SW Minnesota IPM STUFF 2003, vol 9
7/31/03
Soybean aphid
With guest lecturer Ken Ostlie
The flight of alates (winged aphids) into Southwest (and other areas) MN a couple of weeks ago has caused dramatic increases in aphid populations in many fields. A considerable number of acres have been treated in the Minnesota River Valley from Le Sueur north to Granite Falls; a pocket of heavy pressure exists in Watonwan County and also near Mountain Lake. To the north pockets with heavy pressure also exist. At Lamberton, aphid populations reached threshold levels in untreated checks this week. This indicates that other areas to the south and west that have been relatively aphid free early in the season may still have the opportunity to experience yield reducing aphid populations. Additional pockets of infestations exceeding economic threshold continue to develop throughout the region.
A temporary but short-lived lull in aphid populations was observed in some fields as vegetative growth of soybeans stopped. However, rapid population increases have resumed during pod set.
How many aphids are needed before an insecticide treatment is required?
We are still using 250 aphids/plant on more than 80% of the plants as a threshold. This is assumed to be an aphid population that is below the level where unacceptable yield loss occurs but high enough that a high probability of reaching yield-damaging levels exists. Remember that aphids can double populations every two to three days under ideal conditions. Individual plots at Lamberton went from close to zero to over 250 (over 2000 in worst cases) aphids/plant within 2 weeks as a result of heavy immigration and good reproduction.
When counting aphids, count both adults and nymphs; however, avoid counting cast skins and potato leafhoppers. This threshold is based on field averages and not hotspots or field borders. Hot spots often collapse. On the other hand, once a high percentage of the plants in the field have aphid colonies, rapid population increases on an aphid/plant basis can occur.
It may be possible or even likely, that thresholds should change with crop stage, number of nodes (size), or environmental conditions. We do not, however, have the data to allow us to raise or lower thresholds based on crop or environmental factors. Until such data can be generated, we are forced to deal with a one size fits all threshold. It is likely that we are too conservative in some cases and equally likely that we are too liberal in others. If thresholds derived in other states are appropriate for Minnesota they be tested under MN conditions before put them into widespread practice. I know where there are a couple of fields to experiment on.
How do I scout for aphids?
Yield damaging aphid populations can rarely be detected from the road. Early in the season, aphids tend to be located on new growth. Examining upper trifoliates is a quick way to determine the percentage of plants with aphids. As vegetative growth ceases, aphids tend to move down the plant toward pods, stems and leaves lower in the canopy, presumably tracking nitrogen movement into pods. At this time of year, look at non-senescing leaves, especially the undersides, stems and pods throughout the canopy for aphids. Lady beetles, honeydew and ants and cast skins help indicate high aphid populations. Unfortunately, at this time, predators are unlikely to control high aphid infestations. Ants tend to be more abundant on field edges. Within the canopy, large numbers of aphids on the stems and pods indicate populations approaching, if not exceeding, 250/plant.
Which fields are at the most risk?
Yield damaging populations can occur in any field. There are a couple of factors that increase the likelihood of aphid problems. Smaller fields with wooded borders containing buckthorn are most likely to be the first to develop high populations. Plants with iron deficiency, SCN and other stress factors are less likely to develop problems. Late planted soybeans such as beans following peas are often reported to have higher populations. This has not been the case for the second year in a planting date study at Lamberton. This could be due to how immigrating aphids view small plots vs. larger production fields
Isn’t it too late in the season to treat for aphids?
Probably not. In SE Minnesota in 2001, strip trials and yield components analyses pointed out the importance of controlling aphids by pod set. Much of the affected area was under drought stress with aphids leaving fields in mid-August. In contrast, aphid infestations near Fergus Falls in 2002 persisted through pod fill on soybeans under good growing conditions. Similar yield responses (ca. 8-10 bu/acre) to insecticides were reported from applications made through R3-4 while yield component analyses revealed a primary impact on seed size. Approximately 700 aphids/plant peaking at R4 (end of vegetative growth and full pod) to R5 (beginning seed) reduced yield by 20%.
At Lamberton, soybean aphids have tended to leave soybeans the second to third week in August the past two years. However, there are exceptions where populations persisted until frost. When aphids leave soybeans is probably influenced by a combination of photoperiod and other factors such as plant stage or drought stress.
The contrasting scenarios in Minnesota from 2001 and 2002 leave us wondering what will happen with aphids this year.
What about insecticides?
Two factors need to be considered in insectide treatment of aphids:
1) Efficacy or % control and 2) Residual or length time that the plant is protected.
Both factors are influenced by product and rate. The pyrethroids (eg. Warrior, Asana, Baythroid, and Mustang Max) tend to provide longer residual than organophosphates (Lorsban, Parathion). We continue to generate efficacy data. With limited trials the most consistent performers have been Warrior and Asana. On the other hand, aphids are relatively easy to kill and any labeled product might provide effective control with limited migration into the field. Table 2 shows performance from a high-pressure site at Fergus Falls during 2002. Table 3 shows performance at a site where a temporary population crash and mid-August emigration prevented significant yield differences.
Coverage is important during late season aphid insecticide applications because a large percentage of the population is below the canopy. However, good results have been obtained with moderate water (15-20 gallons) and pressure (35-45 lbs), provided proper clearance between the canopy and boom can be maintained. 2002 results indicate that arial (5 gpa) application can provide adequate aphid control equivalent to ground applications. Warrior was the only product tested.
Highly volatile insecticides such as Lorsban may provide less residual if applied to small or open canopies especially under warm temperatures.
Residual is somewhat increased with rate and low end and below label rates are not recommended. Use moderate to high rates of products. Residual becomes less of an issue as the season progresses. Unfortunately, other than a frost, we cannot accurately predict the timing of aphid population crashes. If soybean aphid populations persist late into August, rebound may occur regardless of product and rate.
A final factor that needs to be considered in late season aphid treatment is the pre-harvest interval (PHI) intervals of insecticides (Table 1). For example, Pounce has a 60 day PHI and Warrior should not be applied within 45 days of harvest.
What about payback?
You are not guaranteed a return on insecticide investment. Populations may crash at any time, although forecasted relatively cool weather and rapidly increasing populations suggest an ongoing problems. The longer large populations are allowed to persist on soybean, the greater the chance for an unacceptable yield loss. Similar to potato leafhopper on alfalfa, signs of soybean stress (nutrient deficiency symptoms, stunting or puckered leaves) probably indicate that yield loss is already occurring do not wait until symptoms appear. Dry conditions are likely increase aphid induced yield loss. We do know that these insects are easily capable of causing 10-bushel yield losses. We also know that we can’t replace yield after it is gone.
Table 1. Soybean aphid insecticides. No product endorsements are intended and this chart is not a substitute for reading the label. I have been known to make mistakes.
What could make aphids go away without treatment?
Predicting aphid population collapse is difficult. Heavy rainfall can temporarily reduce aphid populations although this becomes less likely with more mature beans and heavy canopies. Predation or disease may reduce populations although predation functions better at high temperatures and fungal diseases are most effective with prolonged hot, humid weather. Hot temperatures tend to slow the rate of aphid reproduction.
Aphids may also leave on their own for two reasons. Firstly, they may be triggered to produce sexual forms and move to buckthorn. Secondly, they may become crowded and produce winged forms that leave the field. The latter indicates that they have overrun the carrying capacity of the plant; this is probably not good from a yield standpoint. However, delaying treatment and rechecking the field in a couple days is advised if large numbers of alatoid (wing pads) nymphs or diseased (dark colored) aphids are present.
Table 2. Insecticide
performance against soybean aphid. Noetzle, Holen, Holder
and Holen. 2002. Fergus Falls, MN.
Table 3. Insecticide performance
against soybean aphid. Potter. 2002. New
Ulm, MN
What about the potential for bee kills?
It may come as a surprise to many growers that honeybees actually forage soybean and that soybean can be an important resource for honeybees. The importance of soybeans varies with soybean variety, the presence and abundance of competing flowering plants, and moisture. Because insecticides can be lethal to bees and, in some cases, residues carried back to the hive, insecticides labeled for soybean aphid contain warning language similar to the following: Do not apply this product, or allow it to drift, to blooming crops and/or blooming weeds if bees are foraging (visiting) the treatment area. The key challenge is that clause “if bees are foraging”. To minimize the chance for bee kills, follow a few simple precautions:
· Choose a product less likely to be transported back to the hive. Organophosphates (Lorsban 4E, dimethoate, methyl parathion) and carbamates (Furadan 4F) generally pose a greater risk than pyrethroid insecticides (Warrior, Pounce, Mustang Max, Baythroid, or Asana). If in doubt, use products with lower REI.
· Bees forage sun up to sundown with a peak in the late afternoon. Applications near sundown (best option because sprays will dry) and early morning (next best option) will have less impact on foraging bees.
· Contact beekeepers before you spray. If you have questions about honeybee locations, hive locations are registered with MDA. See the licensing information database http://www.mda.state.mn.us/lis/default.htm. Click All Licenses, type in county name and click on Apiary Registration Certificates.
Bean Leaf Beetle
Populations of 1st generation beetle have increased at Lamberton over the past week but are still fairly low (< 2/sweep). Why is this emergence later than other sites to the east? There could be several reasons: 1) Cool weather has extended larval development time magnifying differences in egg lay time. 2) We may be observing a portion of the population attempting to go through a single generation rather than two generations
There are pockets of heavier 1st generation bean leaf beetle. I have also started to observe pod feeding in several fields with higher bean leaf beetle populations. The threshold for pod feeing is 10 % of the pods with feeding or beginning of pod clipping. Continued dry weather is likely to increase the chances of pod feeding.
White mold
In response to the earlier cool rainy weather, high percentages of plants
with white mold are present in some fields. At this time, many lesions are
limited to branches in the lower canopy.
Western Bean Cutworm
Flights of western bean cutworm have started. Lamberton and Fulda Black light and pheromone traps began picking up low numbers of moths early this week.
Bruce Potter
IPM Specialist SW Minnesota
University of Minnesota Extension Service
Department of Entomology
University of Minnesota Southwest Research and Outreach Center
PO Box 428
Lamberton,
MN 56152
Ph: 507.752.5066
Fax: 507.752.5097
E-mail: bpotter@umn.edu