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Extension Entomology

Category: Soybean

Soybean Pests

–by Jeff Whitworth — Field Crop Entomologist

The ongoing hot dry conditions seem to be starting to significantly stress many dryland crops. Then when you add in the stress of an insect pest it can really cause problems. Right now, blister beetles and Dectes stem borers are very active and causing concerns. Many growers seem to be confusing the two–adult Dectes with blister beetles–with good reason, as some blister beetle adults are similarly colored and shaped somewhat like adult stem borers. The two blister beetles most commonly confused with Dectes stem borer adults are shown here at the top, and just above (see fig. 1) a stem borer adult (photo by JR Ewing). Blister beetles may cause some defoliation by eating leaf tissue but it is usually insignificant. However, Dectes are currently still depositing eggs in soybean petioles, and have been for 2-3 weeks, where the small larvae hatch and start boring into the stem, most often causing the stem to die (see fig 2). However, the larvae continue this tunneling/boring into the main stem (see fig 3) and eventually end up just below ground where they remain through the winter.  (Figures 2 & 3 pictures provided by Sean Mills)

Figure 1: Two adult blister beetles on the top-adult and adult Dectes stem borer at the bottom left.

Figure 2: Crop damage from Dectes stem borers

 

Figure 3: Tunneling/boring stem damage

 

 

 

 

 

Garden Webworms

–by Jeff Whitworth — Field Crop Entomologist

Garden webworms (fig. 2) have just about finished feeding on soybeans and/or alfalfa (where they have been much less noticeable). They will pupate in the soil for the next 4-7 days then the moths will emerge to start depositing eggs in acceptable host plants. From a crop perspective this time of year, hopefully they will have little impact as the soybeans have developed far enough that these worms won’t defoliate enough to affect yield. But the really late-planted soybeans or those that may be moisture stressed may still have some concern, so monitoring should include these defoliators starting soon.

 

Figure 2: Garden webworm mature larva (picture by Steve Freach)

 

 

The Soybean Gall Midge

— Anthony Zukoff — Southwest Research and Extension Center–Garden City, KS

 

The Soybean Gall Midge (Resseliella maxima) was first observed in Nebraska in 2011, but was not officially described as a new species until 2018 when this tiny fly established itself as an emerging pest of soybeans in South Dakota, Nebraska, Minnesota and Iowa.  New infestations have been documented every year since and its range has expanded into Missouri.  Soybean gall midge has been documented in Nebraska along the Kansas border as recently as 2021.  To date, soybean gall midge has not been documented in Kansas, however this pest should be actively scouted for during the growing season, especially in counties along the Nebraska border.  Losses from soybean gall midge infestation are due to plant death and lodging (Figure 1 Photo by Justin McMechan, UN-L).  Heavily infested fields have shown the potential for complete yield losses from the edge of the field up to 100 feet into the field and a 20% yield loss from 200 to 400 feet into the field.

Figure 1

Adults: tiny (2-3mm), delicate flies with an orange abdomen, slender bodies and mottled wings. Long legs are banded with alternating light and dark markings (Figure 2 Photo by Mitchell Helton, Iowa State University).

Figure 2

Larvae: small, legless, maggots that are clear to white-colored when young but turn bright orange when mature (Figure 3 Photo by Justin McMechan, UN-L).

 

Figure 3

Soybean gall midge overwinter as larvae in the first few inches of soil. After pupation in the early spring, adult midges emerge and lay their eggs on the lower portions of stems or at the base of soybean plants.  The eggs hatch and the larvae feed within the stems. Infestation does not occur until the V2 stage when natural fissures and cracks appear in stems allowing entry by larvae.  Infestation can continue into the reproductive growth stages. So far, there appears to be at least two generations per growing season.  The adult soybean gall midges do not feed on soybeans.

 

Scouting

Begin scouting soybean plants at the V2 growth stage.  Symptoms of infestation include:

  1. Wilting or dead soybeans along field edges with decreasing damage into the center of the field (Figure 4 Photo by Justin McMechan, UN-L)
  2. Darkening and swelling at the base of stems (Figure 5 Photo by Adam Varenhorst, SDSU)
  3. Brittle stems that break easily near their base
  4. Small orange larvae present in split open stems

Figure 4

Figure 5

Management

Being such a new pest, there are currently no published research-based management recommendations. On-farm studies in impacted states are examining the effects of cultural practices and insecticides on preventing losses.  Seed treatments have not shown to be effective.  Please report any occurrence of soybean gall midge to your local extension professional or contact the K-State Entomology Department.  The Soybean Gall Midge Alert Network (https://soybeangallmidge.org/) can be used to track developments regarding this new pest.

 

Dectes Stem Borer

–by Jeff Whitworth — Field Crops Entomologist

 

 

Dectes stem borer adults have been active for about the past 3 weeks. Females have mated and started depositing eggs in stems for about the last 2 weeks. The small larvae are already causing an impact on young soybean plants (see fig2).

 

Figure 2: Soybean plant damaged by Dectes stem borer

Soybean Pests

–by Jeff Whitworth, Field Crops — Entomologist

 

Soybeans finally seem to be starting to grow and develop a little since receiving some moisture and cooler weather. So, as the beans grow so do the soybean pests. Probably the number one pest all across the Midwest soybean growing regions is the bean leaf beetle (see fig 1). This time of year these beetles are feeding a little, which often causes the characteristic round and/or oblong holes in leaves (see fig 2), but also are actively depositing eggs in the soil around soybean stems. When this next generation of adults emerge then they will become active foliage feeders, and sometimes feed on succulent green pods-which may affect the bean inside the pod.

Figure 1: Bean leaf beetle

 

Figure 2: Leaf damage from bean leaf beetle

 

Green stinkbug adults (see fig 3) are also very active this time of year, much like adult bean leaf beetles, feeding a little but also depositing eggs. These eggs will be hatching over the next few weeks and the small stinkbug nymphs will also be very active sucking juice from host plants. Thus, as these soybeans grow and develop so will the pests. This indicates that monitoring should be initiated then in the next couple of weeks–or at least as the beans start into the reproductive phases, at which time they are at their most vulnerable to pest attack.

Figure 3: Adult green stink bug

(Photos by Cayden Wyckoff)

 

Japanese Beetles

–by Jeff Whitworth — Field Crops, Entomology

Japanese beetles (see fig 1-adults) were 1st recorded in Kansas in 1992. However, they have been in the U.S. since about 1916, and have been a serious pest of ornamental and agricultural crops in several states east of Kansas for many years. They seem to be slowly spreading across Kansas from east to west, and there have been reports of small but dense populations of Japanese beetles extending west past Interstate I-35 (see fig 2-soybean plants defoliated by adult Japanese beetles in Republic Co. in 2021-photo by J. Hammer). Japanese beetle larvae, commonly called white grubs, can now be found in some no-till areas of fields that were infested with the adults in 2021 (see fig 3 by J. Hammer). This is becoming more common because the adults feed for a short time then fly to nearby undisturbed areas to oviposit. Depending upon when these adults emerge they may feed on young corn leaves, silks, and/or young soybean leaves, as far as agricultural crops are concerned.

Figure 1 Adult Japanese beetles

 

Figure 2 Soybean defoliation by Japanese Beetles in 2021, Republic Co., KS

 

Figure 3 Japanese beetle larvae, 2022, from field in Fig. 2.

 

 

Grasshoppers

–by Jeff Whitworth — Field Crops

Grasshoppers continue to be quite numerous in some alfalfa fields, and late-planted soybeans (see pic. 2). As these crops senesce or dry down, these grasshoppers sometimes move into wheat fields when the newly planted wheat starts to germinate where they can be a real hindrance to establishing a good stand.

Adult Grasshopper (there are many different kinds currently in the fields – this is a Differential Grasshopper)  (pic by Cayden Wyckoff)

 

Stinkbugs

–Jeff Whitworth – Field Crops

Stinkbugs are still quite common this time of year and many are still nymphs (see pic. 1). Fortunately, most crops should be past the stage that might be susceptible to stinkbugs. Soybeans are probably the crop of most concern relative to stinkbugs. However, they seem to be very general feeders–sucking juice out of just about any juicy, succulent plant. Their relatively long, but somewhat fragile, mouthparts are used to pierce into plants to suck the fluid that they feed on. This is usually while the plants are actively growing and thus the epidermis is relatively tender or the mouthparts can’t penetrate. Thus, stinkbugs should not be a problem now relative to soybean yield, unless the soybeans are still in the early reproductive stages.

 

Picture 1: Stink Bug Nymph (pic by Cayden Wyckoff)

 

 

WORMS, WORMS, and MORE WORMS (army cutworms, fall armyworms)

–by Jeff Whitworth — Field Crops

 

2021 might be called the “year of the worm”. Starting in late winter/early spring, 2021, there was considerable activity by army cutworms. Most of the problem was caused by the larvae decimating thin strands of wheat and/or alfalfa. Then, since late spring/early summer, a combination of armyworms and fall armyworms have been causing serious concern and damage in lawns, pastures, and alfalfa fields throughout about the eastern 2/3rd’s of the state. Army cutworms spend the summer in the Rocky Mountains but start to migrate back into Kansas in early fall every year. The larvae may feed on just about any plants but mostly affect wheat and alfalfa, as these are usually the only plants actively growing this time of year. Armyworms, probably more so than fall armyworms, may continue to cycle through another generation or even two as they overwinter in Kansas, and thus it will probably take a “hard” frost or freeze to stop them. Fall armyworms, since they don’t usually overwinter in Kansas, may migrate south after this generation become adults-but there could be another, or at least partial generation. Armyworms infest primarily grasses, i.e. sorghum, corn, brome pastures, lawns, and often this time of year, wheat, but occasionally alfalfa, etc. Thus, if armyworms are the problem they could be around through another generation or maybe even two depending upon the weather. So, if armyworms are relatively small (see pic 1) they will probably feed for another 10-14 days then pupate (stop feeding). If they are relatively large (see pic 2) however, they will probably pupate in the next 3-7 days. There will probably be at least one more generation of armyworms. Fall armyworms (see pic 3) have a little wider host range, which includes alfalfa, soybeans, corn, sorghum, wheat, etc., but don’t usually overwinter in Kansas, thus, hopefully, will be heading south after these larvae finish feeding and become moths. Also, in the next 30-60 days army cutworm moths should have returned from their summer Rocky Mountain retreats to deposit eggs throughout at least the western 2/3rd’s of the state and thus, these tiny worms will start feeding on wheat and/or alfalfa all winter.

Picture 1: Small Armyworm (pic by Cayden Wyckoff)

Picture 2: Larger Armyworm (pic by Cayden Wyckoff)

Picture 3: Fall Armyworms (pic by Jay Wisbey)

 

 

Japanese Beetles

–by Jeff Whitworth — Field Crops

 

Japanese beetles have been very active throughout Kansas for the last 2 months. However, they seem to have been more widespread north of Interstate 70 from the Missouri border west to about Smith Center. There were a few “hotspots” south of I-70 but not as widespread. Japanese beetles can be a concern in field crops because they can feed on green silks in corn or on leaf tissue in soybeans. They do have a wide host range but those are the two vulnerable crops and it is just a matter of timing as to which are attacked. If any corn in the vicinity of where the adult beetles emerge is silking, it seems that is highly attractive to the hungry beetles. However, if corn silks are starting to turn brown when the adults are foraging they seem to be attracted to new succulent soybean leaves (see pic 3). Japanese beetles seem to be slowly increasing in numbers and range. The larvae are white grubs and most commonly feed on roots in some area of undisturbed plants, i.e. turf, pastures vineyards, etc. After the larvae pupate, the adults emerge and fly to any nearby food source. This usually starts towards the end of June or in early July. They then feed voraciously for a few days before disbursing to start depositing eggs back in the “nursery” area. They continue this feeding-disbursing, etc. usually for about a month or so. Adults then die, eggs hatch, and the cycle starts all over again.

Picture 3: Japanese Beetle damage (pic provided by Jeff Hammer)