Bill Cissel, Extension Agent – Integrated Pest Management; firstname.lastname@example.org, Phillip Sylvester, Extension Agent – Agriculture, Kent County; email@example.com and David Owens, Extension Entomologist, firstname.lastname@example.org
The old farming adage “Knee high by the 4th of July”, a benchmark used to indicate the corn crop is doing well, seems a little outdated these days. I personally associate the 4th of July as a time when I start noticing some of the earliest planted corn in tassel and that means some are considering whether or not they are going to apply a fungicide. This also begs the question, should an insecticide be included with my fungicide application at tasseling?
Before answering this question, you should ask yourself what you are attempting to control with the insecticide. If you are targeting stink bugs, the first thing you must consider is if you have a stink bug infestation and is the infestation at threshold?
To determine if you have a stink bug infestation, you can scout your corn field by actively searching plants for stink bugs. Scouting efforts should be initially focused on field edges, especially those fields adjacent to wheat and fields bordered by weedy ditches or woods. If you find high levels of stink bugs on the field edge, make sure you are also sampling the field interiors.
We are still capturing a large number of green stink bug adults in some of our black light traps and as wheat harvest winds down throughout the region, experience tells us that this is a time when native brown stink bugs move from wheat to corn. Adding insult to injury, due to the wet spring, field corn is behind in development and at greater risk for yield losses from stink bug feeding.
Two new thresholds have been developed in North Carolina based on the sampling method, partial and entire plant. The partial plant sampling method is based on only sampling a portion of the plant and recording the number of stink bugs per 100 plants. The entire plant sample is based on the percent of infested plants, visually searching the entire plant for stink bugs.
The partial plant threshold is as follows:
V14-VT only sampling the stalk at one leaf above and below the primary ear:
● 9 stink bugs per 100 plants or fewer, do not treat.
● 18 or more stink bugs per 100 plants, treat.
● If the number of stink bugs per 100 plants is between 9 and 18, treat if you have 13 or more bugs per 100 plants.
R1-R4 only sampling the stalk at one leaf above and below the primary ear:
● 35 stink bugs per 100 plants or fewer, do not treat.
● 52 or more stink bugs per 100 plants, treat.
● If the number of stink bugs per 100 plants is between 35 and 52, treat if you have 43 or more bugs per 100 plants.
If you sample the entire plant, the threshold that is being recommended in North Carolina is as follows:
V14-VT sampling entire plant
● <16% of the plants infested, do not treat.
● >26% of the plants infested, treat.
● If the % of the plants infested falls between 16-26%, treat if 21% or more of the plants are infested.
R1-R4 sampling entire plant
● <30% of the plants infested, do not treat.
● >43% of the plants infested, treat.
● If the % of the plants infested falls between 30-43%, treat if 36% or more of the plants are infested.
It should be noted that this threshold was recently published and has not been evaluated in Delaware.
Here is a link for more information on how to use the new stink bug threshold that was developed in North Carolina: https://entomology.ces.ncsu.edu/2018/04/new-stink-bug-thresholds-in-corn/
So what about including an insecticide with fungicide applications at tasseling to target stink bugs?
If your field is below threshold for stink bugs, then the answer is No. Automatically including an insecticide with your fungicide application has not been effective at reducing stink bug infestations and preventing injury to corn for several reasons:
- Stink bug injury on corn is more severe when feeding occurs during earlier growth stages (prior to pollen shed) so these two timings do not necessarily overlap. Waiting until after tasseling to control a threshold population of stink bugs is too late because the most severe damage is already done.
- Controlling stink bugs in corn after tasseling is difficult because of the dense foliage above the ear zone and the habit of stink bugs to seek refuge in leaf axils and in the folds of leaves. The full article can be found here: https://entomology.ces.ncsu.edu/2017/06/manage-stink-bugs-in-corn-before-it-tassels/?src=rss
- Do not expect much if any residual control so if stink bugs aren’t present when you spray, you are not reducing their populations or preventing them from infesting your field.
However, if your field is at threshold for stink bugs, then the answer is Yes, you should include an insecticide. Keep in mind that if you haven’t been scouting your fields prior to tasseling and you have a threshold population of stink bugs, most of the damage is probably already done. The reason for this is the greatest yield loss potential from stink bug feeding occurs prior to pollination so waiting until tasseling to control stink bugs may be too late.
Here is a link to the publication: http://www.bioone.org/doi/pdf/10.1673/031.011.16801
If you are at threshold for stink bugs and need to spray your corn, most pyrethroids should provide control. However, based on a vial bioassay conducted in NC using brown stink bugs collected from wheat, bifenthrin was found to be the most efficacious.
Here is a link to the report: https://entomology.ces.ncsu.edu/2017/06/insecticide-choice-for-stink-bugs-in-corn/?src=rss