Easy to see is easy to fix. That phrase is certainly true for fixing the plumbing in your house. Of course, whenever a leaky pipe starts causing trouble, it’s usually behind the drywall or under the sink. It’s also true for the turf world, at least for the most part. Insects that feed on turf roots are usually much more difficult to deal with than surface feeders, such as aphids, chinch bugs and armyworms.
Roots are important
A case could easily be made that insect damage to roots is twice as bad as damage to shoots, stems and leaves. Not only do roots stabilize the plant, they also draw water and nutrients from the soil. No roots, no turf; it’s that simple.
Out of sight
Since you can’t see them, it’s important to frequently inspect roots for the presence of damaging insects. Fortunately, the tools needed to inspect your turf are readily accessible and inexpensive: a shovel and a garden trowel. A sod spade may actually be easier to use than a traditional shovel, because it is smaller and specifically designed for sod removal. The garden trowel is 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 rootzone for the presence of pests. Turf managers with access to a golf course cup cutter can substitute 4-inch-diameter core samples. If no white grubs are detected but damage is present, examine the turf for other causes of injury, such as disease, excessive thatch, moisture stress, heat damage and sod webworm or billbug feeding.
A “soap flushing” technique can also be helpful in determining if specific insects, such as billbug adults, chinch bugs, mole crickets, sod webworms, armyworms and certain cutworm species, are present in the turf. Mix 1/4 cup of lemon-scented liquid dish soap in 2 gallons of water and sprinkle over 1 square yard of turf. This disclosing solution irritates the insects, causing them to move to the surface in five to 10 minutes, where they can be counted. Flushing with a pyrethroid insecticide is also effective.
If insects are found, you must then decide if sufficient numbers are present to warrant control. In many cases, a few white grubs, billbug larvae or mole crickets will be found, but not in sufficient numbers to cause significant damage to the turf. This is the concept of treatment or action thresholds: the critical number of insects needed to injure the turf and justify control. Remember, the condition of the turf, its value and damage caused by foraging birds and animals may alter these thresholds.
When sampling for turf insects, avoid making hasty treatment decisions after spotting just a few insect pests. It is important to realize that in many, if not most, situations turf problems are caused by multiple factors. Nutritional deficiencies, diseases, excessive thatch, as well as abiotic factors, must be considered along with the presence of insects. It will be difficult to effectively deal with the problem if a multidimensional situation is improperly diagnosed.
White grubs are the larval stage of a group of beetles collectively known as scarabs (Family 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 different scarab species are present in the U.S., but only a relative few cause significant injury to turf. Among the more important 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 similar in appearance. They have C-shaped, creamy white bodies, reddish brown heads, and three pairs of legs. When fully grown they range from .25 to 1.5 inches in length, depending on the species. The best way to tell them apart is to look at the arrangement of hairs and spines on the underside of the last abdominal segment (raster). Distinct patterns can be 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.
White grub control—For all white grub species, control is largely an issue of getting the treatment material to 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 rootzone, 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, aerate the affected area before treatment, mix the insecticide according to label directions, make the application and apply at least .5 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 before the insecticide application should encourage grubs to move closer to the soil surface and enhance the level of white grub control.
Where a preventively applied insecticide has failed to provide adequate control, or when a white grub infestation has gone undetected, a rescue treatment may be the only recourse for the turf manager. Rescue treatments typically employ fast-acting, short-residual products such as such as carbaryl (Sevin) or trichlorfon (Dylox). Post-treatment irrigation is critical in these situations.
Adult billbugs are typical weevils (or snout beetles) with mouthparts located at the end of a curved snout or bill. These insects, which are about .25 to .5 inch long and dark brown to black, are slow moving and frequently “play possum” when disturbed. Billbug larvae (grubs) appear similar to white grubs but are legless. They have cream-colored bodies with brown heads, and when fully developed are about .25 to .5 inch long, depending on the species. Their bodies are slightly curved and resemble a grain of puffed rice.
After emerging from the egg, newly hatched billbug larvae tunnel in grass stems, hollowing out the stem and leaving fine sawdust-like plant debris and excrement called frass. Infested stems discolor and when pulled, readily break away at or near the crown. Subsurface feeding by older larvae can completely destroy the plant’s crown and upper root system, causing the turf to appear drought stressed. Under heavy billbug pressure plants will eventually turn brown and die.
Billbug control—Effective cultural practices can significantly reduce billbug damage. Selection of adapted turfgrass cultivars, as well as proper fertilization and irrigation programs will minimize the impact of billbug infestations. In addition, several endophyte-enhanced, billbug-resistant cultivars of perennial ryegrass and tall fescues are now available.
Billbug infestations can be controlled by reducing the number of overwintered adults before they have an opportunity to deposit their eggs in the spring. An insecticide application is usually justified when visual observation or irritant flushes confirm the presence of one adult per square foot of turf. If warranted, apply insecticides to freshly mowed turfgrass (collect and remove clippings) and irrigate lightly after application to wash the insecticide off grass blades onto the soil surface where billbug adults are active.
Insecticidal control of billbugs can also be achieved by using systemic insecticides, such as Acelepryn, Arena, Meridian or Merit. Application should be made prior to eggs being deposited by females into grass stems. Control is probably warranted if billbug larvae are expected to exceed 25 to 30 per square foot of turf.
Somewhat cricket-like in appearance, these insects have extended forelegs that are used to dig in the soil. The northern mole cricket (Neocurtila hexadactyla) is native to North America and is rarely a significant pest of turfgrasses. The tawny mole cricket (Scapteriscus vicinus) and southern mole cricket (Scapteriscus borellii), however, are serious pests of residential and recreational turf throughout the southeastern U.S. Blade-like projections on the forelegs help distinguish the various mole cricket species. Mole crickets feed on the roots, stems, and leaves of turfgrasses, but their extensive tunneling is often the most damaging to the turf.
Mole cricket control—Effective mole cricket control requires careful monitoring of the pest’s life cycle and timely insecticide applications. Different management strategies are needed depending on the time of year (spring, summer or fall). Spring treatments target adult mole crickets, whereas summer treatments target the nymphs, and fall treatments target both nymphs and adults.
The best time to treat for mole crickets is early summer. At this time, cricket nymphs are still small and spend more time near the surface, making them easier to kill. Most labeled insecticides will provide adequate control at this time. By late summer and fall mole crickets are larger, their activity and damage increases, and they are much more difficult to control. Bait formulations, however, may be effective against larger nymphs in late summer. Because mole crickets are active at night, for best results, insecticides should be applied as late in the day as possible.
John C. Fech is a horticulturist, certified arborist and frequent contributor located in Omaha, Neb. Frederick P. Baxendale is a professor and extension entomologist with the University of Nebraska-Lincoln.