Damage from white grub feeding. Photo by John C. Fech.
Below Deck: A Guide to Subsurface Insects
By John C. Fech and Jonathan L. Larson, Ph.D.
Critters that feed on the all-important root system of turf plants deserve a close look. Sure, leaves do the hard work of photosynthesis, creating sugars and carbohydrates, but half of the heavy lifting is acquiring moisture and nutrients from the soil. Without roots, that doesn’t happen. Keeping root-feeding pests at bay is critical and needs to be a foundational part of the maintenance protocol and budget.
Scouting for subsurface feeders is hard
When it comes to insects that feed underground, you can’t see what might be lurking (and eating) with all of the leaves and thatch in the way. It’s easy to spot stem rust spores on grass blades – the leaves turn orange and leave deposits on your shoes – but not so much with soil-active creatures. In fact, without a proactive approach to pest management, the “out of sight, out of mind” mindset can be pervasive.
Several sampling techniques are effective, starting with recordkeeping. Documentation of insect infestations that have been encountered is invaluable. Records don’t need to be sophisticated; simple phrases in a yearly notebook can suffice. Basic maps of recent outbreaks add real value as well. As a management technique recordkeeping helps to know what to look for from year to year.
The tools for scouting are inexpensive and likely to be on hand in the toolshed. A sod spade (aka square-nosed shovel), heavy-duty pocketknife and a golf cup cutter are easy to use and really helpful for extracting soil, thatch and possibly subsurface-feeding insects. Some of these pests can also be forced from their hiding places by using soap flushes and pouring them on suspect areas. The soap will irritate caterpillars and weevils and drive them to the surface for counting and identification.
Regardless of which tool is used, a “down on your hands and knees” posture should be utilized. In areas previously injured or spots where damage symptoms are observed, remove the sod and start looking/scanning for anything that looks like it could be a responsible culprit. While inspecting, it’s likely that other insects and arthropods such as earthworms, centipedes, pill bugs and sowbugs are likely to be present.
Using your eyes in this manner is particularly important, not only for spotting potential responsible insects, but also for noting the condition of the soil, presence of water drainage limiting layers and the extent of the thatch buildup. All of these factors play a role in overall insect management.
Know what you’re up against
As famous sports broadcaster Keith Jackson would say, grubs are “the granddaddy of them all.” They are sort of the poster child for subsurface feeders. Several species of white grubs can damage sports turf including masked chafers, black turfgrass ataenius, Japanese beetles and May/June beetles. All of these grubs damage turf in their larval stage, except for Japanese beetles, which can cause injury in both the adult and the larval stage.
A good “species separator” for white grubs is to simply measure their size; the smallest one is the black turfgrass ataenius, about half of the size of a small fingernail or 1/4 inch. The next largest is the Japanese beetle’s larval stage which is about twice that size. The largest ones, the masked chafers and May/June beetle, whose larvae start small, reach 3/4 inch in size. You can also familiarize yourself with the grub’s posterior abdominal tip and the spiny hairs that develop there. Different species of grubs have different “raster patterns” that are diagnostic.
Black turfgrass ataenius adults are black (obviously) in color and quite small at about 1/4 inch in size. They overwinter in loose soil, pine needles and leaf litter and begin moving into turf areas in mid-spring. Soon after, larvae hatch and begin feeding on turf roots. Feeding occurs for four to five weeks, then their life stage changes to a resting pupal form for a couple of weeks followed by emergence of adults. In mid-summer, if weather conditions are conducive, a second generation of beetles will develop and prepare to overwinter by mid- to late fall. Large numbers of ataenius larvae are required to cause turf injury.
Japanese beetles have the reputation of being voracious plant feeders and are known to feed on the roots, fruits, flowers and foliage of more than 300 landscape plants, as well as turfgrass. Adults are active from mid- to late summer when egg laying occurs. Their reputation is well deserved – a result of feeding on the roots and rhizomes of most turf species and cultivars. If present in sufficient numbers, severe damage can result from their feeding in a relatively short period of time. Adults are easily identified as hard-shelled beetles with a metallic-green sheen to their wing covers. Larvae appear similar to the other white grub species. Japanese beetle grubs hatch from eggs in mid-summer, feeding through July into September. They will burrow deeper in the soil as fall arrives and overwinter until May the next year when they will pupate and mature into adulthood that summer.
Masked chafer adults are normally present from early to mid-summer. They deposit their eggs into the top 2 inches of soil, usually in small clusters. Hatching occurs in late summer, with immediate root feeding to follow. As the larvae proceed through several stages, the potential for damage grows significantly. As temperatures drop in fall, grubs move deeper into the soil to overwinter. As soil temperatures warm in the spring, they return to the active root zone, feed for a brief period, pupate and emerge as adults to begin a new cycle.
May/June beetles require three years to complete their life cycle. The stages look similar to the other species of white grub, but the process of maturing is extended over a longer period of time. Between the first two years, the grub is feeding and growing. By year three, they will pupate in the early autumn, overwintering in the pupal stage and emerging as an adult the next May or June, hence the name.
White grub management is most often predicated upon the use of insecticides. Managers can consider using either a preventive approach or a curative approach. The choice may come down to the need for perfect turf, budgetary reasons or historical problems. Preventive management is the most common choice and insecticides such as imidacloprid, clothianidin and chlorantraniliprole are frequently used active ingredients. Preventive applications occur in the early part of the growing season, usually being applied between May and July. This can appear counterintuitive as the grubs may not be present or not feeding during this window. These insecticides are systemic and will be inside of the turf for the rest of the growing season, killing the future grubs that may be born in the turf. Spraying these chemicals too early may mean that there won’t be high enough residues of the insecticide later in the season to provide protection. Going out too late means the chemical won’t be transferred throughout the plant when it needs to be. Preventive treatments must also be watered in after application in order for them to work. Core aerification before treatment followed by a uniform application of 1/2- to 3/4-inch of water greatly increases the success rate. Curative treatments, conversely, occur when damage may have already happened. These applications most often use Dylox or clothianidin and are done in August or September.
Many species of billbugs – including hunting, bluegrass and Denver – cause damage to sports fields. They tend to overwinter as adults in sheltered locations in and around infested turf. Hunting billbugs may overwinter as larvae and adults. Most species become active in mid- to late spring, then mate. The females deposit eggs into cavities chewed into plant stems near the crown. Newly hatched larvae feed for two to three weeks within turf stems, then move into the crown and root system and continue feeding on roots and underground stems. It is at this stage when the telltale symptom of stem breakage is evident. To help diagnose damage from billbugs, grab the leaves/stems near the base and give them a sharp tug. If billbugs are responsible, they will easily break off and appear shredded, you may also note sawdust-like material. Billbug larvae look similar to white grubs in coloration, though they are smaller than grubs and are legless.
Depending on the species and overwintering styes, pupation can occur in spring or in mid-summer. In most cases, adult weevils emerge in late summer and move to overwintering sites in hedges, tall grass and tree leaf litter. The greatest injury occurs in early summer as the turf transitions from supportive to stressful conditions for root growth.
Managing billbugs can be complicated when compared with white grub control, and may necessitate more monitoring to be successful. One option is treating for adult weevils that are emerging from their overwintering spots. Reading about billbugs in local Extension factsheets or using pitfall traps can help managers to time their application of pyrethroids, carbamates or organophosphates to turf. These contact products would work by killing adults as they move through them; avoid irrigating these early-season treatments as you don’t want to wash the residue off before it has a chance to kill the weevils. Luckily, billbugs can also be controlled through preventive means. The same products and timing will work to prevent these pests as white grubs, though there can be decreasing success by mid-June, making it a tighter window for management. If you find yourself needing to treat for a recently discovered billbug problem, a curative approach using neonicotinoids, pyrethroids or an insect growth regulator (such as Novaluron) can be continually done according to label directions until the problem has been managed.
While mole crickets can cause damage from feeding on turf roots, stems and leaves of turfgrass, their extensive tunneling is the key objectional behavior as they are disrupters, occupying portions of their lives below ground. Mole crickets can startle some people, they have a somewhat “cricket-like” appearance, but their front legs have been modified into scraper like digging legs, giving them a mole-like look. There are three species that are most commonly encountered; the northern mole cricket (Neocurtila hexadactyla), the tawny mole cricket (Scapteriscus vicinus) and southern mole cricket (Scapteriscus borellii). The northern mole cricket is native to North America and is rarely a pest of turfgrasses. The tawny and southern species are invasive and are serious pests of turf throughout the southeastern United States. Blade-like projections from the forelegs help distinguish between the various mole cricket species. Depending on the location, there can one to two generations of mole crickets per year with the height of damage occurring August-October.
Mole crickets can be managed using insecticidal baits, most often applied in the late spring or early summer. Fipronil, pyrethroids, neonicotinoids and combination products containing both pyrethroids and neonicotinoids are all employed in the management of mole crickets. These can be applied throughout the growing season, with varying lengths of management depending on timing. Monitoring for damage and the crickets can help inform the timing of sprays.
Sod webworms have the unique distinction of being categorized as both surface-feeding and subsurface-feeding pests. Webworm adults are moths that do not damage turf. Larvae feed at night on leaves and stems near the soil surface and hide during the day in silk lined burrows, which extend through the thatch layer into the soil. Small ragged brown spots are the first symptoms of damage; upon closer inspection, these areas have a grazed or scalped appearance. Over time, the spots coalesce and enlarge. While webworm larvae are active from mid-spring to mid-fall, the most serious injury occurs in mid- to late summer when populations are high and root systems are stressed from high soil temperatures.
Pyrethroids are most commonly used when this pest is first found. Applications are most effective when they follow close mowing and removal of the clippings to reduce interference with the product contacting the target species. A thorough irrigation event prior to application will move larvae closer to the surface, in closer contact with the formulation. Granular applications should be followed by a light irrigation to wash the product granules off the leaves and activate the insecticide. Liquid applications may also be enhanced by light applications of water, but, as with all pesticide applications, it’s best to consult the label for specific details that are germane to the product. Chlorantraniliprole, when applied for grub control, would also provide season-long prevention of webworms.
John C. Fech is a horticulturist with the University of Nebraska-Lincoln and certified arborist with the International Society of Arboriculture. The author of two books and more than 400 popular and trade journal articles, he focuses his time on teaching effective landscape maintenance techniques, water conservation, diagnosing turf and ornamental problems, and encouraging effective bilingual communication in the green industry.
Jonathan L. Larson, Ph.D., is an Extension professor for the University of Kentucky who provides insect expertise for people dealing with issues in urban landscapes, turfgrasses, nursery crops, greenhouses, and households. He teaches across a variety of platforms using social media, videos, and traditional Extension publications. He is one of the co-hosts of Arthro-Pod, an entomology podcast.