Soybean aphid colonization of soybean began earlier this spring throughout Minnesota and the Midwest. While nearly every midwestern state reported their earliest calendar dates for aphid appearance in soybean, the colonization is phenologically on-track. Aphids are more prevalent in earlier planted fields, in sheltered fields or fields near buckthorn. Colonization from buckthorn is essentially done, so increasing infestation levels within fields will depend initially on local buildup and re-distribution. Besides the timing, intensity and duration of colonization, the aphid buildup depends on weather (rainfall, temperature), soybean (variety, phenology, drought stress) and natural enemies.

What's the status of soybean aphid in Minnesota?

As of last week, aphids were reported at low levels from most soybean growing areas in the state.

Crookston - Ian MacRae (NWROC) reports populations consistent, more uniform and higher than previously observed at this time of year. In this semi-drought area, the worst fields have 20-30% of plants infested with low number of aphids. The MDA survey also indicates relatively higher levels in NW Minnesota.

Fergus Falls - Locally heavy rains reduced aphid numbers about two weeks ago but aphid infestation levels have increased noticeably over the last week according Doug Holen. Aphids present in most fields with fields ranging from 17% of plants infested to nearly 80% of the plants with low aphid numbers (<20 per plant typically).

Lamberton - Heavy local rains, 8.19 inches since June 5 with 4.26 inches coming June 5, have largely eliminated local aphid populations near Lamberton according to Bruce Potter (SWROC). Nearby areas receiving less rainfall report aphids relatively easy to find and an intensively sampled early planted small field averaged 26% of the plants with aphids and 13 aphids/plant. Infestations were much higher on edges (up to 400/plant) but pockets of infested plants had started to develop in the field. Populations had roughly doubled over the past week and alatoid nymphs were abundant.

Rosemount - Early planted fields are nearing 50% of the plants with aphids at low infestation levels (<20 aphids per infested plant) say Ken Ostlie. Populations are sparse in later planted fields with less than 5% of the plants with aphids. Few winged aphids observed.

Jordan - Aphid populations in sheltered fields along the river are building. Dave Pfarr, county extension educator, called about a field near Jordan where 76% of the plants were infested with an average of 22 per plant on V3-V4 soybean. If this population continues to double every 3-4 days, it could reach treatable levels in 2-3 weeks.

What effects do recent rains have on soybean aphid?

Thunderstorms with heavy downpours have caught the news over the last two weeks. Anecdotal observations indicate heavy, driving rains (>1") on small soybean plants can drastically reduce aphid populations. These observations are backed up research with a rainfall simulator that found a 45% reduction in aphid populations 5 days after a 2" rainfall on V3-V4 soybean. The direct effect of rain diminishes as soybean plant size increases. Areas with heavy early season rainfalls are less likely to have large aphid populations first. With colonization from buckthorn nearly complete, later occurring infestations may still develop because of aphid immigration from other areas with less rainfall. Rainfall may also enhance activity of aphid-attacking fungi and relieve drought stress on soybean (reducing the nutritional quality of the plant to aphids).

When should soybean aphid scouting start?

Historically, scouting on a widespread basis before the end of June has not been needed in Minnesota. Soybean aphids leaving buckthorn can colonize very small soybean plants (V1-V2). However, it takes some time for SBA populations to colonize fields and reach yield-threatening levels. Aphid numbers on individual plants build up and then decline as winged aphids leave to colonize new plants. Meanwhile the frequency of infested plants continues to climb in a field. It's not until most plants in a field are colonized (>80%) that populations began to take off. Now's the time to pick an indicator field, or two, that you could scout on a regular basis to monitor aphid population dynamics.

Which fields should be scouted first?

Previous experience will probably suggest some local fields tend to have aphid problems earlier than others. Initial colonization from buckthorn reflects proximity and planting date. Fields in proximity to buckthorn, especially smaller fields tend to see earlier aphid population development. Earlier planting dates are initially colonized more intensively than later-planted fields. Another common observation is that heavier infestations may occur in fields with coarser textured soils and/ or with lower potassium levels. Moisture stress tends to favor early-season soybean aphid colonization and buildup. Regardless of the reason, you can use these "indicator fields" keep track of aphid population dynamics. Field borders are colonized more quickly than field interiors. Start out by monitoring field edges. A lack of aphids on field edges indicates a low probability of aphids in the field interior. Remember, windbreaks in the center of a field are an edge. Hot spots in a field may develop based on factor listed earlier but remember that you're making decisions for the whole field.

How should I scout for aphids?

Scout the outside rows of a field first. During vegetative and early reproductive stages, look for aphids on the upper two leaves and new leaves on any branches. Ants and ladybird beetles can top you off to the presence of aphids. Unless over 70% of the plants are infested, it's not worth scouting the rest of the field. If over 70% of the plants are infested, use speed scouting to rapidly assess whether the field should be treated. Field testing of speed scouting in 2005 revealed that it prematurely reached treatment decisions, typically ca. 160 aphids per plant rather than the threshold of 250 aphids per plant. To ensure the population is actually increasing, we recommend that the field be re-checked in 3 to 4 days.

How many soybean aphids are too many?

The economic or treatment threshold is reached when an increasing soybean aphid population averages 250 aphids/plant with at least 80% of the plants infested. This threshold refers to a field average, and not field borders or hotspots. The economic threshold is reached well in advance (7 days or more) of when cumulative aphid injury causes yield loss. That leaves some time to verify that populations are increasing and accommodate minor treatment delays from busy applicator schedules or adverse weather. The threshold has been verified under many growing conditions and from vegetative soybean through seed set (R5). Anecdotal yield losses may be also reduced by insecticide applications to heavy aphid infestations during seed fill (R6), but no research has been conducted at these later soybean stages.

Should the treatment threshold be lowered for small plants?

Soybeans have a tremendous capacity to compensate for early season stress. There is no data to suggest that threshold should be lower for young plants. It's not uncommon to see isolated young plants or hot spots where aphid populations exceed threshold, but these populations need to be prevalent throughout the field and persistent to justify insecticide use. On the other end of the season, data suggests that the threshold should be increased as plants near maturity (R6).

What about insurance treatments against low-level infestations?

Insurance treatments for soybean aphids appeal to growers for several reasons: previous losses with tardy aphid control, anxiety about waiting for inevitable, avoidance of later-season infestations, protection of plant health, convenience of tankmixing with a post-emergence herbicides, such as RoundUp. We see little utility for insurance treatments against soybean aphid. Aphid populations vary widely from field to field, as does the timing of infestations. Applying insecticides to unscouted or low-level infestations is a recipe for problems. First, fields may not need treatment so growers waste their money. Second, early applications do not prevent subsequent colonization since any residual toxicity or repellency is gone in less than a week. There is only a temporary reprieve in scouting effort. Don't walk away from any sprayed field for the rest of the season. Third, early applications eliminate natural enemies and may actually increase the chances of needing to treat later in the season. Fourth, unnecessary insecticide use enhances the chance that insecticide resistance will develop in aphids or other soybean pests, such as two-spotted spider mites. My soybean seed was treated with an insecticide. Do I still need to scout? Yes. Research indicates that direct toxicity of neo-nicotinoid seed treatments and effects on aphid reproduction are gone within 40-50 days after planting. While these seed treatments suppress early-season buildup of aphids, they provide no protection against later colonization. Yield-limiting populations can develop in fields treated with neo nicotinoid (e.g. Cruiser, Gaucho) insecticides after mid July. These fields should still be scouted.

How often do I need to scout?

Under ideal conditions aphid populations can double every 2-3 days. Using this maximum reproductive rate allows an estimation of how frequently a field should be scouted. For example, a field with an average of 100aphids/plant can reach economic threshold in 2-3 days. Barring a large number of aphids immigrating into the field, a field with 10 aphids/plant average should not reach economic threshold for 10 days.

Can I mix an insecticide with herbicide?

Insecticide herbicide tank mixes can work if the timing is correct and soybean canopies are still open. Remember that the optimum water volume for aphid control is higher and pressure higher than for drift free herbicide applications.

Bruce Potter
Southwest Research and Outreach Center
and
Ken Ostlie, U of MN Dept of Entomology
 

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