Here's the Dirt: Japanese Beetles

Getting rid of the pesky bug

Champaign, IL - Japanese beetles are a pest of both turf grass and trees.  Their impact to trees (and other plants) which occurs once the adults emerge and begin to feed, which is this time of year.  However, they are considered a major pest of turf as grubs (larvae stage) causing turf damage estimate at over $500 million annually in the US.  
These beetles are originally from Japan and were first detected in the US, in New Jersey in 1916.  Currently most states east of the Mississippi River are infested and approximately 10 states west of the Mississippi have the beetle.
Damage to trees and other plants occurs as the adults emerge and begin to find plants to feed on.  They feed on the upper surfaces of leaves, removing the tissue between veins and giving the leaf a lace-like or “skeletonized” look.  The presence of skeletonized leaves (and the beetles themselves) are the most prevalent signs of an infestation.  The beetle feeds on an estimated 300 spp of plants in the US.
Insecticide treatment options are available, but are not commonly recommended.  Since the grubs hatch from turf areas (which are widespread) in multiple waves, frequent re-treatment is required.  This can be costly and can impact non-target pests, such as pollinators, especially when the host plant in flowering.  The most practical treatment to apply is to physically remove the beetles by knocking them off the limb they are on into a bucket of soapy water.  However, this may not be practical on large trees.  It is important to begin physical removal of the beetles early as other beetles will be attracted to a plant with active feeding. 
Although the damage is unsightly, these beetles very rarely cause dieback in healthy, mature shade trees.  Sometimes treatments may be warranted on newly planted trees that stand to lose a significant portion of their canopy to the beetles.  However, most trees can sustain quite a bit of defoliation during the growing season.
 
Japanese beetles emerge in early July in central Illinois
Beetle activity is most intense for a 4-6 week period
Commonly impacted species: linden trees, willow trees, rose bushes, and some fruit trees
Treatment is generally not recommended unless plants are already in poor health, recent transplants or high value specimens
For additional information please contact the U of I Extension Master Gardener’s Hotline at 217-333-7672
 

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