Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields on a weekly basis for leafhoppers. It is also time to start watching for defoliators in alfalfa, including grasshoppers, corn earworm, webworms and beet armyworm.
Field Corn
During the last week, I have received numerous questions about native brown stink bugs in field corn. The following is review of what I included in the June 23, 2011 issue regarding stink bugs in field corn:
In recent years, we have seen an increase in primarily brown stinkbug damage to developing corn ears, especially when fields are adjacent to wheat fields. Last year, we also saw damage from brown marmorated stink bug (kernel damage not distorted ears) in areas north of the canal in New Castle County. We are continuing to survey fields to evaluate the extent of the damage from all species this season. The following is a summary of information from the University of Georgia on stink bug damage in corn as well as pictures of damage.
http://georgiagraincrops.com/2011/05/25/stink-bug-control-in-ear-stage-corn/
“Corn is most susceptible to stink bug injury during ear formation before tasseling stage (VT). Bugs will feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves usually outward.
Feeding during silking and pollen shed (R1) also will kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages blasts individual kernels, usually causing them to abort.”
Although we have not developed thresholds for our area, the following thresholds are used in Georgia: 25% infested plants (1 bug per 4 plants) as a threshold during ear elongation to pollen shed and 50% infested plants (1 bug per 2 plants) during the later part of pollen shed and blister/milk stage. We also see that initially stink bugs tend to be more prevalent on the field edge, so only a perimeter spray may be needed.
Just this week, we found our first heavy infestation of brown marmorated stink bugs on the edge of one corn field in New Castle County – we have not observed any yet in field corn in Kent and Sussex counties. Heavy populations were found along the field perimeter (200/100 plants) closest to woods. In the field interior, the numbers were low (3 /100 plants). This season, the feasibility of perimeter treatments in corn will be tested at several University of Maryland research farms. Also, graduate students at the Universities of Maryland and Delaware are currently investigating the distribution of BMSB in field and sweet corn fields and their impact on corn grain yield and quality, respectively. Hopefully, this research will help us in the development of sampling and treatment protocols and thresholds specifically for BMSB in corn.
Soybeans
We are starting to see an increase in the levels of bean leaf beetles in full season soybeans. In double crop soybeans, grasshoppers continue to be the predominant defoliator present at this time. Remember, at the bloom to pod fill stage in full season soybeans, the defoliation threshold drops to 10-15% defoliation. Double crop soybeans can not handle as much defoliation as full season fields at the pre-bloom or pod-fill stages. In addition to defoliation, bean leaf beetles can also feed on pods. Bean leaf beetles can clip pods or plant diseases may enter the pod through their feeding sites. This can result in seeds that appear shrunken, discolored, and moldy, causing a reduction in seed quality. Although we have not established thresholds for pod feeding in our area, the following link provides information that is used in the Midwest:
(http://www.ipm.iastate.edu/ipm/icm/2000/8-21-2000/lblroof.html). When possible, a material with residual control should be used for bean leaf beetle control.
Economic levels of spider mites continue to be found in both irrigated and dry land fields throughout the state. It is important to continue to scout the entire field since hot spots can be found throughout fields and edge treatments may not be effective. As a reminder, under heavy mite pressure and extended hot, dry weather, it often takes extended periods of free moisture on leaves, high humidity during the day and cool evening temperatures to get an increase in the fungal pathogens that can significantly reduce exploded mite populations. If egg populations are high at the time of application, two applications will mostly likely be needed. Be sure to read the labels for use rates and restrictions – there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control. Lastly, be sure to consult your crop insurance provider regarding their rulings this year regarding the need to make an attempt to control mites under drought stress conditions.
You should also scout for stink bugs and pods worms as we enter the pod set and pod fill stages.
In New Castle County, we continue to find a few brown marmorated stink bugs (BMSB) in the mix. As a general guideline, current thresholds for stink bugs are set at 1 large nymph/adult (either native brown or native green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans. We currently do not have any thresholds for BMSB in soybeans.
Since corn earworm trap catches are starting to increase again, open canopy blooming fields will be attractive to egg laying earworm moths. In the past, we have used a static treatment threshold of 3 corn earworms per 25 sweeps in narrow fields and 5 corn earworms per 25 sweeps in wide-row fields (20 inches or greater). These thresholds were calculated for a 10-year average soybean bushel value of $6.28. A better approach to determining a threshold is to access the Corn Earworm Calculator (http://www.ipm.vt.edu/cew/), which estimates a threshold based on the actual treatment cost and bushel value you enter.