This article really pertains to insect management to all crops, but I’ll highlight a couple of relevant scenarios. A lot of the comments relate to a common theme. Insect management is often reactive. Spraying too early decreases effectiveness, often increase overall control costs, and unnecessarily select for resistance. Many of our most commonly used insecticides offer effective residual control for 4-5 days, some shorter and shorter yet if rainfall occurs. There are some insecticides that offer better residual control (and these also tend to have better rainfastness). Unfortunately, these insecticides are often fairly specific and/or relatively expensive products that you don’t want to spray ‘willy nilly’ on the chance you might get an infestation.
1) Soybean – applying a insecticide with you fungicide: This is still a common practice that is often done without regard to pest populations. The wiser approach is to sample your soybean fields prior to applying fungicide to determine if insect populations are at least very close to treatment levels. Pyrethroid insecticide are the common tank mix partner with a fungicide. Pyrethroid insecticides may provide effective control of sensitive species, like green stink bug, for a 5-7 days. But a decent rain will put an end to any residual control. The simple truth is that insecticide applications at R3 are usually made too early to control our most common pests such as stink bugs. Worse yet, pyrethroid insecticides can create problems down the road with pest such as soybean loopers because you disrupt populations of beneficial insects. There has been a lot of work done in the Mid South showing that insecticide application timed at R3 seldom increase yield because pest populations are generally below treatment threshold. This same research indicates that there is little relationship between stink bug populations at R3 vs. R5 or R6. In other words, killing a few stink bugs at R3 doesn’t mean you won’t have an infestation at R5 or R6.
It should be remembered the the average soybean field in Tennessee probably needs one insecticide application during the course of a year. That means some will not be sprayed and some may need 2-3 applications. It is very common that early planted/maturing fields will not require an insecticide application. Think before you spray. Scout before you make a fungicide application!
2) Cotton – plant bug control: Some are still looking for that sliver bullet where you can spray and be assured you do not have insects next week. That product does not exist for tarnished plant bugs. Plant bugs can re-infest fields quickly. The phone has been ringing with many recent examples. I’ve had several reports of post-treatment numbers that are higher than pre-treatment numbers, despite the use of good insecticides. This can be especially apparent when adults are migrating into fields. There is a reason that almost everyone recommends 2 applications on a 4-7 day interval when battling heavy plant bugs. This would not be necessary if products truly provided much residual control for 7, 10 or 14 days.
The moral of the story is do not assume a spray this week means you can skip next week. A second moral is to avoid treating before threshold levels are reached in hopes of keeping a population from building. There certainly can be some benefit IF you get lucky on your timing. However, it guarantees you are going to make one application (and likely a second). A far better approach is to wait until pest populations reach the threshold and be prepared to make two applications. You may ultimately may make the same number of applications, but you may actually save an application. Too many people are unwilling to let pest populations reach threshold before treating. You should remember that treatment thresholds are set below population levels causing economic damage. There is a built-in cushion!
