Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
CORN EARWORM ALERT
Soybeans
High levels of corn earworm have been reported in fields throughout the state. As indicated in past newsletters, the combination of drought stressed corn, early dry down of corn and high moth catches has resulted in a high potential for damage this year. Be sure to check all labels for the days from last application to harvest as well as other restrictions.
As of this date, good control has been reported in fields sprayed with the highest labeled rate of a pyrethroid if treatments were applied when worms were small. There are only a few fields so far that may need to be re-treated 7-10 days after application but this is due to a recent hatch – all worms found have been very small in size. As indicated last week, with the extended moth flights and no sign of weather that will help crash populations (i.e. it has just been too dry since you need a combination of rainy weather, cool evenings and mornings with dew and high relative humidity during the day for about a week to get fungal pathogens developing), you may have to treat fields twice under these conditions. A few fields are also reporting larger worms after application but in these cases the highest labeled rate of a pyrethroid was not used. Another word of caution, it will be extremely important to wait and check fields at least 4 days after an application of a pyrethroid. Fields that have been checked at 3 days are still showing worms – however, when those same fields have been visited just 24 hours later we are seeing very good control. As a reminder, although the pyrethroids do have contact activity, the main mode of action is as an ingestion product. So be sure that you do not look at fields too early to make a re-treatment decision. When treating for earworms with pyrethroids, it is important that you make applications to fields that are at threshold levels, apply treatments when the worms are small and use the highest labeled rate. If fields that have been treated with a pyrethroid need to be treated again, you may want to consider using a non-pyrethroid for the second application. In addition, if you are treating for the first time and worms are larger at treatment time, you should consider a non-pyrethroid option. Other labeled options for earworm control include Steward and Larvin. Research trials in VA indicate that Lorsban has only done a fair job and as a reminder it has no residual control.
So – once again, the only way to know if you have an economic level will be to scout and treat when worms are small – do not wait until you see pod damage. With higher soybean prices, the best 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.
We have also seen an increase in soybean aphid populations in fields in New Castle County. As a general guideline, treatment is needed through the R-5 stage (seed is 1/8 inch long in the pod of one of the four uppermost nodes on the main stem) of soybean development if economic levels are present. It may also be beneficial to spray through R-6 stage (pods containing a green seed that fills the pod cavity at one of the four uppermost nodes on the main stem) — reports vary as to the benefit of spraying once plants reach the R-6 stage but in some years and some situations there has been an economic return. Spraying after the R-6 stage has not been documented to increase yield in the Midwest. The suggested treatment threshold from the Midwest is still 250 aphids per plant.