golf green: © valentinarr grub: ©Viter8 | Dreamstime.com

White grubs deliver a one-two punch to superintendents fighting to keep their golf courses healthy and thriving. Initial injury to turf occurs from larval feasting on the roots, which results in infested areas first turning yellow, then brown, and finally dying. It’s the secondary issue, however, that leads to the most damage.

The grubs’ predators are much more destructive than the larvae, itself, says Rob Golembiewski of the Bayer Green Solutions Team.

White grubs are larvae of several beetle species including May-June beetles, green June beetles, masked and European chafers, oriental beetles, Japanese beetles, Asiatic garden beetles and ataenius beetles. They are major pests of higher-cut turf (fairways and roughs) throughout much of the United States, Golembiewski adds, with the greatest occurrence in the Northeast, Mid-Atlantic and Midwest. The May-June beetle will have a 2- or 3-year life cycle depending on species, while other significant species have a 1-year life cycle.

“Egg lying can occur at various times during the season depending on species and region of the country, but usually begins in late June or early July and continues for four to six weeks,” he says. “In most cases, adult emergence occurs in mid-summer, often after significant rainfall or irrigation, followed by mating and egg lying. The eggs hatch and the small larvae begin feeding on roots with molting from first to second instar occurring in a few weeks. Most of the feeding damage is done by the comparatively large third instar larvae, and it is this stage that causes visible turf damage.”

Once a superintendent is aware of a grub issue, identifying which white grub species are present is paramount in remedying the situation. The adult beetle stage of white grubs differ in size, color markings and life cycle, while the larval stage is similar in appearance and can be characterized by the C-shaped position when found in the soil-thatch interface area of the turf, Rob Golembiewski of the Bayer Green Solutions Team says. To identify grub larvae, he recommends using a 10x or 20x hand lens and examine the spines on the underside of the abdomen tip, called the raster. The rastral pattern is different for each grub species and is the most common method of identification.”
© Ohio State University

Overwintering occurs in this third instar stage with larvae moving downward during late October or November into the soil profile for protection from cold weather. The following spring, the larvae move up to the soil-thatch interface to feed and replenish food reserves lost during the winter before moving back down and transforming into the pupal stage. “A one-year cycle will be completed with beetles emerging from this pupal stage a short time later,” Golembiewski says.

And the damage they do can be extensive. When grub populations are heavy, areas of turf are easily lifted from the soil. “Turf damage from white grubs is much less common compared to the damage that results from animals feeding on the white grubs,” Golembiewski says. “Moles, raccoons, skunks and turkeys are the most common animals that destroy turf when feeding on white grubs. Turf can be severely damaged by animal foraging and usually results in reseeding or resodding.”

White grubs thrive in environments with adequate soil moisture and food. Golf courses provide plenty of irrigated organic matter in the soil profile for both larval development and life cycle completion, says Matthew Giese, field technical manager for Syngenta Turf & Landscape.

The term “white grubs” encompasses a significant number of beetle species that affect desirable turf species on any given golf course. “From the largest white grub (green June beetle) to the smallest (black turfgrass ataenius or billbug) and everything in between, each insect has different preferences in terms of what their favorable environment might be for cover (height of cut) and egg laying requirements (turf species),” Giese says.

Turfgrass areas with higher thatch levels are generally preferred, as this provides a significant food source for larval growth, Giese says.

“While it is not always the case, golf course greens tend to have less thatch and therefore, to a lesser degree, lower incidences of white grub infestation,” he adds. “Roughs, fairways, tees and approaches are common desirable areas where adult beetles will mate and lay eggs for optimal larval development and eventually pupation into adult beetles.”

However, Giese reiterates grubs will infest greens. For example, the green June beetle grub – the largest grub species infecting turf – can be found in greens and cause a fair amount of surface damage.

“The bottom line is that if the adult beetle’s basic environmental preferences are met, it’s possible white grubs could be found almost anywhere on the golf course,” he says. “In most cases, the damage is root loss and the inability of the plant to absorb moisture and nutrients for its survival. This type of damage typically results in the visible plant tissues turning yellow or necrotic, and can be mistaken for disease symptoms or even drought stress.”

Predators like raccoons, skunks and birds feed on grubs, resulting in turf damage.
© Rob Golembiewski, Bayer CropScience

If the turfgrass is growing vigorously, then it may withstand root system damage and no visible symptoms will be apparent, Giese says. In this latter scenario, where no surface symptoms are visible, indirect damage may occur – critters digging for a meal.

White grubs management strategies on the golf course are unique from other type of pests a superintendent encounters, Giese says. “Disease and weed-control management will typically require multiple applications for control of these pests,” he says. “Most basic grubs can be controlled with a single well-timed application. The exception here might be billbugs and annual bluegrass weevils, both of which have multiple life cycles that may require more than just a single insecticide application. The key is well-timed.”

Different preventive insecticides have different application timings. Not following the specific recommendations will result in less-than-desired outcomes. Giese says. However, he adds preventive applications are the most effective for season-long control control.

The introduction of the neonicotinoids (Meridian, Merit, etc …) for grub control was seemingly a silver bullet. Make one application prior to egg hatch typically in June or July and expect season-long control.

“If grub breakthrough does occur late in the season, some contact type insecticides are useful, albeit short lived, to suppress white grub feeding,” Giese says. “Within the last five years, the introduction of the anthranilic diamide (Acelepryn) chemistry has provided longer soil residual than the neonicotinoids and a broader spectrum (cutworms, sod webworms, billbugs) of pests found in turfgrass. In addition, this chemistry characterizes a friendlier environmental profile, especially around beneficials and pollinators, and as public and regulatory pressures mount for increased pollinator safety and habitat, it offers an attractive option for preventive white grub control.”

And, unlike other pests, resistance hasn’t been an issue.

“There has never been a confirmed report of neonicotinoid resistance amongst any grub species in turf,” Golembiewski says. “Reduced control is most likely the result of excessive thatch, low use rates, improper application timing, insufficient watering in of applications and/or poor environmental conditions. The impact of thatch on insecticide performance should not be underestimated. A turf stand with thatch layers of 0.75-1 inch may prevent 50-80 percent of any insecticide from reaching the soil.”