You Are Here: Management Updates 2004 Archive April 22, 2004

Date: April 22, 2004
Category: Insects
Subject: White Grub Control Options, Timing

Spring has sprung, it must be grub season again. Many people have been asking about options for control of white grubs during the spring. Here's the situation as I see it:

Until recently turf managers had three basic approaches they could consider for controlling white grubs in New England. They could wait until problems developed and spot treat with a fast-acting material like trichlorfon (Dylox™), they could apply a slow-acting but long lasting material like imidacloprid (Merit™) in the summer to prevent subsequent infestations, or they could use an intermediate material in late summer. The intermediate materials (most of which are either already gone or are being phased out) usually worked within four to seven days after application and remained active for four to six weeks. We have essentially lost the "intermediates" for many aspects of turf management, so my comments today will be restricted to the fast and the slow options.

Imidacloprid, often sold as Merit™ or under other trade names, takes two to three weeks to start working but will remain active two to six months, depending on when it is applied and how often it has been used in a given location. It is not recommended for use in the spring because field trials conducted in Ohio, Pennsylvania, Massachusetts, and other locations over the years have shown that it does not normally have any impact on the grubs that are present in the soil in the spring. (In other words, grub populations are not significantly lower in treated plots than in untreated plots.) This is a source of frustration for us in academia because many formulators of imidacloprid aggressively market it as having "season-long control", which of course implies that an application made in the spring should control grubs that are already present. University research refutes that claim.

Imidacloprid normally should be applied between late June and early August in New England to maximize its effectiveness against the various species of white grubs we experience here. (It can be applied slightly earlier if absolutely necessary, but in most cases the six week window given above is "ideal".) Such an application is in the soil as the eggs begin to hatch and the tiny larvae emerge. Note that imidacloprid is slightly less effective against European chafers than it is against Japanese beetles, and the timing of application becomes that much more critical when the primary species is European chafer. Another product that has a similar window of opportunity is halofenozide, often sold as Mach 2™. This product is a "molt accelerating compound". It is not a traditional nerve poison, like so many other turf insecticides, but instead interferes with a grub's ability to molt successfully, either from the first to the second instar or from the second instar to the third instar. Once a grub reaches the third instar, halofenozide will have no impact on the insect. So late summer or early autumn applications do not make sense because most grubs have already molted to the largest grub stage by then. Similarly, spring applications are not effective because the grubs that are present in the spring are the same large grubs that were present the previous fall. So... applications should be made just as the earliest eggs begin to hatch. That may be as early as late June for European chafers in southern New England or mid August for Japanese beetles in northern New England. (This is yet another example of why it is important to know which species of grubs you have in a given location.)

The only product that provides "after the fact" coverage for white grubs and is still available in the turf market is trichlorfon, usually sold as Dylox™. This material normally will kill grubs within one to three days after application but breaks down in seven to ten days. It is highly soluble and therefore quite mobile, so must be used with care in areas with sandy soils or shallow water tables. But the fact that it breaks down quickly usually minimizes the likelihood that the product will move to groundwater. Dylox™ can be used in the spring to "clean up" grub infestations that become apparent. Applications probably should not be made until soil temperatures (top two inches) are at least 50 °F, and it is possible that temperatures need to be even warmer than that. (I am not aware of any field trials testing the efficacy of Dylox™ in different soil temperatures, so this is a "best guess" on my part.) Late season applications can be made as late as mid to late September, particularly if the autumn is an unusually mild one.

There are two key points: First, any application made to control white grubs MUST be watered in. You should aim for at least 0.1 inch, but in general the more water follows an application, the more effective it will be - up to a point, of course. Up to a half inch of water would be fine for Merit™ or Mach 2™, while up to 0.25 inch would be good for Dylox™.

The second key point is that Merit™ and Mach 2™ normally should NOT be applied in April and May to control white grubs unless there are extenuating circumstances. (Examples of extenuating circumstances would be: a golf course dealing with black turfgrass ataenius and trying to get control of both the BTA and Japanese beetle grubs, or possibly an athletic field complex that cannot water in an application in July or August.) Spring applications of Merit™ or Mach 2™ normally will not reduce spring grub populations significantly and, because they are four to eight weeks earlier than the ideal application dates, may be markedly less effective than the July applications, particularly if the primary species is European chafer or oriental beetle.

- Submitted by: Dr. Pat Vittum