Insect populations are generally lower this week. Loopers can still be found in some fields, as can stink bugs and bean leaf beetle. Defoliation thresholds for R6 beans are considerably higher than earlier R-stage beans. Now that our earlier planted fields are starting to senesce or have dropped leaves completely, this is a good time to take stem samples for Dectes stem borer, especially in fields that had large numbers earlier or have a history of Dectes problems. If your field has a large stem infestation, prioritize that field for as timely a harvest as possible. Lodging loss potential increases with the percent of infested stems and late harvest.
Continue to scout for corn earworm and soybean looper. Last weekend’s storm activity from the south could have brought up more loopers from that region. I have received a couple of reports of pockets of loopers in fields. They often show up in localized sections, typically in drier, less robust parts of fields. Defoliation thresholds prior to R6 are 15%, at R6, plants can tolerate greater defoliation. R7 beans are safe. Also, be sure to check the lower canopy if you do see significant looper activity, they sometimes start first in the lower canopy unlike our other defoliators that concentrate in the upper canopy. NC only recommends Intrepid Edge, VA recommends Intrepid Edge and Steward. Both products are ‘worm’ products and will clean up corn earworm. Steward has some, but not great activity on bean leaf beetle, and neither has stink bug activity. We also had a report of armyworm activity in some isolated fields. Armyworms can feed on pods, but are not as aggressive as corn earworm. There doesn’t seem to be much consensus in different states’ recommendations; some combine them with corn earworm thresholds, others treat them as defoliators, and others use a threshold in between.
Several fields in the SW portion of Sussex County have experienced above threshold corn earworm activity lately. I have not heard of any excessive populations or populations for which pyrethroids did not provide good control. Soybean loopers are present in many fields at low numbers, as are green cloverworm. Its important to distinguish them, cloverworms are easy to kill and are not aggressive defoliators while loopers are less susceptible to many materials. Cloverworms have a narrow body, three sets of prolegs in addition to the last abdominal segment’s prolegs. Larger larvae often have white stripes and hold their last set of prolegs out in a V shape. Cloverworms also wriggle violently when poked. Loopers tend to be narrower near the head and wider near the abdomen, and have two sets of prolegs. There are some reports of unusually high soybean aphid populations in drought stressed fields. The threshold for soybean aphid 250 per plant on 80% of plants with an increasing population. If you hit threshold, you have about a week before they might reach a damaging population. Check back in a couple of days to see if populations continue to increase, they might not if beneficials are present. Soybean aphids cause inconsistent plant injury after R5.5, and are not considered a threat once beans hit R6. Check labels for pre harvest intervals. Pyrethroids all do a good job, some have pretty long PHIs.
Primary insect pests present in fields include green cloverworm, bean leaf beetles, aphids, stink bugs, and yellow striped armyworm. Aphid populations can increase quickly, so keep an eye out. Corn earworm trap captures have been low in some areas, high in others. Within the last few days, counts have come up quite a bit. Be on the lookout for earworm larvae in fields. Fields that are most attractive are later planted open canopy fields at R2. Worm development will coincide with the susceptible R4 to R6 stages. Worms at or before R2 do not typically require treatment. NC State has a very useful threshold calculator that takes into account row spacing, control cost, and price of beans: https://soybeans.ces.ncsu.edu/2015/08/managing-corn-earworm-in-soybeans/.
We have been conducting adult vial tests for monitoring corn earworm pyrethroid resistance, and have observed close to 30% survivorship. If you face a heavy infestation, 2-3 times above threshold, pyrethroids alone might not give enough control. We picked up our first small soybean loopers this week. This can be a significant defoliator. Base treatment decisions on defoliation estimates, and keep a close eye on fields that have loopers in them. Mississippi also uses defoliation and a ballpark figure of 10 loopers per 15 sweeps. Pyrethroid insecticides are generally ineffective on loopers. 2018 Spray trials from NCSU highlight high rates of Prevathon and Intrepid Edge as the most efficacious products. Other products that have resulted in good efficacy from other states include Steward and Radiant.
Bill Cissel, Extension Agent – Integrated Pest Management; firstname.lastname@example.org
Congratulations Bob Leiby for correctly identifying the damage as mechanical and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!
Guess the Pest Week #19 Answer: Hole Puncher
By David Owens, Extension Entomologist
Photo by Joe Deidesheimer, defoliator is Kevin Troyer
This week’s guess the pest was a bit of a trick question, the answer is hole puncher operated by a hard-working student. Soybean canopy defoliation can be a little tricky to estimate, defoliation often appears more severe than it really is because our eyes focus on differences. We are simulating bean leaf beetle feeding injury to R-2 stage soybean by removing approximately 25% of the foliage canopy-wide.
Although this looks really severe, soybeans can compensate for this level of defoliation. Our threshold for defoliation at this soybean stage is 20% CANOPY and FIELD wide. Our most common defoliators right now feed primarily in the upper canopy. So if 25% of the upper canopy of R-stage soybean is defoliated, but only 5% of the lower canopy, total defoliation could be lower than 15% and the plants will not suffer a yield impact. If there is little to no defoliation in the lower canopy, the upper canopy can take a severe beating before canopy-wide defoliation hits 20%. We may start seeing soybean looper later in the season, this species often defoliates from the bottom up.
Vegetative stage soybean can compensate even greater defoliation. Recent work out of Mississippi indicates that 66% of the canopy of VEGETATIVE beans can be lost without a significant yield loss. In the Mississippi study they also defoliated beans during vegetative growth, at R3, and constantly during the season to simulate the impact of multiple sub-threshold ‘dingers’, and found that a constant 17% defoliation did not significantly reduce yields.
Two other important factors that reduce soybean’s compensatory ability are drought and planting date. Late planted beans have less time to recover from severe defoliation and may (but not always) loose yield. Drought stress may also reduce this compensatory ability. The Mississippi defoliation experiments involved a small army of students around the clock picking leaves off of over 100 10-ft plots.
Continue scouting for defoliators and for pod feeding insects. Primary defoliators are green cloverworm, bean leaf beetle, and Japanese beetles. A few more aphids have been found in fields throughout the state, but only isolated colonies. As a reminder, defoliation the threshold in reproductive stage soybean is 15%, and for aphids the threshold is 250 aphids per plant. Stink bugs are generally present in low numbers in R-stage beans, thresholds are 5 bugs per 25 sweep sample. Take multiple samples throughout the field because stink bugs aggregate. A small number of brown marmorated stink bugs have been found so far, this is an edge species that usually does not move into field interiors; check near woodlines, especially those that have tree of heaven. The high humidity has favored pathogens of spider mites. A few hotspots we have visited in the last week have had alarming visual symptoms of mite defoliation, but close inspection revealed that all the mites were dead. Recent humid weather favors fungal pathogens and in some fields, predatory mites have moved in. Dead spider mites will look brown and fuzzy. Predatory mites will be a creamy white to pale orange color, pear shaped, and with no spots or markings. As a heads-up, soybean looper is active in South Carolina. It usually appears in our area near the end of August.
Bill Cissel, Extension Agent – Integrated Pest Management; email@example.com
Congratulations Julie Knudson for correctly identifying the damage in the photo as soybean leafminer damage and for being selected to be entered into the end of season raffle for $100 not once but five times. Everyone else who guessed correctly will also have their name entered into the raffle. Click on the Guess the Pest logo to participate in this week’s Guess the Pest challenge!
Guess the Pest Week #17 Answer: Soybean Leafminer
By David Owens, Extension Entomologist, firstname.lastname@example.org
This week’s Guess the Pest is an interesting but rather unimportant member of the defoliating insect complex. The soybean leafminer (Odontota horni) adult is a beautiful red, flattened, rectangular beetle with red wings and prothorax and black head, antennae, legs, and a black stripe down the middle of the back. The black stripe doesn’t reach all the way to the end of the wings. Adults are active beginning around early to mid-June. They lay eggs on the underside of leaves, and the larvae immediately mine into the leaf. Larvae spend their entire lives between the upper and lower leaf surface, leaving a quarter sized brown blotch. When larvae complete development, they pupate in the mine. Immatures require 30 – 40 days to fully develop into adults. There is only one generation per year. Beetles will continue to lightly skeletonize leaves and over the course of their adult life might feed on the equivalent of one leaflet until they migrate out of fields to find overwintering shelter in late summer. Beetles and larvae are never present in any significant populations.
There are a couple of other beetle leafminers that you may see this summer. The most obvious and abundant is the locust leafminer, Odontota dorsalis. It pretty much stays confined to locust and can cause a large amount of locust defoliation by August. So, as you drive along the highway and notice trees with a brown cast to them, you may be seeing locust leafminer. Locust trees can handle the defoliation and leaf back out.
Some fields are experiencing borderline economic defoliation. The rain this week means that soybeans are growing again and that is going to help them. Many earlier planted fields in the reproductive stages and under irrigation have very good canopies. The primary defoliators are bean leaf beetles, Japanese beetles, and green cloverworm. Some fields are at the cusp of the 10-20% defoliation. Unfortunately, there aren’t very clear answers on the economics of treating vs not treating R-stage beans at this level of defoliation if going over the field with a fungicide. Dr. Dominic Reisig from NCSU cautions in his blog (https://soybeans.ces.ncsu.edu/2018/07/when-to-treat-earworms-in-soybeans/) that recent work in the Midsouth suggests that vegetative thresholds are very conservative. Other work from Virginia Tech stresses canopy development. If the canopy is thick, actively growing, and closed, beans can tolerate quite a bit of defoliation in the reproductive stages (https://pubs.ext.vt.edu/444/444-203/444-203.html). There is a very good extension bulletin from Purdue that discusses defoliation and for evaluating economics of defoliation and crop stage and may be of interest: https://extension.entm.purdue.edu/fieldcropsipm/insects/bean-leaf-beetle.php. If it is a concern for you and you have the flexibility, try a simple experiment on your farm. Treat the field as you normally would and on one or two of the last passes, put an insecticide in. Compare those areas at harvest; and let us know what you think.
Dectes is widespread. Spider mites were beginning to cause significant defoliation last week. Plants are recovering, but as Bill mentioned in his post last week, rain does not automatically ‘cure’ mite populations. Continue to check your fields and evaluate canopy health. Fields with large numbers of Dectes and that have had a history of lodging losses should be prioritized for timely harvest to reduce likelihood of foul weather causing lodging. Easier to say now in July than in October.
Japanese beetles, green cloverworm, bean leafrollers, and grasshoppers are the primary defoliators that we have seen in soybean samples. Bean leaf beetles are also starting to make a reappearance. Thresholds for pre-bloom full season beans is 30% and threshold for reproductive stage soybean is 15%. Double crop fields, especially dry fields, cannot tolerate as much leaf feeding as their full season counterparts.
Dectes stem borer continues to emerge from nearby overwintering sites in last year’s soybean stubble. You can find more information about Dectes stem borer in our fact sheet: http://extension.udel.edu/factsheets/dectes-stem-borer-management-in-soybeans/. Two-spotted spider mites also continue to move into fields, most in low numbers but with warm dry weather, a low number of mites can become a large number of mites in a short period of time. Thrips are also widespread, and abundant in hotspots in some fields. Treatment thresholds for thrips might be triggered if there are more than 8 thrips per leaflet, the beans are drought stressed, and visible ‘cupping’ or distortion is occurring.
Green stink bugs are present in reproductive stage soybean. Feeding is a concern for beans in the beginning pod to filled pod stages. Our general threshold for grain soybean is 5 bugs per 15 sweeps.
The usual defoliators are present in fields, albeit in low numbers this week. We are finding green cloverworms, bean leafrollers, grasshoppers, an occasional bean leaf beetle, thrips, Japanese beetles, and spider mites. First generation bean leaf beetle populations are dwindling, the second generation should emerge from the soil in late July – August.
Two spotted spider mites are showing up in more soybean fields. Mite populations are still low enough to warrant continued monitoring. With hot dry weather (didn’t think I would be saying that), populations can increase rapidly. As a reminder, most pyrethroids tend to ‘flare’ mites by removing natural enemies, although bifenthrin does have activity on TSSM. Zeal and Agri-mek are two other very effective, narrow spectrum products that have excellent spider mite activity. You can find our action thresholds and insecticide recommendations here: http://extension.udel.edu/ag/insect-management/soybeans/. In R-stage soybean, we are also picking up green stink bug adults, egg masses, and first instar nymphs. We picked up our first Dectes stem borer yesterday in a corn field that was soybean last season, emerging from its overwintering chamber in the soybean stubble.