By Scott Stewart, IPM Extension Specialist
I had a client call suggesting I make the following changes to our insect treatment thresholds to make things a little easier. I added the last one myself.
- If you see an airplane, spray 50% of your fields. Please ignore commercial airplanes flying above 30,000 feet.
- If you hear about insect problems “to the South,” spray 75% of your fields.
- If your neighbor sprays, all fields should be treated immediately.
- If you are already driving your sprayer past a field, turn left or right and commence the killing.
I have to confess to similar frustrations where panic and convenience sprays are made with little justification and even less scouting. Field to field variation in insect populations can be tremendous given differences in varieties such as maturity, planting date, GMO traits, fertility, irrigation and previous insecticide use. It is silly to make insect control recommendation without scouting and considerations of pest population levels.
Soybean insects are all over the board right now. The most consistent observations are a general lack of stink bugs and how kudzu bug infestations have failed to develop because of the presence of a fungus killing this insect.
High populations of bean leaf beetles are present in many areas. I’ve had multiple reports of soybean loopers, many cases at low levels, but also those requiring treatment. A lot of these calls are from the most southern counties in Middle and West Tennessee. I’ve also had several people confirm that looper populations are worst where a pyrethroid insecticide was used in the past few weeks.
Another common theme is the presence of saltmarsh caterpillar. Bean leaf beetles, loopers, green cloverworm and saltmarsh caterpillars are part of a defoliating pest complex. There are a few things to consider when dealing with a complex of several species:
- Treatment will often be based more on defoliation levels than the numbers of insects caught in a sweep net. Prior to R6, treat when defoliation exceeds 20-25%. After reaching R6, for the next 10-14 days, treatment is recommended if defoliation exceeds 30-35%.
- It is important to judge defoliation throughout the entire canopy. Bean leaf beetles feed in the top, so people tend to give them too much credit. Loopers tend to start down low and can be overlooked.
- Do not treat if the field is beginning to yellow. That typically occurs at about R7, and the plant is already trying to defoliate itself.
- Choose your insecticide based on the complex of pests you are finding. The presence of many loopers will force you to use one of the “caterpillar specific” insecticides, such as Belt, Besiege, Blackhawk, Intrepid, Intrepid Edge, Prevathon or Steward. The pyrethroid insecticides will not control soybean looper and often make infestations worse.
- It may be necessary to mix insecticides. For example, I often suggest Intrepid at 4 ounces per acre plus a pyrethroid for a complex that includes loopers but also stink bugs, bean leaf beetle or some other pests. However, Besiege already contains a pyrethroid, and a pyrethroid can be added to any of the other caterpillar specific insecticides.