Wednesday, November 4, 2015

Will the Decrease in Rootworm Pressure Impact 2016?

The answer to this question is a definitive yes!  The impact upon regional rootworm pressure will be dramatic.  University specialists fully expect rootworm pressure within our footprint to be as low as it has ever been.  The likelihood of an individual field suffering rootworm injury in 2016 decreased drastically this season. 
            We previously mapped out the reasons why rootworms will be less prevalent throughout the Burrus footprint in2016.   

Does that mean we will not see any issues in 2016? 
            Common sense says this cannot possibly be the case.  Nobody can guarantee zero rootworm pressure and nobody can guarantee zero rootworm injury.  There are too many qualifiers. 
            One of those qualifiers comes in the form of varied rainfall.  Rainfall totals varied across the Burrus footprint. The I-80 corridor provides dramatic evidence.  The eastern portion of that corridor suffered severe crop injury as rainfall transformed fields into swamps.  The western portion received rainfall at just the right rate resulting in a beautiful crop.  Rootworm suffocation would have been an issue toward Kankakee County, but it was a non-issue in Rock Island County.  While much of the Burrus footprint will experience less pressure due to 2015 rainfall, this will not be the case in every region.  There will likely be variation even within saturated regions of the Burrus footprint.
Suspected rootworm root injury near Woodstock, IL
            The second qualifier relates to the subject of rootworm species.  Even though overall rootworm pressure decreased in 2015, beetles were not completely absent.  Burrus noticed more Northern corn rootworm beetles this season than we did Western corn rootworm beetles.  This gave us pause.  We know that Northern corn rootworm beetles, particularly those in the northwest part of our footprint, have occasionally displayed extended diapause (a habit of overwintering eggs for more than one year).  We noted, in this article, that rootworm eggs tend to survive pretty well in saturated soils (mortality increases in cool, wet conditions but it does not completely zero out rootworm eggs).  What if some Northern corn rootworm eggs displayed extended diapause and made it through 2015 unscathed?  While a remote possibility, it is still a possibility.
            These qualifiers keep us from boldly predicting no rootworm pressure in any field in any corner of the Burrus footprint.  We can state with confidence that rootworm pressure will be much less likely across the Burrus footprint – but we dare not proclaim that it is a complete non-issue in 2016.    
What should be our strategy going forward?
            The 2015 growing season was incredibly hard on crops.  Despite some positive harvest surprises, nobody wants to see a season like this again.  It was gut-wrenching.  However, excessive rainfall may have a silver lining associated with it.  Excess rainfall might have acted as a rootworm “reset button” in many fields.  Our best management efforts could never drag rootworm pressure to 2016 lows.  Mother Nature did what our technology was incapable of doing.  Think of this year being a magic wand.  When it was waved over our fields, it pulled many fields back from the edge of resistance.  It did not eliminate resistance concerns, but it did provide us some resistance breathing space.  We may have increased the chances that we can beat this thing.
            Burrus has encouraged growers to rotate traits each growing season.  If they have been relying on only one trait, we have encouraged them to move away from it.  We have encouraged them to use alternative traits in the Burrus lineup.  That recommendation is still sound.  We know it will help us prolong trait life because “it will keep resistance off its’ game.” Many growers have been hesitant to embrace that recommendation though.  “What if I shift to a different trait and it doesn’t work?” 
It is always more comfortable to stay with the familiar, especially when you believe the risk is high.  We do not believe that risk really is high, but after this season, growers should realize that the risks associated with trait rotation are lower than ever before.  There has never been a better time to try a rotation to new traits.

Why Did we See Less Adult Rootworm Beetle Pressure in 2015?

Rootworm larvae need to breathe and our saturated soil kept them from doing so.  The cells of young rootworm larvae initiate a mechanism that allows them to briefly survive when oxygen is low.  During this process, lactic acid is produced as a waste product.  This process is acceptable as long as lactic acid does not reach a critical level.  For the rootworm, more than one day of lactic acid accumulation results in death.  The charts below show that rootworms were repeatedly deprived of oxygen this season.  They were exposed to a very lethal insecticide – their own lactic acid.

What else about 2015 resulted in decreased field to field rootworm numbers?  
 Once again, rainfall is the answer and it impacted every stage of the rootworm lifecycle.  Sometimes that impact resulted in a very direct rootworm decrease.  Sometimes that impact was more indirect.  

Rootworm eggs sit in the soil, overwinter there, and hatch the next growing season.  By their nature, rootworm eggs are pretty hardy when it comes to moisture.  They actually need a little soil moisture to survive and seldom suffocate to death.  However, this changes when waterlogged conditions are accompanied by cool weather.  Put those two together and egg mortality spikes.  The early to middle part of the growing season was unseasonably cool and wet.  While resulting egg mortality is not the primary explanation for decreased pressure this season – it did contribute (ever so slightly) to decreased rootworm numbers.

We already noted that rootworm larvae do “suffocate to death” when subjected to saturated soil.  However, excess soil moisture still impacts those larvae that successfully stumble through wet soil.  Larvae need to find roots, need a pathway to roots, and need adequate roots to graze upon when they get there.  2015 exploited each of those vulnerabilities. 

Rootworm larvae rely upon carbon dioxide and chemicals released by corn roots to hone in on those roots.  This material must filter through soil pores to larvae.  Only then can larvae detect those gases and progress toward the host root material.  However, when soil pores are full of water – these gases can’t get to rootworms.  Survivors literally cannot detect where the roots are.  Survivors run an increased chance of starving to death. 

Rootworms do not really “burrow” through the soil.  Instead, they crawl through soil pores to reach their host.  Saturation literally clogs the small holes/ highways used by rootworms to reach roots.   

Finally, on a more indirect level, saturated soil results in very shallow/skimpy root systems.  During 2015, there was literally less root material distributed throughout the soil profile.  If rootworm larvae somehow found a way to survive and found a way to reach corn roots, there often was a pretty thin feast waiting for them.  
Adult Beetles

  While some “beetle diseases” tend to appear during rainy weather (thus directly reducing beetle numbers), the field to field impact upon beetles is much more indirect.  Researchers know that prolonged rainy, cool weather decreases rootworm field to field migration.  Frankly, if rootworms didn’t survive within a specific field, neighboring fields could not boost that field’s beetle population. (Matt Montgomery)