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Aphids, Aphids Everywhere!! – June 28, 2006

The dry conditions and moderate temperatures over the weekend have been ideal for wheat maturation.  Unfortunately, they’ve also been terrific for aphid population growth!  Both cereal aphid and soybean aphid populations in the central & northern RRV continue to rise. 

The sticky, bumpy, black bits are aphids…. Euclid, MN, 6/27/06.

Cereal Aphids - the later planted fields are at greatest risk of yield loss.  Fields that are in flag or early boot stage still have considerable development ahead of them and the standard recommendation continues to apply: spray as soon as a field reaches threshold (if 80% - 85% of stems have aphids present, there is a strong probability the average in the field is >12/stem).

The current conditions and the immediate forecast continue to favor aphid reproduction and aphid numbers may continue to rise.  In Univ. of Minnesota trials, treating aphids at heading produced an economic return in only two trials – both occurred in years with heavy populations that had been steadily growing over a period of time.  In both circumstances, greater returns were seen with earlier treatments. 

There are several fields in the central RRV that may fit these criteria.  For every 100 cumulative Aphid Days (see last week’s piece just below), the yield loss is estimated to be approximately 0.6 bu/ac.  This rate of yield loss decreases as the plant matures for a number of reasons: physiological changes in the plant and maturation of the grain make it less susceptible to aphid damage, after heading the plant starts to become less suitable as a host, aphid populations start to decline, and natural mortality factors, such as predators, start to impact the population so that aphid populations generally start to decline within 2 weeks after heading.  From heading on, there usually isn’t enough time to accumulate sufficient aphid days to cause the amount of yield loss that would economically justify an insecticide application.   HOWEVER the very heavy populations in combination with the current conditions of dry weather and cooler temperatures may allow enough aphid days to accumulate to provide an economic return (see the photo – June 27, north of Euclid, MN),

The Bottom Line – If plants are pre-heading and at threshold, then immediate treatment is recommended, don’t wait until heading!  If plants are headed, the standard recommendation is not to treat, the exception is if very heavy populations exist a treatment might produce an economic return (100 Aphid Days = ~0.6 bu/ac loss).  Also remember to check the label for Pre-Harvest Intervals!!

Soybean Aphids - Yesterday I saw a 100 ac soybean field that requires treatment; over 90% of plants in this field were infested with soybean aphids exceeded 300/plant on average (and some with far more than 300/plant).  This was apparently a field in which soybean aphids showed up very early in the season and it had a sub-threshold population evenly distributed across the field.  It’s time to

Late vegetative soybean plants with >300 aphids/plant. Euclid, MN 06/27/06.

 scout for soybean aphids!!  Keep a close eye on fields with evenly distributed populations!!

The early appearance and development of soybean aphid populations and the high cost of fuel may make it tempting to tank-mix insecticides with upcoming herbicide applications, especially one of the synthetic pyrethroids which may provide extended aphid suppression.  If the field is already at or near threshold (250/plant) and aphid populations are increasing, fine.  BUT if you’re hoping to get season long aphid suppression, it’s a risky gamble and may have a low probability of paying off!  Be aware: in UMN trials only the high label rates provided extended soybean aphid suppression and the length of suppression most consistently encountered was only 10-14 days.  After this, results were less consistent and may have been influenced by environmental conditions.  In addition, the plants now are still relatively small but are rapidly growing - after the upcoming herbicide application, soybeans will continue to grow, adding a lot of new plant material, all of which will be unprotected by insecticide residue and open to immediate re-infestation by aphids. 

Fuel and chemical costs may also make border treatment seem attractive.  Although there is some data to indicate that soybean aphids do preferentially colonize field borders in western MN and in ND, this appears to be relatively transitory.  Soybean aphids do not appear to be restricted at the field’s edge long enough for border treatments to be a viable control.  By the time populations at field borders are large enough to treat, isolated populations are generally present throughout the field.  This year especially, populations are relatively well-distributed across fields (although still in low numbers in many locations).  Treating just the borders will not prevent populations that are established within the fields from developing and spreading throughout the field and may require re-treatment of the entire field.

Cereal Aphids in Central RRV – June 22, 2006.

Cereal aphids have made an appearance in the central RRV, especially around Polk county in MN and Grand Forks county in ND. Fields in either boot stage or in 6-leaf (pre-jointing) stage seem to harbor the highest populations; this is likely the result of 2 immigration events earlier this summer. Almost all of the infestations are either English grain aphid or bird-cherry oat aphid. English grain aphids are a relatively bright green and can be readily found on the stem. Bird-cherry oat aphids are much darker (may even appear brownish), have an orange or purple spot on the end of their abdomen and can often be found under leaf sheaths (and so are much more difficult to count than English grain aphids).

Previous thresholds have been set at 12-15 aphids per stem, but as aphids are often difficult to count, a modified scouting method can be used; if 80% - 86% of stems in a field have aphids, then the average aphid population in the field probably is > 12/stem. Sample 100 stems from a field, ensuring you get good representation of the entire field, and calculate the percent infested. The best time to scout and treat cereal aphids is at the flag leaf stage, but nature doesn’t always cooperate and we can get later season infestations. Most research indicates that treating for aphids at and after heading doesn’t usually provide an economic return. Exceptions have occurred when aphid populations are very high. However, in those years, greater returns were seen with earlier treatments (i.e. heading stage).

There have been frequent questions about waiting to treat the aphids until fungicides are applied. This depends largely on how long the wait is…. The fields currently at greatest risk are the later-planted fields (i.e. those still in 6-leaf stage) that are up to 2 weeks from fungicide treatment. Aphids damage plants by sucking sap, so yield loss is related not only to how many aphids are on the plant, but how long they’ve been there. Entomologists use the concept of cumulative aphid days (CAD) – 20 aphids on a plant for 1 day = 20 CAD, 20 aphids on a plant for 5 days = 100 CAD, and so on. The concept was borrowed from heat unit calculations. Yield loss in cereals from aphids feeding has been estimated at approximately 0.6 bu/ac/100CAD. Potential minimum yield loss can then easily be calculated by calculating the average number of aphids per stem and multiplying by the length of the wait. I say the potential minimum yield loss because if 6-leaf plants are already at threshold, the aphid populations will likely increase over that 2 week period.

Total CAD isn’t the whole story, however. As wheat progresses through it’s various stages, it is less susceptible to aphid damage. This makes sense if you think of what kind of yield loss the plant can sustain through it’s life cycle. If aphids infest a wheat plant in the 4-6 leaf stages (see growth chart on next page), then damage can include stunting, decreased number of kernels, and decreased kernel weight. By the time the plant is on boot stage, decreasing the number of kernels / head is no longer a threat, but kernel size and weight can still be affected by aphid feeding. By heading, only kernel weight can really be affected. The expected economic return on an insecticide application mirrors this situation. The greatest return on an aphid treatment in wheat is at the Flag leaf stage. By the time wheat has headed, an economic return becomes far less likely.

There is data to suggest that the way CAD accumulate also influences the amount of resulting yield loss. Lower populations of bird-cherry oat aphids that fed over a longer period caused greater yield loss than did higher populations feeding for a short period even though the CAD were about the same. So, generally speaking: the longer they feed, the more damage they do…

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Orange Wheat Blossom Midge in Southern MN? Let’s Find Out! – June 16, 2006

Last year, Orange Wheat Blossom Midge (OWBM) larvae were recovered from a few hard red spring wheat fields in southern Minnesota, especially in and around the Dawson area. This is not necessarily a cause for concern, nor do we recommend treating for this insect without first scouting and establishing the population levels. We do not currently know the extent of the OWBM populations in southern MN, but thought a Heads-Up was in order. Most of the wheat in southern MN is close to heading and because degree day models predict approximately 50% of female midges that may be in that area will have emerged by late next week (June 21-23), it may be prudent to scout fields for OWBM.

Orange Wheat Blossom Midge is a small, pinkish-orange fly resembling a small orange mosquito (Fig. 1). Adults emerge from the soil in last year’s wheat fields over a 4-6 week period in mid-June to mid-July (around wheat head emergence/flowering) (Table 1). During the day, OBWM prefers to rest within the humid crop canopy and flies on warm, calm evenings. Females will fly between wheat heads in the evening when the temperature is above 59F and the wind below 6mph. Eggs are deposited on wheat kernels at or just before anthesis. Eggs hatch in 4-7 days and larvae feed on developing kernels within the glume (making it difficult to directly treat the larvae). Larvae feed for 2-3 weeks, until they are 1/8” – 1/4” long, and then drop to the soil in August after either rain or a heavy dew. They burrow into the soil and overwinter in cocoons in the soil. In spring, the larvae pupate and emerge as adults.

Scouting for OWBM is best done by visually inspecting fields at dusk (9-10pm), when females are flying between plants to deposit eggs. They are poor fliers and only take wing when the wind is below 6mph and temperatures are 59F or higher. Females fly in an irregular pattern over the canopy and tend to flutter between plants. Don’t confuse OWBM with Lauxanid flies, a much stouter yellow fly, more closely resembling a housefly and which has a much stronger, more direct flight pattern. Lauxanids tend to sit on plants horizontally or with their head pointing down where OWBM generally sits on plants with their heads pointing up. Examine at least 4 locations in each field and take an average of the number of females seen per head. If 1 female per 4-5 heads is found, and 30% to 80% of the wheat has headed, insecticide application will likely provide an economic return. Optimum control will be achieved if 70% of wheat has headed. If wheat is 30%-60% headed, some control will be obtained, but will be reduced. If wheat less than 30% of wheat has headed, wait up to 4 days prior to application. If more than 80% of wheat has headed, application is not recommended; either the larvae are already established within the glume and are protected from insecticide application or the wheat is no longer as attractive to adult midge and further infestation is unlikely (table 2). Treatment decisions can only be made from visual scouting, trapping with sticky traps (e.g. paper plates covered with vegetable oil) do reveal if OWBM are present but do not provide a reliable estimate of population levels.

Ian MacRae, Dept. of Entomology &

Jochum Weirsma, Dept. of Agronomy & Plant Genetics,

UMN-NWROC, Crookston, MN

Soybean Aphid Makes A VERY Early Arrival - Crookston, May 30, 2006!!

Soybean aphids have been found in V1 soybean fields between Crookston and Fisher. This is definitely the earliest we’ve ever had soybean aphids in commercial soybean fields in NW MN and is very probably the result of successful overwintering on local buckthorn. The mild temperatures and snow cover during the past winter provided very good overwintering conditions for these aphids and Phil Glogoza has been tracking several overwintering sites north of Moorhead. Aphids were found most frequently on the top trifoliate of the plants.

In the Crookston/Fisher fields, winged females were observed depositing nymphs on leaves and small colonies of several aphids were found on the top trifoliates of several plants. Populations are low at the moment but the weather conditions are conducive for aphid reproduction and we are expecting these ‘good aphid temperatures’ to continue for at least the next week. Remember, at this time of year all soybean aphids are females and reproduction is parthenogenic – asexual reproduction where female aphids are born ‘pregnant’ and when they mature, give birth to live daughters. These nymphs (young aphids) will mature in 3-10 days and then start having daughters of their own. Aphid populations are usually partially controlled by predation and by fungal diseases. However, our recent hot spell may have negatively impacted the fungal diseases that help keep these pests in check and the lack of rain may impair their ability to get established (like any other fungus, these insect killers need some humidity to establish and grow) We’ll have to wait and see if this is the case.

This doesn’t mean it’s time to start spraying soybean aphids yet!! It does mean we need to think about scouting much earlier than we have in the past!

As they say on CNN, more as the situation develops…...

For information on early scouting for soybean aphids, see the link below:

Early Season Scouting for Soybean Aphids

Thanks to Char Hollingsworth and her steady hand for the pics….

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Soybean Aphids – May 18, 2006.

Just a Heads UP – obviously no aphids in soybeans yet (around here most of the soybeans aren’t even up yet!!) BUT - numerous aphids were recorded in Buckthorn in the Red River Valley last fall and there seems to be large numbers of nymphs on these plants this spring. As most of you know, buckthorn is the overwintering plant for soybean aphid. This may be an omen of large, early season populations of soybean aphid in the Red River Valley, something which has not yet occurred. But as this insect continues to throw us curve balls, it might not be a bad idea to scout soybeans a little earlier this year.

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Cutworms in Sugarbeet – May 19, 2006.

Like this time last year, there have been reports of cutworms in sugarbeet in Moorhead, Crookston, Hillsboro and MinnDak.  This is the expected timing for Red-Backed and Dark-Sided cutworms in beets; both occur in the Red River Valley and they have similar life cycles.  They both over-winter as eggs laid by the adult moths in cultivated fields at the end of summer.  The following late May/early June, the eggs hatch and the caterpillars move onto seedling crops. 

Scouting –  Early detection is important with this insect.  Cutworms feed mostly in the evening and at night and so are sometimes difficult to locate in the daytime.  Their feeding, usually at or below ground level, causes wilting, death and sugarbeet seedlings are often cut off near ground level. Scouting for these worrisome worms is best done by looking for wilting or dying plants and then looking in the top 1”-2” of soil at the base of these plants. 

Thresholds & Treatment – An economic return on treatment can be expected when 4% - 5% of beets seedlings are cut in a field.  Pesticides are best applied in the evening as this results in high levels of insecticide being present during the cutworms’ period of greatest feeding activity (evening & night). In addition, in the evening the canopy will tend to have higher humidity and perhaps some dew to carry the insecticide over the plant surfaces and into the soil to increase contact with the insect. A light rain or even mist can also assist in getting insecticide to the cutworms’ location (although heavier rains will likely make re-application necessary!).  For the same reason, liquid formulations tend to be more effective against cutworms.  To assist in delivering the insecticide to the caterpillars, break up severe soil crusting prior to, or during application.  Some insecticides registered for use against cutworms in sugarbeet appear below (remember to always check the label for registration details).

INSECTICIDE	DOSAGE IN LB AI/ACRE	PRODUCT PER ACRE	RESTRICTIONS ON USE
Asana XL RUP	0.03 - 0.05	5.8 - 9.6 fl oz	Use as a seedling spray treatment. Do not exceed 0.15 lb ai/acre per season. Apply in a minimum of 2 gal per acre. Do not apply within 21 days of harvest.
Carbaryl (Sevin)	1.5	variable	This treatment is most effective against cutworms feeding on upper portions of plants. Repeat application as necessary up to 2 times but not more often than very 14 days. Do not apply within 28 days of Harvest. Do not apply more than 3 lb of active ingredient per acre per crop.
Chlorpyrifos Lorsban 4E RUP Nufos 4E RUP Warhawk RUP Yuma 4E RUP	1	2 pts	Do not apply more than 6 pt/acre (broadcast basis) or make more than 3 applications per season. Do not allow meat or dairy animals to graze in treated area or harvest treated beet tops as feed for meat or dairy animals within 30 days of last treatment. Do not apply within 30 days of harvest.
Lorsban 15G	1.5 - 2.0	10.0 -13.3 lb (6.6 – 9 oz / 1,000 row ft)	Granules must be applied behind furrow openers and ahead of press wheels as a 4- to 5-inch band at planting. Do not apply in-furrow or modified in-furrow. Lightly incorporate with chains or tines behind press wheels for best results. Do not apply granules in direct contact with the seed. Do not make more than 1 application per year.
Methyl parathion 4EC RUP	0.25 - 0.38	0.5 - 0.75 pt	Do not apply within 20 days of harvest; or 60 days of feeding tops to animals. Do not enter fields for 48 hours after application.
Mustang Max RUP	0.014 - 0.025	2.24 - 4 oz	Apply as foliar application or in-furrow using a 3- to 4-inch T-band (band over the open furrow) at planting in a minimum of 3 to 5 gals of finished spray per acre. Do not apply more than 0.075 lb active ingredient per acre per season. Registered for 24 (c) special local need sale and use in sugarbeet in ND (SLN no. ND 030003) and MN (SLN no. MN-030001). SLN registration expires before 2008 growing season.

RUP - Restricted use pesticide

ALWAYS check the label.

Table from:

2006 Field Crop Insect Management Guide (Entire Pub PDF 860KB)

Compiled by Janet Knodel, Extension Entomologist,
in cooperation with Mark Boetel, Denise Olson, and Gary Brewer
Department of Entomology
North Dakota State University

North Dakota State University
NDSU Extension Service
E-1143, January 2006
For use in 2006 only

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Selected Past IPM Updates

July 05, 2005 – Armyworm Flight, Crookston

June 06, 2005 – Soybean Aphids – they’re back

May 23, 2005 – Cutworms in Sugarbeets

June 20, 1999 - Potato Leafhopper, Grasshoppers, Cereal Insects
Sept 12, 1999 - Impact of Aster Yellows on Canola May be Greater Than You Think
Aug 31, 1998 - Lygus in Sugarbeets
July/Aug, 2000 - Armyworm Outbreak in RRV
Fall, 2000 - Soybean Aphid, A New Insect Pest in Minnesota Soybeans
Fall, 2001 - Soybean Aphid in NW Minnesota
August, 2001 - Lygus in Sugarbeets
May 2002 - Cutworms in Sugarbeets
July 2002 - Armyworms in Small Grains
July 2002 - Grasshoppers in The Red River Valley
July 2003 - Leafminer in Sugarbeets

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Publications Available for Download

Insects

Grasshoppers -
Minnesota Grasshopper Management - 2002 with color graphics
Minnesota Grasshopper Management Brochure with scouting and control information (available as a PDF file designed to be a tri-fold brochure).

Scouting -
Scouting for Insects in Wheat, Alfalfa, and Soybeans - a manual prepared for the University of Minnesota Extension Service Field School, held at the Northwest Experiment Station, Crookston, MN, July 7-8, 1998.
Early Season Scouting for Soybean Aphids - A guide to scouting early season soybeans for soybean aphid.

Questions? Comments? E-mail us at:

imacrae@tc.umn.edu