July 22, 2004
In this issue
§ Are soybean aphids regulated by weather, natural enemies or both?
§ Manure on wheat stubble
§ News update from Pesticide Education
§ Regional reports
§ Weather news
Since its initial detection in the North Central U.S. in 2000, the soybean aphid has become a key pest of concern in soybean. In 2003, over 42 million acres of soybean in the North Central U.S. were infested and more than seven million acres were treated with insecticides to control the soybean aphid. While 2004 seems to be shaping up as a relatively low aphid year so far, we are just now entering the time when outbreaks typically occur. Many factors contribute to determining if soybean aphids reach outbreak levels or not. Among these are weather and the impact of natural enemies. In this article we focus on the interaction of weather and natural enemies in suppressing soybean aphid.
There has been a longstanding debate among entomologists as to what factors have the greatest influence on pest populations. One school of thought is that weather is the main factor and all other influences play a minor role. Another school of thought contends that competition and the impact of natural enemies are the dominant factors regulating pest populations. As in most such debates the truth is probably somewhere in between. In the case of soybean aphid, a picture is beginning to emerge that suggests that both weather and natural enemies interact to regulate aphid outbreaks.
Cool weather conditions favor soybean aphid outbreaks in several ways. Recent studies in Minnesota show that the soybean aphid reaches its maximum reproductive potential when temperatures are around 81 degrees and that even short exposures to temperatures over 90 degrees greatly decrease aphid reproduction. Coupled with this is the fact that many natural enemies are less active at cooler temperatures. For example, in 2002 many days exceeding 90 degrees and almost no aphid outbreaks were reported throughout the region. In contrast, 2003 was more moderate and aphid outbreaks were extensive. Observations of this sort tend to support the idea that weather is the driving factor.
On the other hand, studies in Michigan and Indiana have documented 22 species of predators in soybean fields that readily feed on soybean aphid. Of these, lady beetles, minute pirate bugs, hoverflies, lacewings, carabid beetles and spiders are common in most fields. Parasitic wasps that are either native or previously introduced are just beginning to show up in some surveys but at present contribute little to soybean aphid biocontrol. Finally, certain predaceous aphid flies and midges lay eggs in the colonies and their larvae help reduce aphid numbers. Thresholds developed for soybean aphid generally suggest that action should be taken once aphids reach 250 per plant. All of these studies have been conducted in open fields with natural enemies present. Another way to think about this is that once aphids reach about 250 per plant, natural enemies are unlikely to stop them before they reach outbreak proportions. Thus, it is important that natural enemies act early to suppress soybean aphid populations rather than just respond to outbreaks.
While it’s pretty hard to experimentally change the weather, we can alter natural enemy numbers by use of exclusion cages. In this way we can ask the question, given the weather conditions in a particular year, are natural enemies an important factor in suppressing soybean aphid? Our experiment consists of establishing 1-meter square plots in soybean fields and infesting them with 10-11 aphids per plant representing a typical early season infestation. We then cage one-half of the plots and leave the other half un-caged (fake cages that allow natural enemies to enter are always included to control for possible cage effects but for simplicity are not discussed here). Pairs of caged and un-caged plots are repeated 6-10 times throughout the field and aphid populations are counted weekly. If weather were the sole driving factor, we would expect to see nearly equal numbers of aphids in caged and un-caged plots over time.
In 2002, aphid populations in open plots (i.e. exposed to natural enemies) never exceeded eight aphids per plant (See figure).
However, when natural enemies were excluded, populations reached 160 aphids per plants by mid-July. Over the next two weeks these numbers reached over 2,000 aphids per plant in exclusion plots, while open plots never exceed 20 aphids per plant. Thus, natural enemies were critical to keeping populations below thresholds in 2002.
In 2003, just one week without natural enemies resulted in rising aphid populations (7/21) and by two weeks (7/28) aphid populations in exclusion plots were out of control (See figure).
However, also notice that in the open plots populations had reached about 200 aphids per plant on 7/28 (i.e. approaching the 250 aphid/plant threshold). We all remember that 2003 was a very bad year for aphid infestations. Thus, while natural enemies suppressed soybean aphid populations and delayed outbreaks, nonetheless, outbreaks did occur in many fields.
In 2004, our data again indicate that natural enemies are very effective in suppressing aphid populations (See figure).
Last week’s counts (7/6) showed over 300 aphids per plant in exclusion plots and less than 10 per plant in open plots. It remains to be seen if natural enemies will keep overall populations below thresholds as in 2002 or if conditions will once again favor aphid outbreaks as in 2003.
In production fields, aphids numbers across Michigan and in surrounding states continue to be very low. The percentage of infested plants in the fields monitored by the MSU Field Crops Entomology lab is less than 5%, and often less than 1%. In other words, the vast majority of plants are still aphid-free, while only a few plants have one or several aphids. In comparison, aphid populations were already building in early July 2003, and by late July, fields were being treated. Even in 2002, the last "low" aphid year, most plants had aphids by the end of July, although in low numbers. Thus, each year from 2000 to 2004 has been a bit different!
In conclusion, it’s both! Weather conditions appear to set the basic trajectory for soybean aphid population growth. Temperatures around 81 degrees favor maximum aphid reproduction, while temperatures over 90 degrees slow population growth. In 2003, aphid populations were growing slowly due to unfavorably hot conditions and natural enemies had no trouble suppressing their populations throughout the season. In contrast, in 2003 more moderate conditions favored the soybean aphid and while natural enemies suppressed and delayed aphid outbreaks for several weeks, they were unable to prevent the eventual build-up of heavy late season populations.
Looking at all three years together it is clear that in the absence of natural enemies, soybean aphids reach outbreak levels regardless of the weather! Our current research is aimed at determining if we can enhance the suppressive effect or our existing predator-dominated communities by augmenting them with parasitoids.
While growers cannot change the weather they can do things to encourage effective natural enemy populations. Most of the natural enemies mentioned above require food (pollen and nectar), alternate prey (aphids or other small insects) and shelter from adverse conditions. Managing habitats on your farm to promote natural enemies may be one way to help ensure that these helpers are always present. For more information on habitat management for natural enemies see the following resources
Farmscaping to Enhance Biological Control. http://attra.ncat.org/attra-pub/farmscape.html
'Naturalize' Your Farming System: A Whole-Farm Approach to Managing Pests. http://www.sare.org/publications/farmpest/index.htm
If manure is being hauled to wheat stubble fields, remember this is a good time to do the following things.
Keep records of manure applications. These should include date, source of manure, rates applied, method of application and/or date of incorporation, acres covered, weather, and field conditions.
Calibrate. Calibrating the rate of manure per acre is vital to knowing the nutrients per acre. Accurate calibration will improve nutrient management, allowing the producer to reduce applied fertilizer to the next crop. Accurate calibration will also ensure that nutrients are not over applied.
Take a manure sample. Manure going to the field is the most representative of the nutrients per 1,000 gallons or per ton. Any agitation or mixing will have occurred and taking a manure sample at hauling time will help build a base line of nutrients applied per acre.
Apply manure as uniformly as possible. Manure nutrients are valuable for future crop production. Applying manure as if it were fertilizer will benefit the next crop.
Ensure that manure stays in the root zone. Manure nutrients that runoff or leach become unavailable to the future crop and pose an environmental risk. Ensure that manure is applied and managed such that it stays in the root zone for future crop uptake.
The following information is from the MSU Pesticide Safety Education Program’s newsletter published by Christina DiFonzo and Carolyn Randall.
An OSHA web page about employing teenagers in agriculture
has safety tips you can use to keep your teen employees safe: http://www.osha.gov/SLTC/youth/agriculture/other.html
The web site identifies hazards presented by working with machinery, the
potential for falls or electrocution, precautions around farm chemicals, and
other farm safety topics that might not be evident to teens.
§ Number of counties with crows testing positive for WNV: 31
§ Number of states with bird or animal positives: 34
§ Number of counties with mosquitoes testing positive for WNV: 5 positive tests from 3 counties (Saginaw, Bay, Midland)
§ First human case this year in Michigan: June 26, 2004
§ Number of states with human cases: 10 (108 cases, concentrated in Arizona, California, and Colorado)
Thirteen counties are now part of the EAB quarantine zone: Shiawassee, Genesee, Lapeer, St. Clair, Ingham, Livingston, Oakland, Macomb, Jackson, Washtenaw, Wayne, Lenawee, and Monroe. In addition, there are a number of sites outside of the quarantine zone where EABs have been found. These sites are under eradication or containment (a ring of trees removed from a prescribed distance from the initial infestation). Also, a trap-tree program has been implemented across the state. The number of trap trees per township depends on the proximity to a known EAB infestation. Trees will be checked throughout the summer.
A battery-operated device designed to protect babies in strollers from mosquitoes was recently dinged by the Federal Trade Commission. The device is called the Love Bug. It clips to a baby strollers and allegedly emits an ultrasonic tone that mimics "the wing-beat of the dragonfly – the mosquito's mortal enemy!"
The marketer, Prince Lionheart Inc, has been required by the FTC to prove its claims, which also include "protecting infants from West Nile Virus." Read more from the FTC at: http://www.ftc.gov/opa/2004/06/lionheart.htm. Incidentally, ultrasonic insect repellers in general do not stand up to scientific scrutiny.
We are seeing European corn borer and adult rootworm activity. Corn began silking last week. Weather for pollination is ideal, with moderate temperatures and continued rainfall. Good weather now will not completely undo early season weather damage. We are looking for corn yields in the lower range of normal.
Last week we finally found soybean aphids and spider mites in Washtenaw County. Rainfall over the weekend has knocked populations back. As mentioned earlier, soybean growth stages range widely because of spread-out plantings. We are concerned about white mold infections. Soil moisture remains good.
Wheat harvest continues to drag on. Scattered rainfall is stretching wheat harvest out. Yield reports continue to be disappointing.
Summertime weather at
last! High temperatures were above 80 degrees 11 out of the last 14 days. Low
temperatures have been in the upper 50’s to lower 60’s. Rainfall totals have
been variable, ranging from one to four inches or more inches over the last two
weeks. Average relative humidity has been in the upper 70’s to upper 80’s.
These are higher humidity levels than what we have been experiencing.
In alfalfa, second and now third cutting harvest is progressing and will continue for the next few weeks as regrowth varies greatly due to delayed harvest schedules. Fields need to be monitored for potato leafhopper. Harvesting dry hay continues to be very difficult everywhere.
Wheat harvest has begun and is progressing slowly.
Grain moisture is high due to a lack of good drying weather. Vomitoxin levels
are variable and generally above 3ppm. Oats and barley are
turning color.
The corn condition overall has
improved through most of the region. Growth stage remains variable. Most corn
is V8 to tassel emergence.
Soybeans, like corn, are variable in condition and growth stage. Potato leafhopper injury has been observed in soybean. Soybean aphid numbers remain low. Dry beans are growing well overall. Potato leafhoppers are an issue for fields that did not receive a planting insecticide.
The region has received scattered showers over the last two weeks. The amounts have varied greatly with most areas receiving about one inch with reports of much higher amounts in isolated areas. The warmer weather has been beneficial but we are still behind in crop development. Some areas of the region were getting dry and the crops were showing drought stress. This week’s rains brought relief. In general, soil moisture is adequate.
The corn crop has been progressing with the warmer weather. We still are behind in crop development. The early-planted corn has been tasseled for about a week with the balance of the crop just beginning to tassel. No real problems in the crop at this time.
Soybeans generally look good. The early-planted fields have set pods on the lower nodes with most fields flowering. Weed control has been a problem in some of the conventional soybeans fields. We have not seen any soybean aphids at this time. We will continue to scout fields for this pest. There have been some reports of leafhopper in soybeans that were mistaken for the aphids. Scout fields regularly and if you find insects that you cannot identify, contact the Extension office or your agribusiness for assistance in correctly identifying pests.
The wheat harvest is just starting in Isabella County this week as the weather conditions allow. The southern part of the region has been harvesting for about a week. Reports of vomitoxin levels have varied greatly. The yields and test weight have been about average. Vomitoxin levels vary from field to field depending on flowering date and weather conditions.
Harvest of second cutting alfalfa is well underway. Some farmers have finished and are reporting very good yields and quality. There is some first cutting still being made. Growers need to scout alfalfa for leafhoppers and treat if this pest is at threshold.
Dry beans went in late but are growing rapidly. Except for the late planting, the crop looks very good. Leafhoppers are present and insecticide applications are being made.
Sugar beets have filled the rows and are looking very good at this time.
Oats have begun to ripen and look pretty good after a slow start. Some oat growers think they will do better with oats this year than their wheat.
It finally feels like summer in this area. Evaporation and moisture uptake by plants is out pacing rainfall. This is a welcome sight.
Some alfalfa fields have been cut for second cutting but most are not yet cut due to late first cutting. Growth of alfalfa remains good due to rainfall.
Sugar beets are looking pretty good in many areas but management is still important. Cercospera leafspot has been confirmed in many areas. This is consistent with the Beetcast leafspot prediction model, which is now showing Disease Severity Values (DSV) over 70 in most areas. Many fields have received a first fungicide application to protect against infection. Rhizoctonia root rot is showing up in some fields also.
In corn, early fields are pollinating now with good conditions. Of course, within these pollinating fields there are areas that haven’t tasseled yet, so pollination of some of these damaged areas may be less than perfect. Growth of late-planted corn is quick with GDU’s accumulating with high temperatures. These conditions (85 degrees and 80 percent humidity) can be pretty tough for us people to deal with, but the corn loves it.
Wheat harvest is well under way in the Saginaw Valley with the Eastern Thumb just getting started. Vomitoxin levels are variable this year. Some loads have been testing less than 0.5 ppm while others are well over 5 ppm. Timing of flowering seems to be the most important factor affecting the level of headscab infection and resulting vomitoxin.
The soybean crop is growing well in most areas. Wet fields are still recovering with slower plant growth while others are growing quickly. Soybean cyst nematode symptoms can be found in problem areas. Foliar diseases are also easy to find on lower leaves of the plants but seem to be staying away from new growth.
This doesn’t appear to be a record-breaking year for dry beans but some fields are shaping up nicely. Potato leafhopper pressure continues to keep most growers scouting and treating where necessary.
Hot temperatures have returned for a while again. Last week (Tuesday, July 20) there was a freak hailstorm that made its way up through the town of Posen. Rainfall ranged from 1.5 inches to close to 5 inches of rain. Hail the size of tennis balls pummeled the area for 10 to 15 minutes, breaking windows in vehicles and houses as well as destroying the south facing siding on over 100 houses. Needless to say the crops in the area six miles long and about 1.5 miles wide did not fair well. Corn was stripped down to a 10-inch stalk, potatoes were stripped and broken, oats and wheat lost over 50 percent due to breakage and heads clipped off, and alfalfa was stripped and broken.
Corn is doing excellent at this point. Later corn is still slow and may struggle to make it the rest of the way. Earlier corn is about a week away from tassle.
Wheat is starting to turn color. Scab is showing up in most fields with some heavy scab levels in some fields.
Soybeans are starting to look good finally. Most fields have been sprayed for weeds now. Early fields are in R1 stage. No signs of aphids yet, but we are looking hard now.
Dry beans are flowering in earlier fields and still at the third trifoliate on later fields. Most fields look pretty good but we need continued warm and weekly rains to get the crop to maturity.
Second cutting is underway in most alfalfa fields. Potato leafhopper is near threshold in some fields, but fields will be cut soon. Weevil is not a problem yet.
Oats are turning with most fields looking good. Potatoes are slow this year. Some earlier fields are in flower with tuber set pretty good. Again, we need good weather later into the fall to get a good crop.
Temperatures gradually warm back up to normal levels by next Tuesday. No rain is forecast Thursday through Tuesday. This pattern continues in the 6-10 day (July 26-30) and 8-14 day (July 28-August 3) periods. Both forecasts call for normal temperatures and below-normal precipitation.