OE wyrtig, adj: Garden-like, full of plants;
On anum wyrtige hamme, Homl. Skt. ii. 30:312



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Japanese Beetles
Popillia japonica


Japanese beetles

While medieval gardeners may not have had to contend with Japanese beetles, the plants in my “medieval” garden definitely do. I first saw them, in very small numbers, two years ago. Last year, we had a truly Biblical infestation – hundreds each day. This year, numbers are down, in part because our control tactics seem (knock on wood) to be working.

A season in the life of...
To manage any pest in your garden, you have to understand how it lives in its world. Beetles begin life as eggs laid about 2-3" deep in the soil. The eggs hatch into white grubs which then spend 10 months below ground -- most often in lawns, golf courses, and parks -- feeding on the roots of grasses and other vegetation.

Japanese beetle larvaTo survive, the grubs must have adequate moisture, and they will travel downwards in the soil to find it. While drought is a problem for the grubs, excess moisture is not, and they survive nicely in very wet years.

When autumn weather lowers soil temperatures to about 60 degrees, the grubs burrow down more deeply, and spend the winter months from 2" to 10" beneath the surface. In the spring, when soil temps warm again to above 50 degrees, the grubs feed for about 6 weeks, pupate, and emerge as beetles. These migrate above ground over a period of several weeks in early to mid summer to devour our plants.

Each beetle has a 30-40 day lifespan. Female Japanese beetles feed, mate, drop to the ground to lay eggs, return to the plant to feed again, and then drop to the ground to lay more eggs. Each can lay 40-60 eggs over the course of her life.

Japanese beetles are awkward fliers, but can travel as much as half a mile to find food. They eat more than 300 different plants, but in my garden, this year, I am finding them only on:

  • Sweet smelling heirloom roses (but not scentless landscape varieties)

  • Yellow (but not pink) hollyhocks

  • Verbascum thapsus "Banana Custard," but not wild Verbascum

  • Purple cone flowers (Echinacea)

  • Stanley plum (their favorite)

  • Corylus contorta

Japanese beetle on skeletanized leaf

When they find a flower or leaf they like, they alight on it and mark it with an “aggregation pheromone” or chemical signal that attracts more beetles, often in large numbers. The beetles often begin feeding about 4-5' above the ground, and work their way down. They eat flower petals, skeletonize leaves, and are quite capable of killing plants both large and small.


Going after the grubs
Because they are travelers, controlling Japanese beetles is a real challenge. One approach is to go after the grubs while they are in the soil. However, because the beetles range so far, soil treatment will be effective only if your neighbors also treat their lawns. That said, you can go after the grubs by applying:

  • Milky spore (Bacillus popillae/Paenibacillus popillia) - A bacteria that doesn't affect beneficial insects, birds, people, or other animals, Milky spore kills the larvae of Japanese beetles. A lawn that has been inoculated will build up a higher concentration of this bacteria over time, thus becoming less successful as a beetle nursery.

  • Nematodes (Heterorhabditis bacteriophora) -  These parasitic roundworms carry a bacteria that infects and kills the larva of some 200 different species, including ants, fleas, moths, beetles, and flies. It is not harmful to earthworms, nor to birds, people, or other animals.

  • Btj (Bacillus thuringiensis japonensis) - Lethal to scarab beetles, including Japanese beetles, this bacteria does not affect other insects, people, or animals.

Controlling adult Japanese Beetles

First, how NOT to control these pests
Don't use Japanese beetle traps or lures. These traps attract beetles with a combination of floral scent and aggregation pheromone. Research has found that they draw many, many more beetles to a garden than they catch and kill. One article suggests that the best way to use one of these traps would be to give it to your neighbor, so that your beetles will be lured to their garden!


The pesticides that kill Japanese beetles often kill beneficial insects as well, including honey bees. Because the beetles emerge over a period of 3-4 weeks, and because they have such a wide range, chemical controls are of limited use.


The best strategy for control in a small garden is prevention. I've had good luck with spraying plants with a solution of two cloves of garlic, 2 cups of water, and a drop of dish detergent. Blend the garlic and water together, and then let it sit for two hours before straining the mixture carefully (or it will clog the nozzle) into your spray bottle, adding the detergent, and sloshing gently to mix.

Spray plants in the morning (after the dew has dried), being careful to spray upper and lower surfaces of the leaves and flowers. Respray every 4 days or so, and after rain. This actually does seem to keep the beetles from alighting on a plant in the first place. Perhaps the garlic scent masks the aggregation pheromone.

That's really important, for once a beetle has landed on a leaf or flower, that part of the plant is marked with the bug's pheromone, which will draw more beetles even after the first is removed. Because of that, and especially if a leaf or flower has had several beetles on it, I often pluck the leaf or flower and dispose of it in a covered container.


But chances are that even with garlic spray, you'll have some beetles appear in your garden, and then the best way to keep the numbers down is to handpick the dang things. What this actually involves is knocking the beetles into a bucket of soapy water..

Some beetles drop off a plant at the first vibration, so you can  just position the bucket below the beetles and then tap the branch. Other beetles (especially later in the day, when it is hotter) will fly away when a branch is tapped. To keep them from escaping, use one hand to position the bucket below the beetles, and then cup your other hand over the bugs as you tap.

I flush the beetles in the bucket down the toilet, because if you pour the drowned bugs out on the ground, even dead they seem to draw other beetles to themselves.


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