Sorghum Midge Threat While Under Sugarcane Aphid Watch
The following was written by Dr. Pat Porter, District 2 Entomologist, about the potential sorghum midge threat this season.
In the July 8th edition of FOCUS I promised to write more detailed information on sorghum midge. This year a lot of sorghum was planted earlier than normal in order to avoid the potential worst problems associated with sugarcane aphid. (Congratulations if you employed this IPM practice! It seems to be paying off now that sugarcane aphid is firmly established in Southern High Plains counties.) In general, sorghum that completes bloom before August 4th or so in our part of the Southern High Plains will escape economic midge damage. However, some of this early sorghum and the abundant Johnsongrass can serve as early hosts for midge and give later populations a head start. It is too soon to know what midge populations will be like this year, but on balance we have plenty of egg-laying hosts in the system.
As I started this article I saw an excellent summary from Angus Catchot and Jeff Gore at Mississippi State University. This article is so good that I am going to link to it as most of what I would have written about sorghum midge: it contains recognition, biology, scouting information and control suggestions: http://www.mississippi-crops.com/2015/07/18/scouting-for-sorghum-midge-with-confidence/ .
One key point about sorghum midge is that it lays eggs in blooming sorghum only on the day the anthers are visible. However, it takes several days for a sorghum plant to flower from the top of the panicle to the bottom and, due to uneven flowering across the field, it may take a week to ten days for the field to complete pollination. Adult midges (tiny flies) live about one day, but there is continual re-infestation of the field each day, so low midge numbers on the first day of flowering might be high midge numbers in subsequent days. And overall midge numbers in the system increase as August progresses.
Sampling should be done in mid-morning, or after temperatures have reached 85 degrees. The treatment threshold depends on sorghum panicles per acre, midges per panicle and cost of control. The threshold calculations can be found in Managing Insect and Mite Pests of Texas Sorghum on page 19 - 20.
Insecticide selection has changed because sugarcane aphid is present in many area sorghum fields. Our management recommendations prior to sugarcane aphid were pyrethroids, Lannate, Malathion and Lorsban. Unfortunately, all of these insecticides kill beneficial insects, the same insects that help slow down the sugarcane aphid. And, to make matters worse, they don't do a good job of killing sugarcane aphids. So the net result of using them might be to help sugarcane aphids rapidly increase in the field. However, it is important to treat midge if it reaches threshold; do not forsake a needed midge treatment out of fear of what might happen with sugarcane aphid.
As a practical matter, scout the field carefully to determine whether there are sugarcane aphids present. If so then you can still use the insecticides listed above, but consider adding Transform or Sivanto (for sugarcane aphid) if you think you need to. Or be prepared to come back with Transform or Sivanto later. Not all midge insecticides will risk flaring sugarcane aphid; Blackhawk has just received a 2ee label on sorghum for midge control and should be used at 1.5 - 3.0 oz per acre. I do not have direct experience with this spinosad product, and in fact have not seen the new label, but Dow says is will work and they stand behind its performance for full control.
I am not sure that we will have an increased midge problem this year, especially since all bets are off due to the very wet spring and early summer. However, I wanted to provide some information on making midge control decisions in light of sugarcane aphid.