Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
The first potato leafhopper adults are now being found so be sure to sample all fields on a weekly basis. Although adults and nymphs both damage alfalfa; it is the nymphs that can quickly cause damage. Once plants are yellow, 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
Although slug damage has been generally light in no-till corn fields, we are hearing reports of fields that have economic levels. We can find small grey garden slugs that have recently hatched in fields with a history of damage, so it will important to continue to scout for slugs, especially in later planted fields.
We continue to find black cutworm leaf feeding as well as cut plants in fields that did not receive a cutworm treatment at planting. Although many of our black cutworms overwinter, we can also see problems from migratory populations. Black cutworm problems are often more common in later planted field corn, so it is important to scout fields on a routine basis starting at emergence for this insect pest. With the continued dry weather, cutworms will tend to feed below the soil surface. 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.
Small Grains
As soon as the earliest planted barley begins to dry down, be sure to watch for head clipping from armyworm. In fields that did not receive an insecticide spray we continue to find armyworm larvae. On barley, significant head clipping from armyworms can quickly occur. As a general guideline, the threshold for armyworms in barely is one per foot of row and for wheat one to two per foot of row. Before making an application of an insecticide, be sure to check all labels for the number of days between last application and harvest.
Cereal leaf beetles continue to be found at economic levels in many fields throughout the state and significant damage has already occurred in some fields. The greatest damage from cereal leaf beetle generally occurs between flowering and the soft dough stage, so be sure to continue to scout for this insect pest. The treatment threshold is 25 eggs and/or larvae per 100 tillers.
Soybeans
Be sure to sample fields starting at emergence for bean leaf beetles and grasshoppers. In the earliest planted and emerged fields, we have started to see an increase in activity of both insects. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage. As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages.