Vol. 20, No. 15, August 11, 2005

Christina DiFonzo, Entomology

Aphids continue to plague growers. Some fields have been sprayed two to three times, others once, still others not at all. Later in the season, when pods begin to fill, we've always known that the threshold for aphids increases from 250 to some higher number. Unfortunately, research data is not available to define that later season number – one rule of thumb that I use in larger, fuller plants is 1000 aphids per plant, but again that has not been confirmed with research, but clearly a plant that is four feet tall can withstand more than 250 small aphids. Instead, when we get later in the season, I evaluate fields using additional characteristics that give me the “feel” of the aphids population, and to define fields that probably do not need to be sprayed immediately – wait a few days and see what happens. Remember, the second half of the aphid spray recommendation: 250 (early in the season) or 1000?? (R5 or later) aphids per plant with increasing populations.

Here is a quick list of such characteristics:

There are less than, say, 1000 aphids per plant. Most aphids are small, newly deposited nymphs. Aphids are not growing in size very quickly, and appear not to be doing very well on the plants. Tiny babies are very common, but they do not appear to “grow.” Instead they appear the same each day (Likely predation is removing many of the older individuals so these really are new babies each day.).

Although aphids are present on every plant, populations do not appear to be increasing. Aphid numbers per plant overall in the field are instead remaining static or increasing slowly.

Beans do not appear stressed: adequate water; green, healthy foliage; tall canopy.

Beans do not have signs of classic aphid infestation, that is, no or little new honeydew, aphids lined up on the stem, etc. There may be signs left over from a previous infestation, for example honeydew and mold, or white skins shed from aphids.

Presence of fungus in the fields is killing aphids. Fungal pathogens are beginning to wipe aphids out at certain locations. Recent foggy mornings, with extended dewiness, are perfect for outbreaks of beneficial fungi. Aphid populations can crash in a matter of days. Aphids attacked by fungus turn pink or tan, and appear fuzzy under a hand lens. In addition to killing aphids, certain fungi also kill spider mites and potato leafhopper.

Many natural enemies are present in the fields.

Fields such as this probably do not merit spraying immediately. If possible they should be resampled in three to four days.

2. Pre-harvest interval of available insecticides is too close. PHIs (days) for some aphid insecticides are the following: Ambush/ Pounce/ permethrin: 60; Warrior: 45; Lorsban and other chlorpyrifos products: 28; Asana, Mustang Max, and dimethoate: 21; Penncap: 20.
These two factors may “clinch” the decision not to spray.

Meanwhile, winged aphid numbers continued to be high in suction traps over the last few weeks with Michigan again leading surrounding states in the number of aphids per trap. During the week ending on July 29, soybean aphid catches per trap were 698 at Kellogg Biological Station, 895 on campus, and a whopping 1840 at the Bean and Beet Farm in Saginaw (a new record). This explains why we continue to see so many small babies on plants.

I realize that many people are frustrated with aphids right now, but this is expected! First, this season has been different in that we had the earliest colonization we've ever seen and a heavy winged aphid flight in midseason. Combine this with some of the earliest planting of soybean, and you get fields that have to be resprayed, simply because of how long aphids have been present in the fields.

Second, due to the threat of rust plus our experience with yield loss due to aphids in 2001/2003, this is really the first season that so many people are actually walking soybean fields on a regular basis. As a researcher sampling aphids for the last four to five seasons, I've experienced frustration towards the end of each season, trying to define later-season threshold, decide which fields should be treated, and when too late is really too late. Now many others are getting to experience to the frustration, too (misery loves company).

But don't despair! If you are frustrated by aphids right now, it’s because you are in the field, watching aphid populations rise and fall for sometimes mysterious reasons. In general, producers have done a great job this year walking fields, following aphid populations, observing other insects in soybeans, asking a lot of questions and making spray decisions. Notice that there are very few "black" soybean fields this year compared to 2001/2003, because people have done a much better job of management, even if it is frustrating at times. The second reason not to despair is that within the next 7 to 14 days, aphid populations will fall. Fungus is going to move into more fields, crashing populations. Maturing beans are a much lower quality host, so aphid increase will slow. And as the days get shorter, temperatures cooler, and plant quality less, the aphid population will switch over to winged males and females that leave soybean for buckthorn. The end is near!

Sandy Perry, SBR Sentinel Plot Coordinator
Jan Byrne, MSU Diagnostic Services

Ten reports were filed from the 20 soybean sentinel plots during the reporting week.

No soybean rust has been found on soybeans or other plants in Michigan or anywhere in the region, including Ontario, Canada.

Plant height: Two plots ranged from 20 to 30 inches, and eight plots ranged between 36 and 48 inches.

Degree of canopy closure: All plots ranged between 95 and 100 percent closure.

Soybean diseases present: Downy mildew is present in half the plots at low to moderate levels but the incidence is increasing. Brown spot remains at low levels in three plots.

Insects present: Two plots have recently been sprayed for soybean aphids, three plots have per plant populations of soybean aphids ranging from 250 to 420 with honeydew and sooty mold present, two plots have low incidence of soybean aphids and three plots report no soybean aphids. Spider mite damage was noted on 5 to 10 percent of the leaves in one plot.

There is no MSU Diagnostic Services report this week.

Ned Birkey

Corn is surprisingly green and tall. A few sample ears showed that pollination was successful, though ear length of the samples were smaller than normal and they were also smaller in diameter. European corn borer numbers are relatively high as we are in the thick of the second generation. Western corn rootworms are present but not in high numbers.

Soybeans are taking a pounding from aphids and spider mites. Most fields are suffering and have been sprayed with insecticides recently. Sudden death syndrome is appearing and soybean cyst nematodes are prevalent. Phytophthora stem rot is also evident, and one farmer is reporting Sclerotinia white mold, though it seems too hot and dry for this disease to become a problem this year. Beneficial insects feeding upon aphids are present, as are the hole-chewing Japanese beetles. Insecticide sprays are not always reaching down into the canopy.

Fred Springborn

Corn maturity is progressing rapidly. European corn borer moth numbers in pheromone traps have increased this week indicating a possible peak of second generation moth flight.

Early planted dry bean fields are starting to turn. Most of this crop is experiencing overall good growing conditions.

Soybean aphid has been observed in high numbers in many fields. Many fields have been treated. Fields that have not been treated with insecticide have started showing a decrease in soybean aphid numbers over the past few days.

Potato leafhoppers are present in alfalfa fields and should continue to be monitored.

Paul Gross

Soybeans are R-4 to R-5 and progressing rapidly with the near-ideal growing conditions. The biggest issue is soybean aphids. The populations are at or over threshold and many fields have been treated. We are seeing white mold in many fields. The region has had almost ideal conditions for mold, and it is beginning to show in several fields.

Wheat harvest is complete with most growers disappointed with yields. The quality was very good.

Oats and barley is harvested with yield and quality generally good.

Harvest of third cutting alfalfa is nearly complete. Yield and quality has been very good. Leafhoppers are present and farmers need to continue to scout for this pest.

Dry beans are doing very well. At this point the yield potential looks very good with good stands and few real problems. We expect harvest to be early this year.

Mark Seamon

The sugar beet crop continues to grow and look good in many areas. Weed escapes have made themselves obvious in the past couple of weeks. Micro rate herbicides worked well in some places while other fields could be considered poor weed control. Hoeing has been active where possible with the limiting factors being number of crews to work and the cost per acre for some of the weedy fields. Cercospera leafspot is easy to find with favorable environmental conditions of high temperatures and plenty of leaf wetness (especially from morning dew and fog). Many growers have made a second fungicide application for control.

The good fields of corn continue to look good while some of the injured fields have improved in appearance. Pollination is pretty much wrapped up with good conditions. Temperatures were a little high during pollination, but short breaks in the mornings and evenings helped to fill ears.

Soybeans are showing good yield potential in most areas. Many fields are more than 36 inches in height and podded up to the top node. Rainfall in the next couple of weeks may be critical to the development of those tops pods. Soybean aphids have boomed again in many areas, which has led to a second insecticide application. Many growers have questions about how long we can expect the aphids to stay around but are cautious about allowing high populations to feed on soybeans developing and filling pods.

Some of the injured dry bean fields have recovered significantly while others continue to thrive. The number of pods per plant and beans per pod point to a good crop but we can never be too confident of this crop until they are in the truck.

Jeff Andresen
Agricultural Meteorology
Geography

After several weeks of a persistent ridging pattern across the central United States, the jet stream will be moving southward during the next couple of days, resulting in cooler and wetter weather for much of the Great Lakes region. The cool front that moved across Michigan Wednesday (August 10) will return northward and linger across the region through early next week, bringing a chance for showers and thunderstorms on an almost daily basis through next Wednesday. Rainfall chances across the state will be greatest overnight Thursday into early Friday, when widespread showers and thunderstorms are a good bet. Most rainfall totals will be in the 0.1- to 0.5-inch range except over southern sections of the state where totals will likely range from 0.5 to 1.0 inches with some isolated 1.0- to 2.0-inch totals possible. A severe thunderstorm or two is possible across southern sections of the state, with the primary threat being high winds. A gradual drying trend is forecast for the Upper and extreme northern Lower Peninsulas by late Friday, continuing into the weekend. Meanwhile, the threat of showers and thunderstorms will continue through the weekend across much of the Lower Peninsula, with the best chances for additional rainfall across far southern sections of the state.

Temperatures during the next few days will remain near or slightly below normal levels with highs ranging from the low and mid-70s north to mid-80s south and lows from the upper 40s north to low 60s south.

In the medium-range time frame, computer model guidance is suggesting that the core of the jet stream, normally near its northward-most range across central Canada at this time of the year, will at lest temporarily shift southward to the United States/Canada border area. This would result in a fast-moving, west-to-east jet stream flow over Michigan with an active, transitional type weather pattern.

The NOAA CPC 6-10 day outlook, covering August 16-20, calls for temperatures to range from below normal in the Upper Peninsula to near normal in Lower Michigan. A wide range of precipitation is forecast, ranging from below normal totals across Upper Michigan to above normal over most of the Lower Peninsula. During the 8-14 day period (covering August 18-24), near normal temperatures are expected statewide, while precipitation totals are forecast to range from below normal over the northwestern 3/4s of the state to below normal across the extreme southeastern Lower Peninsula.