You Are Here: Management Updates 2003 Archive June 17, 2003

Date: June 17, 2003
Category: Insects
Subject: Preventive Control Strategies for White Grubs

Several people have been asking us recently about timing of applications for preventive grub materials, particularly imidacloprid (Merit™). Here are the key points as I see them:

1. It is extremely important to know which grub species is the primary one in a given location: As many of you realize, we have at least four species of grubs causing problems throughout many parts of New England. (See earlier messages from 2003 as well as the grub identification section of the web page for hints as to which species might be active in your location.) European chafers normally develop about two weeks EARLIER than Japanese beetles, and oriental beetles sometimes are slightly earlier as well. Meanwhile each species responds differently to the various insecticides.

2. The Japanese beetle is the easiest grub species to control: If your primary species is Japanese beetle, consider yourself lucky. Japanese beetle grubs are more vulnerable to most turf insecticides than any of the other species. European chafers and oriental beetles are markedly less sensitive to Merit™ than is the Japanese beetle, and the Asiatic garden beetle appears to be virtually untouched by Merit™. In fact, we believe this is why we are seeing an increase in activity of the Asiatic garden beetle in many locations throughout New England. In addition, the oriental beetle is less sensitive to halofenozide (Mach 2™). (Note the Mach 2™ label now has a higher rate of application than in past years, primarily because of confusion in the field about species identification. The older rate was not sufficient to control oriental beetles in field conditions. The new labels provide application rates that should control oriental beetle grubs reasonably well if the applications are well timed.)

3. In situations where Merit™ has been used at least three of the last four years, turf managers should realize that the "application window" is probably shorter than the six to eight week seen when Merit™ first came on the market: Applications should be made to target adult beetles just as they begin to lay eggs (probably late June or early July for European chafers this year, a week or two later for other species). You should get very good results regardless of species with Merit™ applications that are made in July (at least in southern New England). In situations where Merit™ has been used repeatedly, applications that were made in mid to late May may or may not remain active long enough to control grubs that become active in August. (This year in particular has been unusually cool and wet, so adult emergence seems to be delayed about a week. That probably will result in delayed egg-laying during the summer.)

4. The ideal application date for Merit™ varies with location and species of grubs: If the primary species is European chafer, Merit should be applied in July (see above). However, we recognize that commercial lawn care companies often operate on a six week cycle. It is not possible to make applications for every client during the ideal timing. In many instances the June applications will provide adequate control of grubs. When we make suggestions about timing of application, we try to describe the ideal situation. The real world usually is a long way from "ideal".

5. Water remains a critical piece of the puzzle: Whatever chemical you decide to use and whenever you apply it, WATER IT IN. Even though labels of some products imply that post-application watering is not essential, our field data collected over the years confirm the need for water. Aim for at least 0.1 inch, and 0.25 inch of irrigation is even better.

- Submitted by: Dr. Pat Vittum