Diligent Scouting is Key to Minimizing ARMYWORM and POTATO LEAFHOPPER Damage

Cornell Cooperative Extension – Northern New York

Paul Peterson, NNY-Regional Field-Crops Specialist, Cornell Cooperative Extension

June 25, 2012

They’re here!  The true (common) armyworm (AW) and potato leafhopper (PLH) have come far-enough north this late-spring/early-summer to feast on our beautiful field-crops here in Northern New York.  To-date, both pests have been reported mostly in presence, but also in spotty yet significant damage in St. Lawrence, Franklin, and Clinton counties.

Despite limited damage so far, diligent scouting for these insects is essential, particularly in the next couple weeks, as these critters can sneak up very quickly to cause significant, almost-explosive crop damage literally overnight.

Western New York farmers have been severely battered by both pests.  Jefferson-County farmers have had a lot of AW damage, and some PLH damage.  Greatest damage by AW to our southwest has been in winter wheat and mature 1^st-crop grass hay; but also in young corn no-tilled into grass residue, spring oats, and 2^nd-crop grass re-growth.

Staying Ahead of True Armyworms

The larvae of AW chomp on GRASS leaves from the outer margins inward, so our broadleaf crops (e.g., alfalfa, soybean, clover) should escape.  Larvae grow up to 2” long within 7-10 days after hatching, and remain at the most-damaging final larval stage (~80% of all AW damage) for ~1 week prior to pupation (dormancy).  Eggs of the AW are laid in thick, grassy areas by moths blown here from the south; AW doesn’t overwinter in NNY.  The larvae are smooth cylindrical caterpillars, ranging from tan to dark olive green in color, with white and orange stripes running along each side.

True AW is an occasional spring/early-summer pest in NY.  However, a somewhat alarming feature of this year’s invasion is evidence of multiple ‘1^st-generation’ migrations.  In other words, many fields where AW are present continue to have larvae ranging in size from 0.5 to 2”, which suggests more than one wave of 1^st-generation moths.  Thus, diligent scouting is particularly important during the next couple weeks, as well as mid-summer, since NNY can usually anticipate a 2^nd-generation that, although rarely significant, could cause damage this unusual year.

My Franklin Co. CCE colleague Harry Fefee did some thorough scouting throughout Franklin Co. last week and found armyworms on every farm he checked; minimal damage, but legitimate cause to diligently scout grass hay, small grain, and young corn fields at least every other day for the next couple weeks.

If necessary, there are many insecticides labeled for true armyworm for conventional farmers, and several products approved for organic use.  However, most insecticides are broad-spectrum in their insect-killing power, so beneficial insects both known and unknown will be killed as well.   We are fortunate to have a number of good insecticide options in our management arsenal, but I recommend using only as necessary.

Larvae are often parasitized (look for small white parasite eggs on the AW’s neck), and these parasites should be conserved by avoiding unnecessary spraying.  In most years, parasites and other natural factors keep AW numbers low.  It is important to detect problem areas early, while the larvae are small.

For grass hay and haylage, the best management is to harvest or graze as soon as possible and monitor AW populations in re-growth.  For whorl-stage corn, apply insecticide only if most plants show damage and three or more larvae per plant are found.  In small grains and grass hay re-growth, consider insecticide only if three to four or more AW are present per square foot.  Only the infested portion of the field and a 20- to 40-foot border around it need be treated.  This border will prevent AW from invading from an adjacent infested field.  Since larvae are most active at night, apply treatments late in the day.

Staying Ahead of Potato Leafhoppers

Perennial forage stands where I’ve observed significant PLH damage to NNY alfalfa and white clover appear to be old, thinning stands that received a timely 1^st-harvest, or where an old stand of over-mature 1^st-crop alfalfa remains.   The fixed costs to harvest, store, and feed high-quality perennial forage are considerable; so rarely is it profitable to  continue perennial mixed-forage stands that included a significant amount of alfalfa initially.  Even the newest alfalfa varieties lose yield potential after 3 years of harvesting; so take advantage of all that free fixed-N to feed a grass crop in your rotation.

PLH are small, wedge-shaped, green insects about 1/8” long.  They have long hind legs that allow them to hop like a grasshopper and powerful wings that allow them to fly quickly.  Females lay ~3 eggs per day over a ~7-week period.  Since adults are very long-lived and generations continue to overlap, scouting is necessary through mid-August.

So, thinning old perennial-forage stands, particularly those with PLH-damaged alfalfa and/or white clover, warrant thorough termination/rotation consideration.  Summer-seedings of perennial grass/legume mixtures, winter grains for grain or forage, and/or spring oats for high-quality fall oatlage are good options to consider.  All options provide an opportunity for manure fertilization prior to seeding.  Soil moisture is key to any summer-seeding, but NNY climatic summaries suggest high likelihood of mid-summer rain that can enable summer-seeded crops to germinate, emerge, and produce a profitable crop.  Summer seedings also generally face much less weed competition than spring seedings.

Consider whether the stand to be terminated is tall enough to justify machine harvest, grazeable, or left unharvested to leave what alfalfa herbage remains to provide a little more fixed N (~20-30 lb N/ac more) for the grass crop that follows.

Alfalfa stands most susceptible to PLH are those with the most accumulated stress.  Perhaps that’s why many of the denser, vigorous alfalfa stands I’ve seen have no PLH damage.  Drought also increases PLH-damage potential.  Regardless, it’s important to scout diligently for this pest, as considerable  damage has already occurred by the time symptoms appear.

The PLH is a sucking insect, removing plant sap from the vascular system of the plant.  In the process of removing sap, they leave in the plant a toxin that causes ‘hopper burn’, characterized by V-shaped yellowing and reddening of alfalfa leaf-blade tips.  The PLH thus reduces photosynthesis, water-soluble carbohydrates (source of energy and good silage formation) and productivity via stunting; and sometimes kill young seedlings.

If PLH is an annual issue on your farm, consider purchasing alfalfa-varieties with PLH-tolerance.  The newest PLH-tolerant varieties are excellent, usually limiting consideration of insecticide use to just the seedling stage.  With standard alfalfa varieties, the key is to sweep alfalfa fields at least every other day until mid-August to determine whether PLH populations are high enough to warrant insecticide application.

PLH-population thresholds for insecticide application are as follow:

References and Resources

• Paul Peterson, NNY-Region Field-Crops Specialist, Cornell Cooperative Extension-Franklin County Office; 355 West Main Street, Suite 150; Malone, NY 12953; Office 518-483-7403, Cell 518-651-4402, Email pp428@cornell.edu.
• New York State Integrated Pest Management Program, Keith Waldron, Coordinator; http://nysipm.cornell.edu.
• Cornell Cooperative Extension-Northern New York, Joe Lawrence, Coordinator; www.ccenny.com.
• 2012 Cornell Guide for Integrated Field Crop Management, Bill Cox and Mary McKellar, Editors; http://ipmguidelines.org.

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