Kansas State University

–by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa fields sampled this week in north central Kansas had not really changed a great deal with stems anywhere from 2.5 to 9.5 inches tall.  Sampling the taller stems (8 – 9.5”) resulted in a composite infestation of 10%.  Sampling only the smaller stems (2.5 – 4.5”) resulted in a composite infestation of 27%. The lowest overall infestation level detected from any field was 10% and the highest infestation level was 30%.  Smaller stems had smaller larvae with no, or very little damage or defoliation, while larger stems often had evidence of feeding, but no larvae.  Thus, it seems the earlier hatching larvae may have been killed by the recent cold weather, or at least populations were reduced. However, we did find 3^rd instar larvae and even a pupa.  There are also quite a few small 1^st instar larvae.  This probably means eggs are still hatching with more larvae to come, so at least 1 more week of monitoring would be prudent.

Pea aphids are still present in all alfalfa fields sampled but not yet in numbers to be concerned about.  However, there were no beneficials, (lady beetles, lacewings, or parasitic wasps) noted either.

For more information relative to alfalfa insect management, please see the Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

— by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa fields were again sampled 18 April, after the latest bout of sub-freezing temperatures.  Alfalfa weevil larval development does not seem to have progressed much since fields were sampled last week.  All larvae that were collected from plant terminals were still small, 1^st instar larvae.  Like the last two week’s samples, there were live 1^st instar larvae feeding beside dead larvae.  However, a few larger, 2^nd instar larvae were found in the leaf litter under the alfalfa canopy.  Infestation levels varied considerably depending on location.  Open fields with exposure to wind had infestations ranging from 0% (no live larvae found) to a maximum of 10%.  However, one small alfalfa field that was bordered on the north by a windrow had a 35% infestation of live, feeding larvae.

Pea aphids were also detected in every alfalfa field sampled.  Some were already producing nymphs.  Unlike last week, no lady beetles or mummified aphids were noted, but an adult green lacewing was collected from the alfalfa canopy.

For more information on Alfalfa Insect Management, please visit: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

–by Dr Jeff Whitworth and Dr Holly Davis

Sampled alfalfa fields in north central Kansas on 11 April.  Despite the recent cold weather and lack of measurable moisture, the alfalfa did not show obvious signs of stress and had even grown a little since the 4^th and 5^th of April when we last checked these fields.  The recent cold temperatures apparently took a toll on some of the small alfalfa weevil larvae.  However, it did not kill them all as we found live larvae of different sizes still feeding in terminals and often live ones feeding right beside dead ones.

In no case did we find more than a 5% infestation.  No adults or larvae were found in the leaf litter below the canopy.  So, apparently, the recent record cold temperatures killed 40-50% of the small alfalfa weevil larvae that had started feeding.  It remains to be seen how many eggs are still to hatch, but so far infestation levels we have noted are not anywhere near treatment thresholds.

Pea aphids were also noted as were parasitized pea aphids (mummies).  Lady beetles were also active.  For management decisions relative to alfalfa pests, please see the Alfalfa Insect Management Guide, available at: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

— by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa weevils continue to create considerable interest for producers and consultants.  Alfalfa fields sampled on 21 March had small, 1^st instar larvae and a few, very few, signs of feeding.  A few pinprick sized holes and small signs of feeding were noted in the terminals.

Fields checked 4 and 5 April contained considerably fewer larvae, and still only 1^st instars.  Plants were frosty, but no dead or dying larvae were found.  Larvae often crawl down and hide under plant residue on the soil surface during periods of inclement weather and therefore survive.  No fields were sampled that had more than a 10% infestation, yet.  For more information relative to alfalfa weevil management, please see the Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf