VOLUME 22 NO. 12                                                                                                                                                                             July 24, 2008

SUNFLOWER BUD MOTH PROBLEMS

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 unopened 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.

BANDED SUNFLOWER MOTH EGG LAYING UNDERWAY

Eggs of banded sunflower moth have been found on R3 stage sunflower fields. Fields should be scouted now for egg laying activity, especially R3 sunflower fields.  Sampling plans have been developed using both adult and egg counts. Adult moth and egg counts should be made when most of the plants are at the R3 stage.  A low power magnifier is recommended for egg counts. The new sampling system for adult moths should be conducted during the day (late morning or early afternoon). The moths remain quiet, resting on upper or lower surfaces of the leaves of sunflower plants during the day. When disturbed, they flutter from plant to plant. For details on adult moth and egg scouting and economic thresholds go to Banded Sunflower Moth E-823 (Revised) - http://www.ag.ndsu.edu/pubs/plantsci/pests/e823w.htm.  The economic threshold level has changed with the current high market value. When monitoring for adults during daylight hours, one banded moth per 100 plants is a reasonable threshold given the high 2008 sunflower market prices. For egg counts, the threshold level in 2008 is about 2-3 eggs per six bracts. Chemicals should be applied from the late bud stage (R4) to early bloom (R5.1). Border spraying can be an option. Spraying in early morning or late evening is preferred to minimize the impact of pollinators. Some common control mistakes include: not scouting; waiting too long to spray; and perimeter spraying only and not scouting the interior of the field.

SOYBEAN APHIDS INCREASING IN RED RIVER VALLEY

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.  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. 

WHITE MOLD IN BEANS

White mold is a common problem on edible beans, and can be found in many soybean fields as well. Yield loss from white mold is possible in either crop, but white mold is more frequently a problem for edible beans. The disease begins when spores germinate on senescing flower petals, and infection progresses into the stem. An infected stem will take on a dried bone color and may be shredded.  Sometimes a white fuzzy mold appears (where white mold gets its name), and small black survival structures are produced (sclerotia).  Because the infection begins on the senescing flower petals, the most important time to assess disease risk is at early bloom.  The highest risk for white mold occurs when a few environmental factors come together.

Soil saturation. The disease cycle begins when the survival structure of the fungus (sclerotia) germinates and produces a small mushroom-like structure (apothecia) full of spores. These spores are then dispersed, and can land on the senescing flower petals which can result in disease.  For this to happen, researchers have estimated that the soil must be saturated for about 10-14 days.  Canopy wetness.  Once spores are on the petals, a saturated canopy for a prolonged period of time (day or more) is necessary for the disease process to begin.  Rainfall and heavy dews during flowering increase risk of infection.

Temperature.  When it gets hot, white mold is less of a problem. Temperatures above 85 F will inhibit apothecia formation and disease development.  If you believe you may be at high risk numerous fungicides are available. Application timing is important, an application during the early bloom stage is recommended. For further information, consult the 2008 North Dakota Field Crop Fungicide Guide (PP-622), available at:

http://www.ag.ndsu.nodak.edu/extplantpath/fungicide.html.

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.