Though most of the insects you find in your garden won’t cause harm to your plants, there are certainly some that do, especially if their population grows out of control. For homeowners who have lawns, the grub worm is one such pest. Also commonly called grubs, lawn grubs, white grubs, or turf grubs, these critters feed on the roots of lawn grass and can cause significant damage if there are a lot of them infesting a lawn. Before learning how to control grub worms, it’s important to know how to properly identify them and determine how many is too many for your lawn to handle.
What is a grub worm?
No matter what you call them, grub worms aren’t actually worms at all. They are the larval life-stage of several different species of beetles in the scarab family. They are a creamy-white color with a rusty orange head and six legs at the front of their body. Grubs are C-shaped and their bodies appear slick and shiny.
While most people think all lawn grubs are the larvae of Japanese beetles, there are actually several species of beetles that are called grub worms in their larval stage. All have a similar lifecycle and cause the same type of damage to our lawns by eating the roots of the grass. Often Japanese beetles are blamed for the damage of other grub species.
The following four members of the scarab beetle family are known for their turf root-munching activities as larvae. Left unchecked, they are capable of causing conspicuous damage to our lawns (more on what their damage looks like below).
What do grub worms turn into?
Depending on their exact species, grub worms could turn into several different adult beetles. As grubs, they all look really similar, and if you want to tell one type of grub worm apart from the others, you’ll need a magnifying glass and the strange desire to examine the hairs on their butts (no, I’m not kidding). Each type is also subtly different in size right before they turn into an adult, but size shouldn’t be relied on for identification because they grow from egg to pupae over the course of several months, changing size along the way.
Grub worm type 1: Japanese Beetles (Popillia japonica)
This lawn grub is a notorious pest in the Northeast, and their range has now expanded to include isolated populations across much of the continental United States and parts of Canada. Accidentally introduced to North America from Asia in the early 1900s, the 1/2″ adult beetles are metallic green with copper-colored wing covers.
Unlike other types of pest grubs, the last abdominal segment of each Japanese beetle grub has a distinctive V-shaped row of tiny, dark hairs. The larvae grow up to 1-inch in length and spend the winter deep below the soil surface.
Adult Japanese beetles consume the foliage of over 300 different plants, beginning in mid-summer. Though they live for only 30-45 days, the adult beetles can cause a good bit of damage. Don’t ignore newly emerged adult beetles. Early hand-picking goes a long way. Knock the adults into soapy water or squash them.
Our online course Organic Pest Control for the Vegetable Garden, provides even more information about managing pests like Japanese beetles in a series of videos that total 2 hours and 30 minutes of learning time.
Grub worm type 2: May/June Beetles (Phyllophaga species)
Though there are several hundred different species of May/June beetles, only about two dozen of them are considered pests. Adult May/June beetles are brown or black and 1/2- to 1-inch in length. Often found around lights on summer evenings, the adult beetles are nocturnal, and they are active for only a few weeks each year. The adult beetles don’t cause much damage.
The lifecycle of May/June beetles ranges from one to three years depending on the species, and most of their lives are spent underground as larvae. A bit larger than Japanese beetle grub worms, May/June beetles can also be distinguished by two parallel rows of thick, stubby, dark hairs on the underside of their last abdominal segment (see, I told you you’d have to look at grub butts!).
Grub worm type 3: Oriental Beetles (Anomala orientalis, syn. Exomala orientalis)
Since its introduction in the 1920s, this Asian species has become common from Maine to South Carolina and west to Wisconsin. Adult beetles emerge in late June through July and are active for two months. They are similar in size to Japanese beetles but are straw-colored with dark, irregular blotches on their wing covers. Active only at night, the adult beetles feed on flowers and skeletonize leaves. Though they sound intimidating, adult Oriental beetles seldom cause noticeable damage.
The grubs however, can cause substantial injury to the roots turf grass. Often blamed on the more visible Japanese beetle, the damage caused by Oriental beetle grubs generates a brown, patchy lawn, particularly in late summer and fall.
To distinguish this grub worm from other types, look for two parallel rows of dark hairs on their posterior (I know… again with the grub butts….).
Grub worm type 4: Northern & Southern Masked Chafers (Cyclocephala borealis and C. lurida)
Native to North America, the northern masked chafer is found across the northeast. A similar species, the southern masked chafer, is more common in the southern states. There is an imported European species as well.
Adult masked chafer beetles are 1/2-inch long. They are shiny brown with a dark “mask” across the head. Emerging in late June and actively breeding for about a month, adult chafers do not feed. They are nocturnal, and males can be found flying just above the soil surface in search of a mate.
The grub worms of northern masked chafers feed on the roots of cool-season turf grasses while the southern species attacks warm-season and transitional grasses. Their physical appearance is nearly identical to other white grub species, and again, a careful examination of the pattern of hairs on the last abdominal segment is necessary for identification. With this species, the hairs are randomly patterned.
How do you know if you have a grub problem?
No matter which type (or types) of grub worms reside in your landscape, most of the time they don’t cause any problems. Healthy, organic lawns that contain a mixture of grass species and other plants, such as clover and violets, can handle a fairly large population of grubs before showing signs of damage. Grub worm problems tend to develop in lawns that consist of a single grass species or lawns that are over-fertilized and over-irrigated (more on this in a bit). But, when infestations of 15 or more grub worms per square foot of lawn are present, your lawn may develop brown patches that peel back easily like a carpet. When you lift the grass up, you’ll spy the C-shaped grubs in the upper layer of soil beneath it.
Grub worm damage is most evident in spring and fall when the grubs are actively feeding in the upper layer of soil.
Grub worm lifecycle
The exact lifecycle of each type of grub worm is subtly different, but for the most part, the adults are active for a just a few weeks in mid to late summer. Females then lay eggs on or just under the soil surface in your lawn. The eggs hatch several days later and the new grubs begin to burrow down into the ground and feed on plant roots.
They remain as larvae for several months to several years, depending on the species. In the winter, they migrate down deep into the soil, but in the spring and fall, they are found feeding closer to the surface.
How to prevent grubs
There are several things you can do to prevent these insects from becoming a pest.
- Grubs tend to cause the biggest problems in lawns that are fed excessive amounts of chemical fertilizer. Stop using synthetic chemical lawn fertilizers and switch to a natural lawn fertilization program, if you fertilize at all.
- Grub worms thrive in lawns that are frequently, but shallowly, irrigated. Not only do female beetles need soft, damp soil to lay their eggs in late summer, the newly hatched grub worms also need moisture to survive. Stop watering and allow your lawn to go naturally dormant in summer’s heat.
- Adult female beetles prefer tightly-cut lawns with a full-sun exposure for egg laying. To stave off excessive damage, always mow your lawn to a height of three or four inches. Do not cut it shorter.
- Female beetles are more likely to lay eggs in lighter, fluffier soils. Compacted, clay-based soils have lower rates of infestation. For once, compacted soils can be considered a good thing!
How to get rid of grubs organically
Despite your best attempts at preventing grub worms, they still may become troublesome enough to warrant corrective measures if your lawn has brow patches that peel back like a carpet.
Please don’t use grub killers based on synthetic chemicals. Most are made from a class of pesticides called neonictinoids. These chemicals are systemic, meaning they’re absorbed by the roots then carried throughout the plant’s vascular system where they also travel into the pollen and nectar. When you use these products on the lawn, they are also absorbed by nearby trees, shrubs, and flowers where pollinators feed. Avoid using them. They have recently been implicated in the decline of many insect species as well as birds.
Thankfully, all four types of grub worms are susceptible to the following natural product control that doesn’t bring harm to pollinators and other non-target critters.
The best grub worm control: Beneficial nematodes (species Heterorhabditis bacteriophora)
Beneficial nematodes are microscopic predators of all four species of grub worms. Applied in late spring when the soil temperature is above 60 degrees F, these minuscule worm-like creatures seek out and kill grubs throughout the growing season. They do not harm other insects, humans, pets, or the soil. Plus, they’re easy to apply, completely safe, and highly effective. And don’t worry; they don’t look gross. In fact, they just look like powder. To apply, you’ll mix the powder with water and spray the mixture over your lawn in a hose-end sprayer.
Because nematodes are a living organism, purchase fresh stock from a reputable source and store them according to label instructions. The particular species of nematodes used against grubs (Heterorhabditis bacteriophora) is not winter-hardy and should be reapplied every spring if grub damage is present.
Beneficial nematodes acclimate to your lawn best when the soil is moist, so water your lawn both before and after applying the nematodes. Use distilled water to mix the solution and apply the spray in the evening to give the nematodes time to burrow down into the soil before the sun rises. A few weeks after application, look for reddish-brown grubs – a sure sign the nematodes are doing their job!
Another grub worm control
Milky spore (Paenibacillus popilliae, formerly known as Bacillus popilliae) is a bacterium that is applied to the soil in either a powdered or granular form. Japanese beetle grubs consume the spores which then go on to reproduce within the body of the grub, eventually killing it and releasing more spores. Milky spore disease only affects Japanese beetle grubs, though, and leaves other lawn grub species intact.
It is best applied in late August when grubs are actively growing and located in the upper layer of the soil. When applied according to label instructions, milky spore (available for purchase here) can remain effective for ten or more years.
Knowing when to take action
Remember, seeing a few grub worms in your soil is no cause for concern. Unless your lawn develops brown patches that easily peel back or you spy 15 or more grubs per square foot of lawn, just ignore them. They’re a great food source for birds, salamanders, ground beetles, toads, frogs, and other creatures.
For more on caring for your landscape organically, please visit the following articles: