Although every major field crop we grow in Mississippi is responsive to at planting insect protection; field corn is perhaps the most consistent. Why is this? At many of the winter meetings I hear the agronomists talking about maximizing yield of field corn. They mention numerous things that all protect yield such as planter speed, uniformity of seed spacing, seed depth, and the list goes on and on. All of these things can make a huge difference in corn yield. But why? Corn does not have the ability to compensate nearly to the degree as other crops we grow, such as soybeans and certainly cotton for problems. We could go into the whole thing about flex ears and so on, but I will leave that to the agronomists on another day. Point is, you are rewarded for doing all things perfectly with corn more so than any other crop I know of. This is why early season insect protection is so important.
In our area, we can face problems from numerous below ground insect pests such as; sugarcane beetle, seedcorn maggot, white grubs, southern corn rootworm, wireworms, and the list goes on. They can occur alone or in combination, but they will occur to some to degree. We have had in-furrow granular compounds around for a number of years, but now nearly all corn has an insecticide seed treatment. Close to 20 years ago when I was working on my M.S. in corn, we were evaluating the benefit of granular insecticides in corn. There were huge benefits, but many growers did not use them. It was not at all uncommon to see numerous corn stands destroyed by soil insect pests. Since that time seed treatments have been fully adopted and we have seen soil insect problems minimized. Not eliminated, but minimized. As a result we have seen corn yields stabilize to some degree. The question over the last few years has been “Are the low rates of insecticide seed treatments doing enough to fully protect yield”. My data would say sometimes, but not as consistently. While the low rates have done a good job in minimizing across the board soil insect problems, they have left bushels on the table in many situations in our area.
SHEDDING LIGHT ON YIELD LOSS FROM BELOW GROUND DAMAGE
Often it is difficult to determine what insect pest or what degree of damage is responsible for yield responses we are seeing with the different types of at-planting insecticides. The reason is because sometimes the stand looks very healthy, but we see a response in yield. Sometimes the response is quite large, but we never quite see enough damage to adequately explain the yield differences, so we are left wondering where it comes from.
While working on sugarcane beetles in corn, one of our graduate students (Mr. Kevin Lanford) was artificially simulating sugarcane beetle damage to V3 stage corn by removing ½ – ¾ inch of soil and manually damaging the underground portion of the stem (damage was slight stem penetration with a drill bit, simulates sugarcane beetle perfectly). We then replaced the soil and monitored the above ground symptomology that occurred. We developed an above ground rating scale to separate levels of visual above ground damage into categories, hand yielded each plant, and analyzed according to its ranking. We did this in 2 locations for 2 years.
ABOVE GROUND RATING SCALE
0 = no apparent damage, plants look the same as undamaged plants
1 = slight stunting or mild leaf streaking
2 = moderate to severe stunting or severe leaf streaking
3 = dead heart
Plants that showed zero above ground symptomology had 9% shorter ears than plants that were undamaged and yielded 18% less than undamaged plants. Think about this: even slight damage below the soil that does not manifest itself above ground in any way at all may still be enough to cause significant yield reduction in field corn.
This is possibly why we see consistent results in yield protection with at-planting insecticides in field corn in Mississippi even when there appears to be no damage. It is also why we often see even more of a response when we increase the rate or add a granular or in-furrow spray on top of seed treatments. Under significant soil insect pressure it further minimizes below ground insect damage, which results in more yield protection.