Keep an eye out for alfalfa weevil this spring

Keep an eye out for alfalfa weevil this spring

Also watch corn and soybean fields for signs of early season insect pests.

When you are busy planting corn and soybeans, scouting alfalfa fields for insects may not be top of mind. But you need to keep an eye on alfalfa for signs of alfalfa weevil feeding this spring. Brian Lang, Iowa State University Extension field agronomist at Decorah in northeast Iowa, provides the following information.

ALFALFA WEEVIL: This is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults feed on plants, but typically the larvae cause the majority of plant injury. Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized.

As of last week (April 27, 2016) northeast Iowa has now exceeded 250 degree days accumulation (base 48 degrees F, starting Jan. 1), so alfalfa weevil egg hatch is underway. The only question: are there enough weevils present to justify treating with an insecticide? Reaching treatable levels in northeast Iowa is somewhat rare, but can only be determined by scouting. Initial scouting can be accomplished with a sweep net to simply identify if there are any larva present. If they are present, then you should collect 30 stems at random and determine an average number of weevil larva per stem. Look closely at the top folded leaves on these stems as this is a favorable place to find very small larva. 

In a nutshell, it takes about two larvae per stem as an average to be a justified as a treatable level.

What about other insects? Those that can feed on young corn and soybean plants? Bean leaf beetle on soybean is one you should watch for, says Lang. Same goes for black cutworm when corn starts to emerge from the soil. That’s when you need to keep an eye on those early developing plants. Lang offers the following observations.

Bean Leaf Beetle: Normally, with a very mild winter (it improves Bean Leaf Beetle winter survival) and warm conditions in April, we could have potential issues with Bean Leaf Beetle on early soybean development. However, the basically non-existent population of BLB last year doesn’t provide for much of an increasing population threat by this spring. So the threat this spring in northeast Iowa should again be minor and not require an insecticide seed treatment for this pest.

Black Cutworm: Keep an eye on corn as it emerges, and continue to watch fields as the plants are in the seedling stage, for signs of leaf feeding and stalk cutting on the small corn plants. Black cutworm pheromone traps across Iowa have had only a few light catches of black cutworm moths so far. The moths are the adults that lay the egg which hatch into cutworms.

Once the field agronomists and entomologists at Iowa State University Extension get significant catches of black cutworm moths in Iowa they will release the information to the media and will provide information about scouting and thresholds for treatment. The University of Illinois has had some significant catches of moths in Illinois this spring, and Illinois entomologists are predicting initial scouting for Black cutworm to start in northern Illinois around May 21.

Common Stalk Borer: This is a pest that sometimes requires control in corn. For those farmers who have fields that lose corn plants in the first few rows along grassy field borders or grass-back terraces, you may have a problem with common stalk borer.

There are 3 options remaining in this early spring season for controlling infestations of common stalk borer.

1)  Some Bt corn controls or suppresses stalk borer, and some do not. Check the “Handy Bt Trait Table” for those products

2)  You could wait for egg hatch and then treat those grassy areas with an insecticide. Egg hatch starts approximately 575 degree days (base 41, starting Jan. 1).  We are currently ranging about 400 to 530 degree days across northeast Iowa    and increasing on average about 13 degree days per day.

3)  You could wait for larval migration from the grass to the corn at which to apply insecticide on the grass field border and the first few rows of corn. This begins around 1,300 degree days, base 41, January 1. ISU Extension field agronomists will track degree days and let you know when we approach this window. It often occurs around mid-June.

Corn Flea Beetle, Stewart’s Wilt: This insect can survive in Iowa in mild winters and possibly be a threat more so to seed corn production than field corn. The Stevens-Boewe Index predicts potential risk of Stewart's disease based on the sum of the mean temperatures for December, January and February. A sum below 80 indicates a negligible risk; 80 to 85 is considered a low risk; 85 to 90 indicates moderate risk; and greater than 90 is considered a severe risk.

Even though this was a very mild winter, the Stevens-Boewe Index for northeast Iowa is only 72, so it is assumed that this pest is of negligible risk for 2016. For east-central and southeast Iowa, the index is in the high 80s, suggesting a moderate risk. For photos and a discussion about the beetle and how it can spread Stewart’s Wilt disease, go to the 2011 article.

TAGS: Extension
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