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Posted 23 January 2012. PMN Crop News.

What Can Mother Nature Do to Japanese Beetles?

Source: University of Illinois at Urbana-Champaign Press Release.

Urbana-Champaign, Illinois (January 23, 2012)--Our mid-January cold snap has made the winter of 2011-2012 seem a little more "real." In other words, January finally felt like January. However, it was only a few weeks ago that temperatures crept upward enough for anhydrous to be applied in our area. Who could have imagined that January anhydrous applications would even be possible!?!? That recent temperature spike and temperature crash brings up an interesting question regarding our old friend the Japanese beetle. Would a hard winter or a mild winter have any effect on next year's Japanese beetle population? What exactly can Mother Nature do to deter the Japanese beetle?


Any article referencing the Japanese beetle first requires the author to quickly review basic information related to this rather large, green, black and bronze, silk and leaf feeding pest. Readers may remember that Japanese beetles were accidentally introduced into New Jersey in 1916. By the early 1970s, they had spread across the eastern half of the United States, but as of the late 80s their Illinois distribution was typically isolated to urban/metropolitan areas. Some have theorized that the beetles were accidentally spread around as ornamental plant material was distributed across the country thus the initial "urban" habit of the pest. In the latter 1990s, the pest began to move from these pockets and by the turn of the century some Illinois producers were experiencing heart-stopping infestations. Silks were often clipped and leaves were stripped. Producers began to notice Japanese beetle grubs in their fields during the spring while elm trees and fruit trees were completely defoliated each July. By 2009, the beetles had begun to move into the Fulton-Mason-Peoria-Tazewell area. Extension encouraged vigorous and comprehensive scouting to determine if the pest actually needed to be managed because its distribution tends to be clumpy, often but not always restricted to the end rows.

So what about winter weather extremes? Could they knock the pest down a few notches? The answer to that question is "probably not in most cases." Readers will remember that Japanese beetles are present right now in the soil as grubs. Once beetles begin to appear during the latter part of June/early part of July, they begin to mate. The resulting eggs are about 1/16 inch wide. A grub roughly the same size emerges from that egg in about 10 to 20 days depending upon the soil temperature. This first stage grub feeds for about two to four weeks. The second instar follows and feeds for another two to three weeks. The final instar, called the "third instar," migrates in the soil feeding on whatever root material it can find until September or October when soil temperatures begin to drop below 60 degrees. At this point, the grub migrates down in the soil profile. It typically migrates to the bottom of the root zone (about seven inches deep), but it can migrate much deeper. At fifty degrees their activity stops entirely, and as spring soil temperatures creep back to 60 degrees these larvae migrate into shallower regions of the root zone. It is this feature of "going deep for the winter" that tends to save Japanese beetle grubs. The ground typically will not frost deep enough to decimate the grub population.

So what can Mother Nature do to deter this pest? Temperatures may not impact the pest, but moisture does. Torrential rains will not deter the pest, but a drought can. Shutting off the rainfall "tap" can significantly reduce grub counts if the "spigot" is turned off early. Japanese beetle eggs and first instar grubs are very susceptible to desiccation. This means that a dry July and August (the period when eggs and young grubs are most prevalent) can decrease grub survival which decreases adult beetle pressure the following season. Extension typically does not recommend managing grubs with insecticide with the intent of indirectly managing adult beetles. We steer people away from grub control to control adults because grubs are usually eliminated from small areas (individual fields for instance) rather than large geographic regions. Untreated areas allow grubs to survive, adults emerge from these untreated areas, and adults then repopulate grub-treated fields. Money is spent to manage grubs yet adult pressure remains. Area-wide grub management is needed to reduce adult beetle pressure, and Mother Nature can provide that in the form of drought. A drought typically covers a much larger geographic area and does have the potential to eliminate adult beetle pressure because the grubs throughout a region are decimated. Unfortunately, crops usually are too so be careful about asking Mother Nature for help.

Matt Montgomery