The Control Center: Dubbing Them Grubs

To determine which destructive pests are damaging your turf, you need to dig for them first – and then treat accordingly

One of the most common and destructive pests on the golf course is the white grub. Several species can be involved, each with the potential to cause extensive damage to large areas of turf.

However, even though grubs are often perceived as pedestrian, the observed turf injury could be because of several other factors. Many maladies mimic white grub damage, so determining if grubs are present and in sufficient numbers to cause unacceptable damage is the key to effective management. Once this determination has been made, timing, monitoring and effective treatment methods follow.

Obvious and not so obvious

In general, soil-active insects are more damaging to turf than surface feeders because injury occurs below the growing point, making it more difficult for the plant to recover from the setback. In addition, subterranean pests – white grubs, billbug larvae, mole crickets and some species of cutworms – are more difficult to detect because they are not visible from above. Soil dwellers also have an advantage over surface feeders as they are better protected from natural enemies and adverse environmental conditions.

So, what does white grub damage look like? You’ve all seen the stereotypical grub damage – off-color, dried out, loose turf that can sometimes be rolled up like a piece of carpet – but less obvious signs are also possible.

This means you could have grubs in your turf and not even know it, especially if it’s early in the season, the weather has been mild, and the grubs are still small. Instead of being brown and withered, the turf might appear pale, thin or just lackluster.

The only way to know for sure if grubs are the culprit is to closely examine the areas where infestation is suspected. These might include areas where damage has occurred in previous years, areas that are lit at night (near streetlamps, parking lots, clubhouse, etc.), or perhaps turf with a shallow root system. The key being “closely examine.” In other words, if you want to determine if grubs are damaging your turf, ya gotta dig.

A chart of turf diseases and pests that occur in the general time frame when grub damage is possible might look something like this:

Interactions with other factors

If relatively few grubs are found, consider other factors that could be contributing to the turf’s poor appearance.

Drought

– Since white grubs are root feeders, drought can be an important confounding factor in white grub damage. During dry conditions, root numbers tend to be greatly reduced. With fewer roots present, additional root feeding by grubs can result in increased damage to the turf.

Seasonal root retraction

– Cool-season turfgrass, such as creeping bentgrass, tall fescue, fine fescue, perennial ryegrass and Kentucky bluegrass, have a natural annual life cycle, characterized by shallower roots in summer. Shallower roots aren’t as able to extract sufficient water and nutrients from the soil profile, leaving the turf stand in a stressed condition. As with drought, damage from feeding white grubs adds insult to injury.

Compaction

– Compaction from foot traffic, golf cars and machinery reduces the capacity for roots to grow and expand. Constricted or underdeveloped roots that have been consumed by grubs further reduce the thriftiness and vigor of the turf.

Diseases

– As previously mentioned, diseases such as leaf spot, Pythium, dollar spot, rust, summer patch and brown patch occur within the same general time frame as grubs, and can cause similar kinds of damage at first glance. Thinned or diminished crown density is directly related to the number of roots produced, making the stand more vulnerable to an array of causal agents.

Other insects

– Feeding by turf insects, including sod webworms, cutworms, billbugs and mole crickets, can weaken the turf and create a set of conditions where even minor white grub feeding produces more damage than it normally would.

The bottom line, because of turf/pest/environmental interactions, it’s always important to keep in mind that one particular cause may not be the singular causative factor.

Species of white grubs

White grubs are the larval stages of scarab beetles (Scarabaeidae). They are among the most destructive insect pests of turf and where abundant can destroy large areas of turf in a brief period of time.

Many scarab species are present in the U.S., but, fortunately, only a relative few cause significant injury to turf. Among the more important species are masked chafers (Cyclocephala spp.), Japanese beetles (Popillia japonica), European Chafers (Rhizotrogus majalis), Oriental beetles, (Exomala orientalis), Asiatic garden beetles (Maladera castanea), Green June beetles (Cotinis nitida), May/June beetles (Phyllophaga spp.) and the black turfgrass ataenius (Ataenius spretulus). Each of these scarab species has a unique biology and life cycle requiring a specific management approach.

The larval stages of all white grub species are very similar in appearance. They have C-shaped, creamy white bodies, reddish brown heads and three pairs of legs. When fully grown they range from less than .25 to 1.5 inches in length, depending on the species. The best way to tell them apart is to examine the arrangement of hairs and spines on the underside of the last abdominal segment (raster). Distinct patterns can be readily distinguished with the aid of a small hand lens.

After hatching from eggs, white grubs feed on the roots and underground stems of turfgrasses. The first sign of injury is localized patches of pale, discolored and dying grass displaying the symptoms of moisture stress. Damaged areas are small at first, but rapidly enlarge as grubs grow and expand their feeding range. White grub-damaged turf has a spongy feel underfoot and can be easily lifted from the soil surface or rolled like a carpet, revealing the C-shaped white grubs underneath.

Cultural control

Utilizing an integrated pest management (IPM) approach can go a long way toward minimizing white grub infestations and damage on the golf course. An important aspect of IPM involves planning to avoid or minimize future pest problems.

Decisions made during the establishment and maintenance of a turf area can significantly influence pest development. Among these key decisions are the selection of turf species and cultivar; weed, disease and insect control techniques; and irrigation, fertilization, thatch management and other cultural practices that affect the health and vigor of the turfgrass stand.

Utilizing a regular monitoring program is crucial to success in controlling white grubs, especially in areas that are lit at night and with a history of white grub infestations. Successful management of turf insects depends upon the early detection of pests before they reach damaging levels. This can best be accomplished through frequent turf inspections to detect early signs of insects and their damage.

Like most insect pests, white grubs do not distribute themselves evenly throughout the stand, so it’s essential that the turfgrass area be sampled in a consistent, uniform pattern. Enough samples should be taken to assure a reasonably accurate estimate of pest numbers in the sampled area.

Fortunately, the tools needed for inspection are readily accessible and inexpensive: a shovel and a garden trowel. A sod spade may be easier to use than a traditional shovel, because it is smaller and specifically designed for sod removal. The garden trowel can be used to sift through the chunks of soil when looking for insects.

To confirm the presence of soil-active insects such as white grubs and billbug larvae, cut 6-by-6-inch sections of turf on three sides, peel back the sod, and examine the upper 2 inches of root zone for the presence of pests. A golf course cup cutter can also be used by taking 4-inch-diameter turf/soil cores.

Chemical control

For all white grub species, control is largely an issue of timing and ensuring an appropriate insecticide reaches the area of active feeding. Most of the preventively applied insecticides, including chlorantraniliprole (Acelepryn), clothianidin (Arena), imidacloprid (Merit) and thiamethoxam (Meridian), are systemic in nature and will be taken up by the plant and translocated to the root zone where the grubs are feeding.

Curative insecticides, such as carbaryl (Sevin) or trichlorfon (Dylox), must be watered in for acceptable control. To be successful with these products, mix the insecticide according to label directions, make the application, and then apply .5 to .75 inch of water. If conditions have been very hot and dry and grubs are deeper in the soil, a pretreatment irrigation of .5 inch applied 48 hours prior to the insecticide application should encourage grubs to move closer to the soil surface and enhance the level of white grub control.