David Owens, Extension Entomologist, owensd@udel.edu
Soybean
Podworms are showing up in double crop fields, and some fields have been treated earlier this week. As a reminder, the soybean threshold calculator is a good tool to create a ballpark estimate at what level podworms should be treated: https://www.ces.ncsu.edu/wp-content/uploads/2017/08/CEW-calculator-v0.006.html. Pyrethroids are inconsistent for us, and may work in some fields, or they may not work at all or well enough in others. In sweet corn, the best pyrethroid has been Hero at its highest rates. Besiege provides piece of mind and excellent worm control. Last year in spray trials, 6.5 fl oz did well in soybean.
Let’s go back to the threshold calculator. The first box to estimate is the Control Cost. Besiege is probably going to run between 2 and 2.25 per ounce, or 12.50 to 18 dollars in product. Application (machinery, fuel, time, custom ground or aerial spraying) is going to run anywhere from roughly 9-14/acre. The other hidden cost is potential yield loss driving over beans. Dr. David Holshouser in 2008 did a study (abstract here: https://acsess.onlinelibrary.wiley.com/doi/abs/10.1094/CM-2008-0317-02-RS) that suggested that driving on soybeans that had not been driven over previously (you are not re-using the same sprayer track) could result in anywhere from 1-6.4% yield loss depending on boom width (45-120 ft.) and growing conditions. In dryland fields our growing conditions right now are not conducive for compensation. So let’s say we have a field that has potential to make 35 bushels, and a middle of the road yield loss due to wheel driving. We could lose 1.25 bushels, which at current prices would be another 20$. Now if wheel tracks are already present, and if the field is irrigated, then this value is probably going to be lower, but the yield potential higher. You can see this makes aerial application more attractive quickly.
The second box in the threshold calculator is price of soybean. The way this calculator was designed back in 2000, the value of beans maxes out at $10. So what about 13.50 beans? If we plug in 7$, 8$, 9$ and 10$ beans, we can see the value goes down, but not entirely linearly. I plugged this into the computer to get a value around 4 earworms per 15 sweeps with a control cost + yield cost of $45.
If you don’t have that yield cost for driving on soybean, and instead had a control cost of roughly $25, and going through the steps to extrapolate out to 13.50 beans, then a threshold is in the ballpark of 2.3 worms per 15 sweeps. Other mitigating factors include: are earworm counts going to continue increasing? Hitting them earlier assuming an increasing population may be justifiable. Are you applying a fungicide? That might shift where you put the application cost. What about other chemistry?
In 2022, our other best treatments were Intrepid Edge at 5.2 fl oz and Vantacor at 2 fl oz. I do not know what Intrepid Edge or its generics cost, but I strongly suspect Vantacor is going to be more expensive than Besiege. In 2019, we had better results with a pyrethroid + Lannate mix than just a pyrethroid.
Having said all that, once we get close to or above 3 worms per 15 sweeps, it’s probably a good investment to treat the worms, but take into account other potential influencing factors!
Sorghum
Ditto on the soybean discussion, but with an even greater twist. In sorghum, a lot of small small larvae die early. Take a look at the sorghum threshold calculator on Texas A&M’s website: https://extensionentomology.tamu.edu/sorghum-headworm-calculator/. Please note that the value for sorghum is expressed in count weight, not bushels. In 2019 we did a sorghum spray trial (UD spray trail results can be found here: https://www.udel.edu/academics/colleges/canr/cooperative-extension/sustainable-production/pest-management/insect-management-reports/) and achieved good control with carbaryl, Besiege, and Baythroid XL 2 days after application. BUT 6 days after application, the worm counts were very low (see the notes about high natural mortality) and Baythroid XL had 0.5 worms, carbaryl 0.3 worms, and the untreated check 0.75 worms. No significant difference. What certainly did not work was low rates of pyrethroids.
Sorghum that has not begun shedding pollen may be at risk for sugarcane aphid. Their populations are starting to increase in some fields. It is worth scouting for them through the soft dough stages.