action or reflection? As they said in Hitchhikers Guide to the Galaxy….
heard from the field last week …
are present in unifoliate to V3 beans.
per plant vary 6-10/plant, some higher.
Natural enemy populations variable: Some fields no natural enemies (lady
bugs et al) observed, other fields modest numbers of natural enemies
drought conditions in some areas.
growers concerned over need / value of a preemptive insecticide
above scenario prompted a review of resources on SBA management. In
addition, I shared a few Soybean aphid questions with our Midwestern
entomology colleagues who have had considerably more experience with
this insect than we have had in NY.
that soybean aphids have only been in the US since 2000, first
documented in NY in 2001, so our pool of data on it is somewhat limited.
While we have not had many years to watch these insects in action, this
is not the first time we have had soybean aphid (SBA’s) in NY beans this
early. Fortunately, in those situations SBA populations never exceeded
our threshold guidelines. Don’t worry at all? Too early to really tell.
Fortunately, we have worked with SBA's in NY and there is also a
considerable body of information available to us from Land Grant
University research conducted in the Midwest US soybean belt. Based on
previous research and our collective experience, we have reasonable
insights. Now is the time to monitor fields to watch for SBA’s and
document what’s going on. The following is an overview of some pertinent
SBA information and some insights our colleagues provided.
Factors Influencing Soybean Aphid Population Dynamics in Soybean
of colonizing population from buckthorn
Soybean variety and quality
Mortality from: Predators (such as lady bug beetles, lacewing larvae, syrphid fly larvae, and minute pirate bugs), parasitic wasps that cause
aphid mummies, fungal disease outbreaks, heavy thunderstorms
Temperature effects on reproductive rate and survival
re-distribution by winged aphids among fields
Dispersal of alates from fields
of on-line publications describe these factors in more detail. This
particular list is from:
MN Soybean Production
of this article for more soybean aphid information sources.
threshold guideline - 250 per plant at or near R1.
action threshold should be based on an average of aphids per plant over
20-30 plants sampled throughout the field. Regular field visits are
required to determine if aphid populations are increasing. Midwestern
research has found that treating earlier than 250 per plant threshold
guideline in most cases does not pay for itself. But if you have aphids
at flowering, the SBA population is increasing (few or
insufficient numbers of natural enemies present), then yield loss is to
pod abortion and once pods are gone, there is no recovery of yield other
than getting seed a bit bigger. This recommendation from the Midwest has
held up well over the past 4 years.
insecticide treatments now (seedling stage, sub 250 SBA
threshold) pay dividends down the road? Dr. David Ragsdale, entomologist
UMN, recalls data from two aphid outbreak years where a single SBA
treatment at V1 or V3 “resulted in no improvement in yield“.
possible retort… "I'm going over the field with Round Up and the custom
application is $9/acre so why not spike the RoundUp with an aphid
insecticide?" The question is a matter of NEED rather than convenience.
The data we have to date has not shown benefits from insecticide
use at this time. Insecticides cost money. Use them when needed but save
them for when you’ll get the most return on your investment. Several of
the websites at the end of this article discuss this in more detail.
Also, consider the other possible implications should insecticides be
Impacts on natural enemies and why you should care...
past 4 years, our NY SBA research with Drs. John Losey and Ann Hajek
(Entomology, Cornell) has documented good diversity and numbers of
natural enemies in our soybean fields that have SBA populations.
Ladybugs, nabids, carabid beetles, parasitoids and fungal pathogens all
have been found in our NY soybean fields and are known to do a pretty
good job of holding SBA populations in check. Yes, their numbers might
be low in the initial part of the season but their numbers can pick up
fast and…. aphid populations can be very attractive to hungry predators.
Ragsdale says their observation in Minnesota has been aphid populations
below 50 to 100 per plant can be held in check with Natural Enemies
(NE). When aphids are as dense as 50 per plant coccinellids (lady bugs)
will begin to lay eggs, syrphids pay attention, Orius (minute
pirate bug) does its thing, as do a raft of generalist predators. “Take
out aphids and all the NE at V1 and aphids will continue to colonize and
do so in "enemy-free space". A grower who treats with RoundUp and an
insecticide now may be risking a need for a second insecticide treatment
later this season.. Pause, reflect, delay and treat if necessary at the
O’Neil, entomologist from Purdue says: “The generalists (predators)
in fields can do a pretty good job of preventing & delaying SBA
outbreaks. Avoiding sprays below 100 aphids per plant helps give the
natural enemies time to work & the higher SBA densities act to bring in
more predators (and different species.). Spraying below 100 (or the 250
threshold for that matter) holds promise of creating a natural
"enemy-free space"--an open invitation to yearly outbreaks.
Grower-education on the role and potential of natural enemies can be an
effective counter-point to "get 'em early' mentality inherent in tank
Insecticide Use Now and Unexpected Impacts
conditions can be favorable to spider mite problems in soybeans. Dr.
Ragsdale and Dr. Chris DiFonzo (entomologist Michigan State U) urge
caution in selecting insecticides under these conditions. Some SBA
registered insecticides (such as Warrior, Mustang Max, Pyganic, and
others) can also flare spider mite populations.
Many aphid species are resistant to insecticides. Multiple or poorly
timed insecticide applications favor development of resistance. Scout to
ensure insecticides are needed and well timed.
will soybean aphid populations do over the next two months?
time will tell. Over the weekend a number of places across NY
experienced thunderstorms and heavy rains. These have likely had
devastating effects on SBA populations as the tiny insects were pounded
off the seedlings. This also cancelled, at least for now, drought
concerns. If you had fields with SBA concerns last week, recheck them.
practical advice from Drs. David Voegtlin, (IL Natural History Survey)
and Robert O'Neil, (Purdue U) from an article: “Planning for the 205
Soybean Aphid Population” This is a very good overview of the
Midwestern SBA management perspective.
visits to soybean fields to examine even a few plants will help growers
keep abreast of aphid population growth in their crop.
the 250 aphids per plant threshold. To use the threshold properly, it's
important to know if the aphid population is increasing and that's why
regular field visits are necessary.
panic! Just because there are some aphids in the field does not mean SBA
populations will reach the threshold of 250 aphids per plant.
natural enemies in their field are providing free pest control in most
fields and in most years.
Spraying fields before the threshold only helps to disrupt the control
these natural enemies provide and does not guarantee economic control.
temptation to "tank-mix" a little insecticide with an herbicide (or
fungicide) application "to get those aphids before they get me" is to be
Looking for aphids, checking the threshold, and then deciding if you
need to do something, is the key for this year's aphid pest management.
Concluding remarks regarding soybean aphid treatments?
see. We are quite a ways off from 250 aphids per plant and reproductive
soybean growth stages. It is possible that we may need management
intervention later in the season. For now, continue to monitor fields
for soybean aphids and for signs of soybean rust. Also know that there
are lots of people looking for the aphid across our region and the
Midwest and they are networked. If significant changes occur and better
information becomes available it will be made available asap through
local Extension Service and state Land Grant Universities. Keep in
touch, keep informed, and stay tuned.
someone is truly risk averse and feels compelled to treat.... if at
all possible...set up, document, and evaluate a side by side
treatment comparison….. We could all benefit from the data!
information and further discussion on soybean aphids check out:
statement at this site regarding tank mixtures (herbicide, insecticide,
fungicide) and implications for sprayer specifications, nozzle size,
droplet size, spray volume..
Soybean Aphid in Minnesota
Managing Soybean Aphid
U Wisconsin website has had several SBA
for the 2005 Soybean Aphid Population
Voegtlin, Center for Ecological Entomology, ILNatural History Survey;
and R. O'Neil, Dept. Entomology, Purdue)
(NCentral Soybean Research Program's overview of what they know about
factsheet from Ontario, includes images of common natural enemies)
Avoid a Tank
Tri-Mix on Soybeans
York State Soybean Rust Information Center