. Soybean aphids.

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.

Nebraska Observations 2003
In June 2003, extremely low numbers of aphids were found in Nemaha, Cass, Saunders, Douglas, and Burt counties. Aphids were found primarily in fields near wooded river bottoms along the eastern border of Nebraska. Common buckthorn, an overwintering host of the aphid, was found in wooded river bottoms throughout the eastern half of Nebraska. In mid-July, soybean aphid infestations began to be reported from northeast Nebraska. By late July soybean aphids could be found in all soybean production areas of Nebraska (eastern half of Nebraska), with almost all economically damaging populations being in the northeast portion of the state. Populations peaked in mid-August. Peak populations in the northeast ranged from less than 100 aphids/plant to approximately 5000 aphids/plant (field averages). Most infested fields in the northeast had low to moderate populations.

The pattern of soybean aphid colonization in 2003 was similar to 2002. Very few aphids were found until mid-July, with more fields per county infested in the northeast corner of the state. Mid-July colonization coincided with summer storm patterns having high northeast winds. However, population levels were much higher in 2003. In 2002 there were only two reports of fields being treated for soybean aphid. In 2003 many fields were treated in northeast Nebraska, although it is likely that many did not require treatment or were treated after economic damage had been done. A possible explanation for higher numbers in 2003 may be that in 2003 temperatures in the second half of July through the first week of August were rather mild (high 60s to low 80s), which favors soybean aphid reproduction. In 2002 temperatures were high during this period.

Various studies were initiated with support from the Nebraska Soybean Board to examine the effects of the aphid on soybean growth and yield, aphid ecology, etc. In general, if aphid populations reached thresholds and farmers treated in late July or early August, they benefited from treatment. If treatment occurred in mid August, benefit was variable and depended on aphid population size, population dynamics, and predator levels (primarily lady beetles). Late August treatments likely resulted in no benefit, as aphid populations naturally declined.

. Buckthorn Foliage.

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. Although a few aphids have been found this spring on a buckthorn plant in Lincoln, I have not found any on the buckthorn plants I have been monitoring in northeast Nebraska. It remains to be seen how successful the aphid will be at overwintering and colonizing soybean in Nebraska.

Soybean aphid populations can grow to extremely high levels under favorable environmental conditions. Reproduction and development is fastest when temperatures are between 70-80 degrees F. Aphids 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. I believe these summer migrants were the major source of infestations in Nebraska during the last two 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 appear to be most vulnerable to aphid injury during the early reproductive stages. Heavy aphid infestations during these stages can cause reduced pod and seed counts.

Soybean Aphid Management
The aphid is very new to North America and experience with the insect is limited. As we gain more experience with the aphid, recommendations will be refined and developed to manage the aphid under Nebraska conditions.

1. Begin scouting soybean fields once or twice a week in late June - early July. Check 20 to 30 plants per 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. As the season progresses, aphid numbers can change rapidly (populations can double in 2-3 days).
2. The current threshold for late vegetative to R4 stage beans with actively increasing aphid populations is 250 aphids/plant. This gives you about seven days to schedule treatment (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. Thresholds for R5-R6 have yet to be determined, but are likely in the 500-1000 aphids/plant range. Yield response to treatment has been documented during R5 and early R6, but not as consistently as when treatment occurs during R4 or earlier. Treatment after R6 has not been documented to increase yield in the field.
3. 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. One can often find soybean aphids by examining plants where lady beetles are observed. The presence of “fuzzy” aphid carcasses indicates fungal pathogens are present, which can lead to dramatic reductions of aphid populations.
4. 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. 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. If fields are treated, leave an untreated test strip to compare against sprayed sections. This also provides a refuge for beneficial insects.
7. Good insecticide coverage and penetration is required for optimal control of soybean aphid, as 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 acre recommended).
8. Several insecticides are labeled for the soybean aphid (Chinese aphid on some labels). A list of registered insecticides, rates, preharvest intervals, etc. can be found at http://entomology.unl.edu/instabls/soyaphid.htm . Pyrethroids have a relatively long residual, and work best at temperatures below 90 degrees F. Organophosphates have a fuming action, and may work well in heavy canopies or high temperatures. Dimethoate is least effective.
9. Spraying flowering soybean poses a threat to honey bees. Communicate 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.