On the Level
Insects. Everybody's got 'em. Good ones and bad ones. But should you worry about them? You bet. Not only should you worry about the damage they are causing, but also about how many of them are crawling around on your turf. It pays to find out how many you've got — literally. It pays in terms of the cost of insecticides you may buy for treatment, and for the labor needed to apply them. In some cases, you may just leave them alone, even when they are causing damage. In other instances, it's obvious you'll need to treat turf to eliminate them. But if you need help deciding whether to treat, look to established threshold levels. The concept of how many insects are present in a certain unit area of turf and how many it takes to cause unacceptable damage is called a threshold.
SAMPLING AND MONITORING TECHNIQUES
You may have heard the phrase, “Don't guess, soil test.” It refers to the importance of performing soil tests to determine how much fertilizer you need to apply if, for example, the pH is askew or if the organic matter content is too low. The same idea applies to insects: There's no good reason to guess about the numbers in your turf.
If you're standing over an ugly patch of sod and suspect an insect problem, be ready to get dirty. If it's a grub problem or other subsurface insect, you've got to dig. As with other turf management operations, you'll need the appropriate tools for scouting and sampling. For grubs, a sod spade, shovel, light trap and golf course cup cutter will be useful. The first step is to get down on your hands and knees and grab a handful of turf blades. Tug on the blades, pulling upwards. If the sod pulls loose with little effort, then it is likely that grubs have eaten the roots of the turf. Peel away the sod in several directions and look for the underlying grubs. They are easy to spot — large, white and C-shaped — and you'll usually find them at the bottom of the thatch layer or in the upper inch of soil. In the early stages of white grub feeding, roots may not have been completely severed. This is where a sod spade will come in handy. Use it to strip off sections of sod where you see early symptoms of white grub feeding.
Surface insects, such as sod webworms, are tougher to locate. First, they are much smaller — usually only a half-inch or so in length. Second, they are usually the same color as their surroundings: a light green to tan hue. Third, they hide in the thatch and are often burrowed into a tunnel. To coax them to the surface for easier identification, mark off a square yard of turf and mix up a solution of dish washing detergent or the commercial product DetectAid. Use enough detergent to irritate the worms: about ¼ cup per 2 gallons of water should suffice. Pour the mixture on the square yard of turf and let it soak in. If webworms are present, they will wiggle their way to the soil surface in about 10 minutes to avoid the irritation of the mixture.
GETTING YOUR TIMING DOWN
Looking for pests at the right time is also important. Consider the life cycle of each pest when determining the best time to sample. Masked chafers, for example, damage the turf only in the larval stage, while Japanese beetles feed on turf as grubs and on various ornamentals as adults. Neither cause any injury as eggs or pupa.
Annual white grubs cause only one period of injury per season, whereas other species such as the black turfgrass Ataenius can injure turf in both the early and late summer. Therefore, determine when each pest is present in your area of the country and sample accordingly. This information is readily available with a quick call to the local Cooperative Extension office in your county or region.
Sod webworms feed on turf only as larvae (caterpillars). However, there is plenty of opportunity for injury because they may have several generations per season. Most sod webworms species overwinter as larvae, and can cause damage to the turf the following spring.
They complete their life cycle in early summer and transform into the adult (moth) stage. These moths begin laying eggs within a few days. Because the sod webworm's lifecycle is relatively short (about one month), two to four generations per year are common. Considering this, damage from sod webworms is possible several times during the growing season.
HOW MANY IS TOO MANY?
Determining a treatment threshold involves consideration of many factors: the use of the site, the species of turf grown, the maintenance level provided for the turf and the resources available for control. The thresholds described in “Treatment Threshold,” page 16 are estimates of the average number of white grubs necessary per square foot of turf to produce visible injury. They should be helpful when mulling over treatment decisions. Remember that the condition of the turf, its value and the damage caused by birds and animals searching for grubs may alter these thresholds. In general, if white grub numbers exceed these thresholds in non-irrigated turf, an insecticide application is justified. Irrigated areas should be able to withstand substantially more white grub pressure before visual injury occurs. Treatment decisions should be based on average numbers of white grubs detected in the sampled area. If white grub numbers exceed threshold levels in only a few isolated patches, consider controlling these grubs with spot-treatments.
Treatment thresholds for other turf pests such as billbugs, mole crickets, webworms, cutworms, armyworms and chinch bugs are more variable, but will again vary depending on pest species, abundance, and life stage; species and cultivar, vigor and value of the turfgrass; relative effectiveness and cost of control measures; and time of year. The location and function of the site will again have an impact on the treatment threshold. Again, contact you local Cooperative Extension office for treatment thresholds appropriate for your area.
As mentioned above, a threshold can vary depending on a variety of factors. Near the top of the list is the expectation of the customer. Generally, expectations of high turf quality require higher maintenance inputs and often translate into lower insect thresholds. Clients with high expectations include upper-end office complexes, banks and golf courses. Similarly, low expectations require lower maintenance inputs, which equates to higher insect thresholds. Rural cemeteries, city parks and most acreage landscapes fit the lower expectations category. Home lawns are quite variable in terms of customer expectations.
The local microclimate of a given site can have a dramatic effect on the treatment threshold. Proper positioning of plants is critical, as certain plants become stressed when not located according to their need for sun and shade, drainage, wind protection, pH range and nutrient availability. A popular phrase embodies this first consideration: “right plant, right place.” For example, Kentucky bluegrass turf growing on a steep slope in the full sun is a very likely candidate for insect problems. Bluegrass turf in full sun requires an inch or more of water per week to survive. The slope will not facilitate adequate water infiltration; even if an inch is received, most of it will run off. This will decrease the depth of the root system and the density of the turf stand. Sloped turfs are also quite susceptible to scalping injury. These factors create a stressed turf — one that is unable to resist damage from even a few feeding insects. In this scenario, a lower maintenance turf, or possibly a groundcover, would be a better choice. Regardless, the threshold is reduced because of improper positioning.
- Turf Species
The species itself will often influence the threshold level. Sod forming grasses such as bermudagrass and bluegrass have the potential to recuperate from insect feeding injury, while bunch grasses such as perennial ryegrass and tall fescue do not. These grasses can produce new stolons and rhizomes to fill in damaged areas and re-establish the turf. The depth of the root system is also important. Tall fescue turfs usually develop very deep root systems, which provide the turf with an advantage in terms of water absorption and tolerance to insect feeding.
- Time of year
The time of year can affect the threshold in two ways. First, if you notice damage near the end of the feeding period for a particular insect, it is less likely you'll make the decision to treat than if it were closer to the beginning of the period. After all, if you've got a couple dozen annual white grubs per square foot in mid April, you're less likely to treat than if you see the same number in late August. The time of year also can affect rooting depth and, thus, the need for treatment. Creeping bentgrass provides a good example. This species has an annual root system, one that grows relatively vigorously in fall and spring. But as summer temperatures start warming the soil to temperatures above 70°F, the root system shuts down; the roots simply start sloughing off and dying. Damage can occur from only a few pests during this vulnerable stage.
- Proper establishment
Proper establishment of the turf is also important. Soil testing, incorporation of preplant nutrients and organic matter, good seed-to-soil contact and proper follow-up procedures will create a turf with a deep, extensive root system, which offers some level of tolerance to grub and other soil-dwelling insect damage. Establishment can be tedious, but don't underestimate its importance in minimizing insect damage.
Turfgrass maintenance is often overlooked because it can be mundane. However, proper attention to adequate watering, fertilization, aerification and mowing will keep the plants healthy, thrifty and naturally able to resist many insect infestations. For example, a properly mowed, fertilized and watered turf will develop a deep, fibrous root system. If insects become established, turf stands with extensive roots will suffer less damage than those with shallow, weak root systems. In contrast, inadequate irrigation and drought stress may compound damage to turf.
John Fech is an extension educator and Frederick P. Baxendale, Ph.D., is professor of entomology and extension specialist, both at the University of Nebraska (Lincoln, Neb.).
|Grub Species||# per square foot||# per 4-inch core|
|Black Turfgrass Ataenius||30-50||3-5|
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