Agronomic Crop Insects – May 22, 2009

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, there would 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
Slugs continue to be the main pest of concern in many fields. Options to reduce damage and allow plants to grow ahead of the damage include the use of Deadline M-Ps or night time applications of 30% liquid nitrogen when plants are dry (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 – not eliminate — plant injury). Also, 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. Last season, we also had reports of good results in commercial fields where a potash application was needed and slugs were also present in the field. It was probably effective due to its high salt index. Remember that when it comes to slug management all of the available control tactics only reduce the slug activity – buying time to enable the crop to outgrow the problem.

Research conducted by Galen Dively from the University of Maryland in 2003 indicated that “populations of five or more slugs around each plant prior to the 3-¬≠leaf stage may be economic, especially if injury is heavy, plant growth is slow, and cool, wet conditions prevail. If the weather turns hot and dry, 10 or more slugs per plant may be tolerated if the seedlings reach the 3-leaf stage. Generally, if a heavily infested field reaches the 3-leaf stage without severe seedling mortality, the crop has survived the critical period and should outgrow further slug injury, regardless of the population pressure.”

Small Grains
We continue to find armyworms, sawflies and cereal leaf beetles in barley and wheat fields 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. 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 this year and can quickly damage soybeans 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 including a number of pyrethroids, dimethoate, Lorsban, Orthene 97 and Sevin XLR. 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. Combinations of certain formulations, especially emulsifiable concentrates (ECs), can cause significant phytoxicity. Also, it should be noted that the use of Furadan on soybeans has been cancelled. Please refer to the Federal Register notice from EPA regarding existing stocks http://www.epa.gov/opp00001/reregistration/carbofuran/carbofuran_noic.htm#cancel.

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