Turf Tip - July 1, 2009

Japanese beetles and White Grub Damage

Every year starting in June, for the last several years, University of Arkansas Entomologists and Extension personnel begin receiving calls, emails, and questions about Japanese beetles - a destructive beetle in the scarabaeidae family (scarabs).  In Arkansas, June marks the beginning of Japanese beetle adult emergence which will last until mid-summer and peak during a 4-6 week period beginning in late June or early July.  It is this adult stage that causes the most damage in Arkansas, with beetles feeding voraciously on hundreds of plant species, including ornamentals, vegetables, fruits and row crops.  However, many Arkansans are also concerned about the damage caused by the immature stage of Japanese beetles—white grubs (larvae).  White grubs feed within the soil on the roots of turfgrass.  Therefore, it is often difficult to tell where damage may or is occurring until after it is too late and patches of grass are significantly damaged or killed.  Concern about turfgrass damage comes from the logical theory that the high densities of adults Arkansans are seeing will surely yield high densities of white grubs leading to turfgrass damage.  However, a spring 2008 white grub survey conducted in northwest Arkansas suggests that white grub densities are far less than densities requiring treatment.

Japanese beetvle life stages showing egg, larvae, pupa, and adult

During the survey, we collected various white grub species including Japanese beetles, Green June beetles, Masked Chafers, and May/June beetles.  The average density of white grubs in one square foot was less than 2 grubs—well below any white grub treatment threshold.  The density at which significant damage begins to occur will depend on the grub species causing damage, as well as the type and condition of the grass in which grubs are feeding.  Stressed turf can only tolerate around half the white grub densities as healthy turf.

Table 1. White grub treatment thresholds.

 

Treatment Thresholds

Grub Species

Number / Square foot

Number / 4-Inch diameter core

May/June Beetle

3-5

1

Masked Chafer

8-10

1

Japanese Beetle

8-10

1

White grubs can be treated preventatively or curatively, and with various control tactics—e.g., chemical, biological (nematodes), or cultural (tolerant or resistant turfgrass).  Preventative treatments are applied anytime shortly before adult emergence (late May) to shortly after egg hatch (early August), and in high risk areas (e.g., near favorite hosts or areas known to have a history of white grub problems).  In the case of Japanese beetles, favorite hosts include: flowering crabapple, gray birch, hollyhock, Japanese maple, Norway maple, rose of sharon, roses, cherry, plum, peach, black walnut, table grapes, American linden, rose mallow, cigar flower, and crape myrtle.  It is important to note that Japanese beetles prefer to lay eggs in well irrigated areas, making such areas appear as high risk for grub damage.  However, these areas are more likely to be healthy and able to tolerate high densities of grubs, so treatment is recommended only after densities are found to be at the higher end of treatment thresholds—an example of curative treatment.  Curative treatment of white grubs (September/October)  is recommended once grub densities reach treatment thresholds or damage is visible.  This is determined by surveying the turfgrass for white grub damage by either taking random samples, or sampling areas believed to have grubs.

Japanese beetle life cycle
Image from http://www.ca.uky.edu/entomology/entfacts/ef451.asp

A recent study at the University of Arkansas on Japanese beetle egg laying preferences among different turfgrasses indicated that female beetles avoided laying eggs in Tifway hybrid bermudagrass, suggesting an expression of resistance.  This may carry over to other hybrid bermudagrasses as well, and should be taken into consideration when deciding whether to apply a preventative treatment.

The unusually low densities of white grubs found during the spring 2008 white grub survey, lead us to recommend avoiding preventative treatments, and instead apply curative treatments only after turfgrass samples reveal densities reaching or surpassing damaging thresholds.

If adult Japanese beetle densities appear high, why are white grub densities so low?  One theory has to do with the large expanses of grassland for white grub feeding in Arkansas compared to the area occupied by the adult beetles’ favorite hosts.  Two grubs per square foot may not cause damage to the grass, but when they emerge as adults, they can disperse upwards of 10 miles to congregate and voraciously feed on their favorite hosts plants, which are often in our backyards.  The damage caused by these high densities of beetles leads many homeowners to believe that the beetles emerged from their own yard.  In actuality, the beetles may have emerged miles away. 

In Arkansas, chemically treating a lawn, golf course, or park for Japanese beetle larvae (grubs) will not likely be necessary. Additionally, applying uncessary chemicals is not economically or environmentally sound, and is not likely to significantly reduce adult damage since adults are mobile.

For more information on Japanese Beetles, please visit: http://www.uaex.edu/Other_Areas/publications/PDF/FSA-7062.pdf
                                     
For up-to-date information on insecticides for professional and homeowner use, please visit: http://www.uaex.edu/Other_Areas/publications/HTML/MP-144.asp

For a complete summary of recent reserach on beetle oviposition preference, click here.

Tara Wood

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