July 16, 2010

Although we have yet to find soybean aphids in Nebraska soybean fields, it is time to start watching for them in earnest.

Recent high temperatures may have limited population growth of any that were present. Having said that, do not forget that if temperatures become moderate in July, aphid populations can increase very fast. In 2008 we had a relatively cool July (temperatures did not exceed 90º F at Concord), and aphid populations exploded in much of Nebraska. In any case, if you have not begun scouting soybean for soybean aphid, now is the time to start.

Soybean aphid Soybean aphids on a single leaf, 2002. In Minnesota, as many as 13,000 soybean aphids have been found on a single plant.

Soybean Aphid Description

The aphid is light green to pale yellow, less than 1/16th 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.

Life Cycle and Injury to Soybean

The seasonal life cycle of the soybean aphid is complex with up to 18 generations a year under optimal conditions. 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 70s through the mid 80s (degrees F). The aphids do not appear to do well when temperatures are in the 90s, 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 typically 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).

In Nebraska aphid populations can reach economically damaging populations in late July, but most reach economically damaging populations in August, while soybeans are in the mid-reproductive stages (R4-R5). Do not, however, rely solely on this pattern. Soybean aphids in Nebraska can be as variable as our weather. During 2004 there were many fields where the aphid populations peaked in late R5 (beginning seed) to early R6 (full seed). In 2008 we had a relatively cool July and aphid populations started reaching economically damaging populations two to three weeks early than normal. Last year (2009), we had very late developing populations that lasted well into September.

Soybean Aphid Management

1. Scouting. Scout soybean fields once or twice a week. 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 two to three days).
    
2. Economic threshold. 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. Depending on economic conditions, this gives you about five to 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.
   
   
3. Natural enemies. Look for the presence of aphid natural enemies such as lady beetles, minute pirate bugs, 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. (For more information on natural enemies and photos to aid in identification, see the June 29, 2007 CropWatch story.)
    
4. Winged aphids. 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.
   
   
5. Late treatment. 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.
   
   
6. Test strip. If fields are treated, leave an untreated test strip to compare against sprayed sections. This also provides a refuge for beneficial insects.
    
7. Coverage. 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).
   
   
8. Insecticide selection. Several insecticides are labeled for the soybean aphid. A list of registered insecticides, rates, preharvest intervals, and grazing restrictions can be found on a UNL Department of Entomology website at http://entomology.unl.edu/instabls/soyaphid.shtml. Pyrethroids have a relatively long residual. Chlorpyrifos has a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.
   
   
9. Bee safety. 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.
    
10. Not with glyphosates. 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.
     
11. Dual treatment. 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 on the UNL Department of Entomology website at entomology.unl.edu, the CropWatch soybean insect information page,  or through your local UNL Extension office.

Tom Hunt
Extension Entomologist, NEREC Haskell Ag Lab, Concord
Keith Jarvi
Extension Educator, Dixon, Thurston, and Dakota Counties