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Ask a Gardener - Japanese Beetle

June 21, 2014


Question:  Eeek! I just found several Japanese Beetles on roses and evening primrose plants. I've done some reading but there are so many suggestions! What really works? (that won't kill bees?) Is there any hope if the neighbors don't treat too? I knocked off as many as I could but have read such horror stories. What's true in Colorado? 
- Anne

Eeek is right!  Japanese Beetles are some of the most significant garden pests in the country.  They actually are Japanese natives and accidentally snuck into the country in the early 20th century.  Since then, they’ve spread throughout North America becoming serious pests.  The good news is there are a lot of control options available.  The bad news is it can be hard to know what works.  As with most garden pests, it helps to learn a little bit about the Japanese Beetle’s life cycle in order to determine the best control options.  

Adult Japanese Beetles start to emerge from the soil in June through July, and immediately

start to feed.  Female beetles release a pheromone to attract males.  After mating, the female burrows a few inches into the soil and lays a small group of eggs, and returns to the surface to feed.  This cycle of feeding and laying eggs continues until the female lays 40 – 60 eggs during her 4 to 8 week lifespan.  After a couple of weeks, the eggs hatch and the larvae begin to feed on plant roots and other organic material.  They will feed until cool weather drives them deep into the soil, where they remain dormant during the winter.  When the weather begins to warm in the spring, the larvae begin to move to the surface, eventually emerging as adults again in the heat of the summer.  

Japanese Beetles damage plants as both adults and larvae.  Adults feed on soft leaf tissue, leaving skeletonized foliage.  Larvae feed on roots, especially turf grass, leaving dead, brown patches in the lawn.  Complete control depends on addressing both the larvae and the adult bugs. 

Adult Control
Hand picking Japanese beetles can be effective on small outbreaks, if it’s done early.  By catching the first emerging adults, you can prevent them from attracting more beetles to your yard.  Wear gloves and shake the beetles out of the branches into a bucket of soapy water. 


For larger outbreaks, one of the best controls for adult Japanese Beetles is a natural insecticide called spinosad.  Spinosad is a natural, fermentation derived insect control first discovered in abandoned rum distilleries in the Caribbean.  It works on contact and ingestion on Japanese Beetles and many other insects.  When it is first sprayed, spinosad is toxic to bees, but once it dries it has no effect on them.  Spray spinosad in the evening when worker bees have returned to their hive.  By the time they return the next morning, the plants will be dry and safe for them to forage.  

There are Japanese Beetle traps available, however CSU says, “traps are ineffective for control where the Japanese beetle is well established over a large area, common now in many Front Range locations.”  In fact, many pheromone traps can actually attract more beetles to your yard!  So, in general, traps are not recommended.

Grub Control

Grub controls are best applied when the grubs are small and feeding near the surface.  Bayer Complete Insect Killer is one of the most effective insect controls for lawns.  The active ingredient in Bayer Complete is imidicloprid, which is a systemic insecticide and there are some concerns about how systemic insecticides affect bees.  However, since turf grass produces neither pollen nor nectar, bees will not come into contact with Bayer Complete.  Apply Bayer Complete like a fertilizer and give it a good soaking to water it into the lawn.