By Scott Stewart

As much of our corn begins to tassel, some questions have predictably been asked about putting insecticide out with foliar fungicide applications. This is NOT a generally recommended practice. This practice has been tested thoroughly in the Mid-South, and data indicates a negative return on investment in most cases. Below are some points (and exceptions) for consideration.

What pest are you targeting? We often get no answer when asking this question. One common insect mentioned is Japanese beetle. However, our very recent research indicates this is a borderline non-pest unless very high numbers (3 or more per ear) occur during the first week of pollination. It is rare to find an average of 3 beetles per ear, except perhaps on field edges. Test this yourself: Flag some ears where Japanese beetles have ‘gone to town’ on the silks. Go back and look at these ears later.

Another target mentioned is corn earworm. Decades of research has shown that it is not feasible to control corn earworm feeding in ears with a single insecticide application. Our recent data shows that kernel injury can not be reduced by a single insecticide application when corn earworm pressure is high — you kill one and another takes its place. 

We've had a few tests where a single application of at least some insecticides has reduced kernel injury if relatively low numbers of corn earworms are present. Even then, this has often been a short-term phenomenon that largely disappears as other larvae infest ears. And even in these circumstances, this has not translated into a yield increase. 

Some of the newer Bt corn traits are pretty good at reducing kernel injury. However, we haven't been able to demonstrate that this has increased yield. Recent research indicates that much of this kernel damage is to small kernels in the ear tip that never would have made it into the combine. Furthermore, our data suggests that undamaged kernels in the ear can compensate for this injury.

The exception to all of this is treating non-Bt when corn borers are present. This is why it is important to use southwestern corn borer (SWCB) pheromone moth traps to gauge the level of risk in your fields. We’ve had many experiences where a single insecticide application to non-Bt corn has significantly increased yield when SWCB are present at treatment level. Populations tend to be low in many areas of the state, partly due to the widespread use of Bt corn. Frankly, most areas don’t really have a problem. 

Populations of SWCB are also relatively low this year, likely due to the hard winter. UT thresholds for this pest were previously discussed ( Another point to consider is that, as typically occurs around first tassel, moth traps indicate that we are between generations of SWCB. We do not expect the second generation to begin in earnest for 2-3 weeks. Pyrethroid insecticides should certainly not be expected to provide more than a week of residual control (and less if it rains).

Growers should have a better reason to spray insecticides than “I’m going over the field anyhow” or “It really doesn’t cost that much." Besides the added cost, there is baggage from making unnecessary pesticide applications, including potentially flaring secondary pests such as spider mites, selection for insecticide resistance, and potential negative environmental impacts. Have a reason, not excuses!