Extension Agronomy eUpdate Issue 496 March 6th, 2015

Early spring insect control and fertility management practices for alfalfa

As alfalfa breaks dormancy, producers should plan to keep a close watch for insect activity. This is also a time of year when producers can apply lime or fertilizer, if needed.

Insect activity

Alfalfa weevils are probably the first and foremost insect pest to start scouting for at this time of year. Scouting for alfalfa weevil should begin as soon as the plants break dormancy, or see K-State Research and Extension publication Alfalfa Weevils, MF2999, available online at: http://www.ksre.ksu.edu/bookstore/pubs/MF2999.pdf to calculate growing degree days required for egg hatch. Some eggs were deposited last fall. With the warm weather we have had sporadically and which can be expected over the coming weeks, we have now accrued several degree days or thermal units towards hatching these eggs. Heat units accumulate for alfalfa weevils at temperatures above 48 degrees F. However, do not be too quick to treat for alfalfa weevil. Wait until the field reaches the treatment threshold.

The next insect to start watching for would probably be pea aphids. They can also start relatively early in the spring, and can be a problem on first-year stands. If weevil treatments are applied, they will wipe out any beneficial insects -- which normally do a good job of keeping aphid populations under control.

Also, producers will need to keep an eye out for army cutworms as there were some reports of army cutworm activity last fall. Army cutworms will start feeding again anytime temperatures are above 50 degrees F. Armyworms are another potential problem.

Those are the early season pests which have the most potential for damaging alfalfa prior to the first cutting. For more information on control, see K-State publication MF-809, Alfalfa Insect Management 2015, at: http://www.ksre.ksu.edu/bookstore/pubs/mf809.pdf

Fertility decisions

Alfalfa is a crop with high nutrient removal rates, with average values of 10-12 lbs of P 2 O 5 and 60 lbs of K 2 O per ton of alfalfa. Annual fertilizer application of P and K is often needed to maintain soil nutrient levels, which also helps to maintain good stand vigor and therefore the longevity of an alfalfa field.

Source: Soil Test Interpretations and Fertilizer Recommendations, K-State Research and Extension publication MF-2586

Alfalfa also shows responses to some secondary and micronutrients, and in Kansas sulfur and boron can often limit yield potential and should be monitored periodically.

If phosphorus, potassium, sulfur, or boron are needed, how and when should they be applied?

On established stands, broadcasting phosphorus has proven effective on soils low in phosphorus because alfalfa has roots near the soil surface. For nonirrigated stands, topdressing is normally done in the fall, early spring, or even after the first cutting. Irrigated stands can be fertilized in the fall, early spring, or after any cutting because moisture can be supplied to make the top-dressed fertilizer available to plants.

Potassium application times and methods are similar to those for phosphorus, and in most cases,these nutrients will be applied together.

When high nutrient application rates are needed to boost soil fertility, splitting the total required amount into two or more applications is recommended in order to avoid salt injury and luxury consumption beyond the alfalfa nutritional requirement.

For more information, see K-State publication C-683, Alfalfa Production Handbook, at: http://www.ksre.ksu.edu/bookstore/pubs/c683.pdf

Jeff Whitworth, Extension Entomologist

jwhitwor@ksu.edu

Dorivar Ruiz Diaz, Nutrient Management Specialist

ruizdiaz@ksu.edu