I received an email (see note below) earlier this week that had a common question about bermudagrass stem maggots. His question is below, along with my comments in bold. Some parts of the question have been altered to ensure anonymity.
“I have (several) acres of Russell (bermudagrass) in (a county in northwest GA). Stem Maggots started showing damage on 5 Aug. 2017. I did not have the option to cut for hay, so I treated with Lambda (1oz. acre) and plan to treat again within 10 days. I noticed this morning the damage seemed to have (slowed). Damage appears to be about 20-25%. I plan to cut for hay around first week in Sept. Would it be o.k. to treat again with Lambda (1oz. acre) after cutting and removal of hay?”
Yes, if the damage is relatively slight and you caught it in time with the Lambda, then it will likely continue growing. If the damage was heavy (and 20-25% damage is pretty heavy), cut for hay as soon as practicable and then treat the field with a pyrethroid (Lambda is fine) 7-10 days after cutting. You should assess the cost:benefit, though. As we get later in the season, yields of bermudagrass decrease. A September cutting may only produce 0.75 ton/acre. Therefore, the severity of the infestation must be such that the insecticide application will save at least 0.15 ton (i.e., 20% yield loss for that cutting). This assumes the hay will be valued at $100-125/ton to you. If one already has a barn full and, therefore, the hay is of less value, then the cost:benefit of the application will need to be reassessed.