Controlling Japanese Beetles

Japanese Beetle Populations Seem to Have Exploded Recently

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Just when you think maybe they are not coming back this year, you find them devastating a group of plants.  And not just roses!  Japanese Beetles enjoy feasting on a wide variety of plants.  Although we may never be completely rid of them, there are things you can do to control them and minimize the damage.

The Beetles and their Life Cycle

Japanese Beetles are 3/8 to 1/2 inch long and about 1/4 inch wide.  Their rounded backs are shiny metallic green and with copper wings.  Each female can lay about 50 eggs each season, usually in the soil.  The eggs are tiny white ovals that may expand and become rounder if there is enough moisture in the soil.  The eggs hatch grubs that begin to feed.  As the soil gets cold in the fall they burrow deeper into the soil to avoid freezing.  Once matured, the grub enters the pupae stage when it transforms into a beetle.  Initially the beetle is cream colored and ages to reddish brown.  Once an adult beetle, they emerge from the ground in May or June and live for a month or two.  Now they need to eat, and they also notify other emerging beetles where the food is.  They begin the constant cycle of eat, mate and produce eggs.

What do Japanese Beetles Eat?

Adult Japanese Beetles eat the foliage of a wide variety of plants, shrubs and trees.  They seem especially fond of roses, grape vines, apple and crabapple trees, Japanese Maple trees, Norway Maple trees, Pin Oak tress, birch trees, linden trees and raspberries.  But they are not fussy, eating as many as 300 species of plants.  You may find them in your vegetables, annuals, or any shrub, perennial or tree.  Typically they work on the foliage, leaving leaves looking lacy with holes and eventually leaving only a leaf skeleton.  Their favorite food seems to be rose buds, which they burrow into and eat from the inside out.

The grub larvae feed on plant roots and organic matter in the soil.  They particularly like to eat under turfgrass where roots are abundant.  They cause dead patches in the lawn that can be picked up like freshly laid sod.

What Can You Do to Control the Japanese Beetle?

You never know from one year to the next whether a few Japanese Beetles will come to feast, or hoards of them.  Most gardeners these days prefer to avoid chemicals if possible, so depending on the severity of the infestation and damage being done there are a variety of measures that can help.  Always start with the easiest, least expensive and least damaging to the total habitat in your garden or landscape.

Soap:  Pick or knock the beetles off the plants and into a container of soapy water.  If you catch the first few, they may not have notified their friends yet about where the feast is.  Or spray the beetles with insecticidal soap.  Insecticidal soap must directly contact the beetle to be effective.

Plants:  Avoid the plants the Japanese Beetles enjoy most, and instead plant things they do not care for. This may not be entirely possible but perhaps you can interplant with repellant plants such as garlic or chives with a strong odor.  Typically they will not feed on ageratum, arborvitae, ash, baby’s breath, begonia, bleeding heart, boxwood, caladium, carnations, chinese lantern, columbine, coralbells, coralberry, coreopsis, cornflower, daisy, flowering dogwood, dusty miller, euonymous, false cypress, firs, forsythia, foxglove, hemlock, holly, hydrangea, juniper, lilac, lily, magnolia, red and silver maple, nasturtium, red and white oak, pansy, pine, poppy, snapdragon, snowberry, speedwell, sweet William, tulip tree, violets, and yew.  Keeping your plants strong and healthy is also a good defense against the damage that is done.

Cover:  Covering your plants with lightweight floating row covers is very effective,  But for most gardeners, completely unacceptable.  Defeats the purpose of growing beautiful plants if they have to be covered up for two months.  Covering plants may be more acceptable in a vegetable garden, but can make care and harvest inconvenient.  Keep in mind that the Japanese Beetle does not kill your plant, but complete foliar devastation can certainly weaken the health of your plants.

Japanese Beetle traps are not mentioned first because of environmental or chemical concerns, but because they may not be effective, they quite simply attract beetles.  They were originally used to collect and study the beetles.  These traps quite simply invite the beetles over for dinner so if you try them, hang as far away as possible from the plants you are trying to protect..  Many gardeners have been fooled by these traps, thinking that they are catching hundreds of beetles that would have been devastating their gardens.  The ones that didn’t get trapped, ARE now devastating their gardens.  However, if all your neighbors are also using traps and they are positioned away from plants, they HAVE been found to be effective.  Still a lot of studying to be done here.

Neem Oil: A naturally occurring pesticide, neem oil is extracted from seeds of the neem tree.  Neem oil can be formulated into granules, powder or soluble powder and applied to foliage.  It reduces foliar feeding and repels insects, as well as interfering with insect hormone systems, making it difficult to grow and lay eggs.  The process takes time and will not immediately kill on contact, but in the meantime the repellent will minimize damage to foliage.  Neem oil is also effective on aphids, mealybugs, white files and mites, but will not harm beneficial insects such as bees or any other insect that does not eat the leaves.  Neem oil has the added benefit of killing harmful fungus such as powdery mildew, rust and black spot.  Be sure to follow directions carefully and test on a few leaves of each plant.  Certain plants can actually be killed by neem oil, especially if applied too heavily or in full sun.  Some gardeners do consider neem oil a chemical that they will not use.  It is in fact a natural occurring chemical, not man made.  Each gardener needs to draw their own lines on the subject.

Grub Control:  If you have had repeated infestations, treat the soil and turf with grub control.  But keep in mind that they could be over in the neighbors soil, so it may or may not be effective.  A non chemical treatment is bacterial milky disease (Milky Spore).  It will take two or three years for the spores to build up that deter the grubs, and you can not use any insecticide while the bacterium complete their cycle.  This treatment is still being studied.  The effects of controlling grubs with natural nematodes and bacteria are effective over a long period of time, whereas chemical control is only immediately effective so repeated application is necessary.  Be sure to check with your local garden center of university extension service for the best information about natural grub control.

Chemical Pesticides:  Several chemicals are available that kill insects on contact, and some have a residual effect.  Some are “safe” for fruits and vegetables, in other words it won’t kill you if you eat it, but it is still a toxic chemical.  Systemic insecticides for shrubs and trees are taken up by the plant roots and must be applied every year.  Chemical grub controls to apply to lawns and soil are also available.  The chemicals work, there is no doubt about it.  But they kill whatever insects they come in contact with both good and bad.  If you have any animals that nibble the leaves, the chemicals can also be harmful to them.  If your infestation is so bad that you resort to toxic chemicals, at least be informed and use caution.

Go to the next article to see my best and easiest method!

Sharon DwyerComment