Early-Season Scouting
for Soybean Aphid -
By
- Bruce Potter, SWROC and Ken Ostlie, Dept of Entomology
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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?
Aphids are now 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.
