Japanese beetles spreading fast in new parts of the US

Japanese beetles are colonizing new areas of the US, and there is only one thing that can help you protect your garden from them

A Japanese beetle on a leaf
(Image credit: Getty/ Cappi Thompson)

New parts of the US are reporting the spread of the highly invasive Japanese beetle. First brought over to the US in 1912, the beetles quickly spread to Midwestern and Eastern parts of the country, destroying plants and thwarting garden design ideas in those areas ever since. 

However, the critters are now being reported in Washington State and parts of Colorado, with The Washington State Department of Agriculture asking residents to report sightings as far back as last year. So, what's the big problem with Japanese beetles, and what can gardeners unaccustomed to their presence do to protect their gardens?

What are Japanese beetles?

A Japanese beetle on a fern

(Image credit: Getty/Miyako Kondo / EyeEm)

Japanese beetles  (Popillia japonica) are small beetles from the scarab beetle family, characterized by shiny copper bodies and green heads. The grubs overwinter in the ground, emerging in early summer and descending in groups on a huge range of plants. The reason these beetles are so harmful is that they decimate whole plants very quickly, often in a matter of days. Anyone who has seen their rose bushes reduced to skeletons will be able to identify their damage. 

Grubs also damage lawns, feeding on turf root and encouraging further damage from digging by raccoons and other animals. If your lawn is looking patchy with unhealthy brown spots, Japanese beetles may be the cause. Check out our spring lawn care tips to make sure it's not just your lawn needing a bit of love.

The one thing you can do to reduce Japanese beetle numbers

A Japanese beetle on a flower

(Image credit: Robert Thiemann on Unsplash)

Yes, sadly, 'reduce' is as good as its gets in tackling a Japanese beetles infestation. Eric Barrett, The Ohio State University Extension educator for agriculture and natural resources, told the Tribune Chronicle (opens in new tab) that picking off the first beetles that arrive onto your plants is the only thing that works for saving the plants. You can't control their numbers with pesticides, and you certainly won't be able to eradicate them by killing the grubs in your lawn: 'While controlling these grubs in the lawn will reduce the grubs’ feeding on the roots of your lawn (if it is that bad), it will not control the thousands of other grubs in neighboring lawns.'

A Japanese beetle on a leaf

(Image credit: Getty/ Cappi Thompson)

What you need is to do is to anticipate with as much precision as possible the date of the adults' emersion from the soil: 'When they emerge as adults this summer, plan to get the first beetles (the scouts) and eliminate them before they can tell the other beetles where your good tasting plants are located. You can do this by drowning them in a cup of soapy water. Don’t squash them.' 

Adults typically emerge around the 4th of July, but this can be much earlier in different parts of the country. If you live somewhere warmer, you need to start watching for 'scouts', or the first beetles sent out in search of food, from the beginning of June onwards.

If it is roses specifically you're worried about, then consider planting Japanese beetle resistant varieties. Get more advice on how to grow roses in our guide. 

Anna is a keen urban gardener, with David Austin roses and Japanese acers among her favourite plants. She moved into the world of interiors from academic research in the field of literature and urban space a couple of years ago. She's always been interested in how people make houses into homes, and how our concepts of what's stylish change over time.