Agronomic Crop Insects

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 and 30% nitrogen applied at night when the plants are dry (the rate used in past years was 20 gallons/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 a.m. so applications of nitrogen have been most effective when applied between those hours. As indicated in past newsletters, 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. If conditions remain extremely wet, slugs sometimes can absorb enough moisture to compensate for the water lost in mucus production so a second application may be needed. We have also had reports and have seen good results in commercial fields where a potash application was needed and slugs were also present in the field. It has probably been 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.

In 2003 when we had the last significant problem from slugs, the following was reported by Galen Dively from the University of Maryland regarding his research on slug damage in field corn:
“I conducted a field study several years ago to measure the corn plant’s ability to withstand slug damage. The work was done in a no-till field with closed seed slots and an average of ten slugs per plant at emergence. At the 2-3 leaf stage, individual plants were rated for damage and then flagged for later assessments of seedling mortality and plant growth at 1, 2, and 4 weeks after the initial rating. No controls were applied, so the slug population present at plant emergence was allowed to develop and feed on seedlings after the damage ratings were made. The rating categories included: 1= seedling completely severed at ground level; 2= all leaves consumed except one remaining intact (greater than 75% defoliated); 3= all leaves showing moderate damage, but entire plant intact (25 to 50% defoliated); 4= only one leaf showing damage (less than 25% defoliated); 5= no damage. Approximately 100 plants were rated in each category and the following data were obtained:

“The study showed that a considerable amount of slug injury could be tolerated before plant density and growth is severely affected. Although regrowth delayed the production of leaves, 32% of the severed plants and 83% of the plants that were more than 75% defoliated recovered after 2 weeks from the initial onset of injury. All plants in categories 3 (25 to 50% defoliated) and 4 recovered completely and were not significantly different from undamaged plants with respect to the number of leaves and plant height later in the season. Although individual plant yields were not determined, there were no observable differences in plant or ear size at harvest between damaged and undamaged plants; thus, any yield loss from slug damage is probably directly related to stand reduction.

“Based on this study, 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
Fields that were treated with a combination fungicide/insecticide spray and have been re-scouted for insects are still free of insect activity so it appears that those applications are still providing control. However, we continue to find “worms” in barley and wheat fields that were not treated so be sure to check fields as soon as it is dry enough to do a good job scouting. In many cases populations are lower than might be expected with the high trap catches experienced in early May, which indicates that the weather conditions helped to reduce population levels.

Soybeans
As the earliest beans emerge, slugs, bean leaf beetles and grasshoppers can be found feeding on seedling stage plants. After last season, we all know that grasshoppers can be extremely difficult to control and multiple applications will be needed. Unfortunately, the warm winter conditions could result in significant problems again this year so be sure to check for grasshoppers as soon as plants emerge. 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, Furadan (currently under review by EPA for cancellation but FMC Rep says it should be available this year), 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 phytotoxicity.