Both beetles prefer to lay eggs in moist soil and green grass. Weather will play a big role in where the eggs are actually laid. Lots of green grass and damp soils will have the eggs distributed over large areas, lessening the potential need for treatment or any treatment at all.
How bad is it potentially going to be this year? We have the moisture and green grass, yet what is going to be a limiting factor is the actual population of adult Masked Chafers and Japanese Beetles. The big drought of 2012 and the summer drought of 2013 coupled with the very cold winter of 2013 hit the Japanese Beetle larvae very hard and their numbers crashed. Our native grub has adapted to drier summers and colder winters and survives better. Here is proverbial but. Japanese Beetles have become the number one grub, having pushed out our white grub, so even though our native grubs do better, there are a lot less of them now. Populations of both have been climbing up again, yet starting from a low number to begin with.
Another factor of successful egg hatch will be the soil temperatures themselves. Hot, dry soils will limit the egg hatch rate. Soil temperatures above 87 degrees is the threshold. Remember that the adults lay eggs in moist soil for a reason. Eggs hatching will increase in size quickly with enough soil moisture. The grubs must reach a critical size to be able to over winter. Another point about the soil moisture and green grass. The grubs are eating the roots of the grass plants, so if the grass is slow growing or dormant, there is little food available and the grubs will not be large enough to survive the winter.
Grub management is justified if there are more than 10 or 12 young grubs found per square foot. Less than that number means even though they will be feeding heavily, there is not enough of them to cause damage to the lawn. Confirming the need for a treatment should happen in early to mid-august. Currently most of the treatments available are systemic and will last for several weeks. Systemic treatments are often applied ahead of suspected grub damage, yet scouting is the only way to know if a treatment is warranted, saving money if it is not needed. Applying systemic treatments means any plant growing in the lawn will absorb the insecticide. If you have weeds like dandelion or other weed flowers, any beneficial pollinator insects will be killed too.