Soybean Aphid Scouting and Management

July 27, 2015 Leave a comment


By John Wilson, Extension Educator  

Soybean aphids have been found in recent field surveys in northeast Nebraska. I checked a few local fields and found an occasional plant with a few (less than 10) aphids/plant. These numbers are extremely low, which is typical for this time of year, but it does signal it’s time to start scouting.

Aphids will be found on the newest leaves at the top of the plant first. Ths indicates they recently colonized the plant, probably within the last week. The good news is the soybean aphid’s natural enemies have also been found in these field surveys, so they may hold the populations in check, or at least slow their population growth.

Relatively mild weather, between 70 and 85F, favors soybean aphid development, so make sure to check fields at least once a week. Soybean aphid population growth can be quite rapid, and regular monitoring of soybean aphid populations is key to effective management.

Soybean Aphid Description

The soybean aphid is soft bodied, light green to pale yellow, less than 1/16 inch long, and has two black-tipped cornicles, or tailpipes, on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue on the undersides of leaves near the top of recently colonized soybean plants. Later in the season the aphids can be found on all parts of the plant, feeding primarily on the undersides of leaves, but also on the stems and pods.

Soybean Aphid Injury to Soybean

Soybean aphids injure soybeans by sucking out plant sap with their needle-like mouthparts. Symptoms of soybeans infested by soybean aphid may include yellowed, distorted leaves and stunted plants. A charcoal-colored residue also may be present on the plants. This is sooty mold that grows on the honeydew that aphids excrete. Honeydew by itself makes leaves appear shiny. Soybean plants appear to be most vulnerable to aphid injury during the early reproductive stages.

Aphid Scouting Methods

The economic threshold for late vegetative through R5, or pod fill, stage soybeans is 250 aphids per plant with 80% of the plants infested and populations increasing. Begin scouting soybean fields once or twice a week now. Check 20 to 30 randomly selected plants in various areas of each field.

Aphids are most likely to concentrate at the very top of the plant, although they will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. If a tree line or woodlot is adjacent to the soybean field, make sure and include a few sampling locations near these areas. Soybean aphids are often found first in the parts of soybean fields near wooded areas.

Counting aphids is not as difficult as it may at first seem. First, walk to a random spot in the field. Pull a plant and turn it upside down and give it a quick scan to see where the aphids are located. Get a feel for what 10 or 20 aphids look like and count by 10s or 20s.

The current threshold for late vegetative through R5 stage soybean is 250 aphids/plant with 80% of the plants infested and populations increasing. Thresholds for early R6 have yet to be determined, but are likely in the 400-500 aphids/plant range. Insecticide treatment done during or after mid-late R6 has not been documented to increase yield.

Soybean Aphid Management

Look for the presence of aphid natural enemies such as lady beetles, green lacewings, insidious flower bugs, aphid mummies, fuzzy aphids, and other insect predators. Predators and parasitoids may keep low or moderate aphid populations in check. One can often find soybean aphids by examining plants where lady beetles are observed.

Note whether the plants are covered with honeydew or sooty mold, or stunted, and aphids are still present at threshold levels. An insecticide treatment may still be of value but the optimum time for treatment has passed.

Good insecticide coverage and penetration is required for optimal control of soybean aphid because aphids feed on the undersides of the leaves and within the canopy. For ground application use high water volume (15 gallons/acre) and pressure (30 psi). Aerial application works well when high water volume is used (3 gallons/acre).

Several insecticides are labeled for the soybean aphid. Pyrethroids have a relatively long residual, and work best at temperatures below 90ºF. Organophosphates have a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.

Soybean aphids, if they reach economic thresholds, usually do so and require treatment in late July through August. One treatment during this period usually is enough to keep aphid populations from resurging because there is not enough time for populations to build-up before they would naturally leave the fields in late August and early September. The earlier a field is treated, the greater the chance that any surviving aphids can later reproduce or new aphids can repopulate the field.

Remember, insecticide treatment also kills many natural enemies, so any aphids that do re-infest a field are not constrained by predators and other natural controls. Even insecticides with a relatively long residual cannot last when insecticide treatment is done in early or mid-July, particularly during a year when aphid populations are thriving. If one has to treat early, make sure to closely monitor the field until early September.

Another practice that can result in aphid population resurgence is unwarranted insecticide treatment, either because fields were treated well before the threshold was met or fields were treated along with a herbicide (in some cases a fungicide), regardless of aphid presence. These treatments kill natural enemies and are usually done relatively early so there is plenty of time for aphids to resurge, or re-colonize a field.

Aphid populations below or even at the economic threshold do not cause yield loss, so treating before populations reach 250 aphids/plant only increases the probability of aphid resurgence. In addition, we have observed that many fields support a non-increasing, low population of aphids (e.g., less than 100 aphids/plant) through August. Treating these fields would be a waste of time and money.

Tank-mixing insecticides with glyphosate or other herbicides can be problematic because application methods for herbicides (e.g., lower pressures, large droplet producing nozzles) are not optimal for good insecticide efficacy. Tank-mixing with fungicides can be effective because application methods for fungicides and insecticides require high water pressure for adequate penetration and coverage, however, only conduct this practice IF soybean aphid thresholds are met.

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Scout for Soybean Aphids and SCN!

July 25, 2015 Leave a comment


By John Wilson

Extension Educator 

Last week I was scouting fields for soybean aphids and made an unintended discovery. As I pulled up soybean plants to examine the undersides of the leaves for soybean aphids, I had the roots of the plants literally right under my nose. In one of my neighbor’s fields, I noticedcysts from the soybean cyst nematode, or SCN, on the roots of the plants. This is significant because SCN causes more yield losses for Nebraska and U.S. soybean growers than any other pest.

            So when I got back to the house, my phone call went something like this. “Hi Ralph (not his real name – LOL), I just checked your field for soybean aphids and I have good news and I have bad news. The good news is I didn’t find any aphids. The bad news is, I found SCN.” He knew he had SCN in other fields, but not in that one, so he thanked me and will start managing that field for SCN the next time soybeans are planted there. 

            This is the time of year to examine roots of soybean plants for SCN. Cysts will develop on soybean roots about a month after the soybeans emerge. It’s especially important to check if you see problem areas developing in your soybean fields that you can’t explain otherwise. If you have pockets in a field with sudden death syndrome, or SDS, be sure to check these areas for SCN. SDS enters the soybean plant through the roots and is more likely to occur where feeding damage from SCN has occurred.

            The cyst is the only stage in the life cycle of the SCN that can be seen without a microscope. Look for a small, lemon-shaped, white to cream colored “bump” on the outside of the root. These may be confused with the larger, darker brown nodules which contain the nitrogen-fixing bacteria that are normally found on a soybean root.

            The cyst is much smaller, oblong and is lighter in color than nodules. When scouting fields, if you don’t find any cysts, you can not be sure that your field is SCN-free. However, if you DO find cysts, you know you have this pest and need to take action to reduce its buildup in the soil and the yield loss that can occur. That’s why it’s a good idea to take an extra 15 seconds to examine the roots of a soybean plant when you are checking plants for soybean aphids. It takes very little time, but could pay big rewards.

            For a more definitive determination if you have SCN, you need to take a soil sample, much the same as you would sample for fertilizer recommendations. Take 15-25 cores from a field, mix them together, then take a small sample from this mixture. In fact many farmers collect a few more soil cores when sampling for their fertilizer recommendations, then split the sample and send half in for fertility recommendations and the other half for SCN analysis. The Nebraska Soybean Board sponsors a program that pays for the cost of analyzing soil for SCN ($20/sample). You can pick up bags for this free analysis from your local Nebraska Extension office.

            In 2006 through 2013, the University of Nebraska conducted 29 trials looking at the yield advantage of resistant varieties over susceptible varieties in SCN-infested fields. Statewide, there was a 6 bushel yield advantage to the resistant varieties. But if you looked at the yield response in the 5 trials conducted in northeast Nebraska, the yield response was over 12 bushels/acre, double the statewide average. That is why it is so important for soybean growers to check for SCN in this part of the state.

            In almost all of the plots on infested sites over the years, you could not distinguish between susceptible and resistant varieties. Susceptible varieties were not yellow or stunted, all plants looked dark green and healthy. That is why it is so important to scout for SCN now. Many producers have a 10-20% yield loss (or more!) and are not aware that they have fields infested with SCN.

            With other pests, it’s easy to see damage such as shot holes, plants cut off, or leaf and pod feeding. However, with SCN, you may not see any damage to the plants at all! Frequently, the first indication that something is wrong with your soybeans is at harvest. Yields hit a plateau or even drop back for no apparent reason such as weather, weeds, insect or herbicide damage, while corn yields continue to improve in the same field. This isn’t a guarantee that there is SCN in the field, but it should be one of the first things you check. So you do the math, a six bushel/acre yield increase at today’s prices could put a lot of money in your pocket at the end of the season by managing for SCN.

            For more information on scouting for and managing SCN, contact your local Nebraska Extension Office.

John Wilson

John Wilson

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Time to Check For Aphids

August 29, 2014 Leave a comment


By John Wilson, Extension Educator

It’s mid-August and time to check fields for soybean aphids if you haven’t done so already. Even if you checked your fields a week ago, check them again. This is the time of year that aphid populations can build up rapidly. It’s important to remember aphid populations vary greatly from one field to another. So don’t make an insecticide application just because your neighbor did.

I was in a couple soybean fields recently and checked them for aphids. We’ve been fortunate this year that, although there have been a few aphids around, they haven’t built up in our area. Occasionally I’d find a pocket in a field with lots of aphids, but on average, the whole field was below control thresholds. However, I’ve heard reports from other areas in northeast Nebraska that have significant soybean aphid numbers and some fields are being treated in these areas. I’d target my scouting to late planted or replanted fields. These would be the ones most likely to have an aphid problem.

The aphid is light green to pale yellow, less than 1/16th inch long, and has two black-tipped cornicles… which look like tailpipes… on the rear of the abdomen. It has piercing-sucking mouthparts and typically feeds on new tissue near the top of soybean plants on the undersides of leaves. Later in the season the aphids can be found on all parts of the plant.

The only way to know if you need to treat is to check your fields and determine how many aphids are attacking your soybeans. You need to check plants in at least five different locations in a field and determine what is the average number of aphids per plant. It’s not unusual to have a hot spot in one area and very few in another, so get counts from different areas in the field and then average them together.

The threshold for treating is if you have an average of 250 or more aphids per plant and the population is increasing on plants up to R5, or the early pod fill growth stage. You won’t have yield losses at this level, but it should give you 5 to 7 days to make an application before yield losses occur… usually around 650 aphids per plant. If it’s going to take a commercial applicator a few days to get there, continue to monitor your aphid counts. If they level off or start to drop, cancel the application. It’s not unusual for aphid populations to crash around the first of September, but sometimes earlier.

Also, as the plant matures past R5, or if you have pods filling at the top of the plant, you can have higher aphid populations, probably in the range of 400 to 500 aphids per plant, before you need to be concerned about treating. In late August, we usually start getting milder temperatures which is more favorable for aphid populations to increase. So now is the time to get out and check your fields.

A couple final thoughts when scouting for aphids…

 

  • When counting aphids, make sure you only count live aphids. As aphids grow, they shed their skins and these often are left hanging on the under sides of leaves. These are almost white in color and do not move around.
  • Look for the presence of aphid natural enemies such as lady beetles, minute pirate bugs, and other insect predators. Aphid “mummies,” or light brown, swollen aphids, indicate the presence of parasitoids. These predators parasitize aphids and may keep low or moderate populations in check.
  • Look for the presence of winged aphids. If the majority of aphids are winged or developing wings, the aphids may soon leave the field and an insecticide treatment may be unnecessary.

For more information on soybean aphid control, contact your local UNL Extension office.

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