Scouting for Sorghum Midge with Confidence

Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist and Jeff Gore, Research and Extension Entomologist July 18, 2015 08:37

Scouting for Sorghum Midge with Confidence

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When it comes to scouting for sorghum midge I have found that very few people have the confidence to accurately find them. Most believe if they are not finding them that they must be missing them or doing something wrong. The fact is, if you know what you are looking for and are not finding them, they are very likely not there. We know that midge, if present in high numbers, can be extremely devastating to grain sorghum so over the years many have opted for automatic sprays at 25-30% bloom to ensure that the problem was taken care of. This would be fine if the probability of every or even most fields having midge was high but this is not the case. In fact, the majority of fields that are planted early (May 10 and earlier) rarely ever have midge. The further we move the planting dates into may and June the more common they become but even then many fields may not have them at all. There are exceptions of course so that is why it is important to scout.

It is also critical to understand when sorghum is susceptible to midge damage. I have had numerous people telling me about midge applications that are poorly timed and will have zero effect on protecting the crop from midge damage. Timing is critical!

Sorghum flowers from the top of the head down. Individual heads will start to bloom almost as soon as the head emerges from the boot and bloom out in about 4-5 days with the varieties we are growing in MS. You will know the head is blooming when you see the yellow anthers sticking out of the glume (the yellow things that look like they are on a short string all over the head). This is important. The head or only the part of the head that has yellow anthers is susceptible at that time. Once they turn orange that part of the head is no longer susceptible to midge damage and a spray will not protect that part of the head. The reason the bloom is the susceptible stage is because when the glume opens and puts the yellow anther out to pollinate the female midge inserts a single egg into each open glume. Each female midge lays 30-120 eggs. The egg hatches and grub develops inside the kernel until it pupates and emerges about 14 days later. Each female adult midge only lives about 1 day or less.

The picture below is a typical sorghum head flowering from the top down. The first photo shows the top third of the head flowering (yellow anthers present) so this head is about 30% bloom. This is about the time that midge are typically treated. The second photo shows a head that is blooming a little past the middle of the head, so this head is roughly 60% flowering. Notice that the top part is now orange so that portion of the head is no longer susceptible to midge damage. The last photo shows a head that is only yellow on the bottom third. This head is about to be bloomed out but the bottom section where the yellow anthers are is susceptible to midge damage still.

sorghum flowering 2

Now if everything head emerged uniformly this would be cut and dry but the reality is rarely does a sorghum field have uniform head emergence. There are no hard fast rules on how to handle this situation but this reinforces the importance of scouting. Let’s assume that 3o% of the heads in a field are out and are at about 20-30% bloom. If they have midge at threshold I would treat that even though it only represents 30% of the field because it is 30% of the yield potential. If midge are there, don’t wait to try and get 90% or even 60% of the heads out. If 5 days later 30% more of the heads emerge and have threshold, you need to treat those as well. However, if you would have waited on those heads to emerge the first 30% would no longer be susceptible to damage and you would have completely missed protecting them. Pick a group or cohort of heads that are susceptible and treat that group as needed based on scouting. You will likely avoid multiple sprays this way.

The Sorghum Midge:

I have gotten numerous questions about “what exactly does a midge look like?” I have never seen a midge that was not orange or reddish in color, I and I have never seen a sorghum midge that was big as house fly! If you see a small fragile fly about 1/5th the size of a mosquito that is orange on a blooming sorghum head it is likely a midge. Here are a few pictures to help with identification.

Photo by Grad Student Ben Thrash

IMG_0106

midge
Scouting:

When scouting for midge there are basically 2 methods. Visual and the plastic bag method. The visual method is basically just staring at the head and observing it for midge. You can occasionally give the neck a good thump to make them flutter up to see them better. This works well as long as the wind is not blowing hard. The bag method works extremely well unless the heads are wet early in the morning which makes it aggravating. I simply get a gallon zip-loc bag and hold it over the head and give a hard thump or quick shake and the midge will fly up to the top. I have found that it is not necessary to try an quickly close the bag. As long as you don’t turn it upside down they stay at the top. This method works well even in high wind. I have been successful finding midge this year all through the day.

Control:

Up until now we have only recommended the use of pyrethroid insecticides for control of midge. The reason is they are extremely consistent even at very low rates. Many are looking for options that will not flare sugarcane aphids and have wondered about Dimethoate and Lorsban. We have not recommended these in MS because while they have some activity on midge they are less consistent then pyrethroids and I have actually flared aphids with these compounds also. You may have heard about Blackhawk from Dow this year. Blackhawk is a spinosad compound like Tracer. Spinosad does have a track record of having activity on some Diptera. We have been reluctant up until this point to recommend this product for midge control because there was very little data and the company did not actually have a label. The company now claims to have a 2ee label and said they will stand behind this product and the farmer for full control not suppression of sorghum midge. This will be great news because it will not flare aphids and will likely give you some extended control on the headworm complex. Use rates will be 1.5-3 oz per acre.

If you want a one on one in-field scouting demonstration do not hesitate to call us. We have been doing some of these in the Delta and the Hills and have received tremendous feedback. Just give us a call.

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Angus Catchot, Extension Entomologist
By Angus Catchot, Extension Entomologist and Jeff Gore, Research and Extension Entomologist July 18, 2015 08:37
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