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Extension Entomology

Category: Soybean

Soybean Update – Dectes Stem Borer

— by Dr. Jeff Whitworth and Dr. Holly Davis

 

Dectes stem borer adults can usually be found emerging from infested soybean stubble right around the 4th of July.  This year, despite all of the cooler, wet weather throughout north central Kansas earlier this season, is no different.  Dectes adults have been emerging throughout NC KS for about the last week or two.  They usually hang out around the field edges, aggregating for purposes of mating for just a few days up to a couple of weeks.  Then, they will disperse to soybean and/or sunflower fields to start depositing eggs in the stems of either crop.  Eggs hatch within the stems and larvae tunnel into the plant where they eventually bore their way toward the base of the plant and girdle around the interior of the stem.  This is usually completed by late August when they tunnel even further down the stem, below the soil line, for overwintering.

There are no great management tactics to avoid dectes stem borer infestations although the important damage is due to the end of the season girdling, if the plants lodge.  Harvesting fields with significant infestations as early as possible helps greatly to avoid yield losses.

For more information, please see Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

 

Soybean Update – Bean Leaf Beetles and Webworms

— by Dr. Jeff Whitworth and Dr. Holly Davis

 

Adult bean leaf beetles (BLB) are still causing some concern for soybean producers throughout north central Kansas.  These adults have been chewing round or oval holes in the leaves of seedling plants.  However, it seems much of their leaf feeding has slowed and the females are now mostly on the ground around the base of plants depositing eggs.  These eggs will hatch in a few days and the larvae will start feeding on soybean roots/root hairs.  These larvae resemble corn rootworm larvae but BLB larvae do not feed on corn roots just as corn rootworm larvae do not feed on soybean roots.  One other major difference is that the corn rootworms eggs were deposited in fields planted to corn last year whereas the overwintering adult female BLB deposited these eggs after finding seedling soybean plants this season.

Garden webworms have been causing concern because of visible defoliation over the last couple of weeks.  However, most of these webworms, plus thistle caterpillars, have ceased leaf feeding and are in the process of pupating.  The adults will be emerging over the next couple of weeks and the females will be depositing eggs to initiate the next generation.  Thus, this first infestation of larvae of both species was just a “springboard” generation for the next one or two generations to come.

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

Soybean Update – Thistle Caterpillars and Bean Leaf Beetles

–by Dr. Jeff Whitworth and Dr. Holly Davis

                Thistle caterpillars are becoming more evident around north central Kansas as they increase in size and their feeding becomes more visible.  These larvae are the result of eggs deposited by painted lady butterflies that migrated back into the state about two weeks ago.  These larvae will pupate in a couple of weeks and the adults will emerge soon after.  There will probably be even more in the next generation.

 

 

Round and/or oblong holes in seedling soybeans are indicative of adult bean leaf beetle (BLB) feeding.  Remember, these young plants are very resilient at overcoming up to about 50% defoliation in these early vegetative stages.  It takes approximately seven adult BLB/row ft. to achieve that level of defoliation.  However, adult BLB usually don’t feed for more than a few days after locating the seedling soybeans.  While this feeding can cause considerable concern because of the highly visual holes, it typically does not result in much stress to the plants, especially under good growing conditions.

 

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

 

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

 

Alfalfa Update – Alfalfa Weevils, Pea Aphids, Etc.

— by Dr. Jeff Whitworth and Dr. Holly Davis

Alfalfa weevil activity has slowed considerably but has not stopped yet.  Fields that have not been treated at all this season still have relatively large populations of larvae, of all sizes, and increasing numbers of adults.

 

Small larvae along with later, more mature instars, plus pupae and newly emerging adults are still developing because of the recent cooler weather which has slowed down weevil development.  The field shown here withstood a weevil infestation feeding on the leaf tissue since early April but is starting to come back with some regrowth as the larval population matures into pupae, then adults. However, the 1st cutting, at least, has been donated to the alfalfa weevil.  In contrast, fields treated in a timely manner are now being, or are ready to be, swathed as soon as possible.

 

Pea aphid populations are starting to increase in fields treated earlier for alfalfa weevils.  However, this is also allowing beneficials to build up which should be helpful for controlling other aphid populations in other crops throughout the growing season.

 

Another example of alfalfa being a great “sink” for other insects – it is the main habitat for adult bean leaf beetles where they hang out until soybeans start germinating.  They will then migrate from alfalfa to feed on seedling soybeans and begin ovipositing around the base of these seedlings.

Bean leaf beetles are often confused with southern corn rootworms which can also be very common in soybeans but do not have the potential to negatively impact yield.  For more information on alfalfa pest management and/or soybean pest management, please refer to the KSU Insect Management Guides.

KSU Alfalfa Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF809.pdf

KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

Soybean Update – Green Cloverworms and Stink Bugs

–by  Dr. Jeff Whitworth and Dr. Holly Davis

Probably the most common defoliator of soybeans throughout Kansas, for the past 6-7 years, has been the green cloverworm.  This year has been no exception to that although there have not been the significant densities throughout most of the state as in 2015-2017.  Most green cloverworm larvae are relatively mature or have pupated and the adult moths are, and will continue to be, emerging.  In south central and north central KS, these moths are gaining attention as they congregate around lights at night and can be found hiding under boards, logs, rocks, etc. around soybean and/or alfalfa fields during the day.

 

One point of concern is relative to the next generation of larvae.  The adults that are present now are from the 2nd generation.  In KS, we can have 3 generations/year.  So, current adults may oviposit in alfalfa and soybean fields and the larvae may feed for the next few weeks.  However, both alfalfa and soybeans are far enough along that this late season feeding should not impact yields of either crop.  Another concern is about these moths congregating in and around fields and what that means for next year.  Green cloverworms do not overwinter in KS.  So, wherever they migrate, or are blown, in next summer is where they will initiate infestations.  Therefore, infestations next year have nothing to do with infestations this year.

 

Stink bugs are also causing some concern for soybean producers.  The two most common stink bugs found in KS soybean fields are the brown and green stink bugs.  Both lay eggs in groups or clusters and green stink bug nymphs are red, black, yellow, and green while brown stink bug nymphs are generally just yellow to brown.

 

 

 

 

 

 

After emerging from the eggs, nymphs stay in the same vicinity for the 1st couple of molts and then disperse.  So, sampling in areas with clusters of nymphs will often overestimate the density.  They all have sucking mouthparts but must have tender, succulent tissue to suck the juice from.  Treatment should only be considered while beans are still filling in the pods.  Stink bugs have a wide host range of plants including other crops, vegetables, and weeds.  So, make sure they are feeding upon the developing seeds before making treatment decisions.

For more information on management of soybean pests, please refer to the 2018 Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf743.pdf

 

Soybean Update – Defoliators and Podworms

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Soybeans seem to be attracting the attention of many different types of pests, including many defoliators.  Currently, they are mainly green cloverworms, yellowstriped armyworms, and thistle caterpillars.  Fortunately, populations of these species, or any others for that matter, have not reached densities that have warranted insecticide applications, for the most part.

Unfortunately, soybean podworm (a.k.a. corn earworm/sorghum headworm/cotton bollworm) populations are on the increase in south central and north central Kansas.  These worms eat right through the pod to get at, and consume, the seeds within.  So, as the seeds are filling, they are susceptible to being fed upon by these podworm larvae.  They will feed on smaller, more succulent beans for 10-14 days, then cease feeding to pupate.  Since they are feeding directly on the marketable product, it doesn’t take much of this feeding to reduce yield.

 

 

One important point to remember relative to treating for soybean podworms: these are contact insecticides and thus they must physically contact the targeted pest.  Therefore, you need to utilize enough carrier (water) to penetrate throughout the soybean canopy to get to where these larvae are feeding.  But, you need to do this while the larvae are still small and before they have negatively impacted the yield.  There will probably be at least one more generation this year, so monitoring needs to continue as long as plants are adding pods and there is succulent green reproductive tissue to feed on.  For treatment thresholds and insecticide information, please refer to the 2018 Soybean Insect Management Guide available at: https://www.bookstore.ksre.ksu.edu/pubs/mf743.pdf

 

Soybean – Bean Leaf Beetles and Defoliators

–by Dr. Jeff Whitworth and Dr. Holly Davis

Soybean fields around north central Kansas still seem to be relatively unbothered by defoliators.  Bean leaf beetle adults are increasing in numbers and may begin feeding on the succulent pods as they form.

The next generation of corn earworms (coming from sorghum fields) may start feeding on the beans within the pods, although we did not find any pod feeding yet.  The only defoliators sampled this past week were a few green cloverworms, thistle caterpillars, and yellowstriped armyworms.  None were in sufficient numbers to cause concern, individually or collectively.

For more information relative to soybean insect management, please see the 2018 Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf743.pdf

 

Soybean Update

–by Dr. Jeff Whitworth and Dr. Holly Davis

Not much defoliation in soybeans in north central Kansas, so far.  Bean leaf beetle adults are emerging and feeding on leaves.  Photo: Both color phases of the bean leaf beetle and the 12-spotted cucumber beetle or the southern corn rootworm, which is commonly mistaken for a bean leaf beetle.

These adults may start feeding on pods when the plants get to that developmental stage.  All the soybeans we examined this week were still in the late vegetative to early R1 stages.  But, when plants do start setting pods, bean leaf beetle adults need to be closely monitored.

Potato leafhoppers were also very numerous in soybean fields.  They do not seem to be problematic in soybeans yet in KS.  However, these increasing populations will also be in alfalfa.

Dectes stem borers continue to be active in soybeans throughout north central KS, depositing eggs in stems.

For more information on these and other soybean insects, please see the 2018 Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/mf743.pdf

 

Thistle Caterpillars in Soybeans

–by Dr. Jeff Whitworth and Dr. Holly Davis

In 2017, thistle caterpillars caused considerable concern and some defoliation all around the state, especially in double-cropped soybeans.  Thistle caterpillars are the larval stage of the painted lady butterfly.

 

Thistle caterpillars have been a perennial concern in commercial sunflowers, but in 2017, there were significant infestations in soybeans, causing enough defoliation to require many acres to be treated with insecticides. The painted lady butterflies then migrated south, out of KS, last fall as they don’t overwinter here.  However, they are now migrating back into the state and are depositing eggs on sunflowers and soybeans. Thus, thistle caterpillars will soon be webbing together, and feeding on, leaves.  In 2017, there were two generations and there is no reason not to expect the same this year.

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