Kansas State University

— by Dr. Jeff Whitworth and Dr. Holly Davis

Recently mowed alfalfa around north central Kansas still seems to have relatively robust populations of pea aphids.  These aphids tend to be more problematic during the cooler weather of spring and fall, but with the cloudy, cooler temperatures so far this spring, they are still doing quite well. Also, most fields treated for alfalfa weevils have few beneficials yet, although they do appear to be coming back. Warmer temperatures and resurgent densities of beneficials should help control these pea aphid populations without the need of an insecticide application.  However, if summer temperatures are lower than usual, pea aphid populations may persist and thus monitoring should continue.

Potato leafhoppers are increasing in north central Kansas.  Populations sampled were all adults, which means they have recently migrated into the area.  These adults will be depositing eggs in alfalfa stems and the nymphs will emerge to begin sucking plant fluids from the alfalfa.  If potato leafhopper populations are at, or exceed, treatment thresholds and the alfalfa has recently been swathed, so that it is a few weeks prior to the next cutting, an insecticide application may be justified.

Keep in mind that potato leafhopper and pea aphid populations can be greatly impacted by weather so they should continue to be monitored.  For more information regarding these and other alfalfa pests, 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

Alfalfa has mostly passed the alfalfa weevil vulnerable time of year.  However, there are still highly visible signs that there was a significant weevil infestation, i.e. some fields with characteristic striping, which is usually partly attributed to weevil feeding under windows and the “barking” of stems by adults.

— by Dr. Jeff Whitworth and Dr. Holly Davis

Grasshopper nymphs, both longhorned (typically not a pest), and shorthorned are common and they will probably just keep increasing in density for another month or more. Another reminder that the best time to manage them is while they are still small and thus, less mobile.  An application of an insecticide labeled for grasshopper control is most effective, cheaper, and less environmentally disruptive if applied in a timely manner relative to grasshopper development.

— by Dr. Jeff Whitworth and Dr. Holly Davis

These small, lime-green, wedge-shaped, herky-jerky moving insects have already migrated into the state.  Some fields are at or exceeding treatment thresholds.  However, swathing should cure this problem, at least temporarily.  Historically, potato leafhoppers migrate into Kansas a little later in the season, i.e. between the 2^nd and 3^rd cutting. Some “hopperburn” is already becoming evident.

— by Dr. Jeff Whitworth and Dr. Holly Davis

Pea aphids are also abundant at the present time.  Alfalfa weevil insecticide applications decimated most beneficials which usually help control these aphids.  However, lady beetle populations appear to be increasing which should help control these aphids, as should warmer summer weather.

–by Dr. Jeff Whitworth and Dr.Holly Davis

Seems like alfalfa weevils have caused concern for considerably longer this spring than usual as they have been active since early April, at least in north central Kansas.  While there are very few larvae or pupae left, there are adults remaining in fields which is unusual for this late in the year, but not unique.  Adult alfalfa weevils typically remain in the alfalfa canopy until the 1^st cutting.  After swathing, the adults migrate to other areas that provide more shade where they spend the summer.  However, in years when there is a relatively cool spring accompanied by cloudy, rainy weather that delays getting into the fields to harvest, the canopy provides more shelter than when it is swathed in a timely manner.  This results in adult weevils hanging around in the alfalfa fields later into the spring than usual because of the cooler conditions within this canopy.  Adult alfalfa weevils are not the voracious leaf defoliators that the larvae are.  However, they will feed a little on leaf tissue, but more frequently around the exterior of stems.  This is called “barking” and normally does not stress plants under good growing conditions.

One problem with swathing while adults are still active in fields is that when the alfalfa is swathed and then windrowed, it does not kill the weevils, but does concentrate them into smaller areas.  They seem content to remain in the shade provided by these windrows where they may feed on the freshly cut stems, retarding their regrowth.  Often, this adds to the characteristic “striping” in these fields, especially if the windrows cannot be picked up as soon as usual.  There is no management remedy for this situation, other than drier weather.

–by Jeff Whitworth and Holly Davis

This week’s cool, wet weather has kept many growers and applicators out of alfalfa fields.  The cooler weather has slowed down alfalfa weevil activity – but not by much.  Remember, they feed 24/7 as long as the ambient temperature, where they are feeding, is over 48°F. Thus, field monitoring needs to continue until most larvae have pupated or fields are swathed, as weather allows.

If insecticide applications are chosen please remember to consult the label for the pre-harvest interval (PHI) of the insecticide selected.  For more information relative to alfalfa weevil management, please refer to the KSU Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF809.pdf