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White Grubs

August 1, 2001
Adult masked chafers (annual white grubs) emerged in high numbers in central Illinois during the third week of July. They typically begin emerging in central Illinois on July 2 or 3, but this year the first beetle was not reported until July 5. Very few were seen after that for a week or more; however, the recent emergence has made up for that. A similar pattern is being seen in other areas of the state.

One question is: Why are they so late? We must assume that the 3 weeks or so of cooler weather, sometimes called elderberry winter, that we had in late May and early June slowed their development. We had very warm weather in April that pushed many spring-occurring insects forward in their development, which was balanced by the colder May weather so that by mid-June, insect occurrence seemed to be about the same as normal.

In the previous 35 years or so that Roscoe Randell and I have been watching grub emergence, adult masked chafer emergence has deviated more than a few days in only three of those years. In all 3 years, the weather warmed early and stayed warm, resulting in adult emergence 2 to 3 weeks early. In most springs, early warm weather is typically balanced by late cooler weather; early cold weather is typically balanced by later hot weather.

This year, the temperatures in the 70s and 80s that we experienced in April must not have affected the masked chafer grubs. At that time of year, the larvae are deep in the soil, and the soil may not have warmed that deeply. White grubs move upward when the soil temperature around them reaches at least 50 degrees Fahrenheit. Although we had soil temperatures at 4- and 8-inch depths of at least 50 degrees in April, it must not have warmed deeply enough to trigger grub movement. Remember that the winter of 2000-2001 had normal temperature ranges. That is, it was colder than it has been for several years. These cold winter temperatures would have driven the grubs deeper than in previous years.

Thus, an explanation is that the masked chafer grubs were not affected by the April warmth but were up in the surface soil and affected by the May cool weather. This probably caused the late emergence. So why were the Japanese beetle adults only 3 days late in emerging this year? Why weren’t they later? Japanese beetle larvae spend the winter within 11 inches of the soil surface- much more shallowly than masked chafers. The development of the Japanese beetle white grubs probably was sped up by the April warmth and then slowed down by the May cool temperatures. They probably canceled each other out so that the adult beetles emerged very close to the normal time.

The second question is: What does this mean for control of annual white grubs this year? Masked chafer adults do not feed, so they emerge, mate, lay eggs, and die all in about 2 weeks. Thus, egg-laying should continue until about the end of July. White grub eggs take 2 weeks to hatch, so they should hatch by the third week of August instead of the typical first week of August. Damage should also appear from masked chafer grubs about 2 weeks later, so it shouldn’t happen until early September.

However, in areas with Japanese beetles, their larvae will hatch on a more normal schedule: that is, hatch by the first week of August, with associated damage by mid-August. Typically, both species hatch at about the same time, cause first damage at about the same time, and can be controlled with one insecticide application.

People who apply imidacloprid (Merit) or halofenozide (Mach 2) by the end of July should get control of both species with one application because both of these insecticides last for several months. Lawns and other turf areas that are not as heavily irrigated, that are usually scouted in early August, and then treated with trichlorfon (Dylox or Proxol) if numbers are high are not so easily handled. If Japanese beetle grub numbers are high enough in early August to treat, the trichlorfon will be gone by mid-August when the masked chafers hatch. Those areas will need to be scouted again in the third week of August for masked chafers. This year may be one when areas that are not heavily irrigated should be treated by the end of July with Merit or Mach 2 to avoid the problem. Another possibility is to apply diazinon (to home lawns only) or bendiocarb (Turcam) if needed in early August because either lasts a month and controls both the earlier Japanese beetle grubs and the later hatching masked chafers. Some of you may still have some Turcam sitting around for just this type of need.

Author: Phil Nixon


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