japanese beetle

Plant Care

The Japanese Beetle


Japanese Beetles (Popillia japonica)

Is something eating your roses? Or maybe your grapes? Or your apple trees? Is your grass yellowing? Chances are, it's a Japanese Beetle. These beetles are very showy- bright greens, purples, browns, and very shiny. They also like to hang out in clusters together.

Life Cycle:

Japanese Beetles have a one year life cycle. In that one year, they can do quite a bit of damage. They start as larvae living in the roots of grasses, feeding on their roots until around June when adult beetles emerge.

When the adult beetle emerges from the grass, usually around June, they need something new to feed on. They usually opt for roses, grapes, Virginia creeper, raspberries, apples, crabapples, some varieties of maples, as well as a few other varieties of trees. They are usually feeding from June to August.

During this time as well, the female beetles head to moist turfgrass and deposit their eggs a few inches below the surface of the soil. In a 4-8 week egg-laying period, a female Japanese beetle can lay up to 60 eggs.

The eggs stay buried below the roots until they hatch into larvae around September. The larvae will then move to eat the roots of the grass until winter when the temperature is too cold and activity ceases. Once the weather heats up past 60 degrees they go back to their feeding frenzy. At this point, the cycle continues.

Japanese Beetle Damage:

The damage that the larvae do to the turfgrass usually mimics that of drought stress. Even if the grass is getting plenty of water the larvae feed on the roots causing the roots to be unable to take up any water. This can look like the grass is yellowing and drying out. If the damage is too severe, patches of turfgrass can die.

The adult beetles feed on the foliage, buds, and flowers. The chewing pattern from the Japanese beetles is categorized as skeletonizing. This means that the insects are chewing little holes in every part of the leaf but the veins. When they munch the flowers, they leave little holes until there is simply nothing left. While this damage is mostly cosmetic, if the leaves are too badly damaged, the plant itself may struggle to produce new growth in the future.

But what can be done?!

On Plants: There are a few ways to combat these invasive little beetles. The most organic way is to pick them by hand. This is a very simple method, especially if the infestation isn’t too extreme and can help one keep on top of their beetle problem. This should be done in the early morning when the insects are cool and can’t move as easily. One can shake the plant or knock them into a jar.

Another way to treat is to use an insecticide. Since Japanese beetles feed on flowering plants, it is important to choose an insecticide that won’t be damaging to pollinators. It is also important to spray the insecticide when pollination is at its lowest; usually early morning or late evening. Bacillus thuringiensis var. galleriae and chlorantraniliprole (Acelepryn) are both types of insecticides with very low toxicity to pollinators but will still get rid of the Japanese beetles. These insecticides are available at Fossil Creek Nursery. Please note - it is important to read and follow the labels marked on the insecticide bottles for a safe and effective application.

In Lawns: Regular mowing can help deter and rid one's lawn of the Japanese beetle larvae. The reason for this is that the root length increases with the height of the grass. The shorter the grass, the shorter the roots. The shorter the roots, the less there is for the larvae to feed on.

If significant damage has already occurred, nematodes can be released to eat the larvae. This should be done when the larvae are active and present and when the weather is a little cooler. The most effective varieties are those in the genus Heterorhabditis.

An insecticide can also be used on one's affected lawn. Most types of insecticide to treat the eggs and larvae should be applied around June when the larvae are still small and just starting to hatch. There are several insecticides that can be used (available at Fossil Creek Nursery) The insecticide trichlorfon is especially effective after some of the larvae have hatched. Remember, it is important to read and follow the labels marked on the insecticide bottles for a safe and effective application.

Sources: CSU extension
Author: Chesney Babbitt

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