Current Insecticide Approaches for White Grub Control
White grubs (the larvae of various scarab beetles) are common pests of turf in the Northeast. Four species of grubs often cause problems in New England: the European chafer (Rhizotrogus majalis), the Japanese beetle (Popillia japonica), the oriental beetle (Exomala orientalis), and the Asiatic garden beetle (Maladera castanea). The life cycle of each of these species is relatively similar (adults fly in early summer and lay eggs in late June to late July; larvae feed on turf roots from early July through mid autumn and again in the spring; pupae are present in the soil for a week in mid June to mid July; and adults emerge early the following summer to complete the cycle). The life cycle of the European chafer occurs about two weeks earlier than that of the other species in any given location.
There are several insecticides currently on the market that can be very effective against white grubs. Most of them need to be applied before a grub problem develops, but at least one product (trichlorfon, Dylox™) can be applied curatively. However, trichlorfon cannot be used on school grounds in Massachusetts, and several other states in the Northeast have similar restrictions, primarily because the product is extremely soluble in water.
Table 1. Insecticide options currently available for grub control in New England.
|Material||Chemical Class||Application timing|
|chlorantraniliprole (Acelepryn™, GrubEx™)||Anthranilic Diamide||Preventive|
|imidacloprid (Merit™ and several generic products)||Neonicotinoid||Preventive|
|Allectus™ (Merit™ plus Tempo™)||Neonicotinoid plus Pyrethroid||Preventive|
|Aloft™ (proprietary product containing chlothianidin and bifenthrin)||Neonicotinoid plus Pyrethroid||Preventive|
Preventive controls include the neonicotinoids and chlorantraniliprole, but the timing for each approach is very different.
The label directions for most neonicotinoids (products containing chlothianidin, imidacloprid, or thiamethoxam) indicate they should be applied "when females are laying eggs". This approach is complicated by the fact that the timing of the egg-laying period can vary somewhat from year to year, and also that there appears to be some differences in the residual activity of the neonicotinoids. Based on several years of observations in New England (and a few field trials), we have learned that early applications of imidacloprid may not remain active long enough to protect against white grubs that become active in July and August. Chlothianidin appears to have a slightly longer residual, while thiamethoxam appears to be somewhere between the two. Our field trials over the years suggest that imidacloprid should not be applied earlier than mid June in New England if possible, while the other two active ingredients have slightly longer residual activity and could perhaps be applied slightly earlier. For best results, however, neonicotinoids should be applied between mid June and early August in most years so that the active ingredient is released before the grubs become too large.
Note that imidacloprid is not effective against Asiatic garden beetles (AGB). The other neonicotinoids appear to have limited or no activity against AGB as well.
Chlorantraniliprole (Acelepryn™ for commercial applicators, GrubEx™ for homeowners) is a relatively new insecticide from DuPont that has very low toxicity to vertebrates and no activity against bees, ants, or wasps. The product has a very low level of toxicity, so much so that a signal word is not required on the label, not even "Caution". It is extremely effective against all the species of white grubs we have in New England, as well as most caterpillars (such as cutworms, armyworms, and webworms). However, it takes 60 to 90 days to fully dissipate in the soil so for optimum effectiveness against grubs, it should be applied between mid April and early June. Applications after early June may result in reduced efficacy for grub control, but will still provide excellent protection against caterpillars.
If a grub problem develops during the late summer, there are a couple of options to "clean up the mess". Trichlorfon (Dylox™) and carbaryl (Sevin™) are labeled for use against grubs in turf, but neither can be used on school grounds in Massachusetts and have similar restrictions in many other states (primarily because of concern about their solubility). Both break down quickly in water with high pH (anything above 7.2), so if your water source is alkaline, be sure to include a suitable additive to lower or buffer the pH. Ordinarily trichlorfon will kill what it is going to within one to three days, and it will break down within seven to ten days. Carbaryl tends to be very inconsistent, and we sometimes see more grubs in the treated plots than the untreated plots. (We are guessing it is because carbaryl is highly toxic to several beneficial insects.) Carbaryl is also very toxic to honeybees and other bees.
There is evidence that chlothianidin (one of the neonicotinoids) has some curative activity. Our studies indicate that it takes at least 10 days to see an effect on the grubs, but it appears that the grubs begin to die within two weeks after application. While that is not quick enough for me to call it "curative", it is faster than the other neonicotinoids and may be the best option in situations where trichlorfon or carbaryl cannot be used.
Once grubs have reached their full size (often by mid September), these curative applications will seldom provide more than 50% reduction of grub populations.
Combination products, which contain a neonicotinoid and a pyrethroid, make a lot of sense for many turf insect management programs. The neonicotinoid usually is very effective against white grubs if it is applied when the beetles are laying eggs. The pyrethroid component of the product normally provides excellent control against caterpillars and billbug adults, among other things. For these reasons the combination products have become very popular in lawn care and also golf course settings.
So far we have not seen any evidence of white grubs developing resistance to the neonicotinoids, in spite of the pattern of repeated use throughout the Northeast. However, it does appear that the "window of opportunity" for applying imidacloprid when targeting white grubs is shorter than it used to be. In other words, when Merit™ was first available, applications in early May remained active well into autumn and provided excellent control of white grubs. But imidacloprid does not appear to have as long a residual activity against grubs now. Applications of imidacloprid made before early June may not provide the anticipated level of control of the late summer grubs that we saw in the 1990s. As discussed, recent field trials suggest that chlothianidin and thiamethoxam have longer residual activity than does imidacloprid.
Written by: Dr. Pat Vittum