Agronomic Crop Insects

Joanne Whalen, Extension IPM Specialist;

Now that the weather has finally started to warm up, you should begin to sample for alfalfa weevil on a weekly basis. Look for small larvae feeding in the tips of plants producing a round, pinhole type of feeding. Once you detect tip feeding, a full field sample should be taken. In general, no treatment should be needed before you observe 50% of the tips with feeding damage. The most accurate way to time an application and try to avoid multiple insecticide applications is to sample stems and determine the number of weevils per stem. A minimum of 30 stems should be collected per field and placed top first in a bucket to dislodge larvae from the tips. Then count the number of weevils per stem. The following thresholds, based on the height of the alfalfa, should be used as a guideline when making a treatment decision: up to 11 inches tall – 0.7 per stem; 12 inches tall – 1.0 per stem; 13 – 15 inches tall – 1.5 per stem; 16 inches tall – 2.0 per stem; and 17 – 18 inches tall – 2.5 per stem. Numerous pyrethroids are now labeled for alfalfa weevil including Baythroid XL, Mustang MAX, Proaxis, Warrior II and numerous generic pyrethroids. Imidan, Lorsban, Lannate and Steward are also labeled for alfalfa weevil control. Be sure to check all labels for rates, restrictions and days to harvest before application. NOTE: The use of Furadan on alfalfa (as well as all other crops) has been cancelled and existing stocks CAN NOT be used.

Field Corn
In addition to black cutworm (which is generally a pest of later planted corn), we can also have a number of other cutworm species present in corn fields at planting time, including the dingy cutworm, claybacked cutworm and variegated cutworm. Information from the Midwest indicates that the claybacked cutworms can cause economic loss in corn. They overwinter as half-grown larvae in the soil so they can get a “jump” on black cutworms when it comes to cutting each spring. Since they are larger in size earlier in the spring, this species can damage very young corn plants. So, scouting fields at plant emergence is important, even if at-planting materials were used, to catch any potential problems. Just a reminder, if you plan to tank-mix an insecticide with an herbicide for black cutworm control, it should be done at, or immediately following planting.

During the winter meeting season, there was a lot of discussion regarding changes in refuge requirements for corn expressing Bt traits for the upcoming 2010 growing season. As of this date, the refuge in a bag strategy has not been approved for any transgenic corn product. However, there has been one significant change in refuge requirements for 2010 involving SmartStax hybrids. Because these hybrids contain 3 different genes for Lepidopteran control and 3 different genes for rootworm control, the EPA has approved a reduction of the standard 20% refuge to 5% refuge. In the last few weeks, I also received new information from Monsanto regarding placement of the refuge for SmartStax hybrids that applies to our area:

“The common refuge can be within or adjacent to the Genuity™ SmartStax™ field. If adjacent, it can be separated by a road, path, ditch , etc., but not by another field. Monsanto recommends planting the corn refuge for Genuity™SmartStax™ as an in-field or adjacent refuge as explained in this IRM/Grower Guide. However, if corn rootworms are not significant within a region, the common refuge may also be planted as a separate block that is within 1/2 mile of the Genuity™ SmartStax™field. This additional option to plant the refuge as a block within 1/2 mile is only available to farmers in the following States: AK, AL, AR, AZ, CA, CT, DE, FL, GA, HI, ID, LA, MA, MD, ME, MS, MT, NC, NH, NJ, NM, NV, NY, OR, PA, RI, TN, SC, UT, VA, VT, WA, WV, WY.”  Monsanto’s 2010/IRM Insect Resistance Management Grower Guide will be posted to their webpage in the near future. Also, be sure to contact your seed dealers for a full description of refuge requirements for 2010 for all hybrids that you plan to plant.

Since spring green up is underway, be sure to sample fields for cereal rust mite activity. Mites can be found in fields at this time. These mites are very small, so the use of a 20x-magnifying lens may be necessary. If rust mites become a problem, Sevin XLR Plus (which had a 24(c) label on timothy for cereal rust mite management) now has a full section 3 federal label which includes pastures and grasses grown for hay and seed. The following is a link to the new label: Be sure to read the label since there is new information on the number of applications per season as well as the days to harvest. For effective rust mite control with Sevin, the use of the higher labeled rate and at least 25 gal/acre of carrier to get good coverage of leaf surfaces generally results in better control.

It is time to begin sampling fields for cereal leaf beetle activity. We are starting to find evidence of adult feeding, so fields should now be scouted for the presence of egg masses. The threshold for cereal leaf beetle has been adjusted to include sampling for eggs, especially in high management wheat fields or areas where problems were experienced the previous year. The eggs are elliptical, about 1/32 inch long, orange to yellow in color when first laid, changing to a burnt orange prior to hatching. Check our website for pictures of cereal leaf beetle adults, larvae and eggs:

Generally, eggs are laid singly or in small scattered groups (end-to-end) on the upper leaf surface and parallel to the leaf veins. Cereal leaf beetle larvae are brown to black, range in size from 1/32 to 1/4 inch long, and eat streaks of tissue from the upper leaf surface. Since cereal leaf beetle populations are often unevenly distributed within the field, it is important to carefully sample fields so that you do not over or under estimate a potential problem. Eggs and small larvae should be sampled by examining 10 tillers from 10 evenly spaced locations in the field while avoiding field edges. This will result in 100 tillers (stems) per field being examined. Eggs and larvae may be found on leaves near the ground so careful examination is critical. You should also check stems at random while walking through a major portion of the field and sampling 100 stems. The treatment threshold is 25 or more eggs and/or small larvae per 100 tillers. If you are using this threshold, it is important that you wait until at least 50% are in the larval stage (i.e. after 50% egg hatch).