Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
In general, alfalfa weevil populations have been higher in many fields this spring. For alfalfa over 16 inches tall, the threshold increases to 2.5 larvae per stem. Early harvest is often the best option once fields are greater than 16 inches tall and populations exceed 4 larvae per stem; however, there must be enough “stubble heat” after harvest for this technique to provide control. If populations were above threshold before cutting, be sure to check for alfalfa weevil adults and larvae within a week of cutting. Feeding from both stages can hold back re-growth. A stubble treatment will be needed if you find 2 or more weevils per stem and the population levels remain steady.
Field Corn
At this time, we can find both cutworms and slugs feeding in newly emerged corn fields. In addition to black cutworm, which generally attacks later planted corn, we can find a number of other cutworm species present in corn fields at planting time. They include 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, this species can damage very young corn plants. Scouting fields at plant emergence is important, even if at planting materials were used, to catch any potential problems. In addition, a higher rate of an insecticide will be needed to control larger cutworms. As a general guideline, a treatment is recommended if you find 10% leaf feeding or 3% cut plants. If cutworms are feeding below the soil surface, it will be important to treat as late in the day as possible, direct sprays to the base of the plants and use at least 30 gallons of water per acre. For cutworms, fields should be sampled through the 5-leaf stage for damage.
With the continued wet weather, we have seen an increase in slug damage on recently emerged corn. If slugs are damaging plants, you will be able to see “slime trails” on the leaves. Since corn was planted later this year and in many cases fields are just emerging, we could see more injury from slugs that have been hatching and beginning to grow. As indicated in previous newsletters, Deadline M-Ps is available for slug management this year in field corn. This is the only product that we have local experience with in regards to slug management in field corn. We continue to see very good control with Deadline MPs when applied at the 10 lb/A rate as long as you get good distribution of the product (5 pellets per square foot). The best control with the Deadline M-Ps has also been observed when applications were made and there was at least one day of sunny weather after an application. There are also a couple of other products available for slug management in field corn: Sluggo (iron phosphate) and IronFist (sodium ferric FDTA). At this point, we do not have local experience with these two products so will be evaluating them in replicated plots. We hope to get our first applications out today so we should have more information on their efficacy by next week. Please see the following link for comments from Ohio regarding slug management in field crops this season (http://corn.osu.edu/c.o.r.n.-newsletter#2).
Small Grains
Grass sawflies and true armyworms have been found in fields in Kent and Sussex counties. In addition, cereal leaf beetle adults have been found laying eggs and we are seeing the first larvae. Population levels remain variable throughout the state so scouting fields will be the only way to determine if an economic level is present. Depending on the temperature, cereal leaf beetle larvae will feed for up to 3 weeks. Research from Virginia and North Carolina indicates that the greatest damage can occur between flowering and the soft dough stage. Although armyworm can attack both wheat and barley, they can quickly cause significant losses in barley.
You will also need to watch for aphids feeding in the heads of small grains. With the continued cool wet weather and projection for cool weather again next week, populations could continue to increase and beneficial insects will have a hard time keeping up with populations. The treatment threshold is 20-25 aphids per head with low beneficial insect activity.