C.O.R.N. Newsletter 2012-14

Dates Covered: 
May 22, 2012 - May 29, 2012
Editor: 
Glen Arnold
Asiatic Garden Beetle or Grub

Asiatic Garden Beetle or Grub

Last week we received reports of a stand loss in corn following soybeans in northern Ohio near Sandusky, apparently caused by white grubs.  A number of the grubs that were found were brought to us, and consisted of two different types, one being a larger one that was identified as a true white grub which is a multi-year species.  But to our surprise, the smaller of the grubs were identified as the Asiatic garden beetle (AGB) grub, a species that is more associated with being a minor pest in turf.

The Asiatic garden beetle was introduced to the US in the 1920’s on the east coast and have gradually made their way across the country.  It was recently associated as a newer corn pest in northeast Indiana and southwest Michigan around 2006-2008, always in sandier soils and following soybean, both conditions of which this field in Ohio meets.  This grub appears much more damaging than most other grubs, and is considered very aggressive in its feeding habits.  Thus, this “new” corn pest has officially been found causing similar stand losses in our state. 

This pest has acquired a taste for corn based on these findings in those two states and Ohio.  None of the infested fields over the years appear to have been protected by the various seed treatments.  Again, it appears to be found mainly in sandier soils in all three states.

At this time, we would like to get an idea how widespread this pest might be in Ohio in corn.  If you have stand losses in fields that might be grub related, dig around the open areas to look for grubs. AGB grubs are smaller than other grubs likely to be found such as true white grubs and Japanese beetle grubs.  The main characteristic that can be used to identify AGB is the enlarged maxillary palps on the side of their mouthparts, which can be seen in the attached pictures (courtesy of John Obermeyer from Purdue University).  If these grubs are found, please contact us at hammond.5@osu.edu, or else send these to us via your extension educator.  Place them in a container with moist soil for shipping.  Because we would like to hear of any stand reductions by any and all grubs, please collect any grubs that are present so we can correctly identify them.

At the present time, most damage has been in corn following soybeans, although this year a report from one of the other states of injury on corn following corn.  Because this is a new pest, we will provide additional information at a later date.  At the present time, similar to other grub problems, there is not a rescue treatment available, and the only action would be replanting if the necessary.  Remember that most of the grubs should be pupating shortly.

Head Scab Risk Low, but Stripe Rust Showing up

Most of the wheat in the state is now at the early grain fill stage of development. The last three weeks, during the time most of the wheat reached anthesis (flowering), conditions were cool and dry. This has kept the risk for head scab low across the state. However, cool conditions are favorable for other diseases such as rust, and there have been reports of strip rust in some fields.

Three different types of rust, stem, stripe, and leaf rust, affect wheat. Of there, leaf rust is the most common in Ohio. However, so far this season, the level of leaf rust has been very low, but stripe rust is developing in some fields. There is a very clear difference in variety reaction to this disease. Only field planted with highly susceptible varieties are being affected. Unlike leaf rust, which prefers warmer conditions, optimum of 68 to 77F, stripe rust develops best under cool conditions (optimum between 46 and 59F), similar to those we have experienced over the last few weeks.

Symptoms of stripe rust are very characteristic and can be easily distinguished from leaf rust, especially when they occur side-by-side on the same leaf.  Pustules of leaf rust are round or slightly elongated and are commonly scattered on the leaf. Pustules of stripe rust, on the other hand, are small and round and usually occur in groups, forming stripes on the leaf surface, hence the name, stripe rust. In addition, leaf rust pustules are brown, whereas those of stripe rust are yellowish-orange.

Occurring this early during grain fill, stripe rust may still affect yield if high levels develop on the flag leaf. However, the warm, dry weather forecasted for the next few weeks will likely slow this disease down considerably. Scout fields for stripe rust; there still may be time to apply a fungicide to fields planted with susceptible varieties. Read fungicide labels carefully and pay attention to pre-harvest intervals before making a decision to apply a fungicide.

Grass Sawfly

As growers sample their wheat for armyworms or any potential pest, keep in mind of another caterpillar-appearing critter that is more green-yellowish in color.  There might be a concern that these are armyworms, which they are not.  These might be grass sawflies which are fairly common in Ohio, and are not armyworms.  We are getting reports of them from some wheat fields.  They frequently are found on the orchard grasses and other grasses along road sides.  Every few years, we see more of them showing up in wheat and causing grower concern, and perhaps we will see this again this year.  The grass sawfly is actually not a caterpillar, which is the larva of either moths or butterflies.  Sawflies belong to the order Hymenoptera, and are actually related to bees and wasps.  The predominant characteristic that separates them from most caterpillars is that there are more than five prolegs on the abdomen.

Will they cause injury to the wheat?  There is the possibility that they could chew up some leaves and also the possibility that they could clip wheat heads, but as with armyworm it would take quite a few of them to cause significant damage.  Sawflies generally are not considered a significant economic problem in wheat in Ohio. 

True Armyworms

We just received a few reports of true armyworm feeding on various crops, including wheat, corn, and even hay.  This might be a good time to scout fields for their presence and feeding injury, especially on wheat that is still filling their heads and corn if planted into grassy covers such as rye, or in fields with a heavy grass weed pressure.  

On wheat, the primary concern is from feeding on the flag leaf if the heads are not yet filled.  With the flag leaf being the most important leaf on the plant, it is critical that it be protected if the heads are still filling.  As the head is filled and the flag leaf becomes less important, leaf feeding is not as critical.  Significant defoliation after the head has filled will not reduce yield relative to the amount of leaf loss; but only if the heads have filled.

Although head clipping is often a concern, armyworms do not clip as many heads as thought, although they will clip a few looking for a food source.  At that time, armyworms will usually begin their migration to other fields, especially adjacent corn fields, searching for new food sources.  What you might see are armyworms going to the top of the wheat plant, or to the heads, which is often their behavior when they are parasitized or else dying from a pathogen.  Do not assume those larvae found on heads are doing damage; they might be dying.  Look for other larvae present on the heads that have already died and shriveled, or those that are parasitized (small white eggs will be present on their bodies).  In corn, if stand infestation is greater than 20%, consider a rescue treatment when corn is at early-whorl stage and larvae are less than 1 inch long, and you have more than 1 larva per plant or defoliation of infested plants exceeds 50%.

Because the problem is from armyworms moving from adjacent wheat, perhaps only an edge treatment might be necessary.  See http://ohioline.osu.edu/ent-fact/pdf/0036.pdf for more information on the true armyworm on wheat including pictures of parasitized and dying larvae on the head.  See http://entomology.osu.edu/ag/images/Small_Grains_-_Armyworm.pdf and http://entomology.osu.edu/ag/images/Corn_-_Armyworm%281%29.pdf for a list of insecticides available for wheat and corn. 

Two Week Weather Outlook

Temperatures will average 3-5 degrees above normal the next 2 week taking us into early June. Normal temperatures are highs in the mid to upper 70s and lows in the 50s. Expect lots of 80s for highs with a few days of 90s and 70s over the next 2 weeks. Lows will be in the 50s and 60s. The hottest stretch will come over Memorial Day weekend into early next week when highs will likely push 90 except near Lake Erie with little rainfall.

Rainfall will remain generally below normal the next 2 weeks. Normal is a little below 2 inches. The chances of normal by our weather models indicates only a 30% chance of 2 inches so odds favor below normal rainfall except for the lucky spots that get some thunderstorms. The chances of 1 inch over Ohio are about 60% for the next 2 week total. Rainfall will be scattered over the next 2 weeks which is typical of late spring.

Long -Range Outlook: The outlook for summer remains unchanged. Expect a warmer and drier start to summer in June relaxing to near normal conditions by August. We could have a stretch of dry weather that could stress crops in June or July. We will monitor this closely in the coming weeks.

Factors Contributing to Uneven Corn Emergence

Recently, in northern Ohio, multiple corn fields with fair to poor stands were observed. Stands ranged from 14,000 to greater than 28,000 plants per acre. In these fields, stand loss although highly variable, appeared related to past cultural/weather events.  In all fields, surface crusting and soil compaction was evident.  Under the soil crust, a soil compaction zone was noted about 2to 4 inches down from the surface. Corn seminal roots (of emerged plants) were observed growing laterally along the top of the compaction zone. If the plant had not emerged, the corn mesocotyl was often twisted or swollen.

What are some of the factors that may contribute to poor stands? The use of shallow tillage tools (work at soil depths of about 4 inches or less) following a year of record rainfall (2011) and a mild winter without deep soil freezing and thawing may have further added to existing compaction.  Also, some fields were compacted last fall during harvest.  Finally, add heavy rainfall events in this spring and you have multiple factors that will contribute to loss of soil structure with concomitant soil compaction.

Other possible contributing factors to the corn stand loss in these fields include:

1. Rotation; limited rotations without wheat or forage and in some cases multiple year soybeans now followed by corn.

2. Organic matter (OM) of soils in some areas of the fields was less than 2% with a corresponding low CEC.  Low OM soils may be more prone to crusting and compaction

3. Use of pop-up fertilizer applied on corn seed at planting.  Applying fertilizer to the seed is not a recommended practice (Tri- State Fertility Recommendations, page 12 https://agcrops.osu.edu/specialists/fertility/fertility-fact-sheets-and-...). However, if it is done, for soils with CEC greater than 7, maximum salt index is 8  (lbs of N +K20). 

4. Herbicide injury.  For example, products containing cell growth inhibitors may under certain environmental conditions injure corn seedlings.

5. Insect injury.  Various insects such as wireworms, seed corn maggots, grubs, etc. have the ability to reduce plant stands. 

6. Corn seedling diseases caused by various pathogens.

7. Malfunctioning corn planter over seedling depth, fertilizer delivery, etc

The above factors may have been exacerbated by very early planting (first week of April).  Corn was slow to emerge (in some cases 3 weeks or more) and was exposed to wet/dry/cold weather events. For more information go to  http://extension.entm.purdue.edu/pestcrop/2012/index.html.

 

 

 

 

 

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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.