Japanese beetle

What you are looking at

Leaves eaten away between the veins, leaving a lacy skeleton behind rather than ragged holes. The beetles doing it are hard to miss once found: 10 to 12 mm long, with a metallic green body and coppery-bronze wing cases, and they feed in visible clustered groups on the sunniest and uppermost leaves of a plant rather than scattering thinly across it.

Flowers take the same treatment, chewed to shreds or reduced to bare stems, and light-coloured or scented blooms draw the worst of it. Grapes and soft fruit ripening in midsummer get irregular holes eaten straight into the flesh.

Underground, fat white C-shaped grubs feed on grass roots through late summer and autumn, and a lawn with a heavy infestation develops patches of dead turf that peel back like loose carpet, since the roots holding it down have been eaten away.

An old pest with a new front line

This is a well-established, thoroughly studied pest across most of the eastern and central United States and into southern Canada, where it arrived from Japan with nursery stock in the early twentieth century and has been a defining midsummer problem in gardens ever since.

Europe’s situation is different and more urgent. A breeding population was first confirmed in Lombardy, northern Italy, in 2014, almost certainly arriving the same way American populations did a century earlier, and it has since spread into neighbouring Ticino in southern Switzerland. Everywhere else on the continent, including the United Kingdom, remains free of it, and the containment effort treats every new find outside that Italian-Swiss pocket as an emergency, because eradication while numbers are low has worked before and stops working once a population is established and spreading on its own.

Anyone gardening outside northern Italy and southern Switzerland who finds a beetle matching this description should report it rather than treat it, since the response to an isolated find is very different from the response to an established population.

What to do about it

Where the beetle is established, shake plants over soapy water in the cool of the morning through the peak weeks of midsummer, and repeat daily. This is the most effective control available to a home gardener and it also limits the scent cues that draw in further beetles.

Net roses, grapevines and other high-value plants during the flight period rather than relying on a spray programme.

Keep the lawn on the dry side through late summer to discourage egg-laying and grub survival, and consider milky spore or beneficial nematodes as a longer-term, neighbourhood-scale reduction where they are available.

Avoid siting a pheromone trap anywhere near a plant worth protecting; it will draw in more damage than it prevents.

Outside the established range, report a suspected sighting to the national plant health authority and leave the beetle for identification rather than treating it as a garden pest to manage alone.

What it gets mistaken for

A related scarab sold under a similar common name in parts of Europe is a duller green, lacks the coppery wing cases, and feeds alone rather than in the dense working groups that mark Japanese beetle out. A crowd of identical beetles skeletonising the same shrub together, in a place it has established, is the more reliable tell than colour alone.

Leaf miner tunnels stay hidden within the leaf and leave both surfaces intact, nothing like the open lace of a beetle-eaten leaf.