Friday, April 11, 2014

How to read the updated economic threshold table for alfalfa weevil

I recently posted an ICM News article about how scouting for alfalfa weevils in southern Iowa. It doesn't take too many accumulated degree days for them to start moving around and laying eggs. If you grow or scout alfalfa, you will likely see any adults that survived the winter moving in southern Iowa now and northern Iowa next week. In the article, I show an updated economic threshold table based on work from John Tooker at Penn State. It has a lot of numbers in it, and at first, looks kinda confusing. In order to make cost-effective treatment decisions for alfalfa weevil management, three things must be known:

1. what is the expected market value of the hay ($/ton)?
2. how much are the control costs ($/acre)?
3. how tall are the plants (inches)?

Take that information and plug it into the table below. Well, I guess you have to actually go out to the field and sample larvae before consulting the table! The larvae are often tucked into stems and expanding terminals. Pull at least 30-50 stems from different areas of the field and madly shake them into a 5-gallon bucket to dislodge them.


Alfalfa larva (top) and adult. Photos by Clemson University, www.ipmimages.org.

Economic threshold of alfalfa weevil, based on the average number of larvae in a 30-stem sample (Originally published by JohnTooker, Penn State Extension).

In case the table is still confusing, I highlight two examples. Example 1 (look at the orange arrows):  if you are expecting $260/ton, control costs of $14/acre, and plants are 20 inches tall...the economic threshold would be 40 larvae per 30-stem sample. In other words, if you sample 30 stems and get at least 40 larvae, consider taking some management action to protect tonnage and hay quality. Example 2 (look at the red arrows):  if you are expecting $380/ton, control costs of $12/acre, and plants are 28 inches tall...the economic threshold would be 24 larvae per 30-stem sample. In other words, if you sample 30 stems and get at least 24 larvae, consider taking some management action to protect tonnage and hay quality.


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