DAMAGE RELATIONSHIPS OF JAPANESE-BEETLE AND SOUTHERN MASKED CHAFER (COLEOPTERA, SCARABAEIDAE) GRUBS IN COOL-SEASON TURFGRASSES

Relative tolerance to root herbivory by 3rd-instar Japanese beetle, Popillia japonica Newman, and southern masked chafer, Cyclocephala lurida Bland, grubs was evaluated in turfgrass plots of Kentucky bluegrass, Poa pratensis L., perennial ryegrass, Lolium perenne L., creeping bentgrass, Agrostis palustris (Hudson), hard fescue, Festuca ovina L. var. duriuscula, and both Acremonium endophyte-infected and endophyte-free tall fescue, Festuca arundinacea Schreber. Field enclosures and rooting boxes were infested with densities of grubs ranging from 0 to 60 per 0.1 m(2), and effects on aesthetic quality, canopy temperature, foliage yield, and root strength were measured. Damage thresholds were variable but appeared to be much higher in all grasses than the rule-of-thumb estimates commonly used in the turf industry. Initial densities of at least 15-20 grubs per 0.1 m(2) were required to cause any reduction in aesthetic quality of most turfgrasses, and in some cases, initial densities of 60 grubs per 0.1 m(2) caused no apparent damage. Canopy temperatures were not consistently correlated with density of grubs. Damage by C. lurida was greater in endophyte-free than in endophyte-infested tall fescue in some trials and was greater than, or equal to, that of P. japonica in all grasses. Variability in the turfgrass system complicates prediction of turfgrass damage based solely on grub density.