Agronomic Crop Insects – May 23, 2014

Joanne Whalen, Extension IPM Specialist;jwhalen@udel.edu

Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Although adults are the main life stage present, we are starting to see the first nymphs. Both life stages can damage alfalfa but the nymphs can cause damage very quickly. 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 we can find slugs feeding in no-till corn fields, so far damage has been light this season. 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. More information on slug sampling and management can be found at the following link:

http://ohioline.osu.edu/ent-fact/pdf/0020.pdf.

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 we are not running cutworm pheromone traps in Delaware, information from the Midwest indicates that they are having a record year for black cutworm moth catches.

http://extension.entm.purdue.edu/pestcrop/2014/issue6/index.html#moths. Although many of our black cutworms overwinter, we can see problems from migratory populations. In past years, when the Midwest has reported record trap catches, we have experienced higher than normal populations in our area as well. So it is important to scout fields on a routine basis starting at emergence for this insect pest.

Small Grains
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 barley 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.

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.