Agronomic Crop Insects – May 4, 2012

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

Alfalfa
Since weevil populations have been heavier this year and in some cases fields were not sprayed, be sure to check for both adults and larvae within a week of cutting. Feeding from both stages can hold back re-growth. After cutting, there needs to be enough “stubble” heat to control the weevils with a cutting. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.

Field Corn
The recent cooler wet weather in combination with an earlier slug hatch due to the warm winter and March has resulted in an increase in slug damage this past week. We have seen a number of fields with economic levels of slug damage, especially in fields with heavy no-till covers and a history of problems. Options to reduce damage and allow plants to grow ahead of the damage include the use of Deadline M-Ps (or other available metaldehyde baits), Lannate LV ( DuPont issued a 2ee recommendation for slug management in field corn in DE, MD, PA, VA and WV in 2010) or 30% Urea Ammonium Nitrate (UAN) fertilizer. In years past, 30% UAN applied at night when the plants are dry and there is no wind has resulted in variable levels of success (the rate used in past years was 20 gallons per acre of 30% UAN on corn in the spike to one-leaf stage and the mix was cut 50/50 with water to reduce – but not eliminate — plant injury). The best control with the Deadline M-Ps has been observed when there is at least one day of sunny weather after an application. In general slugs stop feeding in 2-3 hours. Although we only have one year of research data with Lannate LV, growers using it this season have reported that they feel it is helping plants to stay ahead of the damage. We are doing a trial again this year to look at application timing. Please see the following link for the Lannate LV 2ee recommendation: http://www.cdms.net/LDat/ld183004.pdf. Remember that when it comes to slug management all of the available control tactics generally reduce the slug activity – buying time to enable the crop to outgrow the problem.

Small Grains
We continue to find armyworms and cereal leaf beetles in barley and wheat fields that were not treated, so be sure to check fields as soon as it is dry enough in the day to do a good job scouting. Population levels remain variable throughout the state so scouting fields will be the only way to determine if an economic level is present. Depending on the temperature, cereal leaf beetle larvae will feed for up to 3 weeks. Research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage so continue to sample carefully for this insect. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. We are also starting to see an increase in sawfly adults laying eggs in wheat fields so hatch may be delayed. Continue to sample for sawflies, especially in areas with a history of problems. Before treatment, be sure to check all labels for the days allowed between last application and harvest.

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