Japanese Beetle Life Cycle: A Fascinating Exploration for Gardeners

The Japanese beetle, Popillia japonica, is an invasive insect native to Japan. This notorious pest has made its way to the United States, rapidly spreading through various regions.

Known to attack a wide range of plants, the Japanese beetle infests landscape trees, shrubs, vegetable and fruit crops, and turfgrass. They feed on the foliage, flowers, and fruits, causing significant damage to the host plants. Understanding the life cycle of these pests can help in managing their damage and controlling their impact on agricultural and horticultural industries.

Japanese Beetle Life Cycle

Egg

• Laid in soil by females
• Moisture-dependent for survival

Japanese beetle eggs are laid in the soil by female beetles. The eggs are dependent on moisture in the soil for their survival.

Larva

• Stage: white grubs
• Feeding: grass roots

In the larval stage, Japanese beetles are known as white grubs. These grubs feed on grass roots and grow as they molt through several stages.

Pupa

• June: pupation
• Soil temperatures influence development

Pupation occurs in June, when larvae transform into pupae. Soil temperatures play a significant role in the developmental speed of the pupae.

Adult

• Late June to July: emergence
• Mating and egg-laying

Adult Japanese beetles emerge from the soil in late June and July. They mate and start laying eggs, beginning a new generation.

Comparison between lifecycle stages:

In most areas, the Japanese beetle life cycle takes one year to complete, but in cooler climates, such as Minnesota, it might take longer for the beetles to develop.

Feeding Habits and Plant Damage

Common Host Plants

Japanese beetles are known to be highly destructive pests as they feed on a wide range of host plants. Both adult beetles and their larval stage (grubs) can cause significant damage to a variety of plants. Some common host plants include:

• Trees: birch, crabapple, linden, Japanese maple, and mountain ash
• Shrubs: rose species
• Fruits: grapes, apples, and cherries
• Vegetables: beans, corn, and tomatoes

Signs of Damage

Adult Beetle Feeding

Adult Japanese beetles feed on more than 300 different ornamental and agricultural plants. They usually target the foliage, flowers, or fruits, leaving behind a distinct type of damage. Common signs of adult beetle feeding include:

• Skeletonized leaves: only the veins remain, giving the leaf a lace-like appearance
• Partially eaten petals and flowers
• Scarring and holes on the surface of fruits

Monitoring: Keep a lookout for adult beetles from late June to early July when they are most active and feeding on host plants.

Grub Feeding

Japanese beetle grubs feed on grass roots, resulting in damaged lawns, golf courses, and pastures. They also contribute significantly to increased turf and white grub populations in the United States. Grub damage typically presents as:

• Brown, dying, or thinning patches of grass that can be easily pulled up (due to weakened roots)
• Increased presence of C-shaped white grubs in the soil

Monitoring: Regularly check the density of grubs in your lawn by examining soil samples. Populations of more than 10 grubs per square foot may indicate significant damage.

Japanese Beetle Control Methods

Manual Removal

Manual removal can be an effective method to control Japanese beetles in small infestations:

• Early morning or late evening are the best times for manual removal.
• Pick beetles off plants and drop them into a container of soapy water.

Pros:

• Environmentally friendly
• Cost-effective

Cons:

• Time-consuming
• Less effective for large infestations

Traps and Pheromones

Japanese beetle traps can help reduce beetle populations:

• Pheromone-based traps attract beetles using synthetic sex pheromones.
• Place traps 30 feet away from susceptible plants to prevent attracting more beetles.

Pros:

• Easy to use
• Effective in reducing beetle numbers

Cons:

• May attract more beetles from surrounding areas
• Need regular cleaning and replacement of pheromone lures

Chemical Control

Insecticides may be used to control Japanese beetles:

• Select an insecticide labeled for use against Japanese beetles.
• Follow label instructions and application timing.

Examples:

• Carbaryl
• Permethrin
• Neem oil (organic option)

Pros:

• Provides quick control
• Can be effective for larger infestations

Cons:

• May harm beneficial insects
• Possible environmental impact

Biological Control

Biological control methods involve the use of natural predators or pathogens:

• Beneficial insects (ladybugs, green lacewings)
• Milky Spore (Bacillus popillae) for controlling white grub stage in turfgrass
• Parasitic flies (Tachinid flies)

Pros:

• Environmentally friendly
• Can provide long-term control

Cons:

• Slow to show results
• May not be effective in all situations

Reader Emails

Over the years, our website, whatsthatbug.com has received hundreds of letters and some interesting images asking us about these insects. Scroll down to have a look at some of them.

Letter 1 – Mating Japanese Beetles

Japanese Beetles
Location: London Ontario
July 12, 2011 2:59 pm
Hello I am seeing these all over the place in Southwestern Ontario, grape vines, crab apple trees, flower beds.
This is the first time I have noticed these beetles and they are in abundance!
Signature: Jeff Kleber

Hi Jeff,
Your photo of mating Japanese Beetles is positively gorgeous, and it is unfortunate this lovely looking beetle is such a major pest of cultivated plants.  They will feed on well over 100 different ornamental plants cultivated in gardens, and they are especially fond of roses.  When the beetles are present, they will gather on a single plant, and when it is defoliated, they will just move to something else.  Since we will be out of the office for several days, we are post dating this submission to go live on Sunday.

Letter 2 – Mating Japanese Beetles

what is this beetle?
Nice website! These guys are on a Virginia Creeper vine in Southern Ontario, Canada. There are a whole bunch of them. At dusk there are a lot flying around the tops of some of the trees. I’m guessing they’re responsible for all the holes in the plant leaves. Any idea what they are? Thanks. Take care,
Cory
Mississauga, Ontario.

Hi Cory,
Japanese Beetles were accidentally introduced to North America in 1916. There are now a serious agricultural pest throughout the east. They will eat many plants and are very fond of roses, primroses, rose of sharon and grapes.

Letter 3 – Mating Japanese Beetles

Mating Beetles
Hi Bugman,
Thought you might like this photo of mating beetles taken August, 2006. These two are mating a hibiscus leaf which they ‘love’ feeding on and other ‘things'<:)) I titled this ‘Dance Of Love’ in my photo gallery at BetterPhoto.com.
Sincerely,
Lorraine Cook
Philadelphia, PA.

Hi Lorraine,
The Japanese Beetles are late in Philadelphia. This pestiferous alien introduction is continuing to expand its range. They decimated my mother’s Ohio garden in July but they have died out for another year. They are especially problematic as they eat such a wide variety of plants. Recently I acquired a pamphlet from the California Department of Food and Agriculture warning of their introduction to California.

Letter 4 – Mating Japanese Beetles

Name these Bugs Please. Ahhh, Bonking and Eating
I was day tripping just north of Lake Erie in Southern Ontario and found these great bugs while picking wild berries. I found your site while trying to identify them. What a great service you are offering. Pat on the back 🙂 I like to imagine what it would be like if bugs were six feet tall! Ahhhh, bonking and eating. Two of life’s finest treasures…so why not do them at the same time. I sent you this photo for identification but thought I’d resubmit it for the Bug Love section of your site which I just found. Location:Southern Ontario just north of Lake Erie in a lovely Conservation Area where I spent the afternoon picking berries. Luckily, the berries were bugless!
Linda

Hi Linda,
Thanks for sending us your photo of mating Japanese Beetles. We have had numerous requests for their identification recently.

Letter 5 – Mating Japanese Beetles

Bug Love
Hi, my name is Brigette and I love your site. I’ve been interested in insects since I was a little kid, and am currently an undergrad studying entomology at McGill University. I love to photograph insects and thought you might enjoy some additions to your ‘bug love’ section. These were taken in my backyard in upstate NY. I have included some japanese beetles, craneflies, horseflies, and ambush bugs (my favorites!). I even have some eggs as a result of the ambush bug matings, I kept several during the fall months. When introduced the males waste no time at all getting busy!
Brigette

Hi Brigette,
Wow!!! Thanks for sending us all your wonderful Bug Love images. They are most excellent.

Letter 6 – Mating Japanese Beetles

Metallic looking beatle skeletonizing grape leaves
Mon, Jul 6, 2009 at 7:34 AM
We have a swarming of metallic looking beatles skeletonizing our grape leaves. It isn’t anything like the pictures of Western skeletonizing bugs shown.
wlarson
Iowa

Hello wlarson,
These are mating Japanese Beetles, Popillia japonica , an invasive exotic species accidentally introduced to New Jersey in 1916. Since that time, the Japanese Beetles have spread throughout much of the eastern U.S. BugGuide does not list any sightings in Iowa, but there are reports from many surrounding states and the westernmost reports on BugGuide are from Kansas. Japanese Beetles feed on the leaves, flowers and fruits of countless ornamental plants, and they are most fond of roses. There are commercial traps available that lure the Japanese Beetles with bait and keep them from feeding on the plants.  Mom in Ohio says the Japanese Beetles make the leaves of her plants look like lace doilies.

Letter 7 – Mating Japanese Beetles

Japanese Beetles
July 1, 2010
Just sharing some Japanese Beetle pictures I took yesterday.
Veronica
Crossville, TN

Hi Veronica,
Your letter is so timely because we just selected the Japanese Beetle as the Bug of the month for July.  It appears your mating Japanese Beetles are on a Rose of Sharon, which mom claims is a magnet for the pestiferous invasive exotic leaf chafer.

Awesome!! I am a new fan of nature photography and seem to get more pictures of bugs than anything. I am loving it!!
Veronica
http://thruveronicaseyes.blogspot.com

Letter 8 – Natural Method to repel Japanese Beetles

Cure for Japanese beetles for small gardens!
April 8, 2011 11:07 pm
My name is Justin and I have a fairly large home garden in Michigan. Every year it seems like the Japanese Beetle population has been growing. We tried everything short of pesticides (I grow organically and I think with a little innovation nature has a cure for all). Recently I read that Japanese beetles rely on their very sensitive sense of smell to find food and each other. I heard that the traps become innefective once they have a fair amount of  beetles in them because of the smell. This gave me an idea. My neighbor hung one of the traps. We sprinkled the dead beetles under the raspberries. We went from a heavy infestation to nonexistant. It smelled unpleasant to the human nose for a couple of days then the smell went away. We did this twice last summer and that protected the entire garden completely. There has been studies that show the beetles may be drawn to the traps but this method works if you are simply trying to protect a small area. Very effective nontoxic approach for the home gardener. Double positive traps some keeps others away. It may not drastically affect populations but it can keep them out of certain areas. Please pass this along before home gardeners use insecticides in their garden being this also kill our beloved beneficial insects.
Signature: Justin Brown

Hi Justin,
Thanks so much for the tip.  It is a few months before we expect to get reports of Japanese Beetles, but we will do our best to feature your tip beginning in June.

Letter 9 – Spined Soldier Bug eats Japanese Beetle

Stink Bug Eating Japanese Beetle
You’re site is terrific – I use it all the time. I’m always looking for ways to rid my yard of Japanese Beetles, so I thought that was wonderful: it looks like a Podisus (?) feasting on the beetle. Do they actually kill their prey or scavenge? I’ve never seen dead japanese beetles laying around like this except where I did the handiwork! It’s from Fort Wayne, Indiana. Thanks much!
Adam

Hi Adam,
You have correctly identified your Spined Soldier Bug, a Predatory Stink Bug in the genus Podisus. Predatory Stink Bugs are true predators, and not scavengers. They need a liquid diet, so they only suck the fluids from the prey, leaving behind a drained dry husk. Gardeners plagued by invasive exotic Japanese Beetles would probably love to be able to purchase Spined Soldier Bugs, and we read on BugGuide that: “P. maculiventris is sold as a biological pest control, and appears to be the most common species in the southeastern United States.”

Letter 10 – Virginia Ctenucha and Japanese Beetle on Milkweed

Found on a Milkweed flower
Location: chicago il area
August 15, 2011 10:55 am
Hi,
Thanks for your site. I am unable to identify this one. It was found on a milkweed bloom. HELP 🙂
Thanks – Steve

Hi Steve,
WE doubt if you need an identification of the Japanese Beetle buried in the blossoms in the upper left of the inflorescence, but perhaps you do.  We suspect you want the identification of the Virginia Ctenucha, a diurnal Tiger Moth.  You can see BugGuide for additional information on the Virginia Ctenucha.

Thanks Daniel,
I adjusted the post on my wordpress site to indicate your help identifying this moth.
Thanks – Steve
http://photoindex.wordpress.com/2011/08/15/bug-on-a-milkweed-bloom-need-to-identify-this-one/