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Flowers, Fruits, and Frass

Local and statewide information on a variety of current topics for home gardeners and market growers.

Japanese Beetle Grub Control

Posted by Kelly Allsup - Bugs

How to Control Japanese Beetles

Japanese beetles are a jeweled menace of plants like roses, lindens, hibiscus and blackberries. They don't come from some elusive place in the south where they have been sunning themselves all winter but rather have hibernated in your lawn, states Kelly Allsup, University of Illinois Extension Horticulture Educator. When spring comes, overwintered full-sized grubs begin to feed on the roots of turf. Right before the adults emerge they pupate. The adult beetles feed and lay eggs for the next two months. Japanese beetles like to lay their eggs on irrigated turf in July and continue into August. Growing grubs begin feeding in August until October. To know this life cycle is key to choosing how you are going to control these most detested garden pests.

"Will control Japanese beetles" on the label of a pesticide in your garden center may prove ineffective if you do not treat at the correct time of year. The second step to take in the battle is to know your active ingredient. The active ingredient is the chemical in the pesticide that controls the pest and can be contained in several pesticides with different trade names. For instance, glysophosate is the active ingredient and Round-Up or Kleenup is the trade name. These active ingredients will be displayed on the front of the bag and in the pesticide label.

Chemicals with the active ingredient of Carbaryl (Sevin) or pyrethroids including cyfluthrin (Tempo) and permethrin (Astro) are effective control on adult beetles, with each spray lasting about two weeks. It is advised to use these only when necessary because these chemicals will kill bees and beneficial insects in your garden. In the case of Carbaryl, bees and beneficial insects will not visit your garden for the rest of the year. A systemic insecticide, imidacloprid (Merit) can be applied as a granular or drench to the root system of susceptible plants to provide control. However, recent reports suspect the use of imidacloprid to damaging the bee populations.

McLean County Master Gardeners are advocates of hand-picking. If you stop by Sarah's Garden and the David Davis Mansion you may see a Master Gardener knocking Japanese beetles off their heirloom roses into a jar of soapy water. Some Master Gardeners will go ahead and trim off all their rose/hibiscus flowers to prevent the infestation. Japanese beetles like sunny locations and are attracted to foliage that has already been eaten due to feeding hormones left behind. It is necessary to remove all of the lacy foliage left behind. To completely exclude planting the over 300 species that are fed on by Japanese beetle adults would leave gardeners without those mid-summer bloomers.

According to Phil Nixon, University of Illinois Extension entomologist, "even heavily attacked trees and shrubs rarely exhibit severe dieback, because the beetles attack after the bulk of food production has already occurred in the leaves." "Photosynthetic production primarily occurs early in the season when the leaves are still soft and pliable. Japanese beetle defoliation occurs later in the growing season," he explained. "This allows one to selectively treat those trees and shrubs in very obvious landscape locations and to ignore the damage on others."

Phil Nixon also suggests to not treat for the grubs unless you feel the infestation is devastating. He suggests this because Japanese beetle adults are good fliers and will travel a long distance to reach your prized rose bush.

There are two types of Japanese beetle grub control: preventative and curative. Preventative products attack grubs while they are small and very young. According to Michigan State University Turfgrass Science Department, products containing imidacloprid, thiamethoxam, halofenozide or chlorantraniloprole will not control grubs in the spring. These work in July when newly hatched grubs are present. These have no effect on grubs present in spring or fall.

Curative treatments for Japanese Beelte grubs are carbaryl and trichlorfon and are used in fall and after labor day to control larger grubs.

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