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In this Issue:

Potato Leafhopper in Alfalfa

Corn Earworm Moth Flight Takes Off!

Potato Leafhopper Scenarios for Late-Summer, Early Fall Decisions, Alfalfa

Bill Hutchison, Dept. of Entomology, University of Minnesota, St. Paul

I have been receiving some good questions about current PLH recommendations for Alfalfa, as we approach the end of summer, early fall. Here, I summarize a few key suggestions, guidelines to consider.

Current Situation:
Good News/Bad News: No New PLH Adults Migrating, but No Rain Either! Although we have not had many recent weather systems conducive to new PLH Adult migrations to MN (i.e., High pressure from the east, colliding with a LOW pressure from the west; front drops rain and PLH into MN), we still have some active, old PLH adults, AND nymphs still hatching from eggs (laid in stems); and current hot/humid weather is conducive to maximum PLH developmental rate and survival (and we do not have many effective natural enemies for PLH). The dry weather could be creating some additional stress for some alfalfa fields.

To Treat or Not to Treat?
Growers must first decide if they think they need to treat, is there an applicator available, who has time (i.e., not busy with soybean aphid sprays).

More seriously, here's a checklist, with some key differences noted for Established Stands vs. New Seedings:

1. Are PLH over threshold in my field? Many fields may still officially be over threshold (> 0.5 to 2.0 adults per sweep, from 5 to 12" tall alfalfa, respectively; AND Nymphs also present). If you have "PLH-Tolerant" Alfalfa, multiply these numbers by 2 (at this point, I cannot attribute more value to the PLH tolerant alfalfa than that). IF PLH adults are less than threshold, OR, IF PLH Nymphs are NOT present (i.e., not an actively breeding infestation), You do NOT need to spray, for either established or new seedings; and will not likely need to spray for remainder of season.

2. Established Stands: If PLH adults are above threshold, but Nymphs Low, or Not Present, I would consider NOT spraying this time of Year. Primarily, because Adult PLH, this time of year, are known to start transitioning to a state of "Reproductive Diapause" (i.e.,stop producing nymphs), in prep. for a return southern migration. Adults keep feeding, but if not laying new eggs, and no new nymphs, their impact lessens. Further, in most years, by the 3rd-4th week of August (MN latitudes), we notice that Adult numbers are declining (and decline is not due to natural enemies). By mid Sept., they are usually gone, close to zero. If Nymphs are High, and/or significant yellowing/stunting exists, And the field is < 2weeks from cutting, then consider a spray; but see additional considerations below (# 8).

Research: Most of this information, confirmed by Elson Shields and Paul Taylor, Cornell Univ., also showed that up to 50% of the adults, as early as July 15th, were beginning to enter reproductive diapause. This is interesting to note for July, but remember the overall impact to a given field reflects total numbers. When we have >10 adults/sweep (as we did this year), even if only 50% are laying eggs, that is still a lot of nymphs per acre! Nevertheless, the Cornell study provides the reason for the natural PLH decline we see in the Midwest in late summer. A two year MN study by one of my former students (Eric Flora) also found that very high adult PLH numbers in August did not have a major carry over effect into the fall, nor into the following spring cutting (did not affect yield or quality, the following spring). We attributed this result to the fact that the alfalfa had a good 4-6 weeks in early fall (much of September), depending on your last cutting, to recover.

3. New 2003 Seeded Stands: If PLH adults are above threshold, Nymphs Present, and you are still more than 2 weeks from taking your 2nd cutting; you should consider spraying (Also look at general guidelines provided below). The same research by Eric Flora showed that high PLH counts severely affected the 1st and 2nd cuttings of a new seeding. Continued non-treated plots, after the 2nd and final cutting of the first year, however, we able to recover.

4. New 2003 Seeded Stands: If PLH adults are above threshold, Nymphs Present, field is Less than 2 weeks from 2nd cutting; AND severe stunting/yellowing is present, I would also Spray; the PLH pressure needs to be removed to achieve maximum yield for the 2nd (and final) cutting of this establishment year; a follow-up spray into Sept. not needed.

5. New 2003 Seeded Stands: If PLH adults are above threshold, Nymphs NOT Present, I would NOT spray; it is very likely that few new nymphs will hatch, and that few new adults will arrive, prior to cutting of final crop; and/or crop will have time to recover in fall.

6. Insecticide recommendations: all the pyrethroids (Warrior, Pounce, Baythroid, Mustang Max), Dimethoate (see specific formulations still labelled; e.g., Dimethoate 400) and Lorsban are recommended; for all materials note the PRE-Harvest Interval (PHI); some PHIs are dependent on the rate used. All products do a good job of initial adult and nymph kill, also at Reduced rates. ONE consideration, for this time of year, if you have lower infestations overall, is to stay at 1/2 of the max. labeled rate, for effectiveness and reduced cost (and/or reduced PHI).

7. What not to use: Malathion 57EC (2pts/ac) will provide initial knockdown kill, but no residual activity after 24 hr. Sevin XLR Plus, will provide initial knockdown kill, but limited residual activity; generally more expensive than other options. (Malathion and Sevin are not RUPs)

8. Other Considerations: Assess overall health of the stand, relative to soil type, other pest pressure (plant bugs?), etc....e.g., what impact might water stress might also have; ALSO, treat/no-treat will come down to local economics, and market for the crop; e.g., how badly does the grower need the current hay for this year (based on current reserves); if reserves are low from previous winter kill, then current crop should be carefully managed; will the hay be sold, for delivery to other customers, "cash" crop? Finally, re: drought stress and PLH: Work by Ted Radcliffe, Entomology, U of M, found that there was not a consistent interaction between PLH stress and drought stress, but that each stress was additive; e.g., significant drought stress will not facilitate a higher level of PLH damage.

9. Resource: For those who wish to download a 2003 Field Crops Insect Pest Management Guide (Wisconsin), a PDF is at: (you can print only the Alfalfa pages you need). http://cecommerce.uwex.edu/pdfs/A3646.PDF

Co-Editors: Bill Hutchison, Department of Entomology, University of Minnesota, hutch002@tc.umn.edu
Jeanne Ciborowski, IPM Program, Minnesota Department of Agriculture, jeanne.ciborowski@state.mn.us
Production Editor: Suzanne Wold, Research Fellow, Department of Entomology, University of Minnesota, woldx018@tc.umn.edu

Last Revised August 25, 2003.
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