Master Tips from a Master Gardener (9/21/17)

Bill Koellner · Wednesday, September 20, 2017
This imported pest was detected in New Jersey in 1916, having been introduced from Japan. The adult beetles feed on over 400 species of broadleaved plants, although only about 50 species are preferred. The grubs will also feed on a wide variety of plant roots including ornamental trees and shrubs, garden and turf grasses. They seem to especially relish Kentucky bluegrass, perennial ryegrass, and tall fescues. The adults are skeletonizers, that is, they eat the leaf tissue between the leaf veins but leave the veins behind. The attacked leaves look like lace that soon withers and dies. The adults will often attack flower buds and fruit. The grubs can kill small seedling plants but most commonly damage turf. The turf first appears off-color as if under water stress. Large populations of grubs kill the turf in irregular patches.

The life stages of the Japanese beetle are typical of white grubs. The white oval eggs from the beetles are usually about 1/16 in size. The beetles lay the eggs in the soil where they absorb moisture and become more roundish.

The adult beetles are a brilliant, metallic green color, generally oval in outline. Larvae that have matured by June as it takes about 1000 degree days of heat and the adult beetles emerge from the last week of June and last through July, and occasionally early August. On warm sunny days the new beetles crawl onto low growing plants and warm for a while before taking flight.

The first beetles out of the ground seek out suitable food plants and begin to feed as soon as possible. These early arrivals begin to release a pheromone or odor which is attractive to adults that emerge later. These odors attract additional adults to gather in masses on the unfortunate plants first selected. Newly emerged females release an additional sex pheromone which attracts males. The first mating usually takes place on turf.

After feeding for a day or two, the females leave feeding sites in the afternoon and burrow into the soil to lay eggs at a depth of 2 to 4 inches. Females may lay 1 to 5 eggs scattered in an area before leaving the soil. These females will leave the following morning or a day or two later and will return to feed and mate. This cycle of feeding, mating and egg laying continues until the female has laid 40 to 60 eggs. About 95% of a population is generally laid by mid-August. This information is from research at Midwest universities.

If the soil is sufficiently moist, eggs will swell in a few days. Egg development takes only 8 to 9 days at 80 to 90 degrees F or as long as 30 days at 65 degrees F, based on studies at Iowa State. The first larvae dig to the soil surface where they feed on roots and organic material. If sufficient food and moisture are available, the first instars can complete development in 17 days at 78 degrees F or as long as 30 days at 68 degrees F. The second take 18 days to mature at 78 degrees F and 56 days at 68 degrees F.

While this development is occurring, grubs may tunnel laterally in search of organic matter and fresh roots. This creates a very spongy feel to the soil and turf. Generally most of the grubs are ready to dig into the soil to hibernate. The grubs burrow 4 to 8 inches into the soil as cold temperatures arrive. At this depth, the soil rarely gets below 25 degrees F and the grubs survive with no difficulty. If the soil begins to cool further, the grubs may dig deeper. The grubs return to the surface in the spring as the soil temperature warms. Generally the grubs can be expected to be active at the surface when the surface soil temperatures are about 60 degrees F.

Since the eggs and young grubs are very susceptible to dry soils, do not irrigate during the time the eggs and first larvae are developing. However, if natural rainfall occurs, this tactic will not work. Do not plant trees and shrubs that are highly attractive to adult Japanese beetles near turf.

Trees and shrubs most attractive to adults include: Japanese and Norway maple, birch, pin oak, horse chestnut, rose of Sharon, sycamore, ornamental apple, plum, cherry, rose, mountain ash, willows, lindens, elms and Virginia creeper.

Trees and shrubs rarely attacked include: red and silver maple, holly, boxwood, euonymus, flowering dogwood, cedar, juniper, arborvitae, red oak, tulip tree, magnolias, red mulberry, forsythia, privet, lilac, spruces, hydrangeas and yew.

The use of mechanical control in trapping has been developed to capture the adults. These traps generally use a mixture of sex pheromones. Those who use the traps indicate that these traps do not significantly reduce grub populations and in some cases may actually contribute to increased foliar plant damage. There has been no correlation between trap captures and reductions in white grub populations in surrounding turf grass areas. In fact, the traps will invite the neighbor’s beetles to your yard.

The use of chemical controls is best done by contacting your local garden center where they have the best pesticides. When using trapping to monitor adult activity, keep in mind that the females lay the majority of their eggs within the first 7 to 10 days of their existence. What are those green garden beetles? Here are tips on how to identify and get rid of Japanese beetles.

Unfortunately, the most effective way of getting rid of Japanese beetles are to hand pick them. It’s time consuming, but it works, especially if you are diligent. When you pick them off, put them in a solution of 1 tablespoon of liquid dishwashing detergent and water, which will cause them to drown.

Neem oil and sprays containing potassium bicarbonate are somewhat effective, especially on roses. The adults ingest a chemical in the neem oil and pass it on in their eggs and the resulting larvae die before they become adults. Neem can be harmful to fish and should be reapplied after rainstorms.

You can buy Japanese beetle traps of all sorts, but most are no more effective than a can of fruit cocktail. Open the can and let it sit in the sun for a week to ferment. Then place it on top of bricks or wood blocks in a light-colored pail, and fill the pail with water to just below the top of the can. Place the pail about 25 feet from the plants you want to protect. The beetles will head for the sweet bait, fall into the water, and drown. If rain dilutes the bait, start over. 

Japanese beetles are attracted to geraniums. They eat the blossoms, promptly get dizzy, fall down, and permit you to dispose of them conveniently with a dustpan and brush. Plant geraniums close to more valuable plants which you wish to save from the ravages of Japanese beetles. 

I have noticed that the very dark colored leaves attract the beetles first, like our ornamental plum, while adjacent to the plum is a serviceberry bush and it was not affected. In addition, our plants that have a waxy surface are not phased by the beetles. This year has been the worst year for the beetle population in our neighborhood.
More Opinions
©2017 West Liberty Index | Web Development by Brian McMillin, LLC