By Dan Wiersma and Ev Thomas

On most dairy farms, there is no clear distinction between a deep, green field of alfalfa and making milk. Alfalfa excels at the junction of high protein fiber and excellent cow health. Seeing any other color than green in an alfalfa field disturbs the natural order of things. When V-shaped yellowing shows up on alfalfa leaves, it is not the sign of victory, but rather an indication of trouble in a field of alfalfa.

Rather easy to spot
Often the first sign of trouble is seen when walking into a nearby alfalfa field, still dripping with morning dew. Within a few steps, potato leafhoppers announce their arrival and at the same time their explosive reproductive capacity.

Where did they come from and how did they get as thick as flies on fresh roadkill? Perhaps most important, what can I do to rid my field of these little sideways-walking, plant-piercing, nutrient-sucking pests?

Part of the history of growing alfalfa involves dealing with the economically important, yield-robbing insect known as the potato leafhopper (PLH). For most of this time, control of leafhoppers has involved periodic insecticide applications to prevent excessive yield loss. More recently, alfalfa breeders introduced PLH-resistant alfalfa varieties to help limit damage.

But questions remain about how best to manage the annual PLH invasion. What factors result in leaf yellowing, or plant stunting? Can PLH-resistant varieties provide the level of protection needed to avoid yield loss? What is the most profitable choice for controlling leafhoppers on your farm?

First off, here comes the bad news about PLH. They are as unpredictable as the weather after mowing first cutting alfalfa. The only consistent thing is the fact that they will arrive sometime during spring planting. Because PLH do not survive winter, arrival depends on spring weather patterns when strong, humid winds from the Gulf Coast area of the U.S. carry PLH adults to the northern regions. This can occur from as early as late April and as late as early to mid-June. Typically, populations of leafhoppers do not build to damaging levels until the second and third crop of the year.

Not always detrimental
The good news about PLH . . . they will not always be a major player in reducing alfalfa productivity during the season. A key factor influencing influencing the severity of this pest in alfalfa is weather conditions which favor explosive population growth and local dispersion of the leafhoppers. For most areas of the Midwest and Northeast, it is at least a 50-50 bet that leafhoppers develop into a moderate to severe infestation level (see figure) with the ability to cause economic damage to alfalfa.

Three to four generations of leafhoppers develop annually. Populations build to very high levels by mid- to late summer, causing their worst damage to alfalfa after first cutting. The adults and nymphs remain active until late summer when populations tend to crash. Hot, dry weather favors buildup of PLH populations.

Damage to alfalfa comes from the feeding activity of PLH nymphs and adults. Nymphs (nonflying, sideways walking, yellowish-green) cause the most serious damage. Plant injury and nutrient loss results from adults or nymphs using their piercing-sucking mouthparts to remove plant juices and inject a toxin. Alfalfa is most susceptible to PLH as a seedling plant. Mature alfalfa plants are most susceptible immediately after harvest. Damage and yield loss compound when plants are under severe stress.

Two strategies
Solutions for preventing or limiting PLH damage fall into two distinct categories — insecticides and alfalfa varietal resistance. Insecticides have been the most common tool used to manage PLH in both scouted and unscouted situations. Growers using either glyphosate for weed control or fungicide for leaf disease frequently apply an insecticide in the same pass. Others only apply an insecticide when field scouting shows threshold levels of leafhoppers or other insects present in a field.

A newer tool in the leafhopper management toolbox is alfalfa varieties with high levels of resistance to PLH injury. These varieties have very small plant hairs (glandular hairs) that exude droplets of a sticky compound. Glandular hairs are a physical deterrent to leafhoppers, and together with antibiosis and better plant tolerance, confer leafhopper resistance.

Varieties differ in the levels of expressed resistance, and all are susceptible to some damage, especially at high-intensity PLH levels. When growing PLH resistant varieties (greater than 50 percent level), insecticide treatment thresholds increase by two to three times based on university research.

Genetics for alfalfa yield and pest resistance have improved tremendously in recent years. Likewise, genetics for PLH resistance have also improved, showing 15 to 20 percent higher yield potential than check varieties in unsprayed trials with moderate to high levels of leafhopper damage (see table). However, these same varieties will lag in yield by an average of 8 to 16 percent against elite conventional alfalfa varieties in trials where leafhoppers are controlled. If your standard practice is using an insecticide for managing potato leafhopper, your best genetic choice will be an elite, adapted alfalfa variety without resistance to PLH.

Based on these numbers, it seems there are a few niche opportunities for PLH-resistant alfalfa varieties. Growers who don’t like to use insecticides, or who are growing for an organic market, will benefit from planting these varieties. Likewise, dairymen with heavy workloads who frequently miss a timely insecticide application can reap the benefits of varietal resistance. Finally, growers who use a nurse crop when planting alfalfa and do not apply an insecticide during the first 60 to 70 days of growth can protect seedling health by using a resistant variety.

At the end of the day, your PLH control strategy is best determined by your farm resources and needs. Which category will your farm fit into, “spray-and-pray” (insecticide no matter what), “scout-then-spray” (monitor and apply insecticide as needed), or “defend-and-spray” (PLH-resistant variety, scout and spray as needed)?