To Scout For Potato Leafhopper In Alfalfa

On June 06, 2020

To Scout For Potato Leafhopper In Alfalfa

Penn State Entomology Specialist John Tooker reports potato leafhopper populations have been found in Lancaster County, and he believes they are present in other counties as well. Alfalfa growers would be wise to keep an eye on populations of this pest species because once their feeding is evident in alfalfa, economic damage has already occurred. Potato leafhoppers tend to be the most damaging pest of alfalfa in Pennsylvania each year. Their feeding, if heavy enough, reduces yield and quality (especially lower protein content), and can decrease stand longevity. 

Potato leafhoppers do not live in Pennsylvania year round, but migrate here each spring on storm fronts, arriving in late May and early June. Potato leafhoppers can be frustrating due to the sporadic nature of their infestations—even in bad leafhopper years, many fields escape damage. Once potato leafhoppers colonize alfalfa fields, adults deposit eggs into stems and leaf veins. In warm weather, these eggs will develop into adults in about three weeks, so populations can increase quickly. Potato leafhoppers have straw like mouthparts and extract plant juices. Heavy feeding disrupts nutrient flow within plants, causing yellow triangles to form at the leaflet tips (known as “hopper burn”), or under heavier pressure cause patches of fields to turn yellow, but this evidence of damage does not develop until 7-10 days after feeding begins.

For non-chemical controls, resistant varieties of alfalfa are valuable. These varieties are covered with fine hairs (the hairs are actually glandular trichomes), which decrease leafhopper feeding, but obviously this option needs to be pursued before establishment. Another option is to mix other forages in with alfalfa. Alfalfa/orchardgrass stands (or other combinations) appear much better at tolerating leafhopper damage than pure stands of alfalfa. Spiders and other natural enemies kill potato leafhoppers, so using integrated pest management (IPM) and spraying insecticides only when economic populations develop will help maintain these allies in pest control. In fact, we have growers in Pennsylvania who do not treat their alfalfa fields with insecticides and claim they do not see potato leaf hopper damage; natural enemies would be the best explanation for this lack of damage.

If leafhopper populations exceed economic thresholds, early harvest or insecticides are the only management choices. Early harvest can stop damage, but regrowth should also be scouted to determine if the next cutting also develops damaging populations. To target leafhoppers most effectively, populations should be sampled and treatment applied only when economic thresholds are exceeded. In our experience in central Pennsylvania, regular scouting and use of economic threshold can limit the need for insecticides to once a summer. In most years, it makes the most sense to start sampling in the first week of June and continue to do so every week or so. 

Four factors help determine whether an insecticide application for potato leafhopper control will be profitable (economic injury threshold): (1) the number of pests present in the field, (2) the development of the plants, (3) the value of the crop, and (4) the costs of the control application.

The variability of each of these factors shows the fluctuation of the economic injury threshold. The pest injury threshold should be adjusted to compensate for each of these changes. These adjustments have been calculated in Table 1.

A few factors should be taken into account when deciding to spray or harvest alfalfa over 12 inches tall. If the leafhopper injury threshold is reached and the alfalfa is less than 50 to 60 percent in bud, a spray is suggested immediately. However, if the crop is 60 percent or more in bud, it will probably be more advantageous to harvest the field within a week rather than spray.

Scouting details and economic thresholds can be found in our potato leaf hopper fact sheet:

Fig. 1. Potato leafhopper adult (with wings) and nymph. (Photo by Art Hower, Penn State)

Fig. 2. Heavy potato leafhopper damage in alfalfa (Photo by Bryan Jensen, University of Wisconsin,

To Control Grassy Weeds In Hay

As first cutting hay comes off, now is a good opportunity to apply herbicides to control certain annual weed species. Extension Agronomist Dwight Ligenfelter explains we often get a lot of questions about weedy grasses being a problem after first cutting; thus we usually suggest the use of Prowl herbicide or another pendimethalin containing product. 

But keep in mind that there are different formulations of pendimethalin, namely the older 3.3EC and the newer water-based formulation, 3.8CS. In alfalfa, alfalfa/grass mixes, and grass hay/pasture, the 3.8CS formulations can be used – these include Prowl H2O and Satellite HydroCap. The 3.3EC formulation (Prowl 3.3, Acumen, Framework, Pavilion, Pin-dee 3.3, Stealth, Satellite 3.3, etc.) can only be used in pure stand alfalfa settings and not in mixed or grass-based stands. Do not use the 3.3EC formulation in grass forages since it tends to cause crop injury (leaf burn). 

As a reminder, pendimethalin (Group 3) provides about 1 month of residual control of many summer annual grasses and some annual broadleaf weeds as they germinate.  It is typically not as effective on winter annual weeds such as annual bluegrass, chickweed, and mustards. It will not control any weeds that have already emerged at the time of application. Adequate rainfall is required after application to activate it. Apply to established alfalfa before weed emergence. Applications should be made prior to the alfalfa reaching 6 inches in regrowth. Some stunting and yellowing of the alfalfa may occur with post applications. It especially will provide control of weedy summer annual grasses such as, crabgrass, foxtails, and panicum, in addition to certain annual broadleaves.

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