Though some insects in your garden may harm your plants, most won't. However, if their population grows out of control, they may cause harm. The grub worm is a pest that affects homeowners with lawns. White grubs are larvae that damage lawn grass by eating the roots. A high infestation can kill the grass. Before you can control grub worms, you need to be able to identify them. You also need to know how many grub worms are too many for your lawn.
What is a grub worm?
Although they are often called worms, grub worms are not actually worms. The larvae of several different species of beetles in the scarab family are in their larval life-stage. They have a cream-white body and a rusty orange head. They have six legs, all located at the front of their body. Grubs have a C-shaped body and their skin appears to be smooth and glossy.
Most people think that all lawn grubs are the larvae of Japanese beetles, but there are actually several species of beetles that are called grub worms in their larval stage. Moss, liverworts and algae are all similar in that they have a similar life cycle and cause damage to our lawns by eating the roots of the grass. People often mistakenly blame Japanese beetles for the damage done by other grub species.
The following four members of the scarab beetle family are known for their turf grass-root-munching activities as larvae. Unless they are kept in check, they can do serious damage to our lawns.
What do grub worms turn into?
Depending on the species, grub worms can develop into various types of adult beetles. To tell one type of grub worm from another, you need to look at the hairs on their butts. There are also size differences among the types of ladybugs right before they turn into adults, but relying on size for identification is not accurate because they grow from eggs to pupae over a period of several months, and their size changes during this time.
Type 1: Japanese Beetles (Popillia japonica)
The white grub is a type of beetle larva that feeds on grass roots, and is a major pest of lawns in the northeastern United States. In recent years, they have been found in isolated populations across much of the continental U.S. and parts of Canada. The metallic green beetle with copper-colored wing covers was introduced to North America from Asia in the early 1900s.
Japanese beetle grubs have a distinctive V-shaped row of tiny, dark hairs on the last abdominal segment, which sets them apart from other pest grubs. The larvae grow up to one inch in length and spend the winter deep below the soil surface.
Adult Japanese beetles consume the foliage of over 300 different plants, with their feeding starting in mid-summer. Even though adult beetles only live for a month or two, they can still do a lot 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.
Type 2: May/June Beetles (Phyllophaga species)
There are hundreds of different species of May/June beetles, but 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, adult beetles are nocturnal. They are active for only a few weeks each year. The adult beetles don’t cause much damage.
Type 3: Oriental Beetles (Anomala orientalis, syn. Exomala orientalis)
Since its introduction in the 1920s, the Asian species has become common from Maine to South Carolina and west to Wisconsin. Adult beetles emerge from late June through July and are active for two months. The beetles are about the size of Japanese ones, but their wing covers are straw-colored with dark, irregular spots. Active only during the night, the adult beetles feed on flowers and eat the leaves until only the skeleton remains. The Oriental beetles that are adults seldom cause any damage that would be noticeable.
While grubs can cause some damage to turf grass roots, it is not usually substantial. The Oriental beetle is often blamed for the brown, patchy lawns that appear in late summer and fall.
To distinguish this grub worm from other types, look for two parallel rows of dark hairs on their back end.
Type 4: Northern & Southern Masked Chafers (Cyclocephala borealis and C. lurida)
The northern masked chafer is a type of beetle that is native to North America. It is found in many northeastern states. The southern masked chafer, a similar species, 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 a shiny brown color with a dark “mask” across their head. Emerging at the end of June, adult chafers breed actively for about a month without feeding. Nocturnal males can be found flying close to the ground in search of a female mate.
The northern masked chafer feeds on the roots of cool-season turf grasses while the southern species attacks warm-season and transitional grasses. The appearance of these grubs is similar to other white grubs, and the best way to identify them is by the pattern of hairs on the last abdominal segment. 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 grubs reside in your landscape, most of the time they generally don't cause any problems. Grubs can damage a healthy lawn if there are too many of them, but a lawn with a mixture of grasses and other plants can handle a larger population. lawns that are made up of a single type of grass are more likely to have grub problems, as are lawns that are over-watered and over-fertilized If there are 15 or more grub worms per square foot of lawn, the lawn may develop brown patches that peel back easily. If you lift the grass, you'll see C-shaped grubs in the upper layer of soil beneath it.
The most damage from grub worms occurs in the spring and fall when they are feeding actively in the upper layer of soil.
Insect Life Cycle
There are four different forms that these beetles go through during their life cycle: egg, grub (larva), pupa, and adult. After the egg hatches, the grubs will develop through three different stages, called instars. Different size grubs are often found in the ground. The grub changes into the pupa, which is the resting stage. The pupa then changes into the adult beetle. This form of life cycle is called complete metamorphism. In this type of metamorphosis, the larval stage looks very different from the adult stage. Grubs that feed on turfgrass will usually be in the shape of a “C” when they are pulled out of the ground. They have a brownish head and three pairs of legs.
Japanese beetles, masked chafers, and green June beetles all have a one–year life cycle in which they spend the majority of their time as larvae before becoming adults. This means that these beetles go through the egg to adult stage in just 12 months. Eggs are laid in the soil during early summer after the male and female have mated. Once the insect has burrowed into the ground, it will go through the different stages of its development until adult beetles come out of the soil the following spring or early summer. Detailed life cycle times are outlined below. The May and June beetles have a life cycle of 2 to 3 years.
Turfgrass Damage
The grubs feed on the roots of most grasses, with one exception. There is no preference for home lawns, golf courses, sports turf, or industrial landscapes. This means that grubs could potentially attack any managed grassy area. When there are enough grubs, they may eat the grass roots, causing the grass to be lifted from the soil. When grass is fed upon by fewer grubs, it may look thin and unhealthy. Many grubs are found in localized areas on lawns. It is important to sample several areas in the lawn or other grassy areas so that you can get an accurate idea of the population of moles in that area.
Monitoring
Scouting for grubs can be a challenging task. When trying to find out if there is grub activity, it can be hard to tell by looking at the condition of the turfgrass because rain and irrigation can hide the damage to the roots. One way to determine the types of animals in an area is to observe the activities of adults in the habitat. A way to determine if adult beetles are around is to see if the plants they typically eat are harmed. They will shortly mate and lay eggs. Within two weeks, the eggs will hatch. If there are fewer adult grubs, it is easier to manage them and control measures can be put into place 3 to 4 weeks after that (usually late June to early July).
The area that must be inspected is where the grubs feed on the turfgrass roots. If your turf is infested, it will likely wilt quickly and turn yellow because the roots are damaged. The soil under the turf may be soft and spongy, and the sod may have lost enough of the root system to be pulled from the soil easily. The easiest way to sample is to cut 3 sides of a 12 inch square, lift the flap back, and inspect the roots and upper 1-3 inches of soil. If there are grubs, they will be found in this area. A grub is the larva of a beetle. To find the average number of grubs per square foot, you would need to sample several areas and count the number of grubs in each square foot.
If your turfgrass is healthy and not under stress, it can probably tolerate 5-7 grubs per square foot. Damaged turfgrass from poor management may be evident at lower infestation levels. The amount of treatment necessary for white grubs can depend on the type of grub present.
Management and Control Practices
Maintaining a healthy turf is important to managing pest problems through cultural means. Proper care of your lawn involves many steps, including picking the right grass for your location, preparing the soil before planting, proper installation, and using recommended management practices to maintain a healthy lawn. However, grubs can damage the best-maintained turf.
These products use living organisms to control pests. There are several products available for controlling pests that use living organisms, called biological control. Pesticides that use organic or biological processes to reduce pest populations are more effective. The most common way to control grub worms is by using milky spore disease. A soil-inhabiting bacterium called Paenibacillus popilliae causes milky spore disease. The disease is spread when grubs eat the bacteria while they are feeding. The bacterium kills the grub, and the grub releases more bacterial spores into the soil when it dries up. The Milky spore disease is not harmful to other animals that are not the intended target in the lawn. The effectiveness of biological controls can vary a lot depending on the location and the environmental conditions there.
Another option for controlling grubs biologically is to use beneficial nematodes such as Heterorhabditis bacteriophora. These will seek grubs out and enter through natural openings in the insect body. After the parasites invade an insect, they release poisonous bacteria that kill the insect quickly. The best time to apply beneficial nematodes is when the grubs are small. Since beneficial nematodes are living organisms, they must be used with care according to the directions on the label. Read the product label for most effective application.
Finally, there has been some research done on using the microbial biological control agent, Bacillus thuringiensis (Bt). There are several products available that are supposed to control grubs, but research has shown that they are not very effective.
There are several insecticides that can be effective in killing grubs in your yard. Grub control is either preventive or curative. The turfgrass takes up preventative insecticides. The insects that hatch from the eggs laid in the summer will die because they will feed on the grass roots that contain insecticide. In contrast, contact insecticides kill grubs that have already infested the soil. The insecticides in this product are not absorbed by the turfgrass, but they kill the grubs when they come in contact with them. Insecticide applications are more effective on smaller, younger grubs. These grubs are present during the early to mid-summer.
The following products contain imidacloprid, chlorantraniliprole, clothianidin, and trichlorfon, which are effective against grubs in residential turfgrass. You can find these products at feed & seed, big box, or landscaper supply stores. Apply treatments to prevent disease in the spring, from April to May, and treatments to cure disease from July to August. After applying granular products to your lawn, you must water it immediately. Read and follow all of the directions on the product's label for specific instructions on how much of the product to use, when to apply it, and how to use it safely.
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