Potato leafhopper (PLH) infestations in alfalfa really took off during the past 5-7 days in southeastern, central and southwestern Minnesota (i.e., just about everywhere we grow alfalfa). We have reports of >4 PLH Adults/Sweep from Lamberton area (Bruce Potter), Rosemount Experiment Station (Eric Burkness) and the Rochester area ( , Crop Consultant).

At this time of year, and because of dry weather, established stands as well as spring-seeded fields, will be in many stages of development. Because of crop variability, local or cash sale purposes for the alfalfa, and variation in PLH infestations, PLH treatment decisions will continue to be needed on a field-by-field basis.

The following economic thresholds are based on Adult PLH counts, and primarily for established stands:

*Notes/Questions:

• Where possible use a 15" diameter sweep net; all threshold research was based on this standard,
  
• Use the Pendulum sweep method; i.e., swing the net in front of you, as you walk through the field (think of the pendulum clock motion).
  
• What is a Sweep? Each time the net comes down through the crop canopy = ONE sweep.
  
• How many Sweeps to take? Our research, and that of Cornell Univ., has shown that a 10-sweep sample unit is much more efficient (accurate and faster) than a 25-sweep unit. Also, for a "typical" 40-ac field, only 4-5 sets of 10 sweeps will be necessary to confirm the infestation.
  
• The net should go about 13-14" into the canopy on each sweep (for 15" net); use a medium-strength sweep motion; IF you are getting over a bale of hay after 10 sweeps, you are sweeping too hard!
  
• When to Sweep? Ideally, you should only sweep when wind-speed is <10 mph; at high wind speeds, we know that sampling efficiency with a net drops off significantly (sampling will underestimate true density).. Experienced consultants can often scale their counts accordingly. Also, must avoid early-morning dew, to keep the net dry. Thus, usually after 10 am, works well.

In addition to the following, final spray decisions, will depend on local economics and needs and uses for the alfalfa (cash crop vs. local dairy use, etc.). However, keep in mind that PLH and water-stress are additive yield reducers (UofM research). If projections for continued dry weather continue (and you can’t irrigate) , control of PLH may become more critical.

• In addition to the standard guidelines, a few notes apply. In general, the treatment decision should be made before the alfalfa is 10-12" tall. However, if you have YELLOW alfalfa that is in the 12" range (and/or possibly stunted by PLH), my recommendation is to spray it now, let it recover, then cut it (vs. cutting now). The alfalfa should quickly respond and green up.
  
• For spring-seeded fields, the above thresholds apply; however, for both new and established fields, if nymphs (immature stages) are present, you have confirmation of an actively/local breeding infestation (use this information to confirm that insecticidal control is needed).
  
• For "PLH-resistant" varieties, under high PLH pressure (such as >4/sweep), insecticide treatment may still be needed (cannot say for sure, because there is such high variability among varieties and their tolerance to PLH); again, decisions must be made on a field-to-field basis. NOTE: some of the more recent PLH varieties will still provide high RFV’s, even without insecticide control, due to high leaf-stem ratios on "moderately stunted" alfalfa.

All insecticides currently labeled will provide good to excellent PLH control (including 10-14 days of residual control of adults, and nymphs as they hatch); if you spray, you will only need to spray once for a typical 30-40 day crop interval. The only exceptions, that I do not recommend, are Sevin (carbaryl) and Malathion; these materials do not provide adequate residual control. Also are recent change is that Penncap-M is NO longer labeled for alfalfa. Check recent labels for recommended rates for the following: Ambush, Pounce, Baythroid, Warrior, Lorsban, dimethoate. Dimethoate is still one of the more inexpensive products, and provides systemic action within the plant. Also check pre-harvest intervals for each.