Agronomic Crop Insects – May 21, 2010

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

Alfalfa
When checking regrowth for damage from weevils, be sure to also consider damage from adults. If economic levels were present before cutting and no spray was applied, both adults and larvae can hold back re-growth. With the cool conditions we have had this week, there may not have been enough “stubble” heat to control the weevils with a cutting. Potato leafhoppers are now present in fields so be sure to sample on a weekly basis after the first cutting. Once the damage is found, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.

Field Corn
With the recent cool, wet weather, slugs continue to be the main pest of concern, especially in fields with 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). In years past, 30% nitrogen applied at night when the plants are dry and there is no wind has also been used with variable success (the rate used in past years was 20 gallons per acre of 30% N 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). Slugs seem to be most active on the plants between midnight and 3 AM so applications of nitrogen have been most effective when applied between those hours. The best control with the Deadline M-Ps has been observed when applications were made and there was at least one day of sunny weather after an application. In general slugs stop feeding in 2-3 hours even though it may take the slugs 2-3 days to die. 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, sawflies 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. Also, with the recent weather patterns, the hatch of armyworm larvae will be staggered –i.e. there will be large and small larvae in fields. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. Before treatment, be sure to check all labels for the days allowed between last application and harvest.

Soybeans
As the earliest beans emerge, be sure to watch carefully for slug damage. Remember, if you had a problem last year, the slugs will still be present in fields and can quickly damage soybeans if present as plants emerge. Be sure to also watch fields carefully for bean leaf beetles and grasshoppers. Early detection and control of small grasshoppers is necessary to achieve control. Numerous products are labeled for grasshopper control. As a reminder, OP insecticides (like dimethoate or Lorsban) cannot be combined with SU/ALS herbicides (like Harmony GT). Since other materials may also state restrictions regarding combinations of insecticide and herbicides, you should be sure to check all labels carefully before combining insecticides and herbicides. Also all uses and tolerances of Furadan have been cancelled so you cannot use existing stocks on any crop in 2010.

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