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Your guide to Japanese beetles

By: Garden Gate staff
You can save your garden from these pests! We’ll show you how. It starts with identifying the beetle and its grub.

One day your zinnias are colorful and bright. The next, you notice gnawed and ragged petals. Upon closer examination, iridescent green bugs, like the ones in the photo above, are tucked way down in the blossoms. Japanese beetles (Popillia japonica) have found your garden! This pest is working its way across North America. So even if you don’t think your area has them, it’s wise to be on the lookout.

Keep scrolling to learn more about the beetles, plants they love and ways to control them.

ht-dwp-japanese-beetle-diagram LG

It’s not easy to spot the Japanese beetle at first. That’s because much of its life cycle takes place underground. After the egg hatches in summer, the grub feeds on roots until the following summer when it emerges as a shiny beetle. Here are the stages in the lifecycle of a Japanese beetle.

1) Time to wake up

After wintering deep in the soil, grubs hatched the previous summer rise to the surface in late spring to feed on roots. This is a good time to apply pesticides.

2) Ready for a change

In late spring, the grub forms a pupa 1 to 3 in. deep in the soil. In a few weeks, the adult hatches and crawls to the surface.

3) Looking for food and a mate

Adults emerge in midsummer to eat flowers and foliage. They release pheromones, or odors, that attract other Japanese beetles for mating.

4) Lots of eggs

After mating, females burrow 2 to 4 in. into the soil to lay 1 to 5 eggs. Then they return to the plants to feed and mate. This cycle continues during the heat of summer until 40 to 60 eggs have been laid.

5) Hungry babies

Eggs hatch in 8 to 14 days, and the small grubs feed on roots and other organic materials. As they grow, they shed their skin several times. Pull back sod in late summer. If you find grubs like the ones in this photo below, it’s a good time to work on a control.

ht-dwp-japanese-beetle-grub

6) Back to sleep

As autumn temperatures drop, the 1-in.-long, fat grubs go 6 to 18 in. into the soil to spend the winter.

Grub damage

In spring and late summer, the larva, or grub, can be as destructive as the adult, finding the roots of your turf to be the perfect food. Without a full root system, grass struggles to gather enough water to survive. One of the best ways to determine grub activity is shown in the photo below. If you can easily pull turf away like a rug and discover grubs underneath, it’s time to work on a control.

Late spring to early fall, coinciding with the times grubs are near the soil surface, are the best times to apply a control. If you prefer a chemical, look for either a spray or a granular material that contains imidacloprid or halofenozide.

Want an organic control? Milky spore, a bacteria you mix with water and sprinkle on the lawn, has killed grubs in some regions. However, studies are still out as to how effective it really is in all areas. If you want to try milky spore, many garden centers carry it or you can purchase it online from Gardener’s Supply Company at www. gardeners.com.

Adult damage

In summer, adults hatch and emerge from the soil to start feeding and mating. Early morning, when they’re still sluggish from the cool night temperatures, is a good time to pluck or shake the beetles from plants. You can squish them or drop them into warm soapy water for a quick kill.

If you’re having a hard time staying ahead of the beetles, you may have to make changes to your garden. Some plants tend to be favorites; others don’t seem to be attractive to them. Scroll down to find out which are which.

What's in your garden?

Japanese beetles feed on lots of plants. But it helps to know their likes and dislikes. If you plant more of the ones they won’t eat and avoid plants from the list of favorites, they may skip your garden altogether!

Beetles won't eat these

  • Coral bells Heuchera spp. and hybrids; Perennial; cold zones 3 to 9, heat zones 9 to 1
  • Coreopsis Coreopsis spp. and hybrids; Perennial; cold zones 3 to 9, heat zones 9 to 1
  • Delphinium Delphinium spp. and hybrids; Perennial; cold zones 3 to 8, heat zones 8 to 1
  • Foxglove Digitalis spp.; Perennial; cold zones 3 to 8, heat zones 8 to 1
  • Hosta Hosta spp. and hybrids; Perennial; cold zones 3 to 9, heat zones 9 to 1
  • Impatiens Impatiens spp. and hybrids; Tender perennial; cold zones 9 to 11, heat zones 12 to 1
  • Lantana Lantana spp. and hybrids; Tender perennial; cold zones 8 to 11, heat zones 12 to 1
  • Nasturtium Tropaeolum majus; Annual; heat zones 12 to 1

Here are some of their favorites

  • Clematis Clematis spp. and hybrids; Perennial; cold zones 3 to 9, heat zones 9 to 1
  • Dahlia Dahlia hybrids; Tender tuber; cold zones 8 to 11, heat zones 12 to 1
  • Hibiscus Hibiscus spp. and hybrids; Shrub, perennial or tropical; cold zones 4 to 11, heat zones 12 to 1
  • Hollyhock Alcea rosea; Biennial; cold zones 2 to 10, heat zones 10 to 1
  • Japanese maple Acer palmatum; Tree; cold zones 5 to 8, heat zones 8 to 1
  • Linden Tilia spp. and hybrids; Tree; cold zones 2 to 8, heat zones 8 to 1
  • Rose Rosa spp. and hybrids; Shrub; cold zones 2 to 10, heat zones 10 to 1
  • Sunflower Helianthus spp. and hybrids; Perennial or annual; cold zones 3 to 9, heat zones 12 to 1
  • Zinnia Zinnia spp. and hybrids; Annual; heat zones 12 to 1

More ways to control the beetles

ht-dwp-japanese-beetle-trap

Not ready to start swapping out plants? There are traps to attract adult beetles. Traps, like the one in the photo above, have good points, but the cons may outweigh the pros. They’re baited with floral scents and sex pheromones to attract Japanese beetles. The bait is so attractive that it tends to draw beetles from as far away as 1,000 ft. Instead of solving the problem, a trap could make it worse unless you can put it far enough away from anything you don’t want the beetles to eat.

And systemic insecticides, such as those containing imidacloprid or thiamethoxam, are absorbed into the plant, giving long-term protection to ornamentals. Contact insecticides like carbaryl, sold as Sevin, can be applied as a spray or a dust. They stick only to the surface of the flower or leaf and need to be applied more often.

Where Japanese beetles have been around for years, natural predators are helping keep them in check. But in areas where they’ve just arrived, these pests can be voracious eating machines. It’s time to prepare your defenses before they devour your garden!

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