Practically all turf managers have heartily accepted the new grub insecticides, Merit® (imidacloprid, Bayer Corporation) and MACH 2® (halofenozide, RohMid). Both of these new insecticides have long residual activity, greatly reduced toxicities and diverse effects on other insect pests. Unfortunately, as with all new products, the salesperson as well as the users claim they have had to adjust their expectations and use practices. Merit and MACH 2 are completely different in their actions on insects.
Merit acts by blocking the receptor sites in an insect’s nervous system. The message is sent by the preceding nerve, but the next nerve doesn’t receive the message or it receives an incomplete message. Therefore, insects that have ingested or absorbed imidacloprid just stop what they are doing or stop normal behavior. The particular receptor site affected is much more common in insects than in vertebrate animals, including humans. This is why imidacloprid has much lower toxicity (LD50 of technical in rats = 450) than many of the currently available organophosphates and carbamates.
This stopping of normal insect behavior has the greatest effect on insects in the early stages of development. A first instar grub may be able to survive for two to three days without feeding while a third instar grub can last for months without feeding.
Halofenozide is an insect molting accelerator. When an immature insect ingests or absorbs this chemical, it turns on the molting switch. The affected insect has no choice, it has to stop what it is doing (usually eating turf) and begin the complicated process of shedding its skin. If the insect is not at its normal state of readiness to undergo this molting, the result is death. Also, insects that are early in their development are more rapidly affected by molting accelerators. Like Merit, the best targets of MACH-2 are early instars of insects. Since vertebrates and many other animals do not undergo the molting process, halofenozide, is “practically non-toxic” (LD50 of technical in rats = 3850 to 5000) to humans, pets and birds.
Both Merit and MACH-2 are top performers in university-based trials for grub control (table 1). While both are producing an average of about 90 percent control of Japanese beetle and masked chafer grubs, neither pesticide has a perfect record. In fact, both have had about 10 percent failures in trials (defined as resulting in less than 70 percent control in a particular study), but this is still much better than our traditional “standards.” The important message to take from these data is that no pesticide gives 100 percent control all of the time. As stated earlier, salesperson claims and advertisements would have customers believe that many products are foolproof and always offers excellent control. Don’t set expectations or the expectations of customers too high. There will always be some failures.
LONG-TERM POWER. There are some other significant differences between Merit and MACH-2. Imidacloprid, especially when applied in a liquid spray is fairly susceptible to photodegradation. If left on grass blade surfaces in full sun, significant loss of the active ingredient can occur. Most people are recommending irrigation within 24 hours of application, but the sooner the better. (Merit-containing granules don’t seem to be as susceptible to this photodegradation.)
Halofenozide is apparently affected very little by direct sunlight. However, as with all grub insecticides, irrigation after the application will help move the active ingredient to the site where the grubs are feeding.
Another unique quality of Merit and MACH 2 is their relatively long residual activity once in the turf-soil-thatch interface. Both appear to have in excess of 100 days of effective residual, though researchers are suggesting that Merit is a bit better in its residual effectiveness. Therefore, manufacturers and most extension turf entomologists are recommending that Merit can be effectively used in May, June, and July through mid-August while MACH 2 should be applied in June, July and August.
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Early evaluations of MACH 2 suggested that halofenozide has significant grub “curative” activity, but this is coming under some debate. Most researchers would now agree that neither Merit nor MACH 2 are real curatives in the sense that Dylox®/Proxol® (trichlorfon) or diazinon are. Merit and MACH 2 are best used as “preventive” applications or as “curatives” only when first and early second instar grubs are present.
Since Merit has been on the market for three years, some turf managers are beginning to question whether they should use the same product for another year without alternating to another grub control product. While alternation of products is never a bad idea, there is no evidence that imida-cloprid is susceptible to enhanced microbial degradation. This phenomenon is still worrisome, especially for turf managers who have been caught in problems with continuous usage of other products. Yet laboratory and field tests suggest that imida-cloprid is not susceptible to enhanced microbial degradation. Such information is not yet available for MACH 2.
We also do not believe that resistance is a factor. Because of the mobility of grub adults and the patchy usage of Merit and MACH 2 in our urban habitats, most grub species are simply not under sufficient pressure to develop resistance.
Another significant result of the registration of Merit and MACH 2 has been a loss of grub insecticides. Crusade® (fonofos) and Triumph® (isazo-phos) have both been voluntarily withdrawn by their manufacturers. In both cases, the manufacturers claimed sales would not justify the cost of continued registration and new data requirements. In some sense, this is rather unfortunate because Crusade and Triumph could be used as curatives against larger grub stages.
There are some significant differences between Merit and MACH 2 and their activity against the full range of grub species that attack turf in North America. The data presented on page 60 is based on the results of tests against Japanese beetles and masked chafers, the two most commonly encountered grubs. However, Oriental beetles and European chafers can be the most important grub species for lawn care operators in New England or Atlantic Coast states. Many of the transition zone states are also battling green June beetle infestations. When targeting the first instar grubs, both Merit and MACH 2 do a satisfactory job, though MACH 2 appears to have less activity against both European chafers and Oriental beetles.
SITUATIONAL KNOWLEDGE. While laboratory tests suggest that MACH 2 is a bit less active on these two grubs, turf managers are telling a different story. Some claim no control of European chafers or Oriental beetles with either Merit or MACH 2. This may be the result of misunderstanding the activity of these new insecticides as well as assuming that European chafers and Oriental beetles are active at the same time that Japanese beetles are.
In many areas, European chafers and Oriental beetles may fly in early to mid-June while Japanese beetles fly in mid- to late July. If Merit or MACH 2 were applied “at the time of egg hatch” with the assumption that this would be in late July and early August, the Oriental beetle and European chafer grubs may already be in their second instar stages. For these two grub species, earlier applications may be much more effective than typical mid-season applications.
On the other hand, the green June beetle adults often fly in late July through August. Their larvae also feed on the turf surface or in the upper layers of the thatch. If Merit or MACH 2 were applied in June, the residues would probably be most concentrated in the lower layer of the thatch, not near the surface. In this case, the green June beetle grubs may simply miss the insecticide. Applications of Merit in late July and early August have proven much more effective controlling green June beetle grubs than earlier applications.
Finally, both Merit and MACH 2 have little effect on non-target invertebrates, especially earthworms, as well as microbial populations. Ecologically, this is good, but in some turf areas, earthworms are becoming a significant nuisance pest. Their castings, mounding of soil and removal of lower leaf blades can be a real headache for people who want smooth, dense turf. As time goes on, we may also be able to determine if our use of traditional pesticides really did influence soil microbial populations, thereby increasing turf diseases. While I have doubts about this claimed phenomenon, contractors should be on the lookout for decreases in the expression of turf disease.
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The grub itself goes through a process of development called moulting. When a grub moults, it actually sheds its skin in preparation for further development. The period of time in between each of the moults is called an instar. After the grub initially hatches from the egg, it is known as a first instar grub. After its first moult, it enters its second instar stage, and following its second moult it becomes a third instar grub. Each moult results in a grub that is larger than the previous instar. The third and final moult results in pupation, which develops into the adult scarab beetle. |
In summary, Merit and MACH 2 are major advances in our pesticides used for grub control. However, lawn care operators must constantly rethink the way that they use grub insecticides and maintain realistic expectations about how effective any insecticide can be so that application effectiveness is maximized and customer satisfaction is delivered.
The author is associate professor of landscape entomology at The Ohio State University, Columbus, Ohio.
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