Agronomic Crop Insects – May 25, 2012

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

Alfalfa
Potato leafhoppers are now present in fields so be sure to sample on a weekly basis. 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
True armyworms can now be found in corn fields. There have also been reports of yellow striped armyworm in fields which I have not seen before. As small grain dries down, be sure to watch for true armyworms moving out of small grain and into adjacent corn fields. You should also scout corn for true armyworms in fields that were planted into a small grain cover. Remember, worms must be less than 1 inch long to achieve effective control. The treatment threshold for true armyworms in corn is 25% infested plants with larvae less than one-inch long. Large larvae feeding deep in the whorls will be difficult to control.

Soybeans
This past week we saw a significant increase in slug damage in no-till soybeans, especially in fields planted into heavy corn stalk or double crop soybean stubble. Slugs are extremely difficult to manage in soybeans because the damage can occur below the ground before plants emerge. Damage to soybean can be more severe than damage to corn because the plant’s growing point is within the emerging cotyledons. If soybean plants are able to emerge, the plant may be able to send out the unifoliate leaves where slug feeding will be noticeable. However, slugs often feed on the cotyledons below ground and/or just as the beans are cracking through the surface feeding on the growing point. This type of feeding results in the death of the plant and significant stand loss. In 2010, we saw significant stand losses from slugs feeding below ground before plants emerged. With the continued cool, wet weather in 2010, the only effective control option was to till fields , then wait until fields dried out and the weather was warmer to encourage quick germination before re-planting. In 2010, it was also extremely difficult to time a bait application. This year, a bait application could be an option if you are scouting fields routinely, plants are just emerging and before there is significant feeding on the growing point. We have had very limited experience with bait applications in soybeans, especially with applications ahead of plant emergence. We will be evaluating fields treated recently to determine the effectiveness and timing of the bait applications.

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