Kansas State University

— by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa weevil development is/has proceeded quite rapidly, and many fields have required an insecticide application.  Pictured here is a representative sample of alfalfa plants from a non-treated field and a close-up from that field.

Using the stem shake bucket method of sampling, this untreated field still had an average of 8+ larvae/stem at different developmental stages.  Plus, pupation had already begun in that field.

In contrast are the plants representative of a field that was treated on 5 April, which had already exceeded the 1 larva/ 2 stems, or 50% infested, treatment threshold.  However, larvae are still hatching and developing, and the feeding damage is starting to become apparent again.

Thus, alfalfa fields should continue to be monitored for at least another 10-14 days for the need of a 2^nd insecticide application or, if possible, swathing.  If insecticide applications are deemed warranted, be sure to check the label for the pre-harvest interval (PHI) requirement for the insecticide used.

For more information regarding alfalfa weevil management please refer to the KSU Alfalfa Insect Management Guide:  https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

–by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa weevil populations seem to have “exploded” around north central Kansas in the past week.  Tiny alfalfa weevil larvae were first detected in NC KS on 5 April, but probably a few started hatching a day or two prior.  However, the infestation levels that were detected on 5 and 6 April were all well below 10%, and mostly less than 1%.  In contrast, fields sampled on 16 April all greatly exceeded 100% infested using the stem shake bucket method and large numbers of different stages of larvae were detected.

To sample using the shake bucket method, randomly select individual alfalfa stems and quickly and vigorously shake them into a small white bucket.  Then, count the number of dislodged larvae in the bucket and divide by the number of stems to get the infestation level.  For example, 15 larvae from 10 stems = an average of 1.5 larvae/stem.  Do this in several areas throughout each field to get a good indication of the alfalfa weevil infestation level and the stage of development of the weevil.  One of the problems with the shake bucket method is that some stems have several larvae/stem while others have none (yet).  Thus, the infestation level may appear to be higher than is the actual infestation.

However, in NC KS, with as many larvae as there are already (with more to come probably) and as much damage as we are starting to see in spots, it may be prudent to treat fields as soon as possible.

For information on insecticides registered for use for alfalfa weevil control, please see the KSU Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

— by Dr. Jeff Whitworth and Dr. Holly Davis

Very small alfalfa weevil larvae were 1^st detected on 5 April, 2019 in north central Kansas.  Every field sampled had at least one newly hatched larva.  There were not enough to sample for a treatment threshold as they are just hatching.  The 1^st indication of these small larvae are visible, but very tiny, pinholes in leaves, or a little chewing on plant terminals.  These tiny larvae are quite difficult to dislodge from their feeding sites when they are this small.  Thus, sampling at this early stage to determine an infestation level is not practical using the bucket-shake method or a sweep net.

— by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa is really living up to its reputation as a ‘sink’ right now for many different insect species, including many beneficials such as lady beetles and green lacewings.  The one and only potentially serious pest that we are still seeing is potato leafhoppers, and they are in densities that exceed treatment threshold in all fields sampled.  These very small, lime green, wedge-shaped insects that move in a herky-jerky manner remove fluid from the alfalfa leaves.  This feeding may also introduce a toxin which initially causes the tips of leaves to turn yellow (hopper burn), but may impact the entire stem, and eventually the whole plant. This can be especially problematic this time of year when the plants need to utilize the foliage to transfer nutrients to the roots before winter.  The potato leafhopper populations will hopefully be diminishing as they don’t overwinter in Kansas and thus should be heading to the southern U.S. soon. Swathing should also help diminish populations.

This time of year fall armyworms may move into alfalfa where they can add to the defoliation caused by other chewing insects already present.  Fall armyworms are more commonly thought of as a pest of corn and sorghum.  This time of year those crops are too mature to support the larvae and therefore the adult moths may oviposit in alfalfa.

We also noticed several green cloverworms along with one larva infected with an entomopathogenic fungus.

For more information relative to insect pest management in alfalfa, please see the KSU 2018 Insect Pest Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

— by Dr. Jeff Whitworth and Dr. Holly Davis

Several alfalfa fields were sampled in north central Kansas this past week.  The alfalfa canopy remains an excellent habitat for many insects, especially those fields not treated for alfalfa weevils this year.  Many of these insects are beneficial.

Very few pests, or potential pests, were detected although there were a few potato leafhoppers present.  These small, lime green, herky-jerky, moving pests are apparently just beginning to migrate into KS, as we didn’t find any earlier in the week, and now are only finding a couple of adults.  For more information relative to potato leafhopper management, please refer to the KSU 2018 Alfalfa Insect Management Guide:  https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

— by Dr. Jeff Whitworth and Dr. Holly Davis

Most alfalfa monitored in north central Kanas in the last week has started flowering and swathing seems to be well underway.  Alfalfa weevil infestations, which we sampled throughout NC KS, never did exceed an overall 30% infestation level (using the cut stem bucket shake method).  This year, any time larval numbers started increasing, a sub-freezing cold spell came through and killed many newly hatched larvae.

Since alfalfa fields were not treated for alfalfa weevils they are currently a great place for many beneficials. Uncut fields sampled in the last week had healthy populations of pea aphids as well as a few spotted alfalfa aphids.

However, as beneficial populations increase, these aphid populations are rapidly declining.  Presently there are many lady beetles, both larvae and adults, and more adults will be active very soon as there are also many pupae.  There are also significant numbers of parasitic wasps parasitizing these aphids as indicated by many mummies.

Additionally, there are a few green lacewings present.  All this beneficial insect activity results in aphid populations declining significantly in the fields we sampled.  On 11 May, pea aphid populations averaged 50-60/stem, dropping to 6-7/stem on 16 May.  Hopefully, after swathing, the beneficials will still be present in sufficient numbers to continue providing aphid control but fields should still be monitored.  For more information relative to alfalfa insect management, please refer to the 2018 Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf809.pdf

–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