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Extension Entomology

Category: Alfalfa

Crop Update – Soybean & Alfalfa

–by Dr. Jeff Whitworth

There doesn’t seem to be a lot of pest activity, at least at the present time, in any of the crops throughout north central Kansas. However, there does seem to be a relatively large number of green cloverworms of different stages (see picture)

 

in both conventionally planted double cropped soybeans. There is enough foliage, however, in all fields that these larvae should not cause any problems, although in some fields they are chewing multiple ragged holes in leaves. There are also a few thistle caterpillars and webworms but again, all defoliators together should not cause enough damage to impact yield. There are also large numbers of potato leafhoppers in both soybeans and alfalfa. They should not cause problems in soybeans. However, they will cause problems in alfalfa if they continue to feed and inject a toxin into the leaves as they do this feeding. Swathing should help mitigate this problem, but monitoring the stubble for leafhoppers should continue to ensure adequate regrowth.

Alfalfa Update

–by Dr. Jeff Whitworth and Dr. Holly Davis

Potato leafhoppers are rapidly increasing throughout alfalfa fields in north central Kansas for two reasons:  1) potato leafhopper adults are still migrating in and 2) the eggs are hatching and nymphs seem to be everywhere.  These nymphs are very small and very shy – which means they are easily under counted as they hop to the underside of leaves, or even off the leaves, at the least disturbance.

Alfalfa weevils mating—(photo by T. Sexton)

Parasitized Alfalfa Caterpillar

Alfalfa weevil adults have mostly migrated out of alfalfa fields in north central Kansas, however there are a few that pupated late and that are just emerging out of their pupal cells.  Interesting, at least to us, was that some of these adults were mating (see picture).  Most of the literature reports alfalfa weevils mating in the late summer, fall/winter  —  not soon after becoming adults.

Alfalfa or garden webworms are also relatively common in alfalfa, where they may cause a problem in new alfalfa, and soybeans.  The next generation will probably be more problematic in small soybeans because there will probably be more webworms as this generation is more of a “spring board” generation.

Alfalfa caterpillars (see picture of larva with attached parasitoid eggs) are also quite common in alfalfa fields as are the white and/or yellow butterflies that they develop into.  However, they have not ever been found in densities great enough to cause any negative impact on yield.

 

Alfalfa Update – Potato Leafhoppers and Pea Aphids

–by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa seems to be growing very well and many fields around north central Kansas have finally dried out enough to swath and remove the hay from the field.  However, potato leafhoppers continue to migrate into the state and will continue to for about another month.  Most are still adults and have been/are now depositing eggs in stems and the tiny nymphs are just starting to emerge.  Thus, potato leafhopper feeding will become more evident as “hopper burn”, the yellowing of leaves which can reduce the health of the plants and the nutritive value of the foliage.  Therefore, if fields were just recently cut, or will be in the near future, while potato leafhoppers are still migrating into the state, they will be very vulnerable to potato leafhopper feeding damage.

Pea aphids are still plentiful throughout alfalfa fields in north central Kansas.  Populations should not reach treatable levels this late in the year, and they are a good host for many beneficial insects.

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

Bean Leaf Beetles

— by Dr. Jeff Whitworth and Dr. Holly Davis

Bean leaf beetle adults that are out and about now have successfully overwintered, probably fed a little in an alfalfa field, and are now eagerly awaiting soybean germination.  These adults are amazing at finding the first, small soybean plants where they begin feeding, causing the characteristic round and/or oblong holes in the small leaves.  These beetles will feed for just a little while and then begin depositing eggs in the soil around the stems of these plants.  These young plants are usually very resilient at overcoming this early season leaf feeding, until it reaches 50% defoliation or more. For more information on bean leaf beetle biology and management, please see Bean Leaf Beetle:  https://www.bookstore.ksre.ksu.edu/pubs/MF2824.pdf

For more information relative to all soybean pests, please see the KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

Pea Aphids and Potato Leafhoppers

— 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

 

Alfalfa Weevils

— 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.

Grasshoppers

— 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.

Potato Leafhoppers

— 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 2nd and 3rd cutting. Some “hopperburn” is already becoming evident.

Pea Aphids

— 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.

 

Alfalfa Weevils

–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 1st 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.

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