Ramsey County

SOYBEAN APHIDS INCREASING

        Soybean aphid numbers are increasing in the Red River Valley area and eastern counties of North Dakota. Most fields still have low numbers, but a few fields are approaching threshold. This is the time to scout and monitor your fields for soybean aphid. The economic threshold for soybean aphid in the upper Midwest is:

        250 aphids/plant in 80% of the field and with soybean aphid numbers increasing.

        The economic threshold of 250 aphids/plant was arrived at through research conducted over a three year period at 19 locations in the northern US including North Dakota. The economic threshold was established to give growers a seven-day lead time for insecticide application before aphid populations reach a level where economic loss actually begins. Even though soybean market prices are high, the 250 aphid/plant threshold is still valid. This is because 250 aphids/plant is lower than the population at which yield loss can be measured and attributed to aphid injury. What this means in practical terms is that the lead time for insecticide application is reduced from seven days to three or four days. Therefore it is critical that growers monitor their fields closely.

        NDSU Extension Entomology has received a few calls from growers asking whether they should tank-mix an insecticide with their last glyphosate application even though soybean aphid numbers are not at threshold. We do NOT recommend this practice, nor do we recommend any insecticide application when the economic threshold has not been reached. Applying insecticides too early to control soybean aphid may result in a second aphid invasion, which will require a second insecticide application. Insecticides also kill natural enemies of soybean aphid (such as lady beetles and damsel bugs), and re-invading aphid populations can increase very rapidly in the absence of natural enemies. Some vendors are offering guarantees on second insecticide applications. While the guarantee covers the chemical, it may not cover application costs. There is also the risk of spider mite flare-up in fields that have been treated twice with pyrethroid insecticides because beneficial mites have also been killed. This may require a third application using an organophosphate insecticide (such as Lorsban) to control the spider mites. Natural enemies keep aphid populations from growing rapidly and may even keep aphid populations from reaching threshold. Conservation of natural enemies is of paramount importance in controlling soybean aphid - don’t spray unless and until you have to!

        Most fields are currently in the R1 growth stage (beginning flowering), though some are still in late vegetative (V) stages. The 250 aphid/plant threshold is valid from late vegetative through the R5 (early seed development) growth stage. Research has shown that insecticide applications at R6 (full seed) and beyond do not give a yield benefit. Therefore, insecticide applications for soybean aphid are not recommended after the R5 growth stage. Pyrethroid insecticides (such as Warrior) offer good control of soybean aphid. Several pyrethroids are labeled for soybean in North Dakota. Be sure to read and follow the label for application rates and pre-harvest interval restrictions.

WHY HERBICIDES KILL WEEDS – WHY NOT THE CROP? - QUICK REVIEW

        Selectivity is the ability of a herbicide to kill one species and not harm another.  We often take selectivity for granted. We apply herbicides at low rates and expect them to give adequate weed control but not kill the targeted crop.

        By what processes do herbicides work or what is their mode of action? Herbicides kill weeds or damage crops through a four step process.

4 Steps in Herbicide Mode of Action

1.  Contact or Retention - contact of the herbicide with the roots or retention of spray droplets on a leaf.

2.  Absorption – uptake of the herbicide molecules into the leaf, shoot, or root.

3.  Translocation – movement of the herbicide from the site of uptake to the location in the plant where it can cause damage such as translocation the roots to the leaves or from sprayed leaves to growing points or roots (contact herbicides don’t need to translocate).

4.  Inhibition at the site of action – typically, the herbicide molecule binds to a specific enzyme, blocking the production of essential products such as amino acids or blocking photosynthesis.

        Why are crops not harmed by herbicides? Simply, some plants are able to metabolize (or detoxify) the herbicide into non-toxic chemicals before they damage the plant. In most cases, the key to selectivity is a plant’s ability to rapidly metabolize the herbicide before it causes damage at the site of action. In these cases, the plant has other enzymes that can cleave off a side chain from the herbicide molecule. This often reduces the toxicity of the herbicide molecule because the molecules no longer has the right configuration or shape to bind tightly at the site of action. The next step in herbicide metabolism is often binding of sugars or amino acids to the molecule, which further detoxifies it. Safeners are added to some herbicides like Dual II Magnum, Harness, Option, and Laudis. These safeners trigger the crop to produce more enzymes to detoxify the herbicides, which increases crop safety. Under normal weather conditions, a crop plant may be able to metabolize a majority of the herbicide in several hours. On the other hand, a sensitive weed will only slowly metabolize the herbicide. During this time, the herbicide is able to bind to the site of action and kill the weed.

        Herbicide selectivity is based on metabolism in most herbicide-crop combinations such as with Accent, Dual, Callisto, atrazine, or Banvel in corn or Valor, Pursuit, Cobra, or Sencor in soybeans. The exception to metabolism-based selectivity is when the crop has an “insensitive” site of action, which means the target enzyme in the crop has a slightly different shape so the herbicide does not bind to it. This is the case with Assure, Select, and Poast Plus on broadleaf crops like soybean and alfalfa. Since these herbicides cannot bind to the site of action, they do not damage these crops.

        Roundup Ready corn and soybean are also resistant to glyphosate based on an insensitive site of action. The gene that was added to these crops produces an altered or insensitive enzyme that glyphosate cannot bind to. Therefore, the enzyme is not blocked and it keeps producing amino acids for the corn or soybeans. The Optimum GAT corn and soybean being developed by Pioneer uses a metabolism-based glyphosate resistance. In this case, a gene (GAT) was added to the crops, which produces a special enzyme to rapidly detoxify glyphosate before it damages the crops. Liberty Link corn and soybean also use metabolism-based resistance where the PAT gene was inserted into the crops, which produces an enzyme to rapidly detoxify Liberty.

        This year we have received a larger amount of crop injury calls. Herbicides from many different chemistries applied alone or on tankmix combinations have produced crop injury. Both soil-applied and post-applied herbicides have caused crop injury. The cool/cold weather we had earlier in the season certainly reduced the metabolism in plants causing the plants to be affected by the herbicides. Multiple herbicide/fungicide combinations require plants to simultaneously degrade pesticide of several different modes of action in the presence of reduce metabolism. In addition, the cool, wet weather allowed plants to form thinner cuticles on leaf surfaces rather than thick cuticles in dry/drought conditions. Thin cuticles allow faster and greater penetration of pesticide active ingredients thus further taxing the metabolic system in plants. I am sure there are other factors involved in the crop injury many of us have seen this year but these are explanations that may help answer why it happened.

Rich Zollinger
Extension Weed Specialist

SUNFLOWER BUD MOTH DAMAGING TO SUNFLOWER BUDS

        Larvae of the sunflower bud moth, Suleima helianthana (Riley), have been observed damaging sunflower buds in R1-R3 sunflowers. Typically, most of damage occurs mostly in the stalk. However, this year sunflower bud moth emergence is delayed and larvae are feeding in the bud causing injury to the developing head.

        The only time yield loss is noticeable is when larvae burrow into unopen buds, preventing proper head development. The larvae normally do not feed on developing seeds but confine feeding activities to the fleshy part of the head. The larva has a dark head capsule with a smooth, cream-colored body and is 0.31 to 0.43 inch (8 to 11 mm) at maturity.

        There are two generations of sunflower bud moth in North Dakota. Adults emerge from overwintering pupae between the last week of May to mid-June. A few days after adult emergence, eggs are deposited on the terminals of immature sunflower or on the receptacle of mature sunflower. Eggs also are deposited in leaf axils. The hatched larvae begin tunneling into the sunflower plant. The initial infestation in mid-June is characterized by an entrance hole surrounded by black frass, or insect excrement. Mature larvae pupate within the sunflower plant. Pupae move to the opening of the entrance holes formed in the stem or head tissue so that adults can emerge easily. The second generation adults appear in August. Infestation by the second generation larvae is not economically important.

        A field monitoring scheme and economic threshold has not been established for this insect since it is not of economic significance most years. Insecticide use is NOT recommended for control of sunflower bud moth, because the larvae are protected when feeding within the sunflower plants. As a result, insecticides will have limited efficacy.

NEW FORMULATION OF LORSBAN^® FOR 2009

        Dow AgroSciences will be releasing a new formulation of chlorpyrifos called “Lorsban Advanced^®” in 2009. It features: lower odor than Lorsban-4E^®, comparable efficacy, water-based formulation, and fewer VOCs (carbon-containing substances that, when exposed to air, volatilize into gases that contribute to ground-level ozone formulation). Lorsban Advanced^® will be registered on the same crops as Lorsban-4E^®, and listed in the North Dakota Field Crop Insect Management Guide for 2009.

NDSU Extension Service, North Dakota State University of Agriculture and  Applied Science, and U.S. Department of Agriculture cooperating.  Duane Hauck, Director, Fargo, North Dakota.  Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914.  We offer our programs and facilities to all persons regardless of race, color, national origin, religion, sex, handicap, age.  Vietnam era veterans status, or sexual orientation; and are an equal opportunity employer.

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Devils Lake  ND  58301
701-662-7027
email - NDSU.Ramsey.Extension@ndsu.edu

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