Response of White Grubs (Coleoptera: Scarabaeidae) to Three Calcium Soil Amendments.

White grubs (Coleoptera: Scarabaeidae) are among the most serious pests of turfgrass, causing destruction to root systems by their feeding activity.  In the Northeast, soils are naturally acidic and liming every 2 to 4 years is common practice to maintain a pH for optimal growth of cool-season turfgrasses.  In previous research conducted over a half century ago, white grub numbers were found to be lower at limed sites and at higher soil pH levels at numerous sites and within various soils in the state of Ohio, suggesting a pH effect on white grub survival and/or adult beetle oviposition.  Later research tended to refute that pH or single heavy lime applications affected larval survival, but the previously seen white grub reductions following lime applications has remained unexplained.  During 2007 and 2008, field trials were conducted to determine the effects of two surface-applied liming agents, a calcium silicate (CaSiO₃) slag by-product (CSS) and dolomitic limestone [CaMg(CO₃)₂], on native white grub populations in New Jersey.  A stand of mixed cool-season turf near Princeton, N.J. on a Keyport silt loam soil (fine, mixed, semiactive, mesic Aquic Hapludult) with an initial pH of 5.0 received early winter applications of surface applied dolomitic limestone (limestone), or CSS for a target pH of 6.5.  In January of 2008, limestone or CSS was applied to the same plots as the previous year’s treatments and gypsum (CaSO₄·2H₂O) was applied to one-half of the control plots at the same total calcium (Ca) rate applied using limestone for both years.  In March 2008, a second mixed cool-season turf site at Long Branch, N.J. on a Freehold sandy loam soil (fine-loamy, mixed, active, mesic Typic Hapludult) with an initial pH of 5.8 received surface applications of limestone, CSS or gypsum for a target pH of 6.8.  In October 2007, white grub densities at the Princeton site were reduced by more than 50% in plots amended with either limestone or CSS when compared with the controls.  Regression analysis showed reduced white grub numbers correlated with increased soil pH, Ca, and phosphorus (P) levels.  However, in 2008 treatment did not affect white grub numbers, and the pH, Ca and P effects seen in 2007 were not exhibited at either site.  Additionally, treatment did not affect soil pH at the Long Branch site in 2008.  Our findings suggest that early winter lime applications are needed to affect white grub densities and that increased Ca and P levels are needed in addition to an increase in pH to between 5.5 and 6.0 to reduce white grub numbers in an acid soil with an initial pH of 5.0.