Soybean Aphid

Agdex#:	622/141
Publication Date:	07/04
Order#:	04-059
Last Reviewed:	07/04
History:	Original Factsheet
Written by:	Tracey Baute -Entomology, Field Crops Program Lead/OMAFRA

Table of Contents

1. Description and Life Cycle
2. Crop Impact
3. Yield Losses
4. Soybean Mosaic Virus
5. Control Options
6. Soybean Aphid Management Recommendations
7. Natural Enemies
8. For More Information

Soybean aphids, (Aphis glycines, SBA) have made their presence known in Ontario since first found in 2001. Suspected to have been introduced into North America from Asia as early as 1995, these small sap-sucking insects have caused significant yield loss in Ontario in both 2001 and 2003. Found in 21 states and 3 provinces, it has become yet another concern for soybean growers.

In response, Canadian and U.S. researchers are working together to find solutions to this new invasive species.

Figure 1. Close up of soybean aphid. Note black-tipped "tailpipes".

Description and Life Cycle

The soybean aphid is a small, pale yellow aphid with black cornicles ("tailpipes") and a pale yellow tail (Figure 1). This insect has 2 primary hosts that it requires to complete its life cycle (Figure 2). The soybean aphid survives overwinter as eggs on the twigs of buckthorn species (Figure 3 and Figure 4). In the spring, nymphs hatch from these eggs and the aphids undergo two generations as wingless females on buckthorn before the third generation develops winged adults for migration to soybean plants.

The aphids then continue to produce wingless generations until the soybean plants become crowded with aphids. Once crowded, winged forms are produced to disperse to less crowded soybean plants. There can be as many as 18 generations of aphids per year.

Like most aphids, the soybean aphids are all female, born pregnant and give birth to live nymphs. Males are only born in the fall so that the females and males can mate to produce the overwintering egg.

To date, no overwintering populations of soybean aphids have been found in Ontario. Factors including short season varieties and fall and spring predation by ladybird beetles on buckthorn may contribute to this. Studies indicate that soybean aphids are quite capable of dealing with Ontario winter temperatures and are known to overwinter in Michigan and other Northern states. Massive migration of winged adults from these states can bring populations into Ontario as early as the first week of June.

Crop Impact

Aphids have a tube-like mouthpart that sucks juices and nutrients from the plant. Plants may yellow and become stunted, reducing pod and seed production if aphid populations become extremely high. Also, aphids can only digest 10% of the nutrients they take in. The rest is secreted out their "tailpipes" as honeydew. This sticky honeydew, in the absence of rain, collects on the surface of the leaves. The honeydew acts as a substrate for sooty mould to develop which turns the leaves black and rubbery (Figure 5).

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Figure 2. Soybean aphid life cycle.
(Adapted from D. Voegtlin)

Buckthorn Characteristics

• small tree to large bush along outer perimeter of woodlots
• note parallel veins
• dark green leaves
• last bush to lose its leaves in the fall
• dark blue to black "cherry like" berries in fall

Figure 3. Close-up of buckthorn. Note parallel veins on leaves.

Figure 4. Twenty-foot tall buckthorn bush on outer perimeter of conservation area.

Figure 5. Sooty mould causes leaves to appear black.
(A. Schaafsma)

Yield Losses

Data from Ontario and the U.S. indicate that yield loss is related to aphid density, crop stage and presence of additional stress factors. Results in 2001 showed inconsistent yield response in spray trials, indicating that several stress factors were involved. On-farm strip trial results from Ontario in 2003 demonstrated that a properly timed insecticide application could result in an average yield benefit of 10 bu/ac (Figure 6). Stress during the pod fill stage (R3) and beyond resulted in smaller seed size.

Yield loss potential is still greatest when aphid populations peak at the flowering stage (R1-R2) because of the impact on pod set. Insecticide applications too early can cause rebounding of aphid populations to levels that may require a second spray during the pod fill stages. Fields under other stresses such as soybean cyst nematode or drought can potentially be more affected by aphid infestations. Late planted fields tend to experience higher aphid infestations as summer migrating adults appear to favour the younger crop stages.

Figure 6. On-farm strip trial results from Ontario in 2003. A positive yield advantage indicates that the yield in the sprayed strips were greater than the yield in the unsprayed check strips. Growth stages at time of spray are also included.

Text equivalent

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Soybean Mosaic Virus

Soybean aphids can also vector viruses into soybeans, in particular, soybean mosaic virus (SMV). Leaves of infected plants are distorted, wrinkled and puckered and have a typical mosaic pattern that is most evident on younger leaves (Figure 7). Infected seeds have a characteristic brown or black discoloration extending in streaks from the hilum region. Virus symptoms are often confused with hormonal herbicide injury. Insecticides are not an effective means of reducing virus transmission by aphids. Planting disease-free seed helps to reduce the incidence of the disease.

Figure 7. Puckering and wrinkling are symptoms of plant viruses.

Control Options

Chemical Control

Dimethoate is the only registered product available for the control of aphids in soybeans in Ontario. As with other organophosphates, there may be some advantage to using Cygon and Lagon when under higher temperatures. Emergency use registration has been granted in the past in Ontario for a pyrethroid product that has shown more consistent control and yield response. Research is underway on testing various insecticides in hopes of having several registered products in place as a sound resistance management strategy.

The greatest concern when spraying aphids is the potential for rebounding, where populations return to levels at or higher than what was there before spraying. Spray according to the Soybean Aphid Management Recommendations (below). Spraying aphids as soon as they appear in a field, before action thresholds are reached, will increase the risk of rebounding and can reduce the likelihood of seeing any yield advantage to insecticide application. Economic benefit of spraying beyond the R4 stage cannot be guaranteed, as research results after that stage have been inconsistent.

If an Emergency Use registration occurs or a new product is registered, information will be posted on the OMAF soybean web page.

Soybean Aphid Management Recommendations

(Consensus recommendations developed by Ontario and U.S. researchers, January 2004)

1. Scout once or twice a week beginning in late June or early July. Fields near buckthorn, the overwintering host, may be colonized at emergence and require earlier scouting. In areas without buckthorn, winged aphids migrate from other areas in mid-season. Check 20 - 30 plants per field. Examine the entire plant, particularly new growth. Scout late-planted fields closely.

2. Use and action threshold of 250 aphids per plant if populations are actively increasing. In replicated research trials, this threshold has worked well in late vegetative (right at first bloom) to R4 soybeans. This threshold incorporates an approximately 7-day lead-time between scouting and treatment to make spray arrangements or handle weather delays. Spraying after R6 has not been documented to increase yield, especially if the crop has grown well through the vegetative stages.

   To determine if an aphid population is actively increasing, check over several visits. Once you have reached 250 aphids per plant, scout field as often as every 2 days. Conditions that favour aphid population growth are cool temperatures, plant stress - particularly drought stress - and a lack of aphid predators.

   • Check for mummies (parasitized aphids, Figure 13) and for winged females. Do not spray if mummies are numerous, or if a majority of the aphids are winged or developing wings, an indication that the aphids will soon leave the field.

   • Plants are likely to be considerably above aphid thresholds if stems or pods are covered with aphids, and honeydew and sooty mould cover the bottom leaves. Insecticide treatment is probably still of value, but the optimal time for treatment (greatest economic return) has past.

3. Consider the product choices for your situation. Aphid kill, residual, and yield gains are not consistent among individual insecticides.
   
   

   • Be aware of the days to harvest interval for registered insecticides. Spraying after R4 may pose residue problems.

   • Good coverage is important. High spray volumes and high pressure help to move the insecticide down into the canopy.

4. Leave an unsprayed check strip to compare against sprayed areas to determine the performance of the insecticide and the value of the treatment.

5. Spraying at early reproductive stages poses a threat to bees. Communicate treatment plans to beekeepers and follow precautions to minimize bee kills. In areas with concern about honeybees, pyrethroids are a better choice for application than other formulations.

Excerpt from Plant Health Initiative Soybean Aphid web site.

Natural Enemies

A long-term strategy for the control of soybean aphid is biological where native and/or introduced predators, parasites and pathogens help to control aphid populations. In fact, because of the strong presence of these natural enemies soybean aphids are only a sporadic pest in their country of origin.

Research conducted in Ontario has found several predators that are playing key roles in keeping the province's aphid population down. Ladybird beetle adults and larvae (Figure 8 and Figure 9), damsel bugs (Figure 10), minute pirate bug nymphs and adults (Figure 11), syrphid fly larvae (Figure 12) and spiders are the most abundant predators found in soybean fields. Aphid mummies (Figure 13) were also discovered in 2003, indicating that tiny parasitic wasps are laying their eggs inside the aphid body, leaving only an empty aphid carcass once the offspring emerge. A natural pathogen found in the soil is also quite effective at killing these aphids. The pathogen only needs moist, warm conditions to become established and develops as a pink fungal mass on the aphid before it explodes and spreads the spores onto other aphids. Its potential for Ontario is under investigation.

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Figure 8. Multi-coloured Asian ladybird beetle adult feeding on soybean aphids.
(Marlin Rice, Iowa State University)

Figure 9. Ladybird beetle larva.
(Howard Russell, MSU Diagnostic Services)

Figure 10. Damsel bug.
(Oklahoma State University)

Figure 11. Minute pirate bug adult feeding on a thrip.
(University of Maryland)

Figure 12. Syrphid or hoover fly larva.
(University of Georgia)

Figure 13. Aphid mummy. Carcass of aphid left after parasitic wasp has emerged.
(University of Florida)

Insecticide applications do kill these natural enemies and may result in rebounding of aphid populations in their absence. Soybean aphids are born pregnant, do not need to mate and give birth to live young, giving them an advantage over their natural enemies who have to mate, lay eggs and need time to mature to their feeding stages (larva and/or adult). It is therefore recommended that management decisions also consider the presence of these natural enemies. Further research is being conducted on these natural enemies to better understand their response in the presence of soybean aphids in hopes of using thresholds that consider their contributions.

For More Information

• Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA)
• OMAFRA Crop Pest Newsletter
  
• Ontario Soybean Growers
  
• OMAFRA Field Crop Specialists CropLine 1-888-449-0937

This Factsheet was created in part using research results funded by the Ontario Soybean Growers and AAFC Matching Investments Initiative. We would like to thank the Ontario soybean growers and agricultural consultants who provided us with their 2003 strip trial results. The author would like to acknowledge the following researchers for their contribution: Wendy Asbil Kemptville College, University of Guelph, Horst Bohner, OMAFRA, Bruce Broadbent, AAFC-London, Robert Foottit, AAFC- Ottawa, David Hunt, AAFC-Harrow, Peter Mason, AAFC-Ottawa, Francois Meloche, AAFC-Ottawa, Roberto Michelutti, AAFC-Harrow, Gilles Quesnel, OMAFRA, Art Schaafsma, Ridgetown College, University of Guelph, Mark Sears, University of Guelph, Albert Tenuta, OMAFRA, Andrew Welsman, University of Guelph and honorary members Chris DiFonzo, Michigan State University and Michele Roy, MAPAQ.

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