How to rid your garden of Japanese Beetles

The gardening season in Vermont is short enough that leaving home for more than a few days means I might miss the emergence of something delicious or beautiful. After a recent summer vacation, I was excited to get back into the garden and see what had popped up in the week since I’d left. What I came home to, however, was disheartening. All that remained of my previously-healthy pole bean plants were the lacy outlines of leaves. A quick examination revealed the culprit: Japanese beetles. Those voracious munchers had gone to town on my bean leaves and did enough damage in that short time to render my garden bean-less. Buggers! 

Identifying them

Japanese beetles made their way to this country in the early part of the 20th century via nursery stock from their native Japan. Like dozens of other non-native species, they spread quickly due to the absence of natural enemies. Unfortunately, these destructive insects are now found in every state east of the Mississippi. The adult beetles, easily identifiable by their ½ inch-long metallic blue-green bodies with copper-colored wings, will consume the flowers and leaves of over 30 different plant species, leaving only skeletal remains and stems. Roses are a favorite. The larvae are plump, c-shaped grubs with dark heads that spend most of the year underground and feed on the roots of many plants, including turf grasses. If your lawn has irregular patches of dead or dying grass, Japanese beetle grubs could be the culprit.

Understanding their lifecycle

Understanding this insect’s 2-year life-cycle is key to successfully getting rid of them. Adult beetles actively feed and mate throughout summer. Females lay up to 60 eggs, usually on grass. Eggs hatch into larvae within 2 weeks. Larvae feed on grass roots until cold weather arrives and then burrow themselves deeper into soil to spend the winter.  Over-wintering larvae make their way toward the warming surface of soil in spring, where they pupate and then emerge as hungry adult beetles by late June.  

Ridding them from your garden safely

Japanese beetles release chemicals that attract others, and they fly. If you see any at all, chances are there will be more arriving soon. Acting quickly at the first sign of adult beetles is key to controlling the population. There are a whole host of products that are marketed to frustrated gardeners wanting to eliminate these pests, and not all of them are effective or safe to use. Chemical pesticides are available that target both adult beetles and grubs, but those pesticides also kill beneficial insects like bees and earthworms. Plus, who wants chemicals on their plants and in their soil? A multi-tiered plan of attack utilizing a combination of natural control methods is your best bet.  Here are just a few options:

1. Pick ’em or shake ’em

Adult beetles can be hand-picked off of leaves of plants they are feeding on or shaken off of plants onto a ground cloth. Dislodged beetles can be tossed into a bucket of soapy water. The best time to manually remove beetles is in the morning, when they are at their slowest and are less likely to fly away upon contact.

2. Neem oil

Neem oil is a commercially available tree seed extract that when sprayed onto plants can provide short-term control of adult Japanese beetles, aphids, and many other pests. Pyrethrin, the active ingredient in the oil, is toxic to leaf-eating insects but all-natural and safe for humans and wildlife. Insects that suck or munch on the oil-sprayed leaves are either repelled by its bitter taste or die from ingesting the treated leaves. Beneficial insects like bees and ladybugs do not ingest the leaves of plants so they are generally unharmed.

3. Floating row cover

Row covers are useful for protecting plants from cold, wind, sun, and insect damage. Drape directly over plants or use with hoops or frames.

4. Beneficial nematodes

Perhaps the most safe and effective defense against Japanese beetle grubs beneficial nematodes are microscopic parasites applied using sprays or soil drenches that will actively attack larva in soil with no harmful effects on plants, earthworms, animals or humans. As they are living creatures, it is crucial to apply them with the perfect environmental conditions needed for their survival: warm and moist. Sufficiently moisten the application site both before and after applying the nematodes and use them when soil temperatures are between 55-90 F (13-32 C). The ideal time to apply nematodes is spring, before beetles emerge. 

5. Phermone traps

Adult Japanese beetles emit pheromones that attract other beetles. Commercially available pheromone traps are effective at attracting and killing adult beetles, but research indicates that the traps may actually attract more beetles than they kill.

6. “Clean” your soil

As winter weather sets in—but before the ground freezes—gently turn up the first few inches of your soil, in beds and around fruit trees, with a border fork to bring grubs toward the surface. (This is a also a great time to work some compost into the soil!) Now exposed, they will meet their demise via the grip of frost or the beaks of hungry birds.

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