In the Southeast there will be a big increase in wheat harvested, some later than usual because of delayed planting. For growers who grow corn in close proximity to wheat, there may be some unexpected and costly guests from the big wheat crop — stink bugs.
The scarcity of soybeans worldwide and the subsequent high price for beans will likely lead some growers in the Southeast to double-crop more wheat acres than usual. This ‘big three’ grain rotation can be economically rewarding, but combined with popular pest management practices this combination may lead to increased problems with sucking insects, stink bugs in particular.
The wide scale use of cotton plants with the bacillus thuriengensis bacteria included in the gene pool has created many benefits for cotton and corn growers over the past few years. Unfortunately, leaving off traditional pyrethroid and organophosphate sprays may be providing an ideal environment for stink bugs and other pests routinely controlled by the traditional sprays.
Brothers Bryan and Wesley Foster grow about 1,500 acres of wheat and double-crop it with soybeans. For a good rotation, they also grow 1,500 acres of corn on their farm near Columbia, N.C.
“We saw stink bug damage last year in some of our corn. Stink bugs bore into the top third of young corn ears, and the corn just didn't recover. You could easily detect the ‘crooked’ ears of corn damaged by stink bugs,” Bryan Foster says.
North Carolina State Entomologist John Van Duyn points out that the use the Bt gene in corn is a powerful tool against insects much more economically damaging and difficult to control than stink bugs. However, in fields where stink bugs are a problem, the grower may not get too much comfort from the many benefits of the Bt technology, he adds.
Stink bugs feed on plants by inserting needlelike mouthparts into cotton bolls, soybean pods or ears of corn. While feeding, they inject enzymes into the plant to aid in sap removal from the fluids on which they feed.
Damage can come in several forms. First, the injection site opens the plant up to a host of other insect and disease damage. Second, the feeding enzymes can cause damage to the plant. Most significantly, regardless of the plant stink bugs feed on, damage is done to the quality and overall yield of the crop.
Stink bug feeding damage isn't always easy to find in young corn. However, there is usually a row of oval holes across the unwrapped leaves of damaged plants. A slimy area may be found in the stalk where sting bugs have fed.
In corn the most dramatic sympton is tillering of plants that have been fed upon by stink bugs. Depending on the time of planting and proximity of stink bugs in other crops, this damage may show up as early as 10 days after planting.
Tillage practices can also have a direct impact on stink bug damage — not good news for many Southeastern farmers who have gone to no-till farming. Stink bug damage is most severe in no-till.
Though stink bugs do feed on conventionally planted corn, the damage to plants is usually lower and most often restricted to border rows between corn and fields of wheat or other host plants.
In Kentucky researchers have found soybean-wheat-corn rotations to be especially vulnerable to stink bug damage in corn.
Stink bug populations build up in soybeans during pod fill. Wheat cover crops provide an ideal environment for insects in the early spring. Some of these insects in some parts of the Southeast can over-winter in wheat stubble, or they may return to these fields in the spring from adjacent over-winter sites.
Adjacent corn then becomes the next most attractive food source for stink bugs. With prices for corn, wheat and soybeans remaining high, it is a good bet many Southeastern farmers will unknowingly provide this perfect environment for stink bugs.
Popular neonicotinoid-containing seed treatments, including Poncho, Gaucho and Cruiser are highly effective in controlling green stink bugs, but only moderately successful in controlling brown stink bugs.
For growers who did not treat their corn seed with one of these insecticides, a number of synthetic pyrethroid insecticides are highly effective in controlling green stink bugs and a similar number of organophosphate-based insecticides are good for brown stink bug control.
The key to effective use of either family of insecticides is to find stink bug populations early and spray in time to prevent damage, because corn will not grow out of early stink bug damage.
For corn in the early silk stage through milk stage, treatment with a pyrethroid or organophosphate sprays may be justified when there is one stink bug per five plants.
From the end of the milk stage through the hard dough stages, one stink bug per plant is a good rule of thumb for treatment. When determining thresholds, only stink bugs one-fourth inch or longer should be considered.
With the harvest of increased wheat acreage in the Southeast coming up, growers should be aware of the increased threat of stink bugs to other crops, particularly corn. Careful scouting and timely insecticide application can manage the problem.
JOHN VAN DUYN, North Carolina State entomologist, says increased wheat acreage may result in more stink bug damage in corn.