Corn or Sorghum in Place of Cotton in 2015: Insect Considerations

Some cotton growers are considering planting corn or sorghum if they can’t get all of the cotton acres planted by the insurance cutoff date. This article is meant to provide a few insect considerations when thinking about corn and sorghum. Dr. Dana Porter, Ag Irrigation Engineer at Lubbock, is preparing an article on irrigation considerations for release here in a few days.

Corn

The three biggest pest threats to late-planted corn on the southern High Plains are fall armyworms, spider mites and mycotoxins (which are affected by fall armyworm and corn earworm).

Any corn planted in June is considered to be “late corn” from an insect perspective. There is still time to reach maturity with some of the less-than-full-season hybrids, so by late I mean only as applied to insects. The first and perhaps most important suggestion is to plant a good Bt hybrid; fall armyworm numbers increase as the season progresses and this insect can cause significant yield loss to non-Bt corn and Bt corn that does not have at least two toxins in it. There are very good hybrids available with two or more toxins and a list can be found here: http://www.texasinsects.org/uploads/4/9/3/0/49304017/traittable2015april6south.pdf). Your seed dealer will be able to make specific hybrid recommendations. I would avoid one toxin corn (Herculex and Yieldgard) because it won’t stand up well to high fall armyworm numbers. Here are my rankings for the efficacy of Bt hybrid types against corn earworm and fall armyworm:

Caterpillar Pest	Bt toxin combination in commercial hybrids
	Cry1F	Cry1Ab	Cry1Ab+ Cry1F	Cry1Ab+ Vip3A	Cry1Ab+ Cry1F+ Vip3A	Cry1A.105+ Cry2Ab2	Cry1A.105+ Cry2Ab2+ Cry1F
Corn earworm	F-P	F-P	F-G	VG	E	G	G-VG
Fall armyworm	F-P	F-P	F-G	VG-E	E	G	VG

P=Poor, F=Fair, G=Good, VG=Very good, E=Excellent
Click here for commercial names of these products. 

Fall armyworm is a major threat. How bad can it be? Our research at Lubbock has shown an average of 0.2 pounds of yield loss per ear when fall armyworm punctures the side of an ear and feeds on kernels. The losses come both from direct ear feeding and from the fungi that come along with the insects and through the wounds they cause. In or trials 52% of the yield loss was from fungi and 48% was from direct kernel damage by the insect. These same research trials showed drastically higher levels of mycotoxins in ears with side puncture FAW damage than ears with only tip damage. Fall armyworm larvae can also damage the ear shank and cause ears to drop to the ground.

Pay particular attention to the Bt corn refuge planting. All Bt corn with two or more toxins planted in the “cotton zone” (basically south of Amarillo) is supposed to have a 20% non-Bt block planted in the field or in a closely adjacent field. Strip refuges of four or more rows are also allowed. Single toxin corn is supposed to have a 50% refuge. Even though our recent research makes us confident that strip refuges have less fall armyworm damage than block refuges (from the toxic Bt pollen falling in the strips and creating toxic kernels), this protection is adequate with only low to moderate populations of fall armyworm. Late planted corn should expect heavier populations of fall armyworm, so a refuge, if it is to be planted, should be a block refuge that can be sprayed if things get bad. Also, even though seed blends are not supposed to be planted in the “cotton zone”, some fields are planted to seed blends. None of the non-Bt seed in a seed blend counts toward refuge in the cotton zone, so a correct refuge, if it is planted, should be calculated as if there was no refuge seed in the seed blend.

We don’t worry about corn earworm (cotton bollworm) much since it is mostly a tip feeder, whereas fall armyworm is a tip feeder, punctures the sides of the ears and does shank damage. Fall armyworm is the biggest caterpillar threat so choose your Bt corn to protect against fall armyworm. 

The next threat, and one that is always present whether corn is Bt or not, is spider mites. Bt corn has no effect on spider mites. Mites can be a very serious problem and occasionally require more than one miticide application. The good news is that late-planted corn is somewhat less prone to reaching the economic threshold for spider mites than is corn planted earlier in the season. Late planted corn should still be scouted, but the spider mite threat is somewhat lower.

Mycotoxins are the next worry in corn. Research has shown that corn growing under too much drought stress is prone to developing high aflatoxin levels, and corn grown with too much water is prone to developing high fumonisin levels. The relationship between water stress and mycotoxins levels is not well understood. There is an insect component, too, because the insects wound the ears and allow points of entry for the fungi that make mycotoxins. If you are going to plant corn then be certain you have enough irrigation capacity to keep it around at least 75% ET during peak water demand; silking through grain fill. Mycotoxins at levels beyond the Federal standards can either cause a dock at the elevator or make the crop unmarketable. Dr. Dana Porter will write about irrigating corn later in the week.

Sorghum

Sorghum is an excellent crop for late planting and the biggest worry being voiced this year is sugarcane aphid. It is true that we have been surprised how quickly the sugarcane aphid moved north 2015, and it is possible it will be an early to mid-season arrival on the southern High Plains this year. (Last year we dealt with it as a late season problem.) However, we have two excellent insecticides to control the aphid and we have solid economic thresholds. If the aphid arrives early to mid-season then it is likely that 2-3 applications will be needed. If it arrives later, say in August, then one application might be sufficient if any treatment is needed at all. There are sorghum hybrids resistant to sugarcane aphid, but the resistance is not all that strong and these fields should be monitored as if they were planted to  susceptible hybrids. The resistant hybrids are Sorghum Partners (Chromatin): SP6929, KS310, NK5418, and K73-J6; Monsanto: DKS37-07 and Pulsar; and Pioneer: 83P56. The sugarcane aphid is not a reason to avoid planting sorghum on the southern High Plains; it is manageable. Statewide sugarcane aphid news can be found here: http://txscan.blogspot.com . 

Sorghum planted in June has a higher chance of needing sorghum midge control than earlier planted sorghum. While pyrethroids are often used for midge control, it should be noted that they can drastically enhance the risk from sugarcane aphid by removing the biological control agents from the field. Non-pyethroid insecticides should be used, and this is also the case if headworms need to be controlled. These complexities are explained in three videos posted here: http://www.texasinsects.org/sorghum-videos.html .