An article in the July 29, 1992, issue of the Horticulture and Home Pest Newsletter explained the background biology and management options for this common and widespread pest of turfgrass. The action threshold, that is, the number of grubs needed to justify insecticide treatment is 10 or more grubs per square foot in healthy turf. It appears most grass is in good health for this time of year because of the abundant rainfall and the higher threshold is still suggested for most lawns.
Rainfall may also have an impact on how severe our grub problem is this year. During normal to dry years the masked chafer beetles concentrate their egg-laying in irrigated areas. We have seen in the drought years of 1988 and 1989 that eggs apparently are laid and grubs do survive in dry lawns, but on average, more eggs are laid in cool, moist soil.
During wet summers, egg-laying tends to be spread out, as a larger area of grass provides the cool, moist conditions the beetles seek. The result is white grubs are not as concentrated, fewer 'spots' or areas within the lawn have populations above the threshold number and most areas do not end up with high enough grub numbers to cause damage.
All of this re-emphasizes the need for scouting before treating (i.e., use an IPM approach). One of the easiest ways to reduce pesticide use is to only treat when necessary rather than blanket an area with preventive insecticide treatments that aren't necessary, or to spray 'just in case.' Scouting will determine if there are sufficient numbers of grubs present to warrant treatment.
This article originally appeared in the August 26, 1992 issue, pp. 144-5.