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4/4/2011 - 4/10/2011

More Bean Leaf Beetles Predicted This Summer

By Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative

The last few years have been harsh on overwintering bean leaf beetles in Iowa. A combination of cold winter temperatures and the increased use of insecticidal seed treatments have curbed bean leaf beetle densities throughout much of the state. There is also decreased risk and incidence of bean pod mottle virus as a result of low beetle populations, as summarized in an April 2010 ICM News article on the topic.

Adults have adapted to Iowa's winters by protecting themselves in leaf litter and under snow cover, but they are still susceptible to cold temperatures. If exposed, adults will die if the temperature is below -10°C. An overwintering survival model was developed by Lam and Pedigo from Iowa State University in 2000, and is helpful for predicting winter mortality based on accumulating subfreezing temperatures. A 20-year historical record of predicted bean leaf beetle mortality in central Iowa is available online. Figure 1 is a map of the predicted 2010-2011 winter mortality in Iowa. In general, Iowa experienced moderate winter temperatures, with predicted mortality ranging from 60-99 percent. These numbers are lower compared to recent winters 2008-2009 and 2009-2010. The mortality prediction model indicates the southern third of the state is likely to see more overwintering adult survival compared to the last few years.

 
Figure 1. Predicted overwintering mortality of bean leaf beetle based on accumulating subfreezing temperatures during the 2010-2011 winter.


Overwintering adults are strongly attracted to soybean and will move into newly emerging fields. Bean leaf beetle is easily disturbed and will drop from plants and seek shelter in soil cracks or under debris. Sampling early in the season requires you to be sneaky to estimate actual densities. Although overwintering beetles rarely cause economic damage, their presence may be an indicator of building first and second generations later in the season.

 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. Hodgson can be contacted at ewh@iastate.edu or phone 515-294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at ajsisson@iastate.edu or by calling 515-294-5899.

Alfalfa Weevil Hatch Happening Now

By Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative

Alfalfa weevil is an important defoliating pest in alfalfa. Heavy infestations can reduce tonnage and forage quality. Adults can feed on plants, but the larvae typically cause the majority of damage.

Newly hatched larvae can be found feeding on terminal leaves, leaving newly expanded leaves skeletonized. Gradually maturing larvae (Fig. 1) move down the plant and begin feeding between leaf veins. Alfalfa weevil larvae have a dark head and pale green body with a white stripe down the back. Alfalfa larvae are about 5/16 inches long. Adults (Fig. 2) eat along the leaf margin, leaving irregular notches. Alfalfa weevil adults have an elongated snout and elbowed antennae. Their wings and body are mottled or brown in color. A heavily infested field will look frosted or silver (Fig. 3).

alfalfa weevil larvae      alfalfa weevil adult
Figure 1. Alfalfa larvae.
                         Figure 2. Alfalfa weevil adult.     
Fig. 1 Photo by Clemson Cooperative Extension Slide Series,
ipmimages.org. Fig. 2 Photo by Joseph Berger, ipmimages.org.


alfalfa weevil damage
Figure 3. Heavily defoliated alfalfa fields appear frosted from a distance. Photo by Whitney Cranshaw, Colorado State University, ipmimages.org.

Alfalfa weevils develop based on temperature or accumulating degree days. Scouting in fields should begin at approximately 200 degree days for areas south of Interstate 80, and 250 degree days north of Interstate 80. Based on accumulated temperatures since January, weevils are likely active now, or will become active in the next several days, in the southern half of the state (Fig. 4). We would expect weevils to become active in northern Iowa by April 20 - 25. To follow accumulating degree days throughout the year, visit the ISU Mesonet website.  The base 48 F degree day map is updated daily at: http://mesonet.agron.iastate.edu/data/summary/gdd48_jan1.png


Figure 4. Accumulated growing degree days (base 48°F) in Iowa from January 1 – April 7, 2011.
Map courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy.


To initially detect alfalfa weevil larvae in the spring, use a sweep net to sample. After finding larvae, collect six alfalfa stems from five locations throughout the field. Take each stem and shake into a bucket to dislodge larvae from the plant. Average the number of larvae per stem and plant height to determine if a foliar insecticide is warranted (Table 1). Remember cutting alfalfa is an effective management tool for alfalfa weevil larvae, and an insecticide application may be avoided if harvesting within a few days.


Table 1. Economic threshold of alfalfa weevil, based on the average number of a 30-stem sample

 

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. Hodgson can be contacted at ewh@iastate.edu or phone 515-294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at ajsisson@iastate.edu or by calling 515-294-5899.

A 2011 Prediction for Stewart's Disease of Corn

By Forrest W. Nutter, Jr., Alison Robertson, and Sharon Eggenberger, Department of Plant Pathology; Erin Hodgson, Department of Entomology

Although December, January and February temperatures were somewhat warmer across Iowa, compared to the 2009/2010 winter, the risk for Stewart’s disease of corn in 2011, based on two predictive models, is negligible to very low throughout most of Iowa.

Stewart’s disease, also known as Stewart’s wilt, is caused by the bacterium Pantoea stewartii. An insect vector, the corn flea beetle (Figure 1), plays a critical role in the overwintering survival and plant-to-plant spread of this microorganism. The bacterium survives the winter months within the gut of hibernating corn flea beetles. If winter temperatures are mild enough for corn flea beetle populations to survive locally, the bacterium will also survive. In the spring, surviving corn flea beetles infested with P. stewartii will emerge from grassy areas near corn fields and, as they feed, transmit the pathogen to corn seedlings.

               corn flea beetle

Fig. 1.  Corn flea beetle on a corn seedling leaf. Photo courtesy of Paul Esker, Univ. of Wisconsin.

Stewart's disease can occur at any stage of corn development, but symptoms are almost always associated with flea beetle feeding. Corn seedlings can wilt rapidly from systemic infection, and seedling death is common, especially in seed corn and sweet corn fields. Plants that survive the seedling wilt phase will be stunted and will serve as a source of the pathogen, for future generations of corn flea beetles to acquire and transmit throughout the crop.

Plant-to-plant spread by overwintering corn flea beetles will continue until late May, when corn flea beetles lay their eggs at the base of corn plants. The overwintering generation of adult corn flea beetles then dies. In early to late June, there is a “beetle-free” period, lasting two to three weeks, which ends as the next generation of adult corn flea beetles (known as the first summer generation) begin to emerge and feed. During this “beetle-free” period, foliar insecticide sprays are not effective.

The first summer generation of adult corn flea beetles emerges in late June, and feeds on infected corn plants. The beetles can acquire the bacterium and facilitate the further spread of the pathogen to healthy corn plants. Later in the growing season, usually after pollination, the leaf blight phase of Stewart’s disease may occur. Diseased plants at this phase first exhibit long, wavy, water-soaked streaks (lesions); diseased leaf tissue then turns yellow and dies (Figure 2). Corn flea beetle feeding scars are usually visible within the lesions. If the disease is severe, whole leaves may wilt and die.

stewarts disease

Fig.2. Early symptoms of Stewart's disease on a corn leaf. Photo by Alison Robertson.

A second generation of corn flea beetles will emerge about mid-August. Insects from this generation overwinter. If the overwintering adults harbor the bacterium, the pathogen can also survive until the next growing season.
   

Disease prediction models
Mild winters during the late 1900s and early 2000s resulted in the occurrence of severe epidemics of Stewart's disease in Iowa. However, severe winters and the widespread adoption of planting insecticide-treated seed have greatly reduced corn flea beetle populations throughout Iowa in recent years. Two disease prediction models are available to predict the seasonal and county-level risk of Stewart’s disease: the Stevens-Boewe Index Model and the Iowa State Mean Monthly Temperature Model. Both models use the monthly mean winter temperatures for December, January and February to predict the degree to which corn flea beetle populations survived the winter.

The Stevens-Boewe Index predicts the severity (how much of the corn leaf tissue is infected) for the mid-to-late season leaf-blight phase of Stewart's disease based on the sum of the mean temperatures for December, January and February. A sum below 80 indicates a negligible risk; 80 to 85 is considered a low risk; 85 to 90 indicates moderate risk; and greater than 90 is considered a severe risk. The summed monthly mean temperatures for the nine Iowa agricultural climate districts are presented in the map below:
 

Stevens-Boewe prediction
Fig. 3. The Stevens-Boewe risk level for the 2011 growing season is “negligible” for all nine agricultural climate districts in Iowa.
Temperature data (summed mean temperatures for the months of December, January, and February, shown above) was obtained from the Bureau of Climatology, Iowa Department of Agriculture and Land Stewardship.


The Iowa State University Stewart’s disease model predicts the prevalence of Stewart's disease, with prevalence being defined as the percentage of fields predicted to be infected by the Stewart’s disease bacterium. A high prevalence of Stewart's disease is predicted if the mean monthly air temperatures for December, January and February are each above 24 degrees F. Eight of the nine agricultural climate districts had either zero months or just one month above 24 degrees F. This indicates that survival of large corn flea beetle populations is highly unlikely this winter, in all but the southwest climate district, which has a moderate risk. The Iowa State Model prediction for 2011 is shown below (Figure 4).

Continuous snow cover in parts of Iowa from late December through February could have functioned as an insulation blanket to protect corn flea beetles from subfreezing temperatures. Even so, we predict that corn flea beetle populations in 2011 will be extremely low and spotty this spring, so the anticipated risk of damage due to Stewart’s wilt based upon both models is negligible-to-low statewide.


ISU prediction 
Fig. 4.  Predicted risk of Stewart's disease in Iowa agricultural climate districts in 2011, calculated using the Iowa State Model.
0 = negligible risk, 1 = low risk, 2 = moderate risk, 3 = high risk.


Insect Management
Corn flea beetle, the vector of Stewart’s disease, can be suppressed with IPM tools such as hybrid selection, scouting and insecticides. Areas with potential risk should incorporate resistant hybrids to minimize adult attraction and subsequent egg laying. Susceptible hybrids planted in historically infected areas should be planted later to discourage adult colonization. Regardless of hybrid selection, all corn fields should be scouted for adult corn flea beetles several times a week during emergence and seedling stages. Look for the shiny, black adults feeding on leaves. Try to walk quietly, as they are easily disturbed and will jump off the plants. Also look for long, light feeding scars on the leaves.

Seed treatments may also provide early season management for the beetle and Stewart’s wilt. A 2000 study at the University of Illinois demonstrated that two insecticides, imidacloprid (Gaucho®) and thiamethoxam (Cruiser®), applied to sweet corn seed, reduced the incidence of Stewart's wilt by 50 to 85 percent under field conditions with naturally occurring populations of corn flea beetles. According to the researchers, these seed-treatment insecticides controlled Stewart's disease during the very early growth of corn seedlings when applications of conventional, foliar insecticides were ineffective. The full article, Control of Stewart’s Wilt in Sweet Corn with Seed Treatment Insecticides, is available online.

Consider using treatment threshold guidelines to protect yield. Corn flea beetle can be controlled with timely applications. Currently labeled products include pyrethroids (Asana XL, Mustang Max, Warrior II), organophosphates (Lorsban 4E, Nufos 4E), and carbamates (Lannate LV, Sevin 4F). See manufacturer's labels for use rates and restrictions. Use the following thresholds for rescue treatments in corn:

  • Field corn--prior to stage V5, 50 percent of plants with severe feeding injury and five or more beetles per plant.
  • Seed corn--on susceptible inbreds, 10 percent of the plants with severe feeding injury and two or more beetles per plant.
     

 

Forrest W. Nutter, Jr. is a professor in the Department of Plant Pathology working on disease risk models for improved disease management. Alison Robertson is an assistant professor in the Department of Plant Pathology with extension and research responsibilities. Sharon Eggenberger is a Research Assistant in the Department of Plant Pathology. Erin Hodgson is an assistant professor in the Department of Entomology with extension and research responsibilities.

Corn Growth Classic Rewritten and Available to Order

By Lori Abendroth and Roger Elmore, Department of Agronomy; Willy Klein, Extension Communications

ISU Extension's corn production team has completed a new publication. “Corn Growth and Development” (PMR 1009) replaces “How a Corn Plant Develops,” the previous Iowa State publication that served as the standard reference on corn growth and development for more than 40 years.

“How a Corn Plant Develops,” written by Iowa State University agronomists of previous eras, established the basics still used today for staging and communicating about crop development. The late John Hanway, a well-known ISU agronomist, wrote the first version in 1966, which was followed by a rewrite in 1982 by Steven Ritchie, Hanway and Garren Benson. 

The team knew from the very beginning that if they were going to remake this classic publication, they would need to contribute something new and fresh and have some very talented people on the team. Otherwise, there wouldn’t be a point to redo it because it already was very useful and popular.

Authors of “Corn Growth and Development” are Lori Abendroth, ISU associate corn agronomist; Roger Elmore, ISU Extension corn specialist; Matthew Boyer, former ISU agronomy graduate student; and Stephanie Marlay, ISU agronomy specialist.

The 2011 publication provides an in-depth look at corn, from the moment the seed is planted all the way to maturity. It takes much of what is known about crop physiology and combines that with field agronomics to provide students, corn growers and agronomists the current and technical information they want and can use.

To develop the new publication, the team conducted multiyear research trials, read piles of research papers, grew hundreds of plants for the photography sessions and spent hours working with editors and designers. One of the first steps was to conduct biomass and nutrient accumulation research, followed by four years of various research trials to fine-tune recommendations and facts in the book, and a year of growing plants for the publication images.

Much of the framework that was successful in the previous versions, including numerous color images and graphics, and descriptive text, has been retained in the new book. “Corn Growth and Development” is more than twice as long as the original, featuring 50 full-color pages, and is based on a new generation of corn hybrids and production research.

The new publication weaves the newest scientific facts regarding corn growth and development throughout the pages in a way that is concise and easily applicable for people in production agriculture. Key features include:

  • more than 90 images, including whole-plant images from emergence to maturity
  • detailed descriptions of vegetative and reproductive development
  • new dry matter and nutrient (N, P and K) accumulations figures
    clarification of corn development staging methods
  • an expansive list of end notes with agronomic research references


“Corn Growth and Development” can be ordered from the ISU Extension Online Store (www.extension.iastate.edu/store) for $14 per copy. Publication images will soon be available to purchase and download from the same location.

 

 

Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production. Abendroth can be contacted by email at labend@iastate.edu or (515) 294-5692.



This article was published originally on 4/11/2011 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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