Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu
Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. Significant damage can occur in grass hay fields from true armyworm and fall armyworm. It is important to catch populations before significant damage has occurred and when larvae are small. In addition to checking labels for rates, be sure to check for all restrictions including, but not limited to, comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. No thresholds are available; however, controls should be applied before significant defoliation occurs.
Soybeans
Corn earworm populations still remain relatively low; however, there are reports of a recent hatch of earworms, especially in double crop soybean. As indicated in last week’s newsletter, this is a result of the increase in trap catches over the past couple of weeks. Therefore, you will need to watch fields closely over the next few weeks to determine if this late hatch will result in economic damage. Although the recent rains in some areas in combination with hot, humid temperatures may help increase the incidence of fungal pathogens that can help to regulate populations — only scouting will tell if this is occurring. It is also important to continue scouting to determine if a treatment is needed since population levels will vary from field to field. Once pods are present, the best approach to making a decision on what threshold to use for corn earworm is to access the Corn Earworm Calculator developed at Virginia Tech (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.
Last week, my colleagues from the south reported that they are seeing an increase in fall armyworm moth catches. This is a migratory pest so although we have had fall armyworms in our area since late June this recent increase in moth populations in the south could also make it to our area. There are two strains – one that prefers to feed on grasses and another that prefers to feed on legumes (e.g. soybeans and alfalfa). However, you cannot tell the strain from trap catches. This week we are finding a few small to medium size fall armyworm larvae in soybeans so be sure to watch for this defoliator. In full season soybeans, the defoliation thresholds are 15% defoliation from bloom through the end of pod fill; and 35% defoliation once fully developed seeds are present. The defoliation threshold in double crop soybeans is 10% defoliation from bloom through pod fill; and 15% defoliation once fully developed seeds are present.
Populations of another defoliator – the soybean looper — are also increasing in states to our south, especially in North Carolina. This too is a migratory pest and although we have been finding low levels for the last month you will need to watch for this defoliating insect pest as well. In addition, it is difficult to control and pyrethroid resistance has been documented in states to our south. Be sure to select a material that lists soybean looper control on the label. Belt, Besiege, Blackhawk, Radiant and Steward all list soybean looper on the label. In most cases, higher labeled rates will be needed so be sure to read all labels for rates and restrictions, including but not limited to days to harvest and rotational restrictions.