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Extension Entomology

Category: Soybean

SOYBEANS – bean leaf beetles, grasshoppers

–by Dr. Jeff Whitworth

Many fields were sampled throughout southcentral and northcentral Kansas over the last couple of days. There seems to be very few insect pests affecting these soybeans, so far. There are some adult bean leaf beetles, which should be monitored as beans start or continue, setting pods as these beetles can start feeding on these new pods. There are a few adult Decte’s stem borers (fig. 1), but not many oviposition holes could be found yet. The only potential problem detected this week were grasshoppers. Weedy/grassy borders adjacent to many fields are loaded with grasshoppers. These areas are still lush and green so far, thus most grasshopper infested areas are still sufficient for these grasshoppers to feed in so they have not yet migrated to crop fields. However, there are some areas that have been treated with herbicides and thus these weeds are/or have died in these areas. Grasshoppers are/have moved into crops–in this case, soybeans (fig 2-3). Continued monitoring is highly recommended and please do not make a pesticide application “just in case”, and please send me an email if soybean aphids are detected.

Figure 1 Ductes Stem Borer  (Cody Wyckoff)

Figure 2   Grasshopper hiding/ feeding (Cody Wyckoff)

 

Figure 3  “Chewed” Soybean leaf  (Cody Wyckoff)

 

Soybeans—false chinch bugs

–by Dr. Jeff Whitworth

We have received several calls relative to false chinch bugs (fig. 5) damage to soybeans.  In most cases false chinch bugs may appear in large numbers in a patch or two in a field, and usually after the soybean field was treated with a herbicide.  The most common natural host of false chinch bugs are plants in the mustard family.  So when these are killed by a herbicide, the false chinch bugs congregate and may feed for a few days before dispersing.  This feeding, by removal of plant juices, usually has little to no effect upon the plants, but occasionally can be very detrimental to soybeans (fig. 6) and sometimes sorghum.

Figure 5: False Chinchbug

Figure 6: Soybeans affected by False Chinchbugs (photos provided by Rod Schaub)

 

DECTES (soybean) Stem Borer

–Dr. Jeff Whitworth

The 1st adult Dectes (soybean) stem borers were observed on 29 June in north central Kansas (see fig. 1). This adult emergence seems to be right on schedule with past years, as we have found adults emerging right around the 4th of July since 1997. These adults usually feed a little while on pollen, then mate for about 7-14 days before disbursing to soybean (or sunflower) fields to deposit their eggs in the stems right at the petiole.

Figure 1 Adult Dectes Stem Borer (BY Cody Wyckoff)

 

BEAN LEAF BEETLES

–by Dr. Jeff Whitworth

Adult bean leaf beetles are very active throughout north central Kansas at the present time. They typically chew round/oblong holes in leaves (note fig. 4 with bean leaf beetle at the tip of the arrow) and deposit eggs in the soil around the base of soybean plants. There are two color phases of adult bean leaf beetles (fig 5), a tan phase and a reddish phase, but both have six black spots surrounded by a black border on their backs. Both color types can be seen in fig 5.

Figure 4 Soybean leaf damage from beetles (Cody Wyckoff)

Figure 5 Bean leaf beetles (Cody Wyckoff)

Soybeans—Bean leaf beetles

–by Dr. Jeff Whitworth

Bean leaf beetles (fig. 6) overwinter in Kansas as adults. They are usually first found in late winter/early spring in alfalfa fields because as the temperatures start to warm these adults become more active and actually feed a little on alfalfa foliage. However, there are not enough to cause any problems in these alfalfa fields. However, every year they are impressive because of their super ability to detect the very first germinating soybean plants, whether volunteer or planted. They can apparently detect these early plants from many miles away. As these first germinating plants are found by these adult beetles from many different overwintering sites, they can do some quite noticeable leaf feeding damage (fig. 7). These plants are usually mainly on border rows as the adults fly to and start to feed on the first plants they find from distant overwintering sites. These oval/oblong holes can cause considerable concern, especially if only border rows are examined and growing conditions are stressful. Please remember these young soybean plants are very resilient at absorbing this early season defoliation without any subsequent impact on the plants or yield, as long as the defoliation is less than about 50% in the vegetative stages and good growing conditions return. These adult beetles then lay eggs in the soil and around the base of these plants where they hatch and the larvae feed on the roots/root hairs. These adults then emerge in mid-summer and start feeding on new leaves and/or, more problematic, sometimes on the pods.

Figure 6. Adult Bean Leaf Beetle (red color phase)

Figure 7. Damaged Soybean Seedlings (Cody Wyckoff

Alfalfa and Soybeans

–by Dr. Jeff Whitworth

Green cloverworm adults (see pic) have been very common in soybean and alfalfa fields the last couple of weeks, and this has caused concern about potential green cloverworm infestations next year. However, green cloverworm adults are, or have been, migrating to the southern US for overwintering. Thus, since they do not overwinter in Kansas, infestations next year will depend on wherever the adults come back to, so predicting future infestations after overwintering adults return from the southern US are not possible.

Adult green cloverworm

 

 

 

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

 

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