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Tuesday, August 20, 2013

Bollworm, Earworm, or Headworm?



                For the past several weeks, we have been expecting a large moth flight this month, based upon regional trap catches and what we, and other area entomologist, have been seeing in the field.  Now that we have had some moonlit nights, we are starting to pick up fresh bollworm egg-lay in our program fields this week.  There has also been some debate about which area crop the bollworm would choose.   With a large amount of late planted corn and sorghum, it was suspected that the bollworms would prefer to lay eggs in late corn first, late sorghum second, and cotton as a last resort.  Early indications from the field show that this is probably correct. 

                Yesterday, August 19th, a field scout reported finding 15 bollworm eggs per plant in a late corn field with a mix of brown and green silks, a prime bollworm habitat.  This morning, August 20th, I scouted a field just starting green silk stage, a field where we would expect to only see the beginnings of moth attractiveness, and found 2 bollworm eggs per plant.  I feel it is likely that this trend of corn preference will continue as long as green corn remains prevalent in the area this fall.   Typically we are not economically concerned about bollworms that choose to be earworms.  Even heavy egg lay of bollworms in corn should not be a major economic concern for Hale & Swisher Counties.  Eventually, the larval caterpillars will cannibalize each other until there will only one worm per ear remains ensuring that the strongest survive.  While we do not like to see even one worm feeding in our corn, it is not economically feasible to attack those few earworms who are protected inside the corn shucks with multiple, predator damaging sprays, especially with spider mites already in the mix. 

                The bollworms that choose to be headworms in sorghum (and millet) are much more of an economic threat.  Mostly because we can make effective sprays to the exposed head of grain, but also headworms do more damage to our sorghum yields proportionally.  Yesterday, August 19th, I scouted a field in milk stage and found 0.5 small headworms per head.  This is sub-economic but something to watch nonetheless.  Roughly half these headworms were FAW (fall army worms), another important species that make up the sorghum headworm complex.

                Any non-BGII cotton is now at a heightened risk, but doubly so if that field is not near any late corn.  This morning, August 20th, I scouted a late and growthy cotton field not near any grass crops and found 7,199 bollworm eggs per acre.  With no corn or sorghum in the vicinity, I would expect bollworms to settle on cotton, the lusher the better for the worms.  So far, we are not finding bollworm eggs in cotton if late corn is in the area. 

 

                We already know that FAW is an economic concern in cotton and sorghum.  Recent research, much of which is still ongoing by Dr. Pat Porter, indicates that FAW should be a concern in corn, especially late corn.  Gary Cross, CEA – Hale, and David Graf, CEA – Swisher, are assisting Dr. Ed Bynum in some area wide moth trapping studies.  Their FAW trap catches remain low, and we are still finding large FAW larva in whorl stage sorghum.  We are also not finding FAW egg masses at any significant level yet.  All of these indicate that FAW are late but may join the mix in the moth flights soon, at least locally.  

 

Please call with any questions, or leave a comment about what you are seeing.  Thanks,

Blayne

Monday, August 12, 2013

Bollworm or Headworm


 
        Corn earworm, sorghum headworm, and cotton bollworm, no matter what you call it, this insect is a nuisance.  We have found a few bollworms in cotton fields this summer, but none at threshold yet.  As the area’s corn begins to harden, we should see the next generation of moths moving into the cotton and / or sorghum to lay eggs.
            The adult bollworm will lay its eggs just about anywhere on the plant these days, but the old textbook states that they prefer the upper 1/3 of the cotton plant on the upper side of the leaf surface.  The eggs are small, oval shaped and pearly white in color, with a slight dent in the tip.  If you have eggs in a field, even huge numbers of eggs, you don’t have to treat yet.  The eggs can be parasitized or eaten by a variety of things and some may even be unviable, so several won’t get to hatch.  Once the caterpillars hatch they begin to feed on whatever is nearby, but move to the fruit as soon as possible.  These worms do live in a world where it is survival of the fittest.  The caterpillars will often cannibalize each other, if their eggs have been laid too close together or they attack the same fruit, ensuring that the strongest larva will survive.  These factors combined are why we usually wait until we find a threshold number of caterpillars (the smaller you catch them the better) before treating a field.  The size of the caterpillar changes the threshold as well, since during the last two instars, the caterpillars consume up to 90% of the total amount they consume before pupating.
            1We really start looking for bollworms in cotton when our blooms have started becoming bolls, but they can infest cotton earlier if their preferred host (corn) is unavailable.  The threshold for this insect in non-Bt cotton with bolls forming is about 10,000 worms ¼ of an inch or smaller per acre and 5,000 worms larger than ¼ of an inch per acre.  In Bt cotton, we look for 5,000 worms larger than ¼ of an inch per acre with 5 t o15% damaged fruit on the plant.  Once the insects reach this threshold, treatment should be considered.
            The threshold for sorghum also varies with the size of the insect and the crop value.  The worry starts when the caterpillars found are ¼ of an inch or larger.  While they are ¼ to ½ of an inch long, the way to determine the threshold by considering the economic injury:

Number of larvae per head =
 

Cost of control in dollars per acre x 9754
Grain value in dollars per cwt x number
of heads per acre x 0.19
 


                                                   
     When they are larger than ½ of an inch, the formula to calculate the threshold by considering economic injury is:
Number of larvae per head =      
Cost of control in dollars per acre x 9754
Grain value in dollars per cwt x number
of heads per acre
 
 

            A sorghum headworm ET threshold calculator is available for Droid phones from
the Google Play Store at http://goo.gl/8mXvv, or for other systems on the internet at http://goo.gl/5k7ZtU.
 
Good Luck!                                                                            Please call or come by the office if
   you have any questions.  Thanks!
        Kate                                                                                                        Blayne
 
References
1          http://amarillo.tamu.edu/files/2010/11/Panhandle-Pest-Update-v4i11-8-24-2012.pdf. Panhandle Pest Update on August 12, 2013
       
 
 

Spider Mites


           The past couple years spider mites have put pressure on our area’s corn and the mites are at it again in 2013.  There are two species we run into commonly, most often we find the Banks grass mite, and occasionally the two spotted spider mite.  The two spotted spider mite has dark spots on either side of the body and can be more damaging than the more solidly colored around the its body edges Banks grass mite.  Hot and dry conditions favor spider mites, and their populations can boom in these conditions.

            Spider mites can be difficult to control, and often beneficials can provide control if enough are present in your field.  Fields that have been treated to control Lepidopteron pests, such as southwestern corn borer and western bean cutworm, are often more likely to develop mite problems.  Insecticide applications to control caterpillars in a field can actually knock back predator populations and cause mite populations to flare.  This summer we have been working on a miticide trial in Hale County, and we actually had to flare the mites by spraying a pyrethroid before the mites started becoming problematic.  We are comparing all possible products labeled for mite control: Portal, Oberon, Onager, and Zeal for best mite control, some at variable rates.  We will be sharing the data from the trial as soon as it becomes available.  We are as eager as you are to see the miticides compared to one another, hopefully before they cause any economic damage for you.  At 3 DAT we had no statistically significant differences between any treatments and the untreated check, but noted several dead mites in all treatments.  This is not uncommon due to the nature of our usable miticides. 

A corn plant can tolerate reasonable mite damage to the lower leaves before mites become economic.  Mites begin to be an economic threat when they near the ear leaf.  When this leaf, the zero leaf, is nearing 50% coverage by mite colonies and the leaves below have heavy colonies and you note some mite movement above, it’s time to seriously consider a miticide treatment.  This is a lower threshold then many of us utilized in years past, but remember all of our newer miticides do not have a very quick knockdown but rather take time to work and rely upon support from beneficials for absolute control.  Many of the older chemistries might still be labeled for mites, but the mites have proven to be resistant to them.  Meanwhile, these newer miticides are much more predator friendly, but if mites do reach ET, treatment needs to be swift.  Only if your corn has reached a solid dent stage, and has near threshold mite populations, you may be able to consider holding off a treatment.  When the corn is starting to dry down, mites do not cause as much economic damage.  However, if the mite populations remain too high, they can cause lodging in the drying crop.  Be sure to consider your options and take a good look at mite populations, plant stage, and predators before treating, but treat quickly if needed.