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Missouri Updates: Pests and Weed Control

Published on Monday, June 12, 2017

Here in Missouri we have welcomed the month of June with open arms! With daily highs in the triple digits, it’s finally feeling more and more like summer. As planting season winds down, we finally have an opportunity to protect the crop we have worked so hard to get in the ground. I wanted to take this opportunity to relay some information on the pests I’ve been seeing out in the field so that you’ll be prepared if you come across them.

Insect Pests

After scouting many different corn and soybean fields throughout different areas of the state, I am honestly a little surprised by the lack of insect pests I’ve seen. Although I haven’t seen many insects, I have compiled a list below of a few of the more common cases I’ve seen so far, and what I’ve received calls about this spring.

White Grub

Pest Type: White grubs are pests in the larval stage of multiple species of beetles. The two most common in Missouri are the Japanese and June Beetles. Japanese Beetles are a pest in both corn and soybean crops, whereas June Beetles are not. 

Injury Type: White grubs may feed on both the seed and roots of seedling corn or soybeans. This year however, I have noticed an increased number of white grubs over many landscape positions that have not been feeding. The high number of white grubs we are experiencing are a result of the mild winter we had. As the season continues, I believe this will result in an increased number of adult Japanese Beetles, as there have already been reports of these adult beetles appearing in southern Missouri.

Control:  White grubs will generally be controlled with seed treatments such as Beck’s Escalate™ yield enhancement system. If you’re looking for an extra layer of protection, an application of an in-furrow insecticide should control them quite well.

Japanese Beetle

Pest Type: A chewing beetle that, at this point in the season, is now starting to pupate from the white grub larvae stage. Japanese beetles are easily identified by their shiny appearance and their distinct white tufts of hair along the perimeter of their wings.

Injury Type:  As previously mentioned, these are chewing insects that can have a major economic impact on a corn crop. This pest focuses on the silks of pollinating corn. They can cause major yield loss if they move into a field at the right time.

Control: An application of a pyrethroid insecticide should be enough to control these beetles. Consult your pesticide label for application rates and control information.

True Armyworm

Pest Type: Armyworms are pests in the larvae stage of the Armyworm moth. These little critters can be devastating, especially in a grass crop. This year we have seen some instances where they severely reduced soybean stands to the point where they needed to be replanted.

Injury Type: These are chewing pest, meaning they eat living plant tissue as opposed to other pests that pierce and suck the plant. In the field pictured above, the soybeans were planted into standing cereal rye that was burnt down at planting. I believe that in this case, because the rye was no longer a viable food source, the armyworms moved to the next viable option…the germinating soybeans. Most of the stand loss in this field was attributed to the armyworms chewing through the hypocotyl, thus removing the growing point and killing the seedling plants.

Control: True Armyworms can very easily be controlled with an application of a pyrethroid insecticide such as Warrior or Baythroid®. Always read your pesticide label to confirm the control of any species.


Pest Type: Slugs are gastropods (snails) that do not have shells. They have an interesting physical attribute. They have a slime that protects them from predators and, unfortunately, insecticides.

Injury Type: The injury symptoms of slugs are very similar to most other “chewing” insects. The distinct marker of the injury they cause is the “divots” they create in either the cotyledons or the hypocotyl.

Control: Slugs are most active during the night and on cloudy days. You can usually find them by removing residue and looking for distinctive “shiny” trails. There aren’t many control options for slugs, other than waiting for conditions that are unsuitable for them to proliferate, such as warm, dry weather on ground without cover from the crop or cover crop residue. The reason these pests have been so widespread this year is because we’ve had a combination of increased cover crop acres, as well as very cool, wet periods.

Weed Control

Over the past few weeks, I’ve noticed a lot of emerging waterhemp as well as cocklebur, morning glory, velvetleaf and other grass species. Many of the fields where this weed emergence is occurring previously received applications of a pre-emergent herbicide. Why are these weeds “breaking” so bad now? Most of today’s pre-emergent herbicide programs have an expected residual of six to eight weeks in ideal conditions. With the conditions we have experienced throughout the spring (warm days followed by weeks of cool, wet weather, followed again by heat) these herbicides have been less effective. Pre-emergent herbicides need a combination of three things to break down in the soil: heat, moisture and microbial activity. We have certainly had all three of those factors and, on top of that, many farmers have made extra trips through a lot of fields for replant, which could have broken the barrier that herbicides provide.

So what does all this mean to you and your farm? As it stands today, we still have a long way to go to get these crops canopied and intercepting enough sunlight to out-compete the weeds I mentioned above. I recommend considering a post-emergent herbicide application that has some residual activity. For corn, consider a group 15 and/or a group 27. For soybeans, a group 15 herbicide application should do the trick. Remember, it’s crucial to make sure you are familiar with your herbicide labels prior to application to avoid making a mistake that could injure you or your neighbor’s crop.

When making herbicide application decisions, the first step you should always take is to scout your fields to see which species of weed you’re dealing with and how tall they have gotten. A good rule of thumb is to apply herbicides to weeds that are less than 4 in. in height. This can be hard to visualize, so I always say that weeds should be sprayed before they are bigger than the height of a soda can.

When dealing with pigweed species such as the Palmer amaranth, pictured above, it all comes down to growing points. As pigweeds develop, they add growing points at an amazing rate. For every 2 in. the plants grow in height, the number of active growing points doubles. The most intimidating factor is that these species can grow up to 2 in. per day!

Remember, the best plan for crop protection is to be proactive. The faster you can identify what the issue is, the faster you can correct it. Or, in the case of residual herbicides, the faster you can prevent the issue in the first place, the better off you will be. 

Happy scouting and have a safe summer!









Escalate™ is a trademark of Beck’s Superior Hybrids, Inc. Baythroid® is a registered trademark of Bayer.

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Author: Alex Long

Categories: Agronomy, Missouri, Field News


Alex Long

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