Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
As alfalfa approaches harvest, the decision to cut instead of treat should be considered as a management option for alfalfa weevil. However, this option should only be used if you plan to cut shortly after you find an economic threshold level of weevils, since damage can occur quickly. Cutting should only be considered as a management option if you can cut within 3- 5 days of finding an economic level. As you get close to harvest, be sure to check labels carefully for time between application and harvest.
Field Corn
Since corn has been planted in some locations for about a week now, be sure to check for cutworm feeding as soon as plants emerge, even if an at-planting insecticide or a Bt corn was used for cutworm control. Although conditions have not been extremely favorable for slugs, we are starting to see an increase in the number of slugs encountered under shingle traps in New Castle County. Once corn emerges, we generally see more problems when conditions remain cooler and soil remains wet. However, it is the generally during the warmer days of April to early May when we start to see significant egg hatch. Although no thresholds are available, past experience in the Mid-Atlantic has indicated that levels of five or more grey garden slugs per square foot have indicated the potential for a problem. As a reminder, Deadline MPs are still available for use in corn and soybeans in 2013.
Small Grains
In barley fields where the heads have emerged, be sure to sample for grass sawfly and armyworm larvae. As indicated last week, we generally see significant outbreaks in years when armyworm larvae are a result of populations coming from a combination of local overwintering populations and migrating moths. Looking at this week’s report from Kentucky, their traps have only caught a few moths but the current year’s capture is in line with the outbreak year of 2008. It is still too early to tell what will happen but it is important to be aware of trap catches in southern states. We will be putting out our traps next week and the first counts will be available by April 30. Remember, armyworm larvae are nocturnal so look for larvae at the base of the plants during the day. As a general guideline, a treatment should be considered if you find one armyworm per foot of row for barley and 1-2 per foot of row for wheat. Since sawflies feed on the plants during the day, small sawfly larvae can often be detected early using a sweep net. However, there is no threshold for sweep net samples. Once sawfly larvae are detected, sample for larvae in 5 foot of row innerspace in 5-10 locations in a field to make a treatment decision. You will need to shake the plants to dislodge sawfly larvae that feed on the plants during the day. As a guideline, a treatment should be applied when you find 2 larvae per 5 foot of row innerspace or 0.4 larvae per foot of row. If armyworms and sawflies are present in the same field, the threshold for each should be reduced by one-half. The higher rates of insecticides are needed for grass sawfly control.
We have started to hear reports of an increase in brown stink bugs in states to our south and each year we get questions about the potential for damage to small grains in our area. In past years, the only species we have seen is the native brown stink bug, very few brown marmorated stink bugs have been encountered. Although information from the south indicates that stink bugs can damage wheat, they also indicated that it only occurs when you find very high numbers. Thresholds used for treatment in some southern states are as high as two stink bugs per ten grain heads. The threshold used in North Carolina is one per ten grain heads. Stink bugs feed on the developing tissue and will damage wheat as the grain is filling. The milk and dough stages are known to be especially susceptible to damage resulting in yield loss.
Besiege – (Syngenta) – It was recently announced that Besiege received a label for use in soybeans. It is a combination of two active ingredients (lambda-cyhalothrin and chlorantraniliprole). Here is a link to the new label: http://www.cdms.net/LDat/ldA7G011.pdf.