Monthly Archives: June 2012

Corn earworm moths already showing high levels of pyrethroid resistance in Virginia

From Ames Herbert via the Virginia Pest Advisory (

As of the end of this week (June 29, 2012) we have tested 372 corn earworm (CEW) moths for pyrethroid susceptibility and have a season average of 31.2% surviving the AVT (adult vial test) challenge (see the attached line graph). We had one sample with over 40% survivorship. These are high numbers for the beginning of the season and compare pretty well to what we had at this time in 2011, if not a bit higher. What does this tell us? We cannot claim pyrethroid resistance based on this kind of random survey of moths, but historically, when we see survival numbers of about 25-30% or higher, we can expect some pyrethroid control problems, especially if moth fights are heavy, and the weather turns dry. That combination would almost guarantee control problems. But, if CEW populations reach only low to moderate numbers and the season continues to get plenty of rainfall, field failures will not be nearly as common. With loss of Larvin, an effective non-pyrethroid for controlling CEW, growers will need to turn to other non-pyrethroids like Belt, Coragen (Prevathon**), Steward, or combinations that include a pyrethroid plus a non-pyrethroid either tank mixed (like a pyrethroid + Orthene) or as a product (like Besiege** which contains Karate and Coragen). (**note, the registration status of these products is not certain at this point)

Make Timely Postemergence Weed Control A Priority

Roundup Ready and LibertyLink soybean varieties have made weed control much simpler.  In addition to being easier, we are generally doing a better job at controlling all weeds, especially those that could not be controlled well with traditional herbicides.  Perennial weed control is especially better with glyphosate.  However, weed resistant to glyphosate continues to get worse.  To date, we have glyphosate resistance in marestail (horseweed), lambsquarters, Palmer amaranth, common ragweed, and johnsongrass in Virginia.  To date, no resistance has been documented for glufosinate (Liberty or Ignite herbicides).  I strongly encourage using preemergence residual herbicides with glyphosate and glufosinate, even in double-cropped soybean.  With glyphosate, tank-mix partners are also in order due to the increasing weed resistance issue.

It is worth noting that glufosinate will not kill as big of weeds as glyphosate, nor is it as effective on grasses.  To me, this may be an advantage of the LibertyLink program.  Why?  First, it almost requires one to put down a preemergence herbicide, if nothing else to buy one a little time for the subsequent postemergence glufosinate application.  Secondly, you generally need to make the glufosinate application before the weeds reach 4 inches in height (height requirements depend on the weed to be controlled; see the label for weed-specific height requirements).  While these two “advantages” may not seem to be advantages, they will help insure that weeds do not become resistant to glufosinate.  In addition, if we let the weeds grow in competition with the soybean much longer, we’ll lose yield (see the discussion below).

On the other hand, since glyphosate will kill larger weeds that glufosinate, we tend to wait too late to spray.  It has been shown that spraying weeds that are too large can encourage herbicide resistance.  That in itself should make timely weed control a priority.  But other than herbicide resistance, I see two main problems with relying on glyphosate weed control systems.  First, we tend to spray later than we should because we want to kill all of the flushes of weeds with one spray.  This is a mistake!  As I said, glyphosate will kill larger weeds than most of our other postemergence herbicides.  So, why not try to wait and only spray once?  While that approach will save time and money if two applications would otherwise be necessary, it could be costing you more money in the long run by letting weeds rob the crop of essential light, water, and nutrients.  The figure below shows the effect of weed competition with soybean as related to application timing and row spacing.  In general, we need to spray our herbicides within 2 to 3 weeks after the weeds emerge.  If we don’t, we’ll suffer the consequences of lower yields and possibly less control.

The second problem is related to the first.  Weeds become more difficult to control the bigger they are, even with glyphosate.  Most of our control failures (and glyphosate resistance problems) are due to allowing weeds to get too large.  Weeds such as marestail, lambsquarters and morningglory are much more difficult to control when they are large.  Again, spray within 2 to 3 weeks after weed emergence.

Plant As Soon As Possible For Maximum Double-Crop Yields

Early planting and good stands are the foundation of high double-crop soybean yields.  Small grain harvest is very early this year.  If we can get soybeans planted by the first week or two of June, there will be very little yield loss due to late planting.  Good wheat and soybean yields plus high prices can make the wheat/soybean cropping system the most profitable in Virginia.

I stress that it is very important that we forge ahead with soybean planting.  After mid-June, we lose about ½ bushel per acre per day with every day delay in planting.

This is not to say that double-crop yields will always be less than full-season yields.  Rainfall during pod and seed development is most important.  For two of the last three years, double-crop yields were equal to or greater than full-season yields.  But, we can’t count on late-season rains every year.  Therefore, it is important that we get the double-crop soybean set up for maximum yields.  That means planting as soon as possible to maximize leaf area production.

For those of you that do not have adequate soil moisture, planting into dry soils is OK, but only if the soil is completely dry.  If the soil has enough moisture to swell but not germinate the seed, the seed may die.  If the soil has enough moisture to initiate germination, but not enough to send a root into soil moisture, it will definitely die.  Considering the cost of seed, my suggestion is not to plant into these dry soils; wait for a rain.  Only if you are sure that there is absolutely no soil moisture in the field and you have hundreds (or thousands) of acres to plant, would I suggest planting into dry soils.  At $40 to 50 per bag, we don’t want to waste any seed.

Keep in mind that my seeding rate recommendation for double-crop soybeans is to plant enough seed to insure a final plant population of 180,000 plants per acre.  Assuming 80% emergence, that’s 225,000 seed per acre (~75 lbs of seed).  Do not attempt to lower seeding rates in double-crop settings. Furthermore, if planting moves into July, we’ll need to increase that seeding rate to 250,000 seed per acre.  On droughty soils and July planting, yield increases will take place up to nearly 275,000 seed per acre.

Slugs Remain in High-Residue Fields

I never expected to be writing an article on slugs in soybean in June , but this year is continuing to bring surprises.  After a generally warm winter and spring, we’ve suddenly cooled off in the first week of June.  And the slug activity in fields with a lot of residue has picked up.  Usually, our slug problem disappears by the time warmer temperatures set in.

The photo to the right was taken 5 days after planting soybean in a no-till field in Suffolk that contains lots of corn and rye cover crop residue.  Stand was only about half of what was expected and feeding scars could be seen on the hypocotyl and cotyledons.  When I dug in the seed furrow, I found slugs more often than not.  Since it was a cloudy afternoon when I was in the field, I even found slugs feeding on soybean plants that were still partially covered by residue (slugs usually only feed at night).  In an adjacent field that had not yet been planted, I found slugs on the underside of corn residue.

Will the soybean crop survive this late-season infestation?  If the crop has emerged and has a couple of leaves on it, I’d say it will.  But if you’re just planted or are now planting soybean and you’re finding slugs, I’d suggest using slug bait/molluscicide.  The only one available (other than in small packages in home improvement/gardening stores) is Deadline®, which contains the active ingredient, metaldehyde.  It is sold at Deadline®M-Ps™ Mini-Pellets (colored with a blue dye) and Deadline®Bullets (dye-free).

What if you have damage and are considering replanting?  Usually, severe damage is only in part of a field, usually wetter areas.  If this is the case and the damage is severe, you may want to consider replanted these areas.  But if it’s the entire field, I’d suggest evaluating the stand of undamaged or slightly-damaged plants, and then following my guidelines for replanted poor stands (in this issue).

For more info, see Purdue Univ. website: or

Ohio State’s pub, Slugs on Field Crops at

Marestail Management in Wheat (Chris Drake, Southampton County Agent)

Over the last couple weeks, I am seeing a lot of Marestails poking through the top of wheat fields throughout the county. This is a major concern for the farmers that have to manage this weed in their bean crops. Once these weeds get the point of being 30 inches or greater in height, control is difficult with any herbicide.

There are a couple options for control after the wheat harvest. One is the use of mechanical cultivation prior to planting beans. Disking, turbo-tilling, or running a dyna-drive will eliminate these weeds that are present. This option does not appeal to many producers due to moisture loss and conservation tillage program payments. The second option is using glufosinate (Liberty formerly called Ignite). An application of Liberty should control or suppress these weeds, but at these sizes two applications may be necessary for complete control. A second application of Liberty will not be possible unless Liberty Link beans are planted. In either scenario, the use of a residual material at planting such as Envive, Prefix, or Boundary is strongly encouraged.

What is causing these escapes? Two things are the likely culprit. One is the lack of use of a burndown/ tillage before planting wheat and two is inadequate weed control in the growing season. Harmony is widely used for weed control in wheat but is not a great product on Marestails. Many applications were made after the weeds were 8 inches in height or greater. Harmony will simply not control this species at this height.

So, what do we do next year to prevent these problems? BURN DOWN the land before planting wheat if you don’t till before planting. Also, in season Marestail control can be obtained using 2-4D after wheat is fully tillered but before jointing. Scouting your fields in late February or early March when you start to see winter annual weeds may be necessary to determine if control measures are needed.