Give grubs the BOOT
By learning a few fundamentals, you can successfully manage white grubs before they become a problem.
White grubs are among the most common and destructive turfgrass pests, especially in cool-season turf and the Transition Zone. Year after year, they probably cause more damage than any other turfgrass insect pest. They are found in all 50 states, on all types of turf, and are among the most difficult insect pests to control. Gone are the days when many grounds managers followed the philosophy of spray early and spray often. However, all is not lost. Learn a few aspects of white grub biology and some management techniques. Also, take advantage of newer grub control products. Empower yourself and take charge of your white-grub control program. Your turf will shine.
Biology and identification
An understanding of white grub biology, behavior and damage will help you develop a successful management program. To do this, you must also learn to accurately identify your pests. At least 8 species of white grubs damage turfgrasses in the United States. The adults (beetles) of these species are dissimilar in many aspects and easy to identify. However, their larvae (white grubs) are nearly identical.
All white grubs have cream-colored, C-shaped bodies, tan to reddish-brown heads and three pairs of short legs behind the head. When fully developed, they range from 0.25 inch to more than 1 inch in length, depending on the species.
Because different white grub species have different biologies, it is essential for you to be able to differentiate them. To do this, consult a textbook that displays the different raster patterns for each species.
If problem species exist in your turf, control options may be necessary. Use the biology of each species to determine when you should treat. You should target grubs when they are young and susceptible to controls. However, the date of this stage varies by species. Check your local cooperative extension office for more information about species prevalent in your area.
Determining the presence of grubs
White grubs feed on the roots and rhizomes of all commonly used turfgrass species, and they are capable of quickly destroying large areas. Look for localized patches of pale, discolored and dying grass displaying symptoms of moisture stress. These areas are small at first, but rapidly enlarge and coalesce as grubs grow and expand their feeding range. Roll back the sod and look for the white grubs underneath. They will be easy to spot. Several animals, including skunks, raccoons and birds, will be attracted to insect infestations in your turf. Look for their signs of foraging. It is a good indicator of white grub activity.Sampling before damage occurs.
Begin sampling early in the predicted grub activity period, before injury is visible. White grubs do not distribute themselves evenly throughout the turf stand, so you must sample the entire area in a consistent, uniform pattern. At each sample site, cut two 6x6-inch sections of turf on three sides. Peel back the sod and examine the upper 2 inches of root zone for the presence of white grubs. You can also use a cup-cutter for sampling, however you will need to convert the data to per square foot. Sample every 10 days or so as the season progresses. If you implement control options, use sampling to determine the need for follow-up treatments.
If you detect white grubs, make your management decisions in the context of thresholds. They are estimates of the average number of white grubs necessary per unit area of turf to produce visible injury that justifies intervention. Thresholds are based on many variables including pest species, life stage, vigor and value of the turfgrass and the time of year. Typically, the threshold of turfgrass under close visual scrutiny (residential, golf course or high-end commercial property) is anywhere between from 5 to more than 25 grubs per square foot, depending on the species. However, other non-scrutinized settings such as industrial sites or cemeteries may have a higher threshold. Threshold guidelines for specific insects on a variety of landscape plants and turfgrasses are available from many sources, including your local cooperative extension office.Examine factors that affect thresholds.
In general, if white-grub numbers exceed threshold levels in non-irrigated turf, you need to make an insecticide application. However, in irrigated areas, turf can withstand higher numbers before visual injury occurs.
Pay attention to your soil, plant selection and maintenance efforts; they affect the vigor of plants and their ability to tolerate grub damage. For example, Kentucky bluegrass turf growing on a steep slope in full sun is a candidate for grub problems. Even under irrigation, the slope will not facilitate adequate water infiltration to maintain plant vigor. This area will have a lower threshold.
Consider spot treatments. Base your treatment decisions on the average number of white grubs that you find in the sampled area. If white grub numbers exceed threshold levels in a few isolated patches, consider spot-treatments to control the infestations.
When using most white grub insecticides, you must get the insecticide down to the root zone where the grubs are feeding. To do this, you must irrigate following treatments. Follow label directions concerning post-treatment irrigation; they differ among products. Also, thatch plays an important role in reducing the efficacy of turf insecticides. If the thatch layer exceeds 0.5 inch, aerify or de-thatch before treatment. Repeat irrigation every 4 or 5 days to continue moving the insecticide into the soil.
When white grubs are deeper in the soil, curative treatments can be more effective when you apply a pretreatment irrigation 48 hours before the insecticide application. This encourages grubs to move closer to the soil surface and enhances the level of control.
Prevention, cures and IPM
When "preventive" grub controls first became available (see section "Preventive controls," below), they challenged the conventional wisdom of integrated pest management (IPM) which says, in effect, don't apply controls before you know you need them.
The new preventive grub controls don't necessarily fit this paradigm for a couple of reasons. First, they simply work better when you apply them early (before or at egg laying or hatch) because they are most effective on the youngest stages. If you waited until grubs grew larger and then decided you needed to treat, you'd likely get better control by using a curative product. And because traditional curative products tend to require considerably higher rates of active ingredient, the net effect of waiting to be sure could mean more pesticide applied to the site, not less - a counterproductive strategy if one of your goals (like many IPM advocates) is to reduce pesticide use.
Further, regardless of the time of application, the narrow target spectrum of the new preventive grub controls reduces non-target effects, such as on beneficial insects.
Nevertheless, you certainly don't want to apply products that you don't need, no matter how benign they may be. If nothing else, it's a waste of money. So what approach should you take with preventive products? Turf managers usually have good success with the traditional strategies: observation and common sense. Grubs tend to re-infest the same sites annually. Watch for adult beetles, which often gather at lights at night. Significant numbers may portend a heavy hatch in the near future. Simple observation and recordkeeping can, therefore, predict with a high likelihood that you can expect grubs to be present in places with a history of infestation. These sites are good candidates for preventive controls.
These strategies do not replace the sampling we describe in this article. You should perform sampling whenever practical. However, they provide some guidance of the use of preventive controls, which is especially important in light of the impending loss of some of the best curative controls
Most white grub insecticides belong to the carbamate and organophosphate chemical class. They are relatively fast-acting with short (2- to 3-week) residual activity. With these controls, you have a narrow treatment window. They are most often used curatively to treat existing infestations.
Following is a list of the controls labeled for white grubs and some information pertaining to their history and use. Also listed are two biological controls that have been used with some success.Bendiocarb (Turcam).
A member of the carbamate family; production of Turcam will stop sometime and 2001. The most effective treatments are in late summer rather than spring.Carbaryl (Sevin and other brands).
The first successful chemical in the carbamate family of insecticides, carbaryl was introduced in 1956. Apply it when grubs are actively feeding and near the surface. Control different species with different rates. If necessary, you can make multiple applications with a minimum of 7 days between treatments. The efficacy of carbaryl is dependent on soil pH so be sure to read label specifications.Chlorpyrifos (Dursban and other brands).
An organophosphate; the EPA halted chlorpyrifos production for use in the residential turf and ornamental market in the summer of 2000, existing stocks are still available though. It is still registered for control of green June beetle grubs in golf course turf.Diazinon.
An organophosphate, diazinon has been in use since 1952. In December 2000, Syngenta, the primary producer of diazinon for the turf and ornamental market announced a voluntary 4-year phase-out. It controls grubs of Japanese beetle, European chafer and Southern chafer. Apply between late July and early October. Irrigate thoroughly after application.Ethoprop (Mocap).
Another organophosphate, ethoprop is only labeled for use on golf courses. It controls grubs of Japanese beetles, chafer beetles, May/June beetles and black turfgrass ataenius. Avoid applications to wet foliage.Isofenphos (Oftanol).
Isofenphos also is an organophosphate. Some labels suggest use as a preventive with applications in mid-April through mid-May. For curative applications - the more typical use - apply in July or early August. Production of isofenphos was phased out at the end of 1999. However, existing stocks should be available at least through the end of 2001. Trichlorfon (Dylox and other brands).
Trichlorfon is another organophosphate. Apply it when larvae are young, actively growing and feeding near the soil surface. Irrigate after treatment. If the thatch layer is greater than 0.5 inch, it must be removed. A second treatment can be applied on mature grubs.Beauveria bassiana (Naturalis T&O).
This is a biological product containing an insect-specific fungus that works through contact action. The addition of a wetting agent helps soil penetration. Irrigation must follow treatment. It controls all major grub species. Because it is a fungus, you cannot apply this product with fungicides.Heterorhabditis bacteriophora (Cruiser).
This also is a biological product consisting of parasitic nematodes. These nematodes are mobile in the soil and seek out insect hosts. It is effective for control of all major grub species and should be applied following egg laying by adult beetles.
The preventive insecticides are typically slower acting (but no less effective) with longer residual activity. Use these treatments when a history of grub infestations or early scouting points toward a probable infestation. They are typified by low application rates and target specificity and are most effective when you apply them early in the season, before eggs hatch or while the grubs are still small.Halofenozide (Mach 2).
Halofenozide is a member of the new diacylhydrazine class of insecticides. Registered for turf grub control in 1997, this compound interferes with larval molting but does not kill grubs immediately. However, feeding cessation does occur within a few hours following ingestion. Halofenozide has a long soil residual so you can make treatments preventively before eggs hatch. The residual will affect hatching grubs for many weeks. It controls most of the major grub species andbecause it is systemic it does not require irrigation for effective control.Imidacloprid (Merit).
Imidacloprid is a member of the chloronicotinyl subclass of the neonicotinoid insecticides. Imidacloprid works by contact and ingestion to affect the insect's nervous system. This chemical has longer residual than traditional grub chemistries and is well suited for applications before or during the egg-laying stage. The residual remains through the hatching period and kills newborn grubs. Irrigate following application.Thiamethoxam (Meridian).
Thiamethoxam is a member of the thianicotinyl subclass of the neonicotinoid insecticides. This chemical is in the final stages of registration and should be available later this year. It acts via contact or ingestion and will provide some control during all developmental stages, except the egg. Thiamethoxam has moderate soil persistence and insects demonstrate affected behavior within an hour of contact.
Refer to the Insecticide Update for a list of brands and manufacturers and other useful information on these grub controls.
Controlling this damaging pest involves more than knowing what chemicals to use. Identify the species you encounter to determine whether they pose a threat. Also, determine thresholds and prioritize treatments based on the location of your turf. Consistently monitor your sites to determine when to make your applications and whether your controls are working. If you follow these fundamentals, you will be able to consistently keep white grubs in their place - the ground.