Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Soybeans
Economic levels of corn earworm can be found in double crop fields in Kent and Sussex counties. We have not heard of reports of economic levels from New Castle County but there were a few larvae in fields last week so fields should be carefully checked throughout the state. Fields in Sussex County that had low levels at the end of last week are above threshold this week. 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.
I have received numerous calls this week regarding chemical selection for earworm control in soybeans. In Delaware, the pyrethroids are still providing effective earworm control as of this date. Although we do not know what will happen in the future, we do know that fields that have been sprayed over the last 7-day period are reporting good control. When treating for earworms, 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 of a pyrethroid. Failures in past years (including a few in 2007) were a result of treating too late (above threshold levels), using too low of a rate, and treating when a mixed population size (too many medium and large larvae) was present. Other labeled options for earworm control include Steward, Larvin and Lorsban. With the sustained moth catches, it will also be important to continue to check fields after treatment because we could see a re-infestation (i.e. new hatch of small larvae). After talking this week to my colleague Ames Herbert from Virginia, he indicated that they are just getting reports of a few fields with pyrethroid failures. The higher rate was used however these applications were made to a mixed worm population – that is medium and large worms were present at application time – and it is occurring in areas where corn was extremely drought stressed. He also indicated that this is only occurring in spotty locations throughout the state.
So – once again the only way to know if you have an economic level will be to scout. Although in the past we have used the treatment threshold of 3 corn earworms per 25 sweeps in narrow fields and 5 corn earworms per 25 sweeps in wide row fields, these are static thresholds based on old soybean prices. 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.
Small Grains
With the increase in no-till wheat acreage, as well as our typical rotation of wheat following corn, it will be important to consider a number of insect pests that can present problems. A recent news article from Kentucky titled “Insect Pest Management Decisions in Preparation for Planting Wheat” does a nice job of addressing the potential pests that pose a threat to wheat in the fall (http://www.uky.edu/Ag/kpn/kpn_08/pn080825.htm#wheins). In addition to the pests listed in this article, we have also seen high true armyworm moth catches in our traps throughout the summer and those dreaded slugs have been known to attack small grains if we have a wet fall.
I have also been asked about seed treatments for aphid control. The main concern in the fall is vectoring of barley yellow dwarf virus (BYDV) by all species and direct damage by greenbug aphids. In talking to the plant pathologists in our area, they still do not feel that we are seeing a significant increase in the incidence of BYDV. However, in areas where you have seen BYDV in the past, where you are planting early, or you have seen damage by greenbug aphids, a seed treatment that controls aphids (i.e. Cruiser and Gaucho) would be a good control option. We have also seen high levels of bird cherry oat aphid in irrigated corn fields in Sussex County. Although we have not seen this in past years, it could mean that we will see higher levels in early planted wheat.