|Ohio Farm News
By Steve Bartels
Now that we have reached mid-April, we are about three weeks away from the time when alfalfa weevil usually reaches its economic threshold here in southwestern Ohio. This is the point where the weevil is doing more damage than the cost of the rescue treatment.
As with any other pest, we can use the calendar to predict when we could expect to have a problem, and watch for its damage. The threshold, however, may be reached many days before we expect or it may never be reached at all. That is why, if you are serious about making quality alfalfa, you need to scout the field for this pest.
The alfalfa weevil larvae are green with a black head and a white stripe down its back. When they are newly hatched from their egg, they can be difficult to see on the plant. The larvae pass through four stages before it reaches the adult stage. The larvae are about 3/8 inch long, in the fourth stage, and are voracious eaters. Both the adult and the larvae feed on the alfalfa leaves, but usually the adult feeding is insignificant.
The eggs may, overwinter here in Butler County, hatch early, and by the time we have accumulated 325 heat units, have reached their peak feeding activity. Most of the larvae you will find in a field hatch from eggs that were laid by overwintering adults. That is why in some springs, you can find larvae in greatly different growth stages in the same fields. Larvae, which hatch in the spring, will not reach growth stage 4 until you have accumulated 575 heat units.
There are beneficial insects and a fungal disease that help control the populations. Three beneficial wasps attack the larvae. We need to try to do everything we can do to protect and increase these populations. The feeding of one larva per stem on 12-inch tall alfalfa will result in a loss of .4 ton of dry hay per acre. If your alfalfa is 16 inches tall, one larva per stem will only cost you .1 ton per acre. Once the alfalfa is 16 inches tall or more, early cutting is a better option than spraying.
To scout the field for damage, the first step is to determine the percentage of stems that are showing damage. If your hay is 6 inches tall or less and you have 25 percent of the leaves showing damage, you need to count the number of larvae per stem. As the alfalfa grows, the percentage of leaf feeding ramps up before you need to check for larvae. If the hay is 9 inches tall, the percentage goes up to 50 percent and if the hay is 12 inches tall, the percentages goes up to 75 percent.
To determine the larvae count, you need to collect 10 stem samples from several locations in the field. We would recommend at least 5 samples. When you have collected 10 stems they should be shaken vigorously in a bucket. Preferably the bucket should not be white or green, so that the small larvae can be easily seen and counted. The larger larvae will be easily dislodged into the bucket. The newly hatched little buggers may hold on tight.
You should look at each individual stem to be sure you see if you have dislodged them all.
If you find one larvae per stem on alfalfa that is 12 inches tall or less, treatment is required. When your alfalfa is between 12 and 16 inches tall, it takes between 2 and 4 larvae per stem, depending on the vigor of the hay, to justify the expense. When the hay is over 16 inches tall and you have 2 to 4 larvae per stem, early harvest is recommended. If the forecast is for wet weather, harvesting as haylage might be your best bet. After early harvest, you should check the hay again in 4 to 6 days. You need to be sure that larvae are not preventing regrowth. If you have 2 or more larvae per crown, you need to rescue the crop.
There are a number of products labeled for control of this insect. Give us a call at 513-887-3722 for the list, or you can go online at ohioline.osu.edu
This farm news was published in the April 26, 2006 issue of Farm World.