If you have ever dug in your garden, and found thick white, soft bodied bugs just under the surface, to several inches down, you have seen grubs. These insects are the larval form of several different species of scarab beetles, which will eventually pupate, and emerge from the ground. There are several species of scarab beetles, including Japanese beetles, June beetles and Northern Masked Chafer beetles. It is interesting to note that the most problematic of these, the Japanese beetle, is a non-native beetle that was first spotted in southern New Jersey, in 1916.
Scarab beetles often occur in numbers that do not affect the appearance of our yards, but sometimes they do become problematic. How do we decide, and what do we do about them when they do become a problem?
As with any pest issue, we advocate the Integrated Pest Management (IPM) approach to control. IPM is a method that identifies the pest, determines action thresholds, works to prevent infestations, and then uses the least toxic controls possible.
The first step, identifying the pest, includes knowing the pest’s life cycle in order to determine when they are most susceptible to control. With grubs, in the Mid-Atlantic area, beetles usually lay their eggs in the ground in summer. The eggs then hatch and grubs remain underground, close to the surface, eating roots of grass and other plants. When soils become cold, the grubs burrow down 2-8” to overwinter, moving back to the surface in the spring. The grubs then pupate, and emerge as adults from May to July, depending on the weather and species of beetle.
Knowing this life cycle, it makes sense that grubs are most vulnerable in late spring, when they are small, and close to the surface.
Now that we have determined when the best time is to treat for grubs, we should set action thresholds. This simply means, we should decide if treatment is warranted at all. Turf managers recommend digging up a square foot area of turf and counting the number of grubs. Although it depends on the species of grub, a good estimate is that if you see 5-10 grubs per square foot of turf, you may want to treat for grub infestation. (Source) Also, if you have areas of grass that are spongy or brown and you can roll back some sod because there are no roots to hold it down, look for grubs and treat if number of grubs meets the threshold. Other signs of grub issues could be skunks or raccoons digging up the lawn, moles, or large number of crows on your turf. Make sure you actually see the grubs before you treat them.
Another IPM step prior to treatment, is prevention. According to Penn State, if turf is allowed to dry out in July and August, beetle eggs have a lower likelihood of survival. Although grass will brown off during the summer if not irrigated, it is simply going dormant and will green back up as temperatures cool.
After we have identified the pest, determined that we have a problem which requires treatment, and instituted preventive measures, we should now select the least toxic means of control. When it comes to grubs, there are two excellent biological control measures that we can use: milky spore, and nematodes.
Milky spore is a bacterium that is an effective control for Japanese beetles. The grubs ingest the bacteria which reproduce inside the grub, eventually killing it. As a pesticide, milky spore is a very selective control because it does not affect humans, pets, birds, or other insects. This control is purchased in powder form and applied in teaspoon amounts every four feet in rows four feet apart creating a grid pattern. It must then be watered in well. Because milky spore is a biological control, it requires a population of Japanese beetle grubs to get established. This establishment period may take 3-5 years.
In the meantime, applications of nematodes can assist in short term control. Nematodes are parasites that also feed on many species of grubs. Although “nematode” can be a four-letter word for gardeners, there are different species of nematodes, and the ones effective in grub control are the good guys. Several species of nematodes can be purchased, and applied by sprayer to the lawn. Because nematodes are live organisms, they require special handling, and have a short shelf life, but if directions are followed carefully, they are a highly effective and safe control for grubs.
As a last resort, there are chemical treatments available to control grubs. These insecticides are generally non-selective, meaning they work on many different insects, and may have an active residual period from several days to 3-4 months “or longer”. Many of these insecticides claim to be safe for the environment and I am sure they are much safer than many previous insecticides because of reduced residual effects, but I am not convinced. In referring to one of the non-selective herbicides, one publication states that it “has selective activity on the insect nervous system. Thus, it poses relatively little hazard to humans, other vertebrate animals, or the environment.” (Source)
Although these chemicals may not be directly toxic to humans or pets, any chemical that is non-selective in its toxicity must be hazardous to the environment.
We advocate responsible land management, by creating beautiful, useful, and healthy spaces for humans and other creatures. This includes a well balanced approach with healthy populations of insects, which are an essential part of the food web. If grubs become an issue in your turf, let’s find safe and effective solutions to control them.