EXTENSION 
INSTITUTE OF AGRICULTURE AND NATURAL RESOURCES         
UNIVERSITY OF NEBRASKA-LINCOLN 
NORTHEAST RESEARCH & EXTENSION CENTER       
601 EAST BENJAMIN AVENUE, SUITE 104
NORFOLK, NE 68701-0812
PHONE: (402) 370-4000 
FAX (402) 370-4010

No Newsletter Next Week

Due to the 4 th of July, we will not have a newsletter next week. The newsletter will resume the week of the 10 th. (KJ)

Current Soybean Aphid Situation in Nebraska

We have not seen the soybean aphid in Nebraska yet this year, but it is possible that there are a few fields that have small populations. We generally find a few in late June in Nebraska , and there have been soybean aphid reports out of Minnesota and a few other states during the last couple of weeks. In any case, if you have not begun scouting soybean for soybean aphid, now is the time to begin.

Soybean Aphid Description - The aphid is light green to pale yellow, less than 1/16 th inch long, and has two black-tipped cornicles (cornicles 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. It is the only aphid in North America that forms colonies on soybean.

Soybean aphid

Life Cycle and Injury to Soybean - The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year. It requires two different species of host plant to complete its life cycle, common buckthorn and soybean. Buckthorn is a woody shrub or tree and is the overwintering host plant of the aphid. Soybean aphids lay eggs on buckthorn in the fall. These eggs overwinter and hatch in the spring, giving rise to wingless females. These females reproduce without mating, producing more females. After two or three generations on buckthorn, winged females are produced that migrate to soybean. Multiple generations of wingless female aphids are produced on soybeans until late summer/fall, when winged females and males are produced that migrate back to buckthorn, where they mate. The females then lay eggs on buckthorn, which overwinter, thus completing the seasonal cycle.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are in the 70's through the mid 80's (degrees F). The aphids do not appear to do well when temperatures are in the 90's, and are reported to begin to die when temperatures reach 95 degrees F. When populations reach high levels during the summer, winged females are produced that migrate to other soybean fields. Like a number of other insect species (e.g. potato leafhoppers), these migrants can be caught up in weather patterns, moved great distances, and end up infesting fields far from their origin. These summer migrants were most likely the major source of infestations in Nebraska during the last couple of years.

Soybean aphids injure soybeans by removing 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 in itself makes leaves appear shiny. Soybean plants are most vulnerable to aphid injury during the early reproductive stages.

Soybean Aphid Occurrence in Nebraska - In much of the soybean aphid's range, significant aphid infestation has often occurred in the early vegetative stages. These infestations then undergo rapid population growth to reach high populations during the flowering stages (R1, R2). During the last few years in Nebraska , however, very few aphids have been found during the vegetative stages. We find a few in late June – early July, but it is usually mid-July before we begin to regularly find aphids, while soybeans are entering or in R3 (beginning pod stage). During 2003 and 2004, a few Nebraska aphid populations reached economically damaging populations in late July, but most reached economically damaging populations in mid to late August, while soybeans were in the mid-reproductive stages (R4-R5). During 2004 there were many fields where the aphid populations peaked in late R5 (beginning seed) to early R6 (full seed).

We did not have enough soybean aphids in 2005 to do a thorough threshold study, but in 2004 we did. A yield trial was conducted at the Haskell Agricultural Laboratory at Concord , Nebraska during 2004 that examined yield loss resulting from late July infestations of soybeans. It was part of a North Central Soybean Research Program Project that has been repeated in several states. In this study 16 soybean plots (irrigated) were infested with very low numbers of soybean aphids on July 23 and allowed to increase in population. Four additional plots were kept aphid free. The aphid populations peaked on approximately August 30, with average peak populations ranging from 952 aphids per plant to 3,634 aphids per plant. The results indicate that late season infestations can result in at least 20% yield loss (see graph). The major portion of the population curves and all population peaks occurred in soybean stage R5 to beginning R6. The data also indicate that the economic injury levels (EILs) would be approximately 1000 aphids per plant, depending on various factors including management costs and crop value. Because a farmer's goal should be to keep the aphid populations from reaching the EIL, the economic threshold will be lower than the EIL and should allow the farmer time to set management tactics in motion. (TH)

Soybean Aphid Management

•  Begin scouting soybean fields once or twice a week in late June to early July. Check 20 to 30 plants per field. Aphids are most likely to concentrate at the very top of the plant early in the season, and will move onto stems and within the canopy as populations grow and/or the plant reaches mid to late reproductive stages. As the season progresses, aphid numbers can change rapidly (populations can double in 2-3 days).

•  The current recommended threshold for late vegetative through R5 stage soybeans is 250 aphids per plant (field average) with 80% of the plants infested and populations increasing. This gives you about seven days to schedule treatment before populations reach damaging levels (if populations do not increase during these seven days, you may be able to eliminate or delay treatment). Determining if the aphid population is actively increasing requires several visits to the field. Factors favorable for aphid increase are relatively cool temps, plant stress (particularly drought), and lack of natural enemies.

•  Look for the presence of aphid natural enemies such as lady beetles, green lacewings, and other insect predators. Aphid “mummies” (light brown, swollen aphids) indicate the presence of parasitoids. These predators and parasitoids may keep low or moderate aphid populations in check (under 200 aphids per plant). One can often find soybean aphids by examining plants where lady beetles are observed.

•  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 treatment can be avoided.

•  If the plants are covered with honeydew or sooty mold, or stunted, an insecticide treatment may still be of value but the optimum time of treatment is past.

•  If fields are treated, leave an untreated test strip to compare against sprayed sections. This also provides a refuge for beneficial insects.

•  Good insecticide coverage and penetration is required for optimal control of soybean aphid, as many aphids feed on the undersides of the leaves and within the canopy. Use high water volume and pressure. Aerial application works well when high water volume is used (5 gallons of water per acres recommended).

•  Several insecticides are labeled for the soybean aphid. A list of registered insecticides, rates, preharvest intervals, and grazing restrictions can be found at http://entomology.unl.edu/instabls/soyaphid.htm . Pyrethroids have a longer residual. Chlorpyrifos has a fuming action, and may work well in heavy canopies or high temperatures.

•  Spraying flowering soybean poses a threat to honey bees. Inform treatment plans to nearby beekeepers and follow precautions to minimize honey bee kills. When there is concern about honey bees, pyrethroids are the better insecticide choice.

•  We do not generally recommend applying an insecticide at glyphosate application. In Nebraska this is usually before the aphids reach damaging levels, or are even in the field. Insecticide treatment at this time would simply rid the field of natural enemies. In addition, application methods for herbicides (e.g. lower pressures) are not optimal for good insecticide efficacy.

•  If soybean rust is present and being sprayed when soybean aphid thresholds also are met, a fungicide/insecticide tank mix should be effective because application methods for both require high water pressure for adequate penetration and coverage.

•  More information can be found at entomology.unl.edu or through your local UNL Cooperative Extension office.

Western Bean Cutworm

Western bean cutworm moths are being caught in our light trap at Concord . Numbers up to now have been low, but we have not yet reached peak flight. As moth numbers increase, mating will commence and the females will begin to lay eggs on corn. The appearance of the first moths provides a signal that farmers and crop consultants should begin to scout fields for the white, dome-shaped eggs. Western bean cutworm moths lay eggs in clusters of five to 200 on the top surface of the upper most leaf of a corn plant and on any leaf surface of dry beans. The eggs require five to seven days to develop, during which time the egg color changes to tan and then to purple immediately before they hatch.

After the small, dark brown larvae hatch on corn plant, they move to the whorl or tassel to feed on the tender yellow leaf tissue or on the tassel itself. Once the tassel emerges or if it has already emerged when the eggs hatch, the larvae will move to the green silks. The developing larvae will feed on the green silks moving down the silk channel until they reach the ear tip. The larvae will feed in the ear tip until they are fully developed. If the infestation on one ear tip is so great that the larvae become crowded, a few individuals may move outside the ear and begin to feed on the side of the ear.

Even though field scouting for western bean cutworm in field corn should begin when the first moths are caught, control decisions should be made shortly after the moth flight peaks. The moth flight usually peaks in early-mid July. When scouting for western bean cutworms in corn, check 20 plants in at least five areas of each field. Look for eggs on the top surface of the upper most leaf or look for larvae in the tassel. If 8% of field corn plants, 5% of seed corn plants or 5% of popcorn plants have egg masses or larvae, consider applying an insecticide. Herculex varieties appear to control the larvae very well, although not perfectly, and should not need treating.

Western bean cutworm moths prefer to lay eggs in corn plants that are in the late whorl stage compared to those that have completely tasseled. Pay particular attention to later planted fields or those with uneven development. Western bean cutworm eggs that hatch when corn plants are in the whorl stage of growth have a high rate of survival. The larvae are well protected in the whorl or tassel.

If an insecticide treatment is warranted in corn, it should be made when 95% of the plants in a field have tasseled. This timing of the application increases the chance that the worms will be exposed to the insecticide resulting in better control. Chemigation has provided very good control of this insect, even at lowest labeled rates. Asana, Ambush, Baythroid, Pounce, Lorsban, Capture, Mustang, Penncap-M, Proaxis, Warrior, and Seven are all labeled for control of western bean cutworm. Many generics are also available that will give good control. A list of registered insecticides, rates, preharvest intervals, and grazing restrictions can be found at http://entomology.unl.edu/instabls. (KJ)