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Extension > Local Extension Offices > Morrison > County Agriculture Educator > Articles > Check Alfalfa Regrowth

Check Alfalfa Regrowth

University of Minnesota Extension, Stearns County News
June 17, 2015

Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Check Alfalfa Regrowth
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (06/04/15) — A few very small alfalfa weevil larva have been found is some fields. Sometimes we see alfalfa weevil larva, armyworms, or cutworms chewing off new growth. With a cool spring, alfalfa weevil are further behind and may not limit regrowth much this year; but could be a problem later in the second crop.  Remember, there are problems with spraying routinely when insects are not present at threshold levels. It’s bad for the bank account, bad for resistance reasons, and bad for the environment.

For all alfalfa insects it is important to consider the pre-harvest window for insecticides. If close to harvest, it could be better to harvest a little earlier if the weather allows and then watch the next crop. Spraying earlier or later in the day can reduce risk to foraging bees and other pollinators. Even when alfalfa isn’t blooming, clovers or other plants mixed in the field can be. Here are some scouting notes.

ALFALFA WEEVIL. Check new regrowth. Inspect 20 one-foot square areas and treat if there are 8 or more larvae per square foot or regrowth is suppressed by feeding. The larvae can be difficult to find. They might be 1/16 to 3/16 of an inch long, a dull creamy-greenish color with a black head. As the larva grow, they will have more of a pale green color with a white stripe down the middle of the back. In a taller crop, we might treat if we find 20 per sweep with a sweep net. We might treat if we find 30% of stems with signs of leaf feeding. Feeding is typically indicated by holes eaten out of the leaf and when severe, you might have just a net of leaf veins left in the leaf.

POTATO LEAFHOPPER (PLH) are small, bright light green and wedge shaped – larger on the head end and tapering to the end of wings on the tail end. The general threshold is finding 1 PLH per 10 sweeps with a 15 inch net for each inch of crop growth. When alfalfa is 3 inches tall, finding 3 PLH in 10 sweeps of a net is enough to merit control. At 10 inches, this means finding 10 PLH in 10 sweeps. With the most advance genetics for PLH resistance, the threshold might be 3 times the normal threshold. PLH suck juices from the leaves. This usually shows up first as a yellow v-shaped patch at the tip of the leaf, and eventually the whole leaf can turn yellowish-brown to purple in color. Extension pest management specialist says PLH hit in a big way starting last weekend in the Lamberton area. 

PLANT BUGS look a lot like boxelder bugs in terms of body structure. There are smaller. Those called alfalfa plant bugs have more green color on them and can be 3/8 inch long. They are more serious. The other common type is called tarnished plant bugs - about ¼ inch long or so and colored more in shades of brown. Plant bugs cause a pinched or crinkled look in the middle of the leaf. 

One threshold is to treat if there are 3 plant bug adults or nymphs per sweep on alfalfa less than 3 inches tall and 5 or more on taller alfalfa. If damage is found within 7 to 10 days of harvest, you might harvest earlier, or carefully consider the pre-harvest interval of insecticides. Another reference says to cut the threshold in half if the alfalfa plant bug is the dominant species or if potato leaf hoppers are also present. For example, if potato leafhoppers and plant bugs are each present at half their threshold, control efforts would be warranted.

WINTER INJURY. This year we are also watching for regrowth issues on fields that showed significant signs of winter injury. If plants are NOT growing back well, and insects are not a problem, it might be helpful to dig some roots around the field and split them to check. If the inside is generally tan or brown, we’d have more concern about how well they will do. If they have more firm white colored flesh through most of the root, that’s better, even if there is some brown streaking in the root.

Some people might decide to till some of these fields after first crop. There should be a pretty good indication of regrowth within a week after harvest. Options for planting something else are largely driven by how you can make the best use of the land for feed or a crop to sell. Corn for silage can still be a good option, and then there are forage sorghums and sorghum/sudan grass crops. Foxtail millet can make a good late planted hay crop that would be similar to harvesting brome grass in 50 to 60 days. There could be other possibilities.

For more information, you’re welcome the U of M Extension Office in Little Falls 632-0161, in Foley 968-5077, in St. Cloud 255-6169 or for Dan Martens with crop production issues 1-800-964-4929.
 

Contacts

Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
marte011@umn.edu
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