Agronomic Crop Insects

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

Alfalfa
As temperatures increase, we will start to see a significant increase in feeding damage from alfalfa weevil. As alfalfa approaches harvest, the decision to cut instead of treat may be considered. However, this option should only be used if you plan to cut shortly after you find an economic threshold level, 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
Although only a small percentage of the corn acreage has been planted, be sure to check for cutworms as plants emerge, even if an at-planting insecticide was used for cutworm control. With the cool, wet weather, slugs can be found under residue in no-till fields. A new fact sheet from Ohio State provides information on slug biology, scouting and management of slugs on field crops:
http://ohioline.osu.edu/ent-fact/pdf/0020.pdf.

Small Grains
As grain heads emerge on barley, be sure to watch for the movement of aphids into grain heads. In many cases, beneficial activity is still not high enough to take care of populations moving from the lower canopy of the plants into the grain heads.

Once grain heads have emerged, you should also begin sampling small grains for sawfly and armyworm larvae. Moth flights are behind compared to past years due to the cooler temperatures over the past few weeks. 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.

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