Insects
Description: Insects that feed under the bark of braches and trunk woord causing major dieback, crown thinning and tree death. Borers on birch and oak trees generally will attack trees that are stressed or weak. Healthy trees can often fend off infestation on their own. Emerald ash borer will attack any ash regardless of health. Healthier trees have a better chance of survival but will require chemical assistance.
Trees Affected: Ash (Emerald ash borer), birch (bronze birch borer), oak (Two-lined chestnut borer)
What to look for: Sunken or “lumpy” areas along branches, exit holes, dieback, and thinning, insect larval galleries under bark.
Treatment Options: Birch and oak trees may need multiple systemic insecticide applications.
Prune out infected limbs. Keep trees as health as possible with proper watering and care.
Ash trees will need continual insecticide trunk applications plus anything to maintain health.
A hint from The Big Oak = Do not treat any ash that does not meet all four of these requirements.
1-Ash tree is large (do not treat small trees)
2-Ash trees is healthy (unhealthy trees will tougher time even with chemicals).
3-Ash tree has solid overall structure (do not treat trees with poor structure
because they are likely to fail).
4-Ash tree growing in “good” location (do not treat trees in poor locations
or areas with highly restricted root zones).
