Novel chemicals provide new grub-control options
Root-feeding white grubs are a familiar headache for golf-course superintendents and lawn and landscape managers. Until recently, curative treatments with short-residual insecticides were your only practical control options. But with the introduction of newer, more-persistent products such as imidacloprid (Bayer's Merit) and halofenozide (RohMid's Mach 2), you now have preventive-control options as well. To understand how these new products work, let's briefly review the biology of white grubs. Then, we'll consider how you can make both curative and preventive approaches to grub control more effective.
Basic grub biology White grubs are the immature, or larval stage of a group of stout-bodied beetles in the family Scarabaeidae (the scarab beetles). Most of the important species, including Japanese beetles, masked chafers, European chafer, green June beetle, Oriental beetle and Asiatic garden beetle, have 1-year life cycles. The beetles are mainly active between June and mid-August. Beetles lay eggs 1 to 2 inches deep in moist soil of turf or pastures. They hatch in about 2 weeks, and the young grubs, not much bigger than a bluegrass seed, begin feeding on grass roots. The grubs grow quickly, molt (shed their skin) twice and become nearly full-sized by autumn. At first frost, the grubs move deeper in the soil for hibernation. They return to the root zone and resume feeding in early spring. When nearly mature (typically May or early June, depending on species and geographic location), grubs form an earthen cell and transform into pupae, the transitional stage between larva and adult. The adult beetles emerge a few weeks later.
Damage from grubs with annual life cycles usually is most severe in late summer when the grubs are vigorously feeding and the turf is otherwise stressed. With severe infestations, there may be 50 or more grubs per square foot and they may completely consume turf roots. Without roots to extract water and anchor the turf, the turfgrass dies and sod easily lifts from the soil. Skunks, raccoons, crows, moles and other varmints may dig or tunnel in the turf to feast on these juicy "land shrimp," as grubs are sometimes called. Grub damage usually is less apparent during the spring-feeding period. Green-June-beetle grubs have somewhat different habits. They feed mainly on organic matter but damage turf by tunneling and pushing up small mounds of soil.
The black turfgrass ataenius, a sporadic pest of golf courses, departs from this pattern. It has two generations per year in southern Ohio and farther south. You will see grubs in late May and June and again in mid- to late summer. In the Great Lakes states, Upstate New York and New England, you will only find one generation of grubs, which will appear between early and mid-summer.
Curative controls Organophosphate or carbamate insecticides, such as bendiocarb (AgrEvo's Turcam), diazinon, trichlorfon (Bayer's Dylox) and isofenphos (Bayer's Oftanol), have been the most common curative treatments for grubs. These insecticides have fairly short residual toxicity (usually 2 to 3 weeks or less), which provides a relatively narrow window for effective control. If you apply them too early, the insecticide residues may degrade before the eggs hatch. Conversely, if you make late applications or "rescue" treatments, severe turf damage may already have occurred.
You should target curative treatments as soon as possible after eggs hatch, when grubs are small. For grub species with 1-year life cycles, the preferred window for curative control usually is late July to mid-August in the Transition Zone or 1 to 3 weeks later in the northern Great Plains, Great Lakes region and in the Northeast. Optimum timing can vary by several weeks, however, depending on grub species and soil temperatures in a given year. Periodically sample with a spade or golf-cup cutter to pinpoint when eggs have hatched and young grubs are present. Concentrate on probable hotspots, such as south-facing slopes, sites that were irrigated during beetle flights, tee and green banks on golf courses and areas with a history of problems. You must be especially observant to detect the small grubs before the damage gets out of hand. Sampling also helps you judge whether grubs are abundant enough to warrant treatment. Healthy turf often will tolerate at least six to eight grubs per square foot with no visib le damage.
Granular and sprayable formulations usually provide comparable control if you irrigate immediately after application to leach the residues into the root zone. However, granules are somewhat more forgiving if you delay post-treatment irrigation. But remember, without any irrigation or rain, you won't get grub control. Be sure to control thatch too. Excessive thatch prevents infiltration of soil insecticides and reduces treatment effectiveness.
Grub infestations sometimes go undetected until brown patches appear in September or skunks and raccoons start to dig. By then, the grubs usually are nearly full-sized, weighing about 80 times as much as newly hatched grubs. These large grubs are much harder to control. In such situations, mow the turf and collect clippings to increase infiltration of the insecticide into the soil, and irrigate beforehand to bring the grubs close to the surface. Use a fast-acting insecticide such as I mentioned earlier, and be sure to water it into the soil. Fall treatments will be ineffective once the grubs have begun to dig down in response to cooler soil temperatures.
Turf managers sometimes apply curative treatments in spring, after overwintered grubs have returned to the root zone. However, several reasons exist why spring generally is not the best time for curative control. Post-overwintering grubs are large and hard to kill. Weather conditions are moderate, turf is vigorous and the grass usually outgrows whatever damage the grubs may do before pupating. Finally, using a short-residual insecticide in the spring affords no protection against reinfestation by egg-laying beetles flying in midsummer. Spring curative treatments may be warranted, however, if you're reseeding grub-damaged areas that you didn't treat the preceding fall.
Preventive control If you are using a preventive grub-control product, you apply the insecticide before a possible grub problem develops. Preventive control is easy to implement and does not require time-consuming monitoring and sampling, or guesswork to pinpoint proper application timing. In addition, you avoid potential damage, which can result in fewer call-backs for lawn-service companies and peace of mind for golf-course superintendents and grounds managers. The downside is that you must decide to treat before you know the full extent of the grub infestation. Because white grubs tend to be localized and sporadic, you may be unnecessarily treating areas that would not otherwise have grub infestations.
The recent registration of imidacloprid and halofenozide opened a new era of preventive grub control. Both of these insecticides are much more persistent in thatch and soil than other turf insecticides. Thus, you can apply them preventively weeks or even as much as 2 to 3 months before the grubs have hatched. Both imidacloprid and halofenozide have low label rates, exhibit low toxicity to humans and other non-insect organisms and pose relatively little hazard to the environment. Understanding the strengths and limitations of these versatile new insecticides helps you use them more effectively.
Imidacloprid belongs to a new class of synthetic insecticides called chloronicotinyls, which have selective activity on insects' nervous systems. It kills grubs by both contact and ingestion. It is highly active against young, newly-hatched grubs but is much less effective against large grubs. Thus, you must apply imidacloprid preventively, before you see symptoms of grub damage. Imidacloprid translocates within plants, providing good control of stem-tunneling larvae of billbugs and annual bluegrass weevil, as well as grubs. It is not, however, especially effective against caterpillars such as sod webworms, cutworms and armyworms (nor does it have labeling for such pests).
? Halofenozide belongs to another new class of synthetic insecticides called molt-accelerating compounds (the trade name, Mach 2, is an acronym for Molt Accelerating Compound Halofenozide). It works by mimicking the action of ecdysone, a hormone that regulates insect molting. Ingestion of even a tiny amount of halofenozide forces susceptible insects to initiate a premature and ultimately lethal molt. Like imidacloprid, halofenozide is most active against young, actively growing grubs. It also works against turf-infesting caterpillars. Halofenozide will control mid-sized and large grubs but not as quickly as organophosphates and carbamates. Although large grubs stop feeding soon after ingesting halofenozide, they may not die and decompose for several weeks. Grounds managers are finding that when you curatively apply halofenozide after damage appears, it may not control large grubs quickly enough to discourage skunks and raccoons from digging.
Both imidacloprid and halofenozide provide residual control of white grubs for up to 2 to 3 months in turf. This provides you with more flexibility in timing applications as well as an opportunity to target multiple pests. For example, golf-course superintendents concerned with black turfgrass ataenius can apply either product in mid-May to control the spring-generation grubs of that pest and still have enough residual to control annual grub species that hatch in midsummer.
However, be forewarned. If your primary target is the major annual grubs such as Japanese beetles and masked chafers, it makes no biological sense to ap ply preventive insecticides in April or May-several months before egg hatch. You will experience poor control if you apply too early because the residues "run out of gas" before the young grubs appear in late July or early August. In general, the optimum window for controlling annual-grub species with imidacloprid or halofenozide is from about 4 to 6 weeks before egg hatch until the first newly-hatched grubs are present. This interval extends from early June to mid-July in the cool-season and Transition zones.
Comparing the new insecticides If you apply them preventively, before egg hatch, both halofenozide and imidacloprid provide excellent control of newly hatched white grubs. Both products require rainfall or irrigation to move them into the root zone, but they provide more leeway than traditional insecticides in this regard. They generally are effective even if you delay irrigation for up to a week. Neither product is especially effective for curative control of large grubs. Once the damage appears, you'll usually get better results with a fast-acting, short-residual insecticide.
Halofenozide is more active against sod webworms, cutworms and armyworms than imidacloprid, which is not labeled for these pests. Imidacloprid, on the other hand, seems to be effective against a wider range of grub species. When you apply them preventively, before egg hatch, both products provide excellent control of masked chafer, Japanese beetle and black turfgrass ataenius. Halofenozide, however, seems to be less effective against European chafer and Asiatic garden beetle, two non-native species that occur mainly in the Northeast.
Imidacloprid and halofenozide already are paving the way for other novel insecticides that pose minimal hazard to humans and the environment. For example, Novartis is working on thiamethoxam, a new soil insecticide that provides good preventive and curative grub control. Novartis expects registration within the next 2 years. Although you will probably see increased federal restrictions on organophosphates and carbamates, new reduced-risk chemistry likely will fill the void. Even with highly effective preventive products, fast-acting, curative insecticides will always have a role for spot treatments, call-backs and to eliminate the food supply where skunks and other predators are digging.
Regardless of whether you use a curative or preventive approach, the current climate regarding pesticides demands that you make every effort to reduce unnecessary insecticide use. This means using selective treatments rather than routine, "fence-to-fence" applications. Good record keeping is a must. Keep a close watch on areas that had grub problems in the past, because those sites are likely to be reinfested. Learn to recognize adults of the predominant grub species in your area. Sites with abundant beetles are more likely to have a grub problem in late summer. Japanese-beetle grubs often are more abundant in the vicinity of favored adult food plants such as lindens, sassafras and Norway maples. Preventive treatments can be selective and cost-effective if you target them at high-risk sites or where perennial infestations occur.
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