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Controlling Adult Japanese Beetles

Adult Japanese beetles usually appear in mid-to late June and their feeding can virtually destroy your favorite plants overnight. We know that beetles will feast upon roses (all types), ornamental cherries, crape myrtles, linden trees and birch trees, just to name a few. Don’t waste money on those hormone traps–rather, “pretreat” your landscape plants with a systemic insecticide, known as “imidacloprid”, sold under many different brand names, including Merit® and Dominion®. Ornamental plants “protected” with imidacloprid will may exhibit a modest amount of feeding damage (“skeletonized leaves”) by Japanese beetles, but remember, the beetle has to take a few bites of leaf tissue to ingest enough of the insecticide.

Protect your trees with either of the two imidacloprid products sold by THE Turf Plan®, timing the applications around June 1st each year in St. Louis. For those of you in other states, check with your local horticultural extension department, and time your imidacloprid treatments at least 3 weeks prior to the early side of beetle emergence.

This insecticide needs to be taken up by plant roots, so it’s important to provide irrigation for the first few days after application. By keeping the soil moist for 7 to 10 days after you apply imidacloprid, you’ll encourage active root growth, and therefore, adequate uptake of the insecticide by roots. Apply Merit® 0-0-7 to the root zone of those plants most favored by adult beetles at a rate of 2 TBL/10 sq ft, or 1 cup/100 sq ft, no later than June 1st of each season. If you prefer to use the liquid concentrate, Dominion™ 2L, apply as a foliar spray or a drench. If you use Dominion as a foliar treatment, add our Kalo® DRI nonionic surfactant at a rate of 1 pack per 2 gallons of spray. Small (<15 ft tall) shrubs and trees can be protected by a foliar spray or a root drench.

Note that the timing for Japanese beetles is no later than June 1st of each summer. But this insecticide controls at least a dozen important insect pests that attack our landscape ornamentals, if applied in the early spring. Perhaps the most significant benefit of using imidacloprid is that it will control all of those boring beetles, such as the emerald ash borer, lilac borer, and the rose cane borer, just to name a few! Here’s a partial listing of the insect pests controlled: aphids, lacebugs, mealybugs, thrips, whiteflies, leafhoppers, leaf beetles, sawfly larvae, leafminers, scales and larvae of boring beetle species.

THE VERY BEST PROTECTION IS TO APPLY THE PRODUCT IN MARCH, BEFORE SIGNIFICANT BUD BREAK! It takes at least 3 weeks for the active ingredient to make it from the roots to the tops of large trees!

To protect large trees, where you can’t spray the entire crown, follow the directions below (directly from the product label).

How To Calculate Root Drench Rates

For large trees, the amount of product applied is dependent upon the girth (diameter) of the tree trunk. For small trees, you can accurately assess the diameter by “eye balling” it. For huge trees, however, you’re better off measuring the circumference, and then dividing that number by “pi” (3.14) to calculate the trees’ diameter. We’ll use an actual pin oak in Jeff’s front yard, to calculate the dose of Dominion to apply as a root drench.

  1. The drench rate for Dominion® 2L is 3 to 6 mL per “inch of trunk diameter.” Let’s err on the high side of the dose, or 6 mL per inch.

  2. Measure trunk diameter on small trees directly with a tape measure.

    1. To calculate the diameter of a large tree, determine the tree trunk circumference by wrapping a tape measure around the tree trunk at 4.5 ft above ground level. For those of you into useless trivia, the forestry industry calculates tree size based upon the diameter at breast height, or “DBH” typically at 4.5 ft above the ground. Ask any of the hot ladies in the neighborhood to stand next to the tree trunk to be sure you’re accurately measuring at DBH. Be very careful if said hot woman is NOT your wife. Try not to get slapped.

    2. Divide the circumference by pi (3.14) and that’s the trunk diameter (in inches).

    3. Multiply the that number times 6 (mL per inch), to determine the total amount (in mL of concentrate)

    4. There are approximately 30 mL per ounce, so divide the product in step 2C by 30, to convert mL to fluid ounces of concentrate. Now you know how much of the liquid concentrate needs to be applied UNDERNEATH the huge tree.

  3. You can look at the product label for different ways to apply the required amount of insecticide, but we’ll explain a very effective, and easy, way to make the soil drench. It is important to apply the drench to the entire area (360 degrees) around the tree trunk.

Jeff’s Pin Oak Example

The picture below shows the tree to have a diameter of 86 inches. Dividing 86 inches by pi (3.14) = 27.4 inches

  1. 27.4 inches times 6 mL per inch = 164 mL

  2. 164 mL divided by 30 mL per oz = 5.5 oz

  3. Jeff recommends that you add 2 oz of Dominion into our Fertilome® hose-end sprayer (a sweet little $5 sprayer that is PERFECT for this) and fill it up to the top with water.

  4. Turn on the hose and spray until the contents of the sprayer are gone, moving around under the tree’s dripline the entire time. Be sure to shake the sprayer every minute or so to keep the insecticide throughly mixed with the water. You’ll have 6 gallons of spray using the recommended sprayer, which should be enough to completely treat the soil area underneath this huge tree.

  5. When the sprayer is empty, repeat the process. Add another 2 oz of product, fill to the top with water, and repeat. Your total product applied under this tree is now 4 oz.

  6. Repeat the process a final time, but this time, use 1.5 oz, to achieve the required dose of 5.5 oz concentrate (for THIS example!). In steps 4 to 7, you’ll have applied 5.5 oz in a total of 18 gallons of water.

  7. Keep the area moist by watering it every other day for a week, to ensure uptake.


(Note: This blog was originally published Sept. 11, 2012)

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