Agronomic Crop Insects – May 20, 2011

Joanne Whalen, Extension IPM Specialist;

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, rainy 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 – Cutworm Alert
During the past week, we have seen a steady increase in the number of no-till fields with economic damage from cutworms. Damage has mainly occurred in fields that were not treated with a cutworm product at planting. In some cases, cutting damage has been well above the threshold of 3% cut plants in spike to 2-leaf stage corn. Be sure to scout fields carefully for cutworms – in some cases you will need to check fields twice a week to be sure you do not miss an economic population. In addition to cut plants, be sure to watch for leaf feeding which can be an indication of the potential for significant cutting damage and yield loss.

With the recent cool, wet weather, slugs continue to be a problem in no-till corn, especially in fields with a history of problems. In fields where Deadline MPs have been applied at a rate of 10 lbs per acre and the distribution of pellets is at 5 per square foot, control has been good and plants have been able to grow ahead of the damage. 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. There is also a 2ee recommendation for Lannate LV for slug management that was issued for corn last season. In the past, evening applications of 30% nitrogen when the plants are dry and wind conditions are low has resulted in varying levels of slug control. The rate used in past years was 20 gallons per acre of 30% N on corn in the spike to one-leaf stage. 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. 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. Population levels remain variable throughout the state so scouting fields will be the only way to determine if an economic level is present. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley. Heavy defoliation of the flag leaf can result in significant economic loss in wheat and barley. Armyworms generally begin head clipping when all vegetation is consumed and the last succulent part of the plant is the stem just below the grain head. Larvae can feed on the kernel tips of the wheat, resulting in premature ripening and lower test weight.

As barley and wheat approach harvest, the treatment options change due to the pre-harvest interval (the waiting period between application and harvest). In addition, not all materials are labeled on both crops so be sure to carefully read all labels. Lannate has a 7 day pre-harvest interval (PHI); therefore, it is the only labeled option on barley at this time. All other products labeled on barley are not an option since they have a 30 day PHI. In addition to Lannate, there are a number of pyrethroids labeled on wheat with varying PHIs: (a) beta-cyfluthrin (Baythroid XL;30 day PHI), (b) cyfluthrin (Tombstone; 30 day PHI) ), (c) gamma-cyhalothrin (Proaxis;30 day PHI), (d) lambda-cyhalothrin (Warrior II and generics; 30 day PHI); and (e) zeta-cypermethrin (Mustang MAX, Respect; 14 day PHI). It should also be noted that some pyrethroid labels say control of small larvae only so be sure to carefully read the label. Remember, the label is the law so be sure to read the label for rates as well as all restrictions.

As the earliest beans emerge, be sure to watch carefully for slug damage. Remember, if you had a problem in past years, the slugs may 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. Small grasshoppers have already been detected in fields before planting. Early detection and control of small grasshoppers is necessary to achieve control. As a reminder, OP insecticides (e.g. 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.


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