White grubs are the larval (immature) stage of several different scarab beetles. In South Carolina, as well as the rest of the southeastern United States, turfgrass can be attacked by the grub stage of Japanese beetles, masked chafers, May and June beetles, and green June beetles. From year to year, the Japanese beetle consistently causes the most damage to both turfgrass and ornamentals. There are two additional scarab beetles that attack turf and are relatively new to the southeastern United States. The black turfgrass Ataenius beetle and the oriental beetle are slowly expanding their range and can soon be a problem for turfgrass managers in South Carolina.
Insect Life Cycle
All of these beetles go through four distinct forms during their life cycle: egg, grub (larva), pupa, and adult. Once the egg hatches, the grubs will go through three larval stages or instars during their development. Because of this, various size grubs are commonly found in the soil. The grub changes into the pupa, which is the resting stage, and the pupa then changes into the adult beetle. This form of life cycle is called complete metamorphism. With this form of metamorphosis, the larval stage looks completely different from the adult stage. Most of the grubs that feed on turfgrass will assume a “C” shape when they are dug out of the soil. They have a well–developed brownish head and three pairs of well–developed legs.
Japanese beetles, masked chafers, and green June beetles all have a one–year life cycle. This means that these beetles go through the egg to adult stage once during a calendar year. Eggs are laid in the soil during early summer after the mating flights have occurred. The insect will then go through its various developmental stages with the adult beetles emerging from the soil the next spring or early summer. Detailed life cycle times are outlined below. May and June beetles both have a 2 to 3–year life cycle.
With one exception, the grubs feed on the roots of warm and cool–season grasses. They show no preference for home lawns, golf courses, sports turf, or industrial landscapes. All managed grass areas are potentially susceptible to grub attack. When grub numbers are high enough, the grass may be lifted from the soil because the grubs have eaten the grass roots. With feeding by lower numbers of grubs, the grass may appear thin and unhealthy. Many times, grubs are concentrated in localized areas of a lawn. For this reason, it is very important to sample several areas in the lawn or other grassy areas.
Scouting for grubs can be a challenging task. When looking for grub activity, it can be difficult by observing the turfgrass condition because rainfall and irrigation can mask the damage to the root system. One technique is to monitor adult activity in the landscape. One way to determine if adult beetles are in the area is to observe if plants they commonly feed on are damaged. They will shortly mate and lay eggs. Ten days to 2 weeks later the eggs will hatch. Smaller grubs are easier to control, so based on adult activity, control measures can be implemented 3 to 4 weeks later (usually late June to early July).
Since grubs feed on turfgrass roots, this is the area that must be inspected. Infested turf will often wilt quickly and turn yellow due to root damage. The soil under the turf may feel spongy, and the sod may have lost enough of the root system to easily be pulled from the soil. The simplest method of sampling is to make cuts on three sides of a 12–inch square with a stout knife or a shovel, pry this flap back, and carefully inspect the root zone and the upper 1-3 inches of soil. If grubs are present, they will be found in this area. Sample several areas and determine the average number of grubs per square foot.
Most healthy turfgrass that is not under stress can tolerate 5-7 grubs/square foot. Poorly managed turfgrass may show damage at lower infestation levels. These treatment thresholds can vary depending on the kind of white grub present. Treatment thresholds and identification tips are discussed below in the description of each major type of grub.
Japanese Beetle: This introduced pest is now found in most states east of the Mississippi River except Florida. In South Carolina, they are well established over the entire state. The Japanese beetle was unintentionally introduced into the United Sates in the early 1900’s probably in the soil of imported plants.
The 0.3- to 0.5-inch long adults have a metallic green head and thorax (area behind the head), and coppery brown wing covers. There are five tufts of white hairs on the sides of the abdomen and two tufts at the tip of the abdomen. The adults are active daytime fliers that can fly half a mile. They feed on over 300 different kinds of plants, including ornamentals, vegetables, and field crops. During feeding periods, Japanese beetles can cause considerable damage to the foliage, fruit, and flowers.
The female beetle lays eggs in the top 2-3 inches of soil. Eggs hatch in 10-14 days into 0.1–inch grubs, which are nearly translucent. These tiny grubs begin feeding on grass roots. After feeding, the posterior end of the body becomes black or gray. After 2-3 weeks, the grubs molt into second instars, which feed for 3-4 weeks before molting to the final or third instar. Mature grubs are about 1–inch long. The head is yellow-brown. Feeding continues through the fall until the beginning of winter. At this time, the grubs move deeper into the soil. In March or early April, depending on the current climate trends, the grubs move back into the root zone, feed for a short time and pupate. After 1-3 weeks, the adults emerge from the soil, mate, feed, and start the cycle again by laying eggs.
Healthy turfgrass that is not under stress usually can tolerate 10 grubs per square foot. However, skunks, armadillos, raccoons, and other animals may dig up the turfgrass when grub populations are present and cause significant damage. Six to eight grubs per square foot may be a safer treatment threshold for healthy turfgrass. Treatment threshold levels are predetermined insect levels where a control measure is recommended.
Southern Masked Chafer: This native beetle is found throughout South Carolina. The adult has a shiny, light red-brown body, and a chocolate brown head. The size is about 0.5 inch long and 0.25 wide. The adult beetles do not feed. They are often attracted to lights at night. Newly hatched grubs are about 0.12 inch long. The grubs are white when they first hatch, but turn gray after feeding. Mature third instar grubs are about 1 inch long. The head is chestnut brown.
Masked chafers have a one-year life cycle. The egg stage lasts 14 to 21 days, the larval stage 10 to 11 months, the pre-pupal and pupal stage 15 to 21 days, and adults 5-25 days. The feeding cycle of the grubs is very similar to that of Japanese beetles. However, the grubs tend to feed on the roots just below the thatch layer. Damage is most severe in the late summer and fall. Drought conditions and hot weather during this period can intensify the damage symptoms. Populations of 40 or more grubs per square foot are fairly common. At these levels, dead patches of turfgrass often appear and can be easily lifted. Skunks, armadillos, and raccoons often will dig in the turfgrass for the grubs.
Well–managed turfgrass may be able to tolerate 20 grubs per square foot. Stressed turfgrass will develop symptoms when eight or more grubs per square foot are present.
May & June Beetles: This large group of native beetles belongs to the genus Phyllophaga and are commonly known as May and June beetles. Found throughout the United States, several species are established in South Carolina. Generally, adults are somewhat oval, are 0.5 to 1 inch in length, and light to dark brown. Adults are active at night and are attracted to lights. They feed on the leaves of herbaceous plants, shrubs, and trees.
May and June beetles usually have a two to three year life cycle. The eggs are laid singly in cells in the upper soil layers. Hatching occurs in a few days and the young grubs begin to feed on grass roots. The first and second instars are present for only a short time. Most of the life cycle is spent as a third–instar grub. Just before pupation, the mature grub stops feeding and empties its gut. This gives the rear end of the grub a shrunken appearance. This is referred to as the pre-pupal stage. After a few days, pupation occurs. Adult beetles emerge in May and June, depending on the species. Because nearly a year is spent in the third instar, considerable turfgrass damage can be done by these grubs.
Populations of more than seven grubs per square foot can result in complete loss of roots. The usual treatment threshold is four to five grubs per square foot. Controls should be aimed at the first and second instar grubs because they are easier to kill and cause less damage.
Green June Beetle: This native beetle is found throughout the eastern half of the United States. Adults are active during the day and range from 0.75 to 1.0 inch in length. The upper surface of the body is a forest green and usually has a tan stripe along the margin of the wing covers. The underside of the body is a bright, metallic green. Adults feed on overripe fruit, among other things. Large numbers may cluster on a ripe peach or plum. Adults are often seen swarming just above the surface of the turfgrass on bright, sunny days. Their beelike buzzing may alarm people who do not know what they are.
Green June beetles have a one-year life cycle. Adults are present from mid to late June through early August. Eggs are laid in the upper soil layer. The egg and first two grub stages are quite short. By the end of August, nearly all the grubs will be in the third instar. Mature grubs are two inches long. These grubs differ from the other white grubs. Green June grubs have short legs, short, stiff bristles on the back, and they usually lie on their back in an extended position and rarely assume a “C” shape. They feed primarily on decaying organic matter instead of turfgrass roots. The young grubs construct horizontal tunnels in the top two inches of the soil, while third–instar grubs construct vertical tunnels in the soil and come to the surface at night to feed. The tunneling can disrupt the root system, and under drought conditions, this can cause the turfgrass to die out. Piles of soil are pushed up at the entrance to the tunnel, which can become unsightly. While feeding on the surface, the grubs may move around considerably. Often they are found on paved areas in the early morning or may be trapped in street gutters. If the grubs become trapped on hot pavement, they die and smell bad as they decay.
Although the grubs do not feed on turfgrass, their tunneling can cause problems if there are more than five to seven grubs per square foot. After treatment, the grubs will come to the surface within 12 hours. As they decay, the odor can be very strong. With high populations, it may be necessary to remove the grubs from the treated area to prevent odor problems.
Grubs found in turf areas can be identified by the arrangement of spines and bare areas around the anal opening, which are called the rastral pattern. The grub must be placed on its back, and the area on the underside of the last abdominal segment washed off with water so the arrangement of the spines can be seen. A 10X hand lens will help in determining the pattern.
The Japanese beetle is fairly easy to identify because of the distinct “V” shaped arrangement of smaller spines near the anal opening. The southern masked chafer has a fairly random arrangement of 25 to 30 coarse, hooked spines around the anal opening. May and June beetles have two longitudinal rows of spines surrounded by more or less randomly arranged, stouter spines. These first three grubs also will assume the “C” shape when removed from the soil. The green June beetle has two rather faint rows of longitudinal spines surrounded by numerous, randomly arranged short spines. In addition, the green June beetle grub will more or less lie straight instead of curling up.
If grubs are to be sent in for identification, it is important to preserve them properly. The best way to kill the grubs is to bring a container of water to a boil and then remove the container from the heat. Immediately drop the grubs into the water and leave them in for three or four minutes. Remove the grubs from the water and dry them on a paper towel. When dry, place them in a container filled with rubbing alcohol.
This “cooking” action kills the enzymes and bacteria present in the gut. If not killed in this manner, the grubs will turn black, making identification much more difficult. The alcohol will then preserve the grubs.
Management and Control Practices
Cultural Control: Maintaining a healthy turf is important to culturally manage pest problems in any situation. This will involve proper soil preparation prior to turfgrass establishment, proper turfgrass selection for the site, proper installation, and using recommended management practices to assure a healthy lawn. However, grubs can damage the best-maintained turf.
Biological Control: There are several products available that fall into the category of biological control. These types of pesticides will reduce pest populations using other organic or biological processes. The most common and widely used biological control of grub worms is milky spore disease. Milky spore disease is a soil inhabiting bacterium (Paenibacillus popilliae) that is ingested by the grub during normal feeding. The bacterium then kills the grub and upon desiccation, the grub will release more bacterial spores into the soil. Milky spore disease is nontoxic to non-target species in the lawn. As with all biological controls, efficacy can be highly variable depending on the site and the environmental conditions at that site.
Another option in biological control of grubs are beneficial nematodes, such as Heterorhabditis bacteriophora. These microscopic organisms will seek out grubs and enter through natural openings in the insect body. Once inside, they release toxic bacteria that kill the insect rather quickly. The best time to apply beneficial nematodes is when the grubs are in an early larval stage, i.e., when they are small. Being living organisms, beneficial nematodes must be handled and applied with care according to label directions. 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). Although there are several commercially available products of Bt for grub control, research has shown control to be marginal.
Chemical Control: There are several insecticides that are labeled for use in residential turf, which provide effective control of grubs. Grub control is either preventative or curative. The turfgrass takes up insecticides that are preventative in their mode of action. Grubs that later hatch from eggs laid that summer will feed upon the turfgrass roots that contains these insecticides, and die. By contrast, contact insecticides give curative control of grubs that were already present in the soil at the time of application. These contact insecticides are not systemic within the turfgrass, but when they are applied, they move down through the soil and kill the grubs upon contact. Contact insecticide applications are more effective on smaller, younger grubs, which are present during the early to mid-summer.
Products that contain the active ingredients imidacloprid, chlorantraniliprole, clothianidin, and triclorfon are labeled for grub control in residential turfgrass, and various brands are found at feed & seed, big box, or landscaper supply stores (see Table 1 for examples of brands). Apply preventative applications in the spring, April through May, and curative treatments during July through August. Granular products must be watered into the lawn immediately after application. Read and follow all label directions on the product for specific instructions on rate, application, and safe use.
Table 1. Insecticides for Residential Lawn Grub Control.
|Brand & Product||Active Ingredient(s)||Form, Sizes, & Rates|
|Best insecticides for grub prevention: apply during April or May|
|Bonide Systemic Insect Spray with Systemaxx Ready to Spray (RTS1)||Imidacloprid 1.47%||RTS1: 32 fl oz for 5333 sq ft|
|Bayer Advanced Season Long Grub Control RTS1||Imidacloprid 1.47%||RTS1: 32 fl oz for
5000 sq ft
|Hi-Yield Systemic Insect Spray Concentrate; & RTS1||Imidacloprid 1.47%||RTS1: 32 fl oz for 3560 sq ft; Conc.: 16 fl oz|
|Bonide Dura Turf Insect & Grub Control Granules||Imidacloprid 0.5%||Granules: 6 lbs for 5000 sq ft; & 18 lbs|
|Southern Ag Grubs Away Imidacloprid 0.5G||Imidacloprid 0.5%||Granules: 1.4 to 1.8 lbs/1000 sq ft; 9 & 30 lb bags|
|Hi-Yield Grub Free Zone III||Imidacloprid 0.5%||Granules: 10 lbs for 7000 sq ft|
|Bayer Advanced Season Long Grub Control Plus Turf Revitalizer Granules||Imidacloprid 0.25%
& 6-0-1 fertilizer
|Granules: 12 lbs / 5000 sq ft; 24 lbs for 10,000 sq ft|
|Hi-Yield Grub Free Zone II||Imidacloprid 0.2%||Granules: 15 for 5000 sq ft; 30 lbs for 10,000 sq ft|
|Gordon’s Grub No More Granules||Imidacloprid 0.2%||Granules: 30 lbs for 10,000 sq ft|
|Bayer Advanced Complete Insect Killer Concentrate; & RTS1||Imidacloprid 0.72%
& Beta-cyfluthrin 0.36%
|RTS: 32 fl oz for 5334 sq ft; Conc.: 40 fl oz|
|Bayer Allectus G||Imidacloprid 0.20%
& Bifenthrin 0.16%
|Granules: 2.9 lbs per 1000 sq ft; 50 lb bag|
|Bayer Advanced Complete Insect Killer Granules||Imidacloprid 0.15%
& Beta-cyfluthrin 0.05%
|Granules: 10 lb for 5000 sq ft; & 20 lb for 10,000 sq ft|
|Caravan G||Thiamethroxam 0.22% & Azoxystrobin 0.31 %||Granules: 30 lb for 10,700 sq ft|
|Acelepryn G||Chlorantraniliprole 0.2%||Granules: 2.3 lbs/ 1000 sq ft; 25 lb bag|
|Scotts Grub Ex Season Long Grub Killer||Chlorantraniliprole 0.08%||Granules: for 5000 or 10,000 sq ft|
|Roundup For Lawns Bug Destroyer||Chlorantraniliprole 0.06%
& Bifenthrin 0.115%
|Granules: 2500 sq ft. Maximum 2 applic./year|
|Best insecticides for early grub curative (contact) control: apply during July or August|
|Bayer Advanced 24 Hour Grub Killer Plus||Trichlorfon 9.3%
(same a.i. as Dylox)
|Granules: 10 lbs for 5000 sq ft; & 20 lbs for 10,000|
|Bayer Dylox 6.2 Granular Insecticide||Trichlorfon 6.2%||Granules: 30 lb for 10,000 sq ft|
|Best insecticides for grub prevention & early curative control: apply May through July|
|Aloft LC G||Clothianidin 0.250%
& Bifenthrin 0.125%
|Granules: 3.6 lbs per 1000 sq ft|
|1RTS = Ready to Spray (Hose-end spray bottles)
Note: Most products have a maximum application rate per 1000 square feet per year. Read the product label for best rate for grub control.