White Grub Identification, Biology and Management

“White grub” is the common name for root-damaging turf pests in the family Scarabaeidae, or scarab beetles. Scarabs, like all beetles, undergo complete metamorphosis, transitioning through egg, larva, pupa, and adult stages. All white grubs appear as plump, pale, typically C-shaped larvae that feed in the thatch layer of grass stands. White grubs have six true legs and no abdominal leg-like structures (prolegs) like caterpillars. Late instars have highly developed chewing mouthparts. After pupating, white grubs become stout-bodied, short-lived beetles. White grub species vary in their size, life cycle, and distribution but are otherwise similar in their biology, the damage they cause, and the methods used to manage them.

The various white grub species are distinct as adults, having varied coloration and body sizes (Figure 1). The larvae (grubs) are comparatively uniform, varying in final instar size (Figure 2) and their raster pattern (Figure 3). This raster pattern consists of a group of hairs and spines on the lower surface of their abdominal tip and is most easily observed using a dissecting microscope. The raster pattern is unique for most white grub genera, so it can be used to identify which species is causing the infestation.

Biology and Life Cycle

White grubs hatch from eggs that adult female scarabs deposit in moist soil. The eggs are shiny, white, and oval shaped, expanding into spheres as they take on moisture and the embryo inside develops. After hatching, the larvae (grubs) will begin to feed on the organic thatch layer below the turf surface. While they do not target grass roots directly, their feeding severs roots and destroys turf crowns. Their constant tunnelling can dislodge grass plants and open channels into the soil, increasing desiccation. While most white grubs remain under the surface until pupation, green June beetle grubs are unusual because they emerge from established burrows at night to feed on grass blades while creeping upside down. Unlike other grubs, green June beetle grubs crawl on their backs, undulating their bodies to carry themselves with surprising speed.

Adult scarabs do not damage turf. Some feed on plant leaves or ripening fruits (e.g., May/June beetles, Japanese beetles, green June beetles, and Oriental beetles). The rest do not feed as adults.

Most white grub species are univoltine, meaning they produce a single generation each year. Annual white grubs overwinter as larvae, cease feeding, and burrow deep into the soil once temperatures begin to drop. They resume feeding in the spring, with adults emerging in the early summer months. May/June beetles are unusual in that they need 2-3 years to complete a life cycle, while black turfgrass ataenius produces two generations per year.

Distribution

Crop and turf-damaging scarab species exist all over the world. In the United States, every state has at least one species of significance, with many states having over a dozen. Numerous species of white grubs damage turfgrass in Ohio, with Japanese beetles and masked chafers being most abundant.

Damage

White grubs damage turf by feeding on the thatch layer, dislodging and chewing through roots and crowns in the process. Direct feeding damage resembles drought stress, appearing as irregular patches of dried dead grass (Figure 4, left). Infested grass patches will feel spongy underfoot due to the grub’s tunnelling. In cases of severe damage, the turfgrass can be pulled and rolled up like a carpet, a result of feeding activity severing the roots from the underlying soil. Often the first sign of white grub activity is animal digging (Figure 4, right). Many animals, especially birds, racoons, and skunks, will dig and rip at the turf surface to feed on the grubs beneath. Damage from animal digging is often far more noticeable than grub damage alone.

Monitoring/Sampling

Japanese beetles can be trapped using pheromone lures in the daytime, while some beetles (such as masked chafers and European chafers) are active at night and can be captured using blacklight traps. Large numbers of beetles can be captured in these traps to identify peak flights and predict egg-laying periods. These traps are not used to control populations as they can attract more beetles to nearby areas than they capture.

Where severe infestations are suspected, the “tug method” can be used to verify where grub populations are present. Use both hands to grip the turf canopy and attempt to pull the turfgrass out. Where grass roots have been damaged by feeding, the turf will pull back like a carpet, often revealing the grubs beneath (Figure 5). In the early stage of infestation or when damage is not evident, white grub populations can be estimated by soil sampling using a cup-cutter or a shovel. Take soil samples every 10–15 feet in a grid pattern on home lawns and sports fields. Samples can also be taken in a zigzag/transect pattern on golf courses where grubs are suspected due to historical infestation, or in locations with high adult beetle populations based on monitoring. The threshold level of a grub population that requires action varies by grub species, the turfgrass type, and conditions (i.e., fertility, irrigation, light intensity). Usually, for annual species, if the grub population exceeds 10 per square foot, control measures need to be applied to prevent grub damage and discourage animal digging. Threshold levels of smaller grubs such as black turfgrass ataenius are higher, with approximately 30–80 per square foot in spring and 15–40 per square foot in summer. Healthy turf tends to tolerate more grub feeding than turf that is under environmental and/or disease stresses.

Control

Cultural Control

Cultural management practices can help alleviate grub damage. Thick thatch can attract more egg-laying, slow water movement, and may compromise the efficacy of pesticides that have low water solubility and high binding capacity to organic matter. Thus, reducing thatch via dethatching or aeration is recommended when the thatch thickness exceeds half an inch. These practices can improve air circulation, increase water movement, and promote root growth.

Providing adequate irrigation and utilizing fertilizers when necessary helps promote stronger and deeper root systems in the turfgrass, making the plants less susceptible to grub feeding and allowing them to recover faster from damage. On the other hand, excessive irrigation can worsen white grub infestations and sometimes attracts more adult beetles into that irrigated area, since their eggs require higher levels of moisture in the soil to hatch.

Biological Control

Various biological control agents can be utilized in conjunction with cultural—and some chemical—control tactics.

Tiny roundworms called entomopathogenic nematodes are often found naturally occurring in the soil, and a few species can be purchased commercially for biological control of white grubs. The effectiveness of these beneficial nematodes can vary with species and target hosts. Among the commercialized species of nematodes, Heterorhabditis bacteriophora (Figure 6, left) is most broadly used for grub control; for example, against Japanese beetle larvae it can be as effective as the standard chemical insecticides (organophosphates and carbamates) used in early years if applied under suitable conditions. Another species, Steinernema scarabaei (Figure 6, right), can provide good control of grubs, including Japanese beetle, masked chafers, Asiatic garden beetle, European chafer, and Oriental beetle larvae. Stages of white grubs may have different susceptibility, varying with grub and nematode species. In general, nematodes are more effective against younger grubs than older instars, except for a few species such as S. scarabaei maintaining high efficacy against mature grubs.

As these nematodes require moist conditions to be effective, they need to be watered in immediately after application. Daily watering should continue for a week. Also, nematodes are highly susceptible to extreme heat and UV radiation, so application should be avoided at midday during hot and sunny conditions. Some chemical insecticides pair well with nematode applications. Insecticides containing the active ingredients imidacloprid and chlorpyrifos are compatible with nematodes and can work synergistically with the nematodes to provide enhanced control of white grubs.

Green and white muscardine fungus are known to naturally infect white grubs (Figure 7). Two species of entomopathogenic fungi, Metarhizium brunneum and Beauveria bassiana, demonstrate insecticidal activity against many species of white grubs, and both fungi are commercially available in numerous formulations. These fungi can provide some level of grub control, but their efficacy may be inadequate and incomparable to chemical control. Like entomopathogenic nematodes, the fungi are susceptible to adverse environmental conditions (e.g., heat, drought, UV). Also, the fungi need direct contact with target hosts to take effect, while they mostly bind to thatch if applied on the surface, which limits their effectiveness against soil pests such as grubs. The endophytic fungi, which are naturally present in some turfgrass types (tall fescue and perennial ryegrass), are largely ineffective against white grubs because endophyte toxins do not reach the soil-thatch zone.

Bacterial agents, such as Bacillus thuringiensis (Bt), can successfully control white grubs, but the species of white grubs must be known if using these agents. Different strains of Bt have been commercially produced for pest control. Among them, only the Bt subspecies japonensis and galleriae have efficacy against white grubs. In particular, Bt japonensis is only effective against Japanese beetle grubs. Another bacterium, Paenibacillus popilliae, naturally infects white grubs (Figure 8). The hemolymph of infected grubs turns a milky color (forming spores), hence, infected grubs are said to have milky disease. It is slow acting, killing grubs in about 4 weeks. Most grub species have their own strain, and the only commercial strain available controls Japanese beetle.

Finally, natural enemies such as predatory insects and parasitoid wasps can serve as naturally occurring biological-control agents. Most white grub species have at least one species of parasitoid wasp or fly that targets them, except for the European chafer, which has no known parasitoids in our region. Common parasitoids include tachinid flies that attack Japanese beetle, as well as Tiphia and Scolia wasps that parasitize white grubs (Figure 9). Predatory insects such as robber flies usually target adult beetles rather than the larvae, although some insects, such as the larvae of bee flies, attack white grub pupae. To promote populations of these natural enemies, provide pollinator gardens or conservation strips to give them supplemental food and refuge away from chemical application areas.

Chemical Control

Chemical insecticides can be used for preventive and/or curative control of white grubs, as listed in Table 1. It is worthwhile to note that pyrethroids are not effective for grub control because they are contact insecticides with high binding capacity to thatch and other organic matter. Thus, for the pyrethroid and neonicotinoid combination products, only the neonicotinoid affects the white grubs.

Where previous white grub activity has occurred or sampling reveals potentially damaging populations, preventive treatments are recommended before damage occurs. Anthranilic diamide and neonicotinoid insecticides have systemic properties and possess relatively long residual activities, making them ideal for preventive grub control. Preventive insecticides are often applied from late May to mid-August, depending on the residual time of their chemicals. For example, imidacloprid and thiamethoxam have a residual period of up to 8 weeks and need to be applied no earlier than mid-June, while clothianidin and chlorantraniliprole last for up to 12 weeks and can be applied slightly earlier but not earlier than late May. Chemicals (such as carbamates and organophosphates) used for curative control usually have short residuals and need to be applied when grubs are present, with the exception of clothianidin that is often applied for preventive control but can also provide curative efficacy when applied at higher rates. Usually, smaller and younger grubs are more susceptible to insecticides than larger and older ones.

The choice to use preventive or curative control depends on various factors, such as budget, labor, infestation history, adult monitoring, etc. Preventive control is often conducted over a larger area than is necessary to prevent potential damage and reduce or eliminate the labor needed for grub scouting. On the other hand, curative control allows for spot treatment on a needed basis, reducing the use of insecticides and their non-target effects on natural enemies and pollinators. Spot treatment does, however, require intensive scouting of grub populations and has an increased risk of grub damage to turf. For sites with a history of grub infestations or areas identified as having high numbers of adult flights, preventive control may be required.

For curative control of grubs, the green June beetle is an exceptional case. Unlike most grubs only found underground, green June beetle grubs may emerge to the surface to search for food at night and thus are vulnerable to both below- and above-ground insecticide treatments. However, treatment made for large green June beetle grubs warrants caution. After treatment with insecticides, green June beetle grubs are triggered to surface and die, and the dead grubs generate foul smells due to their defensive scent glands. It is recommended to remove these cadavers using a rake or other tools prior to mowing to avoid clogging or damaging the mower.

For all other white grubs, insecticide treatments need to be watered in with a quarter to half inch of irrigation soon after treatment to optimize the efficacy. Pre-treatment irrigation may be needed for curative insecticide application when the soil is dry. Granular insecticides may need more irrigation and take a longer time to be effective. Reducing the thatch layer (as was reviewed with cultural control) can help facilitate the movement of insecticides into soil and better reach targeted grubs.

Overall, integrated management practices are recommended for controlling white grub populations, based on our knowledge of grub species and their life history. Scouting and threshold-based chemical control is suggested for applications made at the appropriate time, using methods that optimize control and minimize environmental impacts.

Disclaimer: Pesticide recommendations are subject to change with time and are provided only as a guide. It is the pesticide applicator’s responsibility, by law, to read and follow current label instructions for the specific pesticide being used. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author and Ohio State University Extension assume no liability resulting from the use of these recommendations.

Additional Resources

• Application technology and environmental considerations for use of entomopathogenic nematodes in biological control.
  (doi.org/10.1016/j.biocontrol.2005.09.005)
• Microbial control of insect pests of turfgrass.
  (sciencedirect.com/science/chapter/edited-volume/abs/pii/B9780128035276000226)
• OLCA News.
  (cdn.ymaws.com/www.ohiolca.org/resource/resmgr/newsletter/olca_newsletter_-_fall_24-co.pdf)
• Turfgrass insects of the United States and Canada.
  (cornellpress.cornell.edu/book/9781501747960/turfgrass-insects-of-the-united-states-and-canada/#bookTabs=1)
• Variation in the laboratory susceptibility of turf-infesting white grubs (Coleoptera: Scarabaeidae) to biological, biorational and chemical control products.
  (doi.org/10.1002/ps.1835)
• White grub control in turfgrass.
  (doi.org/10.1093/amt/30.1.G44)