No way to sugar coat it: Japanese beetles (Popillia japonica) are terrible for plants and gardeners. And I mean Terrible, with a capital “T,” that rhymes with “D,” which stands for Disgust, Detest and Despise.
Count your blessings and enjoy each minute you have this summer and every outdoor growing season that your landscape is mostly JB free. Their plant-munching march is making progress to new neighborhoods every year. The latest are in Jefferson County and communities north to Fort Collins.
I have had Japanese beetles in my yard for at least 10 years. The first couple of years was not difficult to manage; it was easy to dunk and drown these iridescent green-copper winged beetles into a small cup of soapy water in the morning or evening. Each new gardening season, however, more insects arrived and I found my negative attitude toward them became almost personal. They voraciously ate my plants, and I detested their intrusion into my gardening enjoyment. What was I to do?
Please join me in a deep breath as we tackle the understanding of how to manage these small beetle beasts going forward. We are halfway through their summer eating carnage, and there are deeds for their destruction we still need to do.
Just the facts
Numerous online sources recommend practices, sprays and trapping of Japanese beetles that simply do not match the research of what actually works for homeowner management. Avoid these sites if possible; the false hope, time and money spent compared to the end results can be disappointing.
Japanese beetles arrived on the East Coast in 1916 and got well-established in a few decades before moving to the Midwest and eventually to Colorado in the new century. Gardeners in these areas continue to battle them every summer like we do; you may have grown up with them before coming to Colorado.
Fortunately, there have been decades of university and government research on what methods and products produce results, ie., fewer beetles. My attitude about dealing with Japanese beetles got much better once I understood what the experts learned and have shared with us.
Japanese beetle refresher
Japanese beetles have a one-year complete life cycle from egg, larva, pupa, adult. Adults usually emerge from grass turf where they finished their underground life cycle in June to early July. Once out of the ground, female beetles fly to nearby plants for a quick lunch before mating (sometimes they skip lunch and mate first).
Through their short life as adults (approximately two months), females burrow 3 inches down in the grass soil and lay up to 60 eggs (not all at once). They like well-irrigated lawns. Cool-season grasses like Kentucky blue grass, ryegrass and fescue — the majority of grasses grown along the Front Range — are their preferred turf for egg-laying. Eggs grow to the white grub (larvae) stage in a couple of weeks and will continue growing and living in turf roots until next spring, when they emerge as adults for the cycle to begin again.
Winter white grub kill from cold and snow is minimal; to avoid severe cold, larvae will move deeper in the soil, and do the same to find moisture in dry winters.
We know the damage that adult Japanese beetles cause: lacey chewing patterns on foliage, tearing and defoliation on many flowers and fruits (over 300 plant species, including agricultural crops). The white grubs need to eat, too, so they dine on turf roots as they grow to their maximum size and weight to help them survive through the winter.
Adult Japanese beetles can fly up to 5 miles looking for more eating opportunities in new neighborhoods. The volatile oils released from chewed plants attract more adults to the plants being eaten. Their lives are simple, really: They are late-morning loungers on plants, so they don’t get started eating, mating and egg-laying until it warms up later in the morning. They idle down again in the evening after several hours of plant destruction and procreation. Morning and evening, when they are sluggish, are the best times to tap them into soapy water or simply squish to your heart’s desire (I wear gloves).
Effectively reducing Japanese beetle numbers in landscapes every summer requires addressing both life stages: adult beetles on plants this summer and their offspring (white grubs or larvae) in lawns, next summer’s generation.
Garden practices to reduce adult and white grub numbers in August and early September
If you are to the point where adult Japanese beetle numbers are out of control, and drowning or hand crushing is no longer a workable daily option, consider other controls. The non-chemical ones include removing plants they devour and avoid planting ones they will eat. Taking out plants, even cool-season lawn grass, is a big step and could be costly and sad when you love the same plants they attack.
Plant coverings like bridal veil can work for the weeks in the summer when they are doing the most damage. I covered those options in a previous column.
RELATED: Japanese beetles are heading back to our gardens. Here’s how to fight them off.
Keep lawns taller (3 or more inches high) to deter female egg-laying. Eggs need moisture to grow to the white grub stage. Keeping the lawn on the drier side during July and August can lead to egg mortality. Be careful where tree roots are growing under and throughout lawns; reducing water too much can be a detriment to tree health. Resume regular watering and maybe a bit more in September to help dry lawns recover.
Beetle traps
The pheromone lures in traps are highly attractive and will draw in beetles from long distances. But researchers say to not use them. Why bring more beetles to your yard and on your plants? I think traps are like offering free, fresh-baked apple pie on the patio all summer. Friends (beetles) will quickly show up for a piece or two, then invite some of their friends and neighbors to enjoy a slice. And then what happens? They never leave, since the word is out that your house is the place for free eats.
While it is gratifying to see the number of dead beetles in the trap and tempting to put one up each summer, just say “no.”
It would make sense for everyone with a lawn impacted by Japanese beetles to consider killing laid eggs every summer. This was the plan successfully carried out in Palisade a few years ago. They got ahead of the beetle and the possible destruction of their grape and peach industries.
They held planning meetings that included businesses, municipalities, homeowners and state and local officials. The plan agreed upon by all residents and entities included the use of products to kill adult beetles, eggs and larvae. Lawns were watered less where feasible. Beetle traps were used to keep track of where severe outbreaks were happening and to gauge when the numbers had waned enough to declare the program a success. (Read more about their eradication plan here.)
Will the Palisade model of ridding Japanese beetles work in metro Denver, or anywhere else along the Front Range where they have taken hold areas? Probably not. Our urban areas provide a banquet of plants they like to eat. Our irrigation practices are necessary to keep our lawns growing and green through hot summer months. We have set the table for them with a very hospitable place for their survival.
There are a handful of effective recommended adult beetle and grub control products on the market for homeowner purchase at garden centers and online. Licensed lawn care and landscape companies also offer Japanese beetle management services. Use caution with products, since some can harm pollinators and beneficial insects. Apply products late in the day when the good bugs have gone home for the night. For researched insecticide and biological control options, click here.
Apply lawn products through mid-August to kill eggs and young larvae. Most products will be effective for several weeks, so only one lawn treatment is needed per summer. Use care and read all package instructions before applying lawn products to treat white grubs. Protect bees by first mowing away blooming white clover or dandelions in lawns before treatment.
When the final Japanese beetle has fallen dead sometime in September, smile and enjoy the rest of the fall season. There is hope on the far horizon to battle this ravenous foe with the controlled release of three beneficial agents that specifically target Japanese beetles and white grubs. I’ll be writing more on this soon; stay tuned.
More beetle resources
- Biology of the Japanese beetle.
- Japanese beetle management Colorado.
- Questions and answers about Japanese beetles in Colorado.