In this issue
§ Asian soybean rust sentinel plot scouting report for July 20-27
§ Breaking insect updates: soybean aphid, armyworm and corn rootworm
§ Aphid sucker part 2: Aphid counts
§ You might have mites
§ Orthene 2ee label for soybean aphid
§ Management of annual weeds following wheat harvest
§ Regional reports
Fourteen reports were filed from the 20 plots during the reporting week.
No soybean rust has been found in
Growth stage: Eight plots=R3, 5 plots=R4, 1 plot= R5.
Plant height: Ranged from 20 inches to 40 inches, 4 plots = 20 in., 1 plot = 22 in., 2 plots = 24 in., 3 plots = 30 in., and one plot each at 35, 36, 38 and 40 inches.
Degree of canopy closure: Ranged between 90 and 100 percent.
Soybean diseases present: Eleven plots = low brown spot, 2 plots = low bacterial blight, 7 plots = low downy mildew, 2 plots = low Phytophthora stem canker.
Insects present: Four plots = a few soybean aphids per plant, 8 plots = 0-50 per leaf, 2 plots = 0-10 spider mites per leaf, and 2 plots = low levels of Japanese beetles. One plot had been recently sprayed for soybean aphids and 3 plots reported that soybean aphid numbers were increasing.
No soybean samples were submitted to MSU Diagnostic Services to be assayed for soybean rust during the reporting period.
MSU Diagnostic Services reports the following pests and problems found on field crop samples submitted in the last 2 weeks:
|
Kidney bean |
Common bacterial blight (Xanthomonas campestris subsp. phaseoli) |
|
Soybean |
Downy mildew (Peronospora manshurica) |
|
|
Manganese deficiency |
|
|
Rhizoctonia |
|
|
Two-spotted spider mite injury |
|
Sugar beet |
Rhizoctonia root rot |
Confirmed soybean rust detections have occurred in eight
Fields range from a few soybean aphids to “dripping” with aphids, even on the same farm.
Hundreds of tiny, white newborn babies are being deposited by winged aphids (See related article on the aphid suction trap in this issue) on calm mornings and afternoons. This is our first experience with such heavy deposition of babies over such a long period – now stretching into several weeks. In a single morning, a field can essentially go over the 250 threshold if 50 to 100 winged aphids land and deposit four to five babies each! The key to dealing with the babies is to remember the second half of the current aphid recommendation: 250 aphids per plant with increasing aphid populations. Interestingly, in many fields colonized by babies, the tiny babies are not growing into adult aphids, which would only take a matter of days. Instead, it appears that predators are cleaning up babies and that new babies are simply deposited each day (predation is balancing deposition). Thus the plants continue to have small baby aphids, but no or few adults. My best guess is that fields that are exhibiting this type of situation do not have an “increasing” aphid population – they simply have a static, recycling population of babies. However, watch these fields carefully to see if the situation changes and babies begin to survive to adulthood. In that case, a treatment decision may have to be made on short notice.
Armyworm is reported in corn from
In some cases, no ground equipment or aircraft is available to spray – these fields cannot be treated, and the hope is that the defoliation will end soon. The reported armyworms appear to be at least one inch in length, so they will pupate soon and stop feeding. Also, when outbreaks occur, we often see beneficial organisms such as diseases and parasitic flies reducing populations (See figure). In cases where equipment is available, pyrethroids such as Ambush, Asana, Baythroid, Capture, Mustang, Pounce and Warrior should do a good job of killing larvae. Sprays do not need to penetrate down to the ground, as larvae will climb up the plant to feed in the evening. Corn can partially recover from armyworm defoliation if the growing point is unaffected, but the amount of recovery will depend on the stage of the corn, as well as how much leaf material is left.
Corn rootworm beetle emergence is tremendous this year. There are numerous reports of downed corn as storm fronts move across the state. Now is the time to dig and evaluate roots to determine:
1) The cause of downed corn
2) The efficacy of insecticides and transgenic corn
3) The presence of variant feeding on first-year, rotated corn.
Beetles are also heavy in soybean in southern
Defoliation is present in many fields, caused by a complex of insects: Japanese beetles skeletonize leaves, creating a lacy appearance. Bean leaf beetle and corn rootworm beetle chew round holes in leaves. Grasshoppers often chew from the leaf margin. The threshold for defoliation during pod fill is 25 percent. The human eye often overestimates defoliation. It is common to find pockets of worse defoliation on upper leaves, particularly where Japanese beetles cluster to mate in a small area and feed heavily. When assessing defoliation, examine more than the top leaves. Plants can compensate for feeding, and light can also filter down to leaves in the mid-canopy. One thing to watch for is pod feeding by bean leaf beetle, rather than simply defoliation. Pod feeding can directly impact yield and quality, and the threshold is only 10 percent of pods damaged. This type of feeding usually occurs later into August.
As promised in the July 14 Field Crop CAT Alert, below are the
counts from one of the three suction traps running in
The table also lists some of the other aphid species captured in the trap. I provide notes about the host range (Where are these aphids coming from?) as well as some of the important viruses that are transmitted by each.
|
Aphid species * notes on host range and virus transmission |
Trap count ending the week of: |
|||
|
June 24 |
July 1 |
July 8 |
July 15 |
|
|
Bird cherry-oat aphid *Hosts: small grains and corn *Vectors: barley yellow dwarf |
1 |
1 |
0 |
6 |
|
Black legume aphid * Hosts: legume crops and weeds * Vectors: non-persistent viruses in many crops, including cucurbits. |
0 |
2 |
7 |
31 |
|
Buckthorn - potato aphid *Host: potato [note, this native aphid over- winters on buckthorn] *Vectors: potato virus Y |
0 |
1 |
0 |
13 |
|
Corn leaf aphid *Hosts: corn, small grains *Vectors: barley yellow dwarf |
0 |
0 |
0 |
0 |
|
English grain aphid *Hosts: small grains *Vectors: barley yellow dwarf |
0 |
1 |
0 |
2 |
|
Green peach aphid *Hosts: potato, peppers, beans, beets, tobacco, *Vectors: Highly efficient vector |
0 |
2 |
1 |
0 |
|
Greenbug *Host: small grains *Vectors: barley yellow dwarf |
0 |
2 |
0 |
1 |
|
Pea aphid *Hosts: alfalfa, other legumes *Vectors: alfalfa mosaic |
2 |
21 |
20 |
9 |
|
Soybean aphid *Host: soybean *Vectors: soybean mosaic, bean common mosaic potato virus Y, CMV, WMV2, ZYMV |
2 |
48 |
96 |
853 |
|
Spiraea aphid *Hosts: polyphagous, numerous hosts. *Vectors: CMV, WM2, ZYMV, PVY |
0 |
8 |
22 |
18 |
|
Spotted alfalfa aphid *Hosts: alfalfa, other legumes *Vectors: alfalfa mosaic |
0 |
15 |
5 |
12 |
|
Turnip aphid *Hosts: Cruciferous weeds and crops *Vectors: CMV, turnip mosaic |
4 |
22 |
1 |
7 |
How do the counts
from
Here is a quote from Dr. Dave Voegtlin,
who is sorting the catches from the regional trapping network, “I am sure that
you don't have to tell Chris that
Reports of spider mite damage are coming in from across
Spider mites overwinter as adults. In spring, they move to new plant growth and lay eggs on the undersides of leaves. Mites can be blown by wind, so initial colonization of a field will often occur in the direction of prevailing winds, or along landscape features that disrupt airflow such as tree lines, houses or even telephone poles. Infestations usually start on dusty edges of fields. It is thought that dust dries the leaf surface, protecting mites from disease, or perhaps that the dust provides a surface for the mites to anchor their webs. With a hand lens or microscope, you can see that webbing acts like an interstate highway with mites moving back and forth above the leaf surface.
Eggs hatch in a few days and small mites begin to feed. Immature mites resemble the adults, except that they are smaller in size. They grow by molting, and if you look closely at a mite-infested leaf, you sometimes see the shed skins of immature mites. A leaf heavily colonized by mites will have eggs, immatures, adults, shed skins and webbing.
Spider mites feed on numerous crops, and under certain conditions, will increase to the point where they affect yield. This is especially true under dry conditions or on sandy soils where water stress is an issue. Mites pierce individual plant cells and suck out the contents, initially causing tiny yellow spots (called stippling) on leaves. This feeding also increases water loss from the plant. As populations increase, symptoms include yellowing of leaves and in more severe cases browning, bronzing or death of foliage. As leaves become yellow and die from mite damage, the photosynthetic ability of the plant is reduced. With the additional water loss, leaf drop and reduced photosynthesis, yield and quality of crops is reduced by severe mite feeding.
If an infestation is identified early, spot treatments of
the affected area plus a border strip may be enough to take care of mites.
Otherwise, if mites spread across a field, the treatment threshold for dry bean
is 25 percent of the plants infested with mites with yellowing. For soybean,
estimate the percentage of leaf surface damaged by mites. Treatment thresholds
(based on percent yellowing) vary with plant stage: pre-bloom = 40%; bloom R1 to pod fill R5 = 15%; R5 to early maturity R7 =
25%; after R7 = Do Not Spray, damage at this point has little impact on yield. A
handy evaluation scheme (originally from
(Based on an article from the late Dr. Hal Willson, originally printed in June 14, 1999, C.O.R.N. Newsletter)
|
Presence of mites |
Damage |
Assessment |
|
Barely detected on undersides of leaves in dry locations or on edges of fields. |
Barely detected. |
1 - Non-economic |
|
Easily detected on undersides of leaves in dry locations or on edges of fields. Difficult to find on leaves within the field. |
Foliage green, but stippling injury detectable on undersides of leaves, although not on every plant. |
2 - Non-economic, but keep monitoring |
|
All plants are infested when examined closely. |
All plants in field exhibit varying levels of stippling, even on healthy leaves. Some speckling and discoloration of lower leaves. Field margins and dry areas exhibit severe damage. |
3 - Rescue treatment is warranted, especially if many immatures/ eggs are present. |
|
All plants heavily infested when examined closely. |
Discolored and wilted leaves easily found throughout the field. Severe damage evident. |
4 - Effective rescue treatment will save field. |
|
Extremely high numbers. |
Field discolored, leaves drying down. Significant foliage and stand loss. |
5 - Rescue treatment may not save field. However, new growth may resume if treated. |
Mite control is difficult, and more than one application may be needed. Unfortunately, multiple applications can lead to resistance. Later in the season, spraying obviously becomes more difficult due to closing of the canopy, but spraying is also less effective as plants mature (yield is no longer affected).
** Dry bean: dimethoate (0.5 – 1 pint/ acre). Will also control potato leafhopper.
** Sugarbeets: Lorsban (1-2 pints/ acre)
** Soybeans: Lorsban (0.5 – 1 pint/ acre) or dimethoate (1 pint/ acre)
This season, many fields have both spider mite and soybean aphids. Ironically, one way to get rid of soybean aphid appears to be having a mite infestation. Aphids feed gently on the leaves, and you cannot see wounds on the plant after an aphid has fed. On the other hand, mites feed by piercing plant cells and damaging the leaf surface. In 2001, when aphids and mites were present together during a drought year, soybean leaves heavily infested with mites appeared to be too physically damaged to support soybean aphid feeding. As the season progressed, plants with many mites eventually had few or no aphids. So watch for a reduction of aphids in fields with mites.
Another consideration when both mites and aphids are present is choice of products for control.
§ If aphids are your target, both organophosphate [OP] insecticides (Lorsban, dimethoate, Orthene) and pyrethroids (Asana, Baythroid, Mustang, Pounce, Warrior, etc) can be used.
§ If mites are the target, then an OP is preferred. OPs tend to work better against mites than pyrethroids, which tend to flare mite populations.
§ If both pests are present, OPs again are the preferred option to reduce risk of flaring mites. However, Lorsban is the preferred option within the OPs. Although dimethoate is a low cost, effective mite control, it is less effective for aphid control compared to Lorsban.
What about natural controls? A heavy rain may reduce the population a little, but don't depend on rain alone. The real key is high humidity, since the fungal pathogens that kill mites require high, sustained humidity to grow and spread through the population. A brief rain, followed by quick drying, does not provide a long enough period for diseases to spread and kill a significant portion of the mite population. Instead, you need a sustained rainy pattern, or repeated dewy wet mornings.
Iowa State Entomology Image gallery for spider mite:
http://www.ent.iastate.edu/imagegal/plantpath/soybean/spidermite/mitedamfield.html
Orthene 90s from Valent now has a 2ee label for soybean aphid control in soybeans. The label rate is 0.56 to 1.1 lb per acre. The reentry interval is 14 days, and the preharvest interval is 14 days. The label also allows for tank mixing with certain herbicides and soybean fungicides.
Based on my past experiences working in potato, Orthene should work well on soybean aphid. In the plant, Orthene is systemic and breaks down into a compound that is even more toxic to sucking insects than the original active ingredient. It is an OP (like Lorsban and dimethoate), however, I do not know if it has the “fuming” action under hot conditions that we see with Lorsban. Like the other OPs, Orthene breaks down in days (rather than weeks) which allows beneficial insects to recolonize a field, OR allows recolonizing aphids to survive (However you want to look at it!). Orthene is listed as a moderate miticide and would be preferred over a pyrethroid if mite populations are heavy in a field along with aphids. Orthene is one of the products in our spray trials this summer, so we will have efficacy data available at winter Extension meetings.
Annual weed densities are very high in wheat stubble this year. These weeds must be managed soon in order to prevent seed production. Several effective options are available for management of annual weeds in wheat stubble. However, these practices must be implemented before seed production to avoid significant additions of seeds to the weed seed bank.
After wheat harvest, weeds take advantage of available resources, especially newfound sunlight, and will grow rapidly, especially in years with adequate soil moisture in July and August. These weeds are capable of producing large amounts of seed prior to a killing frost.
Common ragweed, velvetleaf, common lambsquarters, redroot pigweed and foxtail species are examples of common summer annual weeds in wheat stubble. At this time of the year these weeds are completing their life cycles by flowering and producing seed. The main goal following wheat harvest is to prevent or reduce weed seed production. Post-harvest weed control options include tillage, mowing, and herbicides. In general, management of annual weeds in wheat stubble is not difficult, however timeliness is critical.
Tillage is a viable option for control of both broadleaf and grass weeds. Mowing will reduce seed production of most grass and broadleaf weeds. However, mowing is often less effective on grasses since seeds can be produced on tillers low on the plant. Glyphosate and 2,4-D are common herbicides used for post-harvest weed management in wheat stubble. Refer to the herbicide labels for appropriate rates, adjuvants and tank mixes.
It has been extremely hot with wildly scattered, and at times locally, very heavy rain over the past two weeks. Most areas have had rain with some areas reporting five inches. We may well end up with average or even above-average rainfall for July, though it has come down in “buckets” and late in the month. We have had 24 days of 88 to 97 degrees since June 1. A “small” tornado blew down three barns Tuesday evening (July 26) during one of these local storms.
Farmers are now optimistic that corn and soybean yields will be “decent” with the recent rains. It is hard to imagine that August weather can be worse on crops or temperaments than has been June and early July weather.
Alfalfa fields need to have the second, or in at least one case, the third cutting removed as soon as possible because of leafhopper damage and plants are past full bloom stage and are even blown over in the storms. Potato leafhopper damage and boron deficiency are in most fields.
Corn has tasseled and is pollinating, regardless of plant height. European corn borer trap counts are increasing and corn rootworms and Japanese beetles are actively feeding. We have already had more corn borer damage than in recent years. Farmers should carefully evaluate fields to see which fields to harvest early to avoid lodging losses.
Oat harvest has just finished with average yields. Oats are still in demand for feed and some people like the oat straw.
Soybeans have grown a foot or more in some fields over the past two weeks. Since fields were planted perhaps 30 days earlier than last year, farmers are pleased with the recent growth. Many fields are now waist high with new growth and are at the R2 to R3 growth stage. Aphid populations are skyrocketing – 1,000 per plant in one case. No variant Western corn rootworms have been found in one set of yellow, unbaited, sticky traps, but walking in the same field you can find an occasional Western corn rootworm. How do you get good spray coverage for aphids in 36-inch tall soybeans? Where is the target height for the sprayer boom? One farmer is using 100 psi, another 70 psi. With aphids and, in the future rust, farmers will have to do more thinking about boom height, spray pressure, nozzle selection, etc. Japanese beetle populations seem highest since the late 90s. Bean leaf beetles and grasshoppers are also evident.
Wheat harvest is finished with yields down 10 to 35 percent.
We are cooperating with ADM Grain Company of
Grain prices have had
some good, if temporary, rallies. The August 12 USDA crop report will be very
important as a substantial area of the
The weather this week continues to be more summer-like than
what we have seen in the past few years. High temperatures have been in the
upper 80s to lower 90s F. Low air temperatures have been in the 60s and 70s F.
Average humidity levels have also been rather high with the averages in the 70s
and 80s. Scattered rain showers have cropped up across the region over the past
two weeks generally yielding only a few tenths of an inch of rain in localized
areas. Soil conditions continue to be dry west and north of
Oats are turning.
Wheat harvest is just about to
begin.
Corn growth has progressed. Much of the crop is about to put out the last leaf before tassel emergence. Leaf rolling is prevalent in dryland corn in the northern and western areas of the region and on the very sandy soils elsewhere. European corn borer moth numbers in pheromone traps are very low as second generation has not yet emerged.
Dry beans are growing rapidly under irrigation. Most of this crop is experiencing overall good growing conditions.
In soybeans, soybean aphid has been observed generally below threshold.
Second cutting alfalfa is complete. Potato leafhoppers are present in alfalfa fields and should continue to be monitored.
The region has received over four inches of rain over the past two weeks. Some came all at once some came over several hours. Soil moisture is adequate to surplus in most areas. There is water standing in some fields. Most farmers are very optimistic that these recent rains will nearly make the corn crop.
The corn crop is nearly through the pollination stage. It appears that yield potential will be very good based on near-ideal temperatures and moisture during this critical stage. There are large populations of corn rootworm adults. Root feeding has been a bigger problem this year than we have seen in several years. There are fields that were nearly flattened by the wind and rains of a week ago as a result of severe root pruning. It will be critical that growers scout fields for adults to help make decisions on managing this pest next year. Another pest that has been observed in isolated areas is armyworm in corn. Severe defoliation has occurred. This is another reason to get out into your fields and scout for potential problems.
Soybeans look very good and the yield potential is giving farmers reasons to be optimistic. Soybean aphids are in nearly every field at various populations. It is very important that you get out and scout your fields and know what the numbers are and what the thresholds are. We are also seeing fields over threshold for spider mites and are being treated. Continue to scout for aphids because the population can grow rapidly.
Wheat harvest is nearly complete. Most of the white wheat has been harvested and the red will be finished when the weather conditions allow. Quality has been very good but the yields have been disappointing with most fields in the 50 to 60 bushel range. Little or no scab has been reported.
Harvest of third cutting alfalfa will begin this week. There are still some farmers making second cutting. After a light first cutting most yields of second cutting has been very good. There should be surplus supplies in the region this year because we have had timely rains throughout the growing season. Leafhoppers are a problem in many fields. Scout for this pest.
Sugar beet fields look good where there are good stands. The recent rains will really benefit fields that were replanted and are a little behind.
Harvest of oats and barley is just getting underway.