It’s that time of year again – adult Japanese beetles (Papillia japonica) have emerged! In short time, you’ll likely notice significant feeding damage from these pesky beetles. We field a lot of concerns about the Japanese beetle each year, but oftentimes they are not detrimental to plants.
Your plants will likely survive damage
Japanese beetles are considered one of the worst landscape pests in the U.S. as they can cause damage in both their larval and adult life stages. As larvae, Japanese beetles will feed on the roots of trees, shrubs and turfgrass; as adults they feed on the flowers, fruit and foliage of more than 300 types of plants. Linden trees are one of their favorites and it’s not uncommon to see dozens of adult beetles devouring a single plant at a time. Foliage is characteristically consumed by eating the tissue between the veins of the leaves, known as skeletonization. Plants will often turn brown and look dead due to heavy feeding by adult beetles. This may sound bleak, but don’t worry! While heavy feeding and defoliation of otherwise healthy trees may cause them to look bad, this will not kill the tree. Trees will be stressed but often recover the following spring.
Japanese beetles have an annual life cycle. Eggs are laid in moist sod near the soil surface July through August. Shortly after, eggs hatch and the larvae burrow underground. Japanese beetles spend most of their lives as a large white, “C” shaped grub approximately 1.25 inches in size. Grubs remain in the soil over winter and the next generation of adults begin emergence in June with peak emergence mid-July. Populations are highly variable from place to place and across years.
How to protect your landscape from Japanese beetles
Spend your money on watering
Control of Japanese beetles is difficult! Both adults and larvae cause damage and since adults can fly up to 15 miles in search of food, controlling one life stage does not prevent problems with the other. If you cannot tolerate the cosmetic damage caused by Japanese beetles, your best bet is physically removing or excluding them. Plants are more resilient to pests and disease if they are properly selected for their location and if they are cared for properly (watered).
Choose plant varieties that are more resistant to feeding damage. While adults are known to feed on 300+ varieties of plants, some are more susceptible to damage from the beetle including: roses, grapes, linden, sassafras, Norway maple, and Japanese maple. However, some varieties are more resistant such as Sterling Silver Linden or Northern Red Oak (See Table 1: Landscape Plants Likely to be Attached by Adult Japanese Beetles and Table 2: Landscape Plants Seldom Damaged by Adult Japanese Beetles from University of Kentucky College of Agriculture for more information: http://entomology.ca.uky.edu/ef451)
Physical Removal & Exclusion
For small plants, it may be practical to remove adult beetles by hand, especially if populations are small. Simply shake beetles into a bucket of soapy water early in the morning when temperatures are cooler and beetles are less active. Highly valuable plants can be covered with netting or cheesecloth to protect them from feeding damage during peak feeding activity. These strategies may be impractical if the plant is too large (like a tree) or you are trying to protect a large area.
Japanese Beetle Traps:
You’ve probably seen these at your local garden store. There are several types, usually with yellow vanes and a baggie containing pheromone lures. These actually draw more beetles to your yard than they trap, often INCREASING feeding damage on plants in your landscape. Not to mention, these traps smell awful when full. We’d suggest saving your money, but if you use a trap, place it as far away from your most prized plants as possible.
Pesticides are usually unnecessary or ineffective
“Anytime you can do nothing is best. Leave nature alone.” – Chuck Conner, Community Forester, Missouri Department of Conservation
Rather than searching for a poison to fight against nature, we recommend Integrated Pest Management (IPM). IPM is a more holistic, ecosystem-based, strategy that focuses on long-term prevention of pests or their damage through a combination of techniques that minimize risks to human health, beneficial insects (like bees!), other non-target organisms and the environment. Pesticides are used only as a last resort after monitoring indicates they are needed and they are applied according to established guidelines. A good IPM plan uses a combination of biological, cultural, mechanical and/or chemical controls to keep pest populations below tolerable levels. Because tolerance for damage varies among individuals, strategies will change and no program will look exactly the same.
There are many products labeled for use on Japanese beetles, but most are ineffective. Using grub killer will not protect plants from adult feeding and spraying for adults will not protect your turf from grub damage later in the season. If you’d like to explore chemical controls, we suggest reaching out to your local extension agent to learn more about your options and timing. Remember to always read and follow the label when using a pesticide – the label is the law!
The Japanese Beetle – Iowa State University Extension
Managing the Japanese Beetle: A Homeowners Handbook – USDA APHIS
Japanese Beetles in the Urban Landscape – University of Kentucky Entomology
Japanese Beetles – Missouri Department of Conservation