Determine threshold levels for turf pests
It's virtually impossible to eliminate pests. But knowing when to treat can keep their numbers down to an acceptable level.
As I sit here looking out the window at several feet of snow on the ground, spring and summer seem like a distant memory. And while most insects are quiet during this time of year, it is a good time to evaluate your turfgrass pest decision-making process. Particularly, you should determine treatment thresholds: at what point you should treat for turf pests before they cause economic damage.
Defining thresholds Historically, treatment thresholds originated in agriculture. More recently, treatment thresholds were developed for turfgrass. In pest management, a threshold is defined as the line between acceptable and unacceptable pest populations or injury. In production agriculture, thresholds have an economic basis and include an economic injury level (EIL) and an economic threshold (ET). According to V.M. Stern, an IPM researcher, an EIL can be thought of as the lowest pest population density that will cause economic damage. In other words, the cost of controlling the pest equals that amount of damage caused by the pest. Stern defines the ET as the number of pests present at which you should employ pest-management options to prevent the population from reaching or exceeding the EIL. The ET and EIL may be very different because the most economical way to prevent pests from reaching the EIL is to treat well before pests approach that population level.
Turf and ornamental thresholds In landscapes and turfgrass settings, thresholds take a little different form and, according to arboriculturist R.W. Harris, may be based on the intensity of a pest population that affects the overall health and longevity of the plant (commonly called action thresholds). Another major aspect of ornamental horticulture is the appearance of plants or aesthetics. In certain situations, pest populations may not reach levels that affect the health of the plant (action thresholds), but the appearance of the plant is unsatisfactory and unacceptable to the client. These thresholds are commonly known as aesthetic thresholds and are defined as the highest level of a pest population or damage that would be acceptable to most of the people who use the affected area.
While thresholds can be helpful in pest-management decision making, accurate pest identification and monitoring are essential and a prerequisite to using thresholds. If you cannot identify the insect responsible for the observed damage, or if you do not have a grasp of insect pest density (i.e. grubs per square foot), thresholds will be of little value.
Once you have properly identified the pest, you have the basis with which to decide whether treatment is necessary. In the paragraphs that follow, I will discuss how to determine the thresholds for three prominent turfgrass pests.
Grubs One of the major groups of turfgrass pests are "grubs," or the larval stage of scarab beetles. Common examples include the annual white grub, true white grub (larvae of adult May/June beetles), Japanese beetle grub and the black turfgrass ataenius. The grubs of all of these insect pests feed on the root system of turfgrasses including, but not limited to, Kentucky bluegrass, bentgrass and annual bluegrass. Roots are pruned off by the grubs and large brown patches of turfgrass will appear.
m Identification. The annual white grub and Japanese beetle grub have a 1-year lifecycle while the true white grub has a 2- to 4-year lifecycle with an average life of 3 years. Consequently, depending on your situation, it is possible to have all three of these species present at a given location. The black turfgrass ataenius differs from the previous pests in that it has two generations per year with potential grub damage occurring all season long.
How do we tell one grub from another? You have to be willing to look at the south end of a north-facing grub. You can identify beetle grubs by using the pattern of stiff setae or hairlike structures, known as raster patterns, on the underside of the last segment of their abdomen. In the case of the annual white grub, their raster pattern is random (i.e. no pattern), while the raster pattern of a true white grub resembles a zipper, and the Japanese beetle grub has an inverted "V" raster pattern. Black turfgrass ataenius grubs lack a definite raster pattern, but have two distinct pad-like structures at the end of their abdomen.
m Monitoring. Once you have determined the grub species or mix of grub species, you can begin the assessment process to determine whether treatments are necessary. However, before you start making plans to put down chemical treatments, make sure you sample the damaged area. Do not base your decision to treat on just one sample. Take a number of samples; the more the better. This will give you a better and bigger picture of what your grub population is and where the "hot spots" are located. Sampling for grubs is reasonably simple. If you can pull the sod up in large patches or sections, it is quite likely that you have grubs. Look at the root system and examine the soil directly underneath the affected areas. The grubs will lie right on the soil surface and look up at you (be sure to smile back)! If the brown patches or damaged areas are firmly rooted, grubs are not your problem.
m Mapping. Make a map of the affected areas. Compare it to the unaffected turf so that you can gain a better picture of the distribution of grub populations and potential trouble spots. Also, use sampling to determine the effectiveness of treatments and make appropriate pest-management decisions.
m Thresholds. Treatment thresholds for the annual white, true white and Japanese beetle grubs generally are 10 to 12 grubs per square foot. Populations greater than this have the potential to cause dieback of the turf, resulting in economic damage. The black turfgrass ataenius differs. Because of their small size, it takes more of them to cause damage. The damage threshold for these grubs is 50 grubs per square foot. Keep in mind that with insects like the black turfgrass ataenius, which has two generations per year, damage may be more severe during the second generation compared to first generation. This is seen quite commonly among insect pest populations that have multiple generations per year.
Sod webworms m Identification. Another example of a turfgrass pest with two generations per year is the sod webworm. This insect feeds as a larva by clipping the grass blades about 1/8 inch above the crown and then transporting the foliage back to its silken bag to consume it. In some areas, a third generation may develop, depending on the weather. Second-generation feeding damage is usually more severe.
m Monitoring. How do you determine if sod webworms are the culprit? Because they tend to inhabit the area near the crown of the grass, visual inspection is possible but is usually tedious and time consuming. A simpler method involves watching for adult moths. The adults are weak and erratic fliers. They typically will fly up in front of mowers and even foot traffic. Make a mental note of these events because, approximately 2 weeks after a heavy moth flight, you should be on the lookout for larval damage. Cool and damp weather will reduce larval populations and potential damage. Many times birds, such as starlings, will be highly attracted to turf areas where larvae are present. Holes that are 1 to 2 inches deep and 1/2 inch in diameter are associated with bird feeding. Another easy technique for larval monitoring is applying an irrigating drench of pyrethrum (1 teaspoon of 6-percent pyrethrum per gallon) or liquid detergent (1 ounce of detergent per gallon of water) to the affected area. The drench will not kill the larvae, but it will irritate them and bring them to the surface where you can accurately assess larval populations.
m Thresholds. Because of their hardy appetite, particularly as the larvae mature, it takes only about two larvae per square foot to cause economic damage. Therefore, treatments may be justified if any of the sampling measures indicate sod webworm presence.
Greenbugs m Identification. An insect pest that has the potential to cause aesthetic damage to turf is a sap-feeding insect known as the "greenbug" or grain aphid. This insect causes the turf to turn yellow or brown as result of feeding activity. Because of their migratory nature, damage is more severe in some years than other years.
m Monitoring. Visually examine the affected areas, particularly under trees or other vertical objects. Greenbugs like the shade and do not thrive in locations that receive intense sunlight, so greenbug damage may be apparent to the east or north of a building, fence or other shade-producing structure. Proper monitoring for this insect is important. Greenbugs are commonly found in areas between dried-out, brown turf and green healthy turf.
m Thresholds. There are no specific damage thresholds for aphids. If your monitoring program indicates aphid presence, be on the lookout for the signs of damage. If aphids are present, apply effective treatments.
You make the call The treatment suggestions outlined in this article are tools, not recipes for treatment, and are only as good as the person using them. Client expectations, weather and natural factors will all impact insect pest populations. You will have to make adjustments depending on your location. High-end residential or commercial properties may not tolerate any aesthetic damage. For these clients, you may need to make preventative treatments before insect populations reach aesthetic thresholds.
What I have provided here are suggestions to aid you in making good choices. The more you use them, the more you will learn about your most common pests, their potential to cause serious damage and the effectiveness of your treatments. Experience and time will contribute to your knowledge and make you a more informed and effective turf manager.
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