Dr. Doug Richmond
My last entry in this newsletter explored what 20 years of data say about the efficacy of two different white grub insecticides (imidacloprid and chlorantraniliprole) when applied at different times during the white grub development cycle. Here, I’d like to share some additional data that I’ve been able to summarize. These data cover those two initial active ingredients plus three more (clothianidin, cyantraniliprole, and tetraniliprole).
As I’ve mentioned previously, insecticide programming is a combination of logistics and the application of scientific information. We all have our favorite products and approaches, but operational constraints often dictate how much flexibility we have in developing insecticide programs. Since many of our best white grub insecticides are long residual, systemic products, turfgrass professionals have a lot of flexibility in their approach for managing white grubs effectively. We have a wealth of great insecticides available to us and the residual activity of these products provides an extended application window that allows for applications to be made over a longer period of time. As a result, most professionals have landed on a preventive approach that puts the active ingredient in place well ahead of any potential grub infestation. But, unless you are targeting other insects, like billbugs, that occur earlier in the season with a multiple targeting approach (one application, multiple insect targets), there really isn’t any good reason to make grub applications so far in advance. Our most effective grub insecticides do not require a preventive approach. In fact, timing applications closer to the appearance of grubs in the soil, or shortly thereafter, may be the best approach. Why leave a grub material exposed to the elements and slowly degrading for weeks or months before white grub eggs even begin to hatch?
In the figure below, I’ve summarized all of the white grub insecticide trials my lab has generated over the last 20 years to highlight how application timing (think management strategy) influences the efficacy of five widely used white grub insecticides; chlorantraniliprole, clothianidin, cyantraniliprole, imidacloprid and tetraniliprole (Figure 1). For this analysis I considered applications made during May and June as preventive applications. These applications were made well in advance of the time white grubs are typically hatching from the eggs. Applications made during July and August were considered early curative applications and were typically made after egg hatch, but prior to the appearance of destructive, late instar larvae in the soil. Application made during September and October were considered late curative applications and were directed toward late instar larvae that are capable of causing the most damage.
When controlling for application rate, chlorantraniliprole, clothianidin and imidaclorpid performed equally well when used in a preventive strategy (Figure 2), with cyantraniliprole and tetraniliprole providing good control, but lagging just a bit compared to the top performers. Neither chlorantraniliprole nor imidacloprid saw any reductions in performance when applied during the early curative window, which is noteworthy since this window provides professionals with even greater flexibility, expanding the application window by another 8 weeks! Also, both cyantraniliprole and tetraniliprole saw a slight numerical bump in efficacy when applied during this window, matching the efficacy of clothianidin. It’s also worth mentioning that there was no relationship between efficacy and application date during these two application windows, regardless of active ingredient.
Now for the most interesting part of this —the late curative window. This is the most challenging time to manage grubs because they are large, may have already caused damage to the turf, and cooler temperatures can force them deeper into the soil profile at any time; putting them out of the reach of any insecticide. These data clearly show a decrease in efficacy for chlorantraniliprole and imidacloprid compared to the levels of control provided by these two products when used preventively, but control is still hovering around the 80% mark for both. Cyantraniliprole performed better during this late curative window than it did at any other time and only tetraniliprole dropped below the 70% control mark. These are good numbers at this late stage of the game, but the later you go in this window, the less effective these products will be. As a matter of fact, it is only during this window that we see any relationship between % control and application date.
As a reminder, these data were not collected from just one efficacy trial, but from many replicated trials conducted over the last 20 years (n= the number of trials for each treatment). The take home message is that while several products perform well when used preventively, early curative timing seems to be optimal regardless of active ingredient. It is also a bit surprising how well many of these products perform later in the season. These data indicate that the time frame for good grub control effectively runs from May to September for most white grub materials. You have much more time to get your grub applications out, so don’t kill yourself to get them out by the end of June!
Figure 1. Influence of application timing on the efficacy of five insecticides against white grubs in turfgrass. Preventive = May-June, Early Curative = July-August, Late Curative = September-October. N = the number of replicated field trials including each treatment (material x application timing).
Figure 2. Life cycle of a typical white grub species (Japanese beetle as a model) and application windows for three different chemical management strategies. Preventive = May-June, Early Curative = July-August, Late Curative = September-October.