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Extension Entomology

Category: Soybean

Soybeans

–by Dr. Jeff Whitworth

Most soybeans in north central Kansas, even double cropped fields, are getting to the stage where the pods are hardened enough to protect the beans inside (see picture of pod feeding scar by bean leaf beetles and an adult bean leaf beetle (pic1). Woollybear caterpillars (pic 2) are becoming more noticeable as the soybean leaves start to senesce, the caterpillars are getting larger and thus more visible, and as they move to the ground, looking for overwintering sites.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Soybean Update—podworms, corn earworms, sorghum headworms

–by Dr. Jeff Whitworth

 

Most soybeans around north central Kansas are now well into the mid to late reproductive stages. There doesn’t seem to be much potential for any massive infestations by defoliators and even if there was, as the soybeans get farther along in their development the leaf tissue becomes less needed. However, the direct pests, those that feed on the marketable product, are still very active and some are even increasing. Bean leaf beetle adults are active, feeding on pods, however, there do not seem to be as many as in past years. Soybean podworms, i.e., corn earworms/sorghum headworms, are very common and seem to be increasing in numbers in some areas. Treatment thresholds are usually considered to be 1 larva/row foot, with small worms, i.e., less than ½ inch (see pic1) and they are feeding on the seeds (beans). These larvae feed for roughly 2 weeks before pupating. As the larvae develop larger than the one shown here (see pic 1) they consume more as they get bigger and this feeding will continue for about another 7-10 days at these temperatures

 

 

 

Soybean Pest Update

–by Dr. Jeff Whitworth

Defoliators are still present in most soybean fields throughout north central and south central Kansas, especially in double cropped fields. However, infestation levels are still relatively low and growing conditions still seem to be really good.

Podworms (corn earworms/sorghum headworms) are just starting to move into soybeans from sorghum as the sorghum gets past the soft dough stage. Thus, as the soybeans are in the reproductive stages, with new succulent pods being added to the plants, these larvae, plus adult bean leaf beetles and possibly stink bugs, may start feeding on them, which can impact yield pretty quickly. Therefore, using a drop cloth and vigorously shaking the plants over it to count the bugs that fall on it is highly recommended to quantify the pests present, which is necessary to determine management options.

 

Also, the results of Dectes stem borer tunneling is becoming visible as scattered petioles start to die. Most of the larvae sampled were still relatively small (see pic), i.e., probably only 1/4 – 1/3 grown.

Dectes stem borer

 

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.

Soybean Update

–by Dr. Jeff Whitworth

The usual soybean defoliators, i.e., thistle caterpillars, garden webworms, green cloverworms, etc. have just started feeding again as small larvae. However, soybeans should have enough foliage, and the growing conditions are good enough, that this feeding should be negligible. Please note, Ms. Rene Hessel, a soybean researcher for KSU, and a soybean aphid detector extraordinaire, reported finding the first soybean aphids in Riley Co. on 7 August (see pic). Soybean aphids have migrated into Kansas every year since 2002, however, there has only been a couple years in that time that conditions were conducive enough to allow the aphid populations to build up to treatable levels. Most conventionally planted soybeans are in the early reproductive stages throughout south central and north central Kansas, so these aphids warrant periodic monitoring.

Also, just FYI. There seems to be a healthy number of stinkbugs (see pic) in most soybean fields at the present time. Remember, when pods are filling, the beans are vulnerable to both “podworms” and these stinkbugs.

Soybean aphid (photo by Rene Hessel)

Adult Green Stinkbug

 

Soybean Pest Update

–by Dr. Jeff Whitworth and Dr. Holly Davis

Soybeans appear to be relatively pest free at the present time – but looks can be deceiving.  There are a few small green cloverworms, thistle caterpillars, soybean podworms (corn earworms) and webworms, along with stink bugs and spider mites.

These pests are probably only going to increase in the next few weeks.  Bean leaf beetle adults will also likely be emerging and showing up in fields soon.  As the beans continue to develop so will the pests, thus monitoring should continue until beans senesce.

 

MORE – Japanese Beetles

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Japanese beetle adults have also been emerging over the past week or two.  These adults will feed on just about any pollen, nectar, or succulent plant source for a few days then disperse to begin ovipositing into the soil.  The adults may attack emerging silks in corn or new succulent leaves in soybeans, but typically only around the edges of fields.

Dectes Stem Borers

–by Dr. Jeff Whitworth and Dr. Holly Schwarting

Dectes stem borer adults continue to emerge throughout north central Kansas.  These adults are currently aggregating mainly around field borders and most commonly on ragweed.  They will start dispersing into soybean fields within the next week to 10 days, as they do every year, to begin oviposition.  The adult females are relatively mobile and move from plant to plant inserting eggs into, or just below, the petioles of many plants.  This oviposition period may last for four weeks or more and may be spread throughout the field! This is one reason why controlling dectes stem borers with an insecticide is so difficult – timing of application.

Eggs hatch in the stem and the small larvae start feeding/boring their way to the main stem and then down this stem to the soil surface.  They usually reach the soil line in late August and larvae girdle their way around the inside of the stem, weakening the stem and often leading to lodging, especially if there are strong winds.  This lodging is responsible for most yield loss.  For more information regarding dectes stem borers, please see Dectes Stem Borer, MF2581: https://www.bookstore.ksre.ksu.edu/pubs/MF2581.pdf

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

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

 

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