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Extension Entomology

Category: Soybean

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.

Soybean Update

— by Dr. Jeff Whitworth and Dr. Holly Davis

The first adult Dectes stem borer was detected on 23 June, from north central Kansas (specifically, DK Co.).  These beetles traditionally spend approximately 7-14 days congregating or aggregating around the borders of stubble fields near where they overwintered.  Then, they disperse throughout soybean and sunflower fields and begin depositing eggs in plants of either crop.  Several more have been collected since the 23rd.

Bean leaf beetle adults have been and will continue chewing characteristic round or oblong holes in soybean leaves.  However, at least around north central Kansas, populations seem reduced from recent years.

 

So far, other than a few small grasshopper nymphs, there seem to be less defoliators than usual in either alfalfa or soybeans.  However, there is still time left for significant populations to develop.  A few garden webworms and yellowstriped armyworms were collected from a couple of fields and many of the soybeans are still very small, in the 3-5 trifoliate stage.

 

Green June Beetles vs. Japanese Beetles

— by Dr. Jeff Whitworth and Dr. Holly Davis

Adults of both green June beetles and Japanese beetles seem to be ramping up their activity throughout eastern Kansas.  These two relatively large, conspicuous beetles are being confused.  Green June beetles, at 1 inch long, are considerably larger than Japanese beetles.  Also, green June beetles are green to copperish green in color and more pointed toward the anterior (head) end.  Japanese beetles are probably only 1/3 to ½ as big as the green June beetle.  They also have small, but highly visible, little white tufts of hair on both sides of the abdomen sticking out from under the elytra.

 

 

Japanese beetles may be found feeding on silks in corn fields and/or pollen or leaves in soybeans while green June beetles are more confined to feeding on nectar from flowering bushes or trees close to where the larval stage, i.e. grubs, were developing in the soil.  Green June beetles are not an agricultural concern while Japanese beetles occasionally can be.

 

Insect Management Guides, 2018

–by Jeff Whitworth and Holly Davis

 

 

 

Ms. Donna Sheffield, Communications Department, recently sent the links to the 2018 Insect Management Guides which can be found as follows:

Alfalfa, MF809: https://www.bookstore.ksre.k-state.edu/Item.aspx?catId=42&pubId=1492

Corn, MF810: https://www.bookstore.ksre.ksu.edu/Item.aspx?catId=221&pubId=20262

Cotton, MF2674: https://www.bookstore.ksre.k-state.edu/Item.aspx?catId=1081&pubId=20259

Sorghum, MF742: https://www.bookstore.ksre.k-state.edu/Item.aspx?catId=281&pubId=20260

Soybean, MF743: https://www.bookstore.ksre.k-state.edu/Item.aspx?catId=281&pubId=20261

Wheat, MF745, https://www.bookstore.ksre.ksu.edu/Item.aspx?catId=299&pubId=1463

 

Soybean Stem Borers

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Dectes (soybean) stem borer damage is becoming more and more apparent as soybean harvest is delayed throughout north central Kansas.  The dectes stem borer was first detected infesting soybeans in Kansas in about 1985 in south central parts of the state.  Annually, the adults emerge from soybean stubble, where they overwinter as larvae, around the 4th of July.  They mate and deposit eggs around soybean leaf petioles.  The small larvae bore into the plants and feed, making their way into the main stem.

Dectes stem borer larvae are cannibalistic, so as these larvae come into contact with others only one survives to tunnel down the main stem to the base where they girdle around the interior of the stem, typically just prior to harvest.  This girdling activity often goes unnoticed.  However, the stems are weakened and wind will blow girdled plants over.  Fields with significant infestations will have serious lodging, as we are seeing now in parts of NC KS.  Harvesting these lodged plants is very difficult at best, and if harvest is delayed further, the grain lying on the ground may be lost to rodents, disease, etc.  Infested fields should be harvested as soon as possible, hopefully before any additional lodging occurs!  There is no preventative or rescue treatment available for dectes stem borers in soybeans.

For more information on the biology of the dectes stem borer, please visit Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

 

Soybean Update — Green Cloverworms

— by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Green cloverworm larvae have been very numerous throughout north central and south central Kansas for the past 30-45 days.  However, the larval stage and the leaf defoliation that they’re known for, has pretty much ceased as most larvae have entered, or are entering, pupation.

 

 

A few green cloverworm larvae can still be found in late planted beans and alfalfa, but for the most part, their feeding will soon cease.  The most common question is, with all these green cloverworm adults around, will they be in the same fields next year?  They do not overwinter in Kansas and most soybean fields will be rotated, so the answer is no.  If the same fields are infested next year, it will not be because of the green cloverworms that survived the winter in that field.

 

Soybean Update – Green cloverworms and Stink bugs

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Green cloverworm adults are quite numerous and are laying eggs in alfalfa and soybeans.  So, there are, or will soon be, small larvae present.  Feeding by green cloverworms will probably not impact alfalfa or most soybean fields unless there are significant larval populations in really late planted fields.

 

 

Stink bug populations seem to be increasing in north central Kansas but most beans should be far enough along in their development that stink bugs should be of little concern.

 

For management decisions for all soybean pests, please refer to the 2017 Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

Sugarcane aphid in Kansas sorghum, 2017

by –J.P. Michaud, Brian McCornack, Wendy Johnson and Sarah Zukoff

 

As we reach the midpoint of September, it is becoming clear that the impact of sugarcane aphid (SCA) in Kansas will be only a fraction of what it was in 2015 or 2016.  Multi-state monitoring efforts using myfields.info to track SCA have reported SCA in 138 different counties in 8 states in 2017; the first record in Kansas was on August 1 in Sumner Co. You can track county movement by visiting the myFields distribution map, or sign up for an account to receive an email alert when SCA has been detected in your area. Only southwestern Kansas has had some fields with infestations heavy enough to warrant treatment, and many others have remained below threshold (see our Scouting Card for more information).  A large proportion of the earlier planted fields are now mature enough to be safe from yield losses, even though SCA may be able to survive on these plants for some time.  At this point, only the latest planted fields that have not yet filled grain remain at risk, and lower overnight temperatures are slowing aphid activity.  Remember, SCA can survive overnight freezes and continue to feed on plants as long as they have any green tissue remaining, although without any further impact on yield if grain fill is complete.

 

Decreased acreage

 

A substantial decrease in sorghum acreage this year, especially in the regions of central Kansas that were most affected in 2016, has likely impeded northerly movement of the aphid this year.  Reduced sorghum acreage, much of it converted to soybeans and dryland corn, has meant the aphid must traverse longer distances to reach suitable plants on which it can establish populations capable of producing the winged migrants that enable further spread.  But several other factors have likely been even more important.

 

Improved management

 

There has been a much higher level of awareness among sorghum growers, and much better preventive and remedial management of the aphid in the southern regions that are the source of aphids for Kansas infestations.  The widespread adoption of seed treatments in south Texas effectively prevents the infestation of young plants for up to a month or longer.  An increase in the acreage planted to the many hybrid varieties expressing resistance to the aphid has greatly impeded its ability to produce large populations so quickly.  Timely scouting and identification by concerned growers has resulted in the early discovery and effective treatment of fields that did exceed economic thresholds, which in turn reduced the number and size of alate swarms that dispersed northward in 2017. Look for more help on scouting and determining treatable infestation levels here: KSU Scout Card

 

Evolving natural enemies

 

Just as pest species can evolve new behaviors (for example, attacking a new host plant), so beneficial species can quickly evolve new pest/host plant associations.  An example of this is provided by the Asian multicolored lady beetle, Harmonia axyridis, which last summer produced very large populations in Kansas sorghum for the first time, feeding primarily on SCA.  This lady beetle was not found in sorghum previously, but was drawn into fields by abundant SCA, and is now responding to sorghum as a habitat containing many types of suitable prey.  This year we have found it regularly feeding on corn leaf aphids and greenbugs, in the absence of SCA, something we had not previously observed.  Similarly, H. axyridis did not frequent soybeans until the arrival of soybean aphid in 2002, whereupon it quickly became a key source of mortality for this aphid, and has remained a regular resident of soybean fields ever since.  While the example of H. axyridis is quite obvious and visible, many changes in the responses of other aphid natural enemies in the sorghum agroecosystem are more subtle, but also important.  For example, the greenbug parasitoid, Lysiphlebus testaceipes, appears to be gradually overcoming SCA immunity to parasitism, and we are starting to find some successfully mummified SCA.  Aphid natural enemies are now colonizing sorghum faster, and in greater numbers, in response to SCA.  Surveys for SCA in central Kansas revealed many small colonies of greenbug, corn leaf aphids, yellow sugarcane aphids, and English grain aphids, all approaching extinction due to heavy predation and parasitism.  Lacewings and hoverflies were especially abundant, with adults flying everywhere and several lacewing eggs on almost every lower leaf, independent of the presence of any aphids.

 

In summary, we are clearly advancing from the epidemic phase of the SCA invasion to the attenuation phase, and considerably faster than we might have expected.  Vigilance will be required for the next few years, and appropriate monitoring and management will need to be maintained, but it is quite possible that 2016 will mark the high point for SCA problems in Kansas and we will not see another year that bad again.

 

Photo caption:

 

A colony of sugarcane aphid showing evidence of substantial predation.  Note ‘bloodstains’ (aphid hemolymph) along leaf midrib and the fact that aphids are widely scattered rather than forming a compact colony.

Soybean Update – Defoliators, Pod and Bean Feeders

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

 

Soybeans continue to be the focus of attention of many defoliators, as well as bean and pod feeders, throughout north central Kansas. Thistle caterpillar populations seem to be largely late instar larvae and/or pupating inside of their very characteristic chrysalis.  Green cloverworms also seem to be increasing in both size and densities.  These are the main two species feeding on the leaves, but throw in a few yellowstriped armyworms and blister beetles and they all add up to a formidable group of very active defoliators.  Fortunately, soybeans are very resilient at coping with the loss of leaf tissue.  However, periodic monitoring should be continued, and if considering insecticide applications, please consult the 2017 KSU Soybean Insect Management Guide for treatment thresholds and insecticides labeled for these pests.

 

 

Pod and bean feeders seem to be just getting started. Bean leaf beetle adults may feed on the pods and will continue to do so until pods dry.  Soybean podworms (aka corn earworms) may feed upon the bean within the pod and thus both may reduce yields relatively quickly.  Fortunately, podworms will only feed on the beans for about two weeks.  However, bean leaf beetle adults may feed on the pods as long as they remain green.  Stink bug populations are also increasing and these may also feed directly on the developing beans within the pods.  Again, for management decisions, please refer to the 2017 KSU Soybean Insect Management Guide: https://www.bookstore.ksre.ksu.edu/pubs/MF743.pdf

 

 

 

 

 

 

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