Agronomic Crop Insects – April 20, 2012

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to scout fields for both alfalfa weevil and pea aphids. 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
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 been extremely dry for slug damage, with the predicted rains this weekend and into early next week there is always the chance that we could see damage in fields with high populations before planting. When sampling this spring, we did find a significant numbers of grey garden, marsh and banded slugs under shingle traps, especially in fields with heavy double crop stubble. Although we see more problems in corn when conditions remain cooler and soil remains wet, it is the generally during the warmer days of April 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. Be sure to read the following fact sheet from Ohio for more information on slug management: http://ohioline.osu.edu/ent-fact/pdf/0020.pdf

Small Grains
Low levels of grass sawfly and armyworm larvae can be found in fields throughout the state. As indicated last week, outbreak years most often occur when we have higher levels of local overwintering populations as well as high populations of moths migrating from the south. A recent report from Kentucky indicated that although moth captures did begin very early this year and the first several weeks appeared to be greater than the rolling 5-year average, recent counts have dropped below the 5-year average. Our first week of black light trapping shows low levels of moth activity, but only time and weekly scouting will tell if we have economic levels in our fields: http://ag.udel.edu/extension/IPM/traps/currentblttaw.html. 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. Also, the higher rates of insecticides are needed for grass sawfly control.

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