Thursday, May 21, 2015
Written by David Shetlar, Ph.D. (the BugDoc)
The Ohio State University Department of Entomology
My neighbor called me over last week to show me a cup with some grubs in it! Of course, their question was, "What should I do about these?” My answer was brief – NOTHING! Of course, that had to be followed with an explanation.
First, I saw no evidence of white grub damage to the lawn last October (a neighbor down the street had some browning and raccoon digging!). The most damaging period for all white grubs is in September and October when the populations of third instar grubs are the highest. Most grub populations decrease during the winter months – winter is tough! Second, finding a half dozen grubs after digging up several square feet of turf to convert into flower beds isn’t enough to worry about. Most healthy lawns can withstand up to 8-10 grubs per square foot before showing signs of damage (wilting or animal digging).
Second, treating grubs in the spring has always been an "iffy” situation. The grubs are coming to the soil-thatch interface to finish feeding and each grub may feed briefly or for a couple of weeks before they stop feeding, dig back down into the soil and pupate. So, at any one time, you have only some of the grubs feeding and others below the zone where the insecticides are likely active. I often joke with homeowners who have "discovered” a few grubs that they will probably kill more grubs by dropping the insecticide bag on the grubs rather than applying the insecticide!
Third, having grubs in numbers that are easy to find now suggests that a preventive treatment should be considered. We had a "great” grub season last year because we had lots of rain in July when the eggs were laid. This greatly increased the grub populations. If the same thing happens this July and early August, there WILL be enough grubs to damage the lawn. Again, the targets of the preventive applications (whether using Acelepryn from April to August, Arena – clothianidin – from May to August, or Merit – imidacloprid – or Meridian – thiamethoxam – from June into August) are the first instar grubs that hatch and move to the soil-thatch interface to feed from mid-July to mid-August!
Grubs fact sheet: http://ohioline.osu.edu/ent-fact/pdf/ENT_51_14.pdf