To stop Japanese beetles in Colorado, hand-pick them into soapy water early in the morning, protect the plants you care most about with fine netting, and do not hang a Japanese beetle trap anywhere near your garden. Traps pull in far more beetles than they catch — you end up feeding your neighbourhood’s population on your own roses.
Why This Matters in Colorado
Japanese beetles are relatively new along the Front Range and they are spreading. They arrived with nursery stock and irrigated turf, and irrigated turf is exactly what they need — the grubs develop in moist, well-watered lawns, which is why they thrive in suburban Denver and are almost absent from dry, unirrigated ground.
Adults feed for roughly six to eight weeks in mid-summer, skeletonising leaves and leaving the veins behind. Crabapples, lindens, roses, grapes and Virginia creeper take the worst of it. The damage looks alarming, but a mature, established tree will almost always survive a season of it.
How to Control Japanese Beetles in Colorado
- Hand-pick in the cool of the morning. Beetles are sluggish before the day warms up. Hold a tub of soapy water underneath and tap the branch — they drop straight in. Ten minutes a day through the flight period genuinely reduces the numbers.
- Net what matters most. Fine mesh over a young grapevine or a favourite rose for those few weeks is more effective than anything you can spray.
- Do not hang traps. The lures are strong enough to draw beetles in from a long way off. If a neighbour insists on one, ask them to put it at the far end of the property, downwind and away from anything valuable.
- Water the lawn less, or less often. Grubs need consistently moist soil in mid to late summer. Letting turf go a little drier in July and August makes it a poorer nursery.
- Be cautious with sprays on anything in bloom. Most products that kill beetles also kill bees. If you spray at all, do it in the evening when pollinators are not working, and follow the label exactly.
Common Mistakes
- Hanging a trap. The most common and most counter-productive thing people do.
- Panicking about a skeletonised tree. Ugly is not the same as dying. A healthy established tree recovers.
- Spraying open flowers. You lose the pollinators and keep the beetles.
- Treating for grubs on a calendar rather than a count. Grub treatments are worth it only where damage is real and confirmed by digging a square foot of turf and counting.
Related Garden Wise Resources
- Critter Confrontations — repellents, barriers and scare tactics
- Native plants and nativars for Colorado gardens
- Twelve July garden tips for Colorado
Still have a gardening question?
Keith answers Colorado gardening questions on the radio show and online. Tell him what you are growing, where in Colorado you are growing it, and what is going wrong.
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