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ISSUE 11   July 19, 2007

WHEAT STEM MAGGOT CAUSING WHITE HEADS IN WHEAT

Wheat stem maggot has been observed causing over 20% white heads in wheat fields, especially in western and north central North Dakota (see map).

Wheat Stem Maggot in Wheat map

The "white" heads (see photo) are caused by a single larvae (see photo) mining internally in the stem. The plant stem pulls out very easily and if larvae are not found, the stem is usually chewed off. Due to the large volume of recent calls, it fair to say that "Producers are increasingly concerned about the high amount of white heads in fields and their potential for yield loss."

White wheat heads            Wheat stem maggot larvae
"White" wheat head                                                Wheat stem maggot larvae

Wheat stem maggots overwinter in the larval stage, inside the lower parts of grass stems. In the spring, the larvae pupate and adults emerge in June. The adults are yellowish-white flies, about 1/5 inch long, with three conspicuous black stripes on the thorax and abdomen and bright green eyes (see photo).

Wheat stem maggot adult
Wheat stem maggot adult

After mating, females deposit their eggs on the leaves or stems of grasses. The young maggot crawls down beneath a leaf sheath and tunnels into the stem. The stem is partially severed causing the head to turn white. The larva pupates within a cigar-shaped, pale green puparium. The adults emerge about midsummer and lay their eggs on wild grasses or volunteer grain. The resulting larvae overwinter in the stems of the wild grasses and volunteer grain. The principal cultivated crop hosts of the wheat stem maggot are wheat (bread and durum), rye and barley, with wheat being preferred. It also attacks bluegrass, timothy, quackgrass, slender and western wheat grass, wild barley, bromegrass, green and yellow foxtail and bluestem grass.

No chemical control is currently recommend for management of the wheat stem maggot. However, preliminary insecticide trials conducted by agronomy dealers indicate that in severe infestations can be reduced by an early spray timing. However, research is still needed to validate these studies, to understand proper insecticide spray timings in relation to emergence /densities of wheat stem maggots, and to determine if there is an economic benefit to spraying.

 

SCOUTING FOR ADULT BANDED SUNFLOWER MOTHS

Another method for scouting for banded sunflower moths besides egg sampling is scouting for adult moths. During the day (late morning to early afternoon) the moths remain quiet, resting on upper or lower surfaces of the leaves of sunflower plants. When disturbed, they flutter from plant to plant. When sampling for moths during the day, the decision to treat is based on comparing the mean number of adult moths in the field to the EIL for moths. The EIL is the number of moths per head that will, if not managed, result in seed damage with a value equal to the cost of treatment. Use the following formula based on treatment costs, plant population and market price to determine the adult EIL for day sampling.

EIL
(moths per 100 plants)

=

(

(Treatment Cost ($) / Market Price)

)

x 582.9

)

- 0.7

Plant Population

The constants in the formula simplify the calculation and include the amount of loss attributable to each banded sunflower moth larva produced per moth.

A sample calculation of the EIL based on moth sampling for the following conditions is given below.

Insecticide treatment cost = $8.00/acre
Market price = $0.17/lb.
Plant Population = 20,000/acre

EIL

=

((

($8.00 / $0.17)

)

x 582.9

)

- 0.7

= 0.67 moths per 100 plants

20,000

For this set of variables, an infestation of about 0.67 moth (or 1 moth) per 100 plants will result in sufficient larvae to destroy seeds in the sunflower head equal to the $8.00 treatment cost per acre in a field of 20,000 plants per acre with a market value of 17 cents per pound. If the adult population has reached or exceeded this level, then the grower should consider the use of a chemical insecticide to prevent larval seed damage.

 

SCOUT FOR EUROPEAN CORN BORER EGG MASSES AND YOUNG LARVAE

Corn should be monitored weekly for least five weeks once plants exceed an extended leaf height of 17 inches. At this point, corn borer larvae will be able to survive on the plant. Inspect plants for the presence of egg masses, whorl feeding, and active larvae. Observing moth activity around field margins or within the field may alert you to developing infestations. Recent corn borer infestations in ND developed in mid to late July as a result of the late emergence of the numerous single generation type borers.

The females deposit their eggs (see photo) on the undersides of the leaves near the midvein.


European corn borer egg masses

It takes 3 to 7 days for eggs to hatch, depending on temperatures. The egg masses are 1/8 to 3/16 inches long and contain about 20 to 30 eggs. The eggs, when laid, overlap like fish scales. As the eggs develop, they change from white to a creamy color. Just prior to hatching, the black heads of the larvae become visible through the shell; this stage is referred to as the "black-head" stage. After hatching, larvae move quickly into the whorl and begin to feed. Their feeding results in shot-holing of the leaves. This damage becomes more apparent as the leaves lengthen and emerge further from the whorl. When larvae are about 10 days old, they reach a length about equal to the diameter of a dime and begin to tunnel into the midvein of the leaf, then burrow into the stalk. Once inside the stalk, it is too late for corn borer control.

 

APPLICATION TIMING FOR CONTROL OF BANDED SUNFLOWER MOTH

Although scouting for eggs or moths is conducted, treatment is directed at the larval stage of the banded sunflower moth which is the actual damaging stage. Once the decision to treat has been made, it is critical to correctly time the spray application to get maximum control. The best sunflower plant stage to treat is when banded sunflower moth eggs have hatched and larvae are present, usually close to the R5.1 growth stage (beginning of flowering). At this time the larvae are beginning to feed on the disk flowers, are exposed on the head, and are susceptible to the insecticide treatment. On older plants where the seeds have started maturing, most larvae will be feeding within the seeds or under the protection of the florets and will be protected from the insecticide. By then, much of the feeding damage has already occurred.

Insecticides should be applied early in the morning or late in the day to minimize the adverse effect of the chemical on bees and other pollinators. For a list of current insecticides registered for banded sunflower moth control in North Dakota, please consult the North Dakota Field Crop Insect Management Guide website: http://www.ext.nodak.edu/extpubs/plantsci/pests/e1143w1.htm.

 

SCOUTING FOR SOYBEAN APHIDS CRITICAL DURING REPRODUCTIVE STAGES OF SOYBEAN

Although soybean aphid populations have been low, cooler temperatures this past week may accelerate aphid reproduction. The early reproductive (R1 to R5) stages are critical for optimal soybean development. Above 250 aphids per plant and increasing populations (economic threshold) during the R1 to R5 can result in a significant loss of yield and quality of soybeans. Scout 20-30 plants per field covering at least 80% of the field. To determine if populations are actively increasing, check field over several visits. This threshold provides a 7-day lead time between scouting and insecticide treatment. Spraying at R6 has not demonstrated a yield increase.

Two sources of weekly soybean aphid reports from multiple locations in North Dakota are available:

In PIPE, sentinel plots are monitored throughout the U.S. for soybean rust and soybean aphid. State commentaries are also provided.

 

BIOLOGICAL CONTROL IN ACTION: SOYBEAN APHID

The following article is extracted from an email of Ian MacRae (UMN).

Thought I'd share an excellent example of the importance of predation in preventing the development of soybean aphid populations. An 3' x 8' cage was established randomly in a soybean field near Fergus Falls, MN about 4 weeks ago (actually just to test the materials, not to gather aphid population data). When it was placed into the field, there were ~10 aphids in the entire caged area (far fewer than 1/plant). Since then, aphids in the surrounding field have become slightly more numerous, plants averaged ~1.8 aphids/plant. Inside the cage, however, plants averaged ~780 aphids/plant. The cage excluded predators and parasitoids while those outside the cage were open to this mortality. Several different predators, including lacewings and ladybugs, were noted in the field when the cage was established, and more now. The difference was truly striking (a statistics professor of mine used to refer to these situations as inter-ocular hypotheses - the difference is so great it hits you between the eyes!)

The first picture below is of a plant taken from inside the cage, where plants averaged ~780 aphids/plant. The second image is of a plant growing outside the cage (from a row immediately adjacent to the cage), plants outside the cage had ~1.8 aphids/plant (note no aphids visible on the stem or leaves). Inside the cage, cast skins and honeydew were becoming quite common on the leaf surfaces, plants outside the cage had no such evidence of aphid presence.

       

Same colonizing population, very different population development. Bottom line, natural enemies are having a significant impact on the establishment and development of soybean aphid populations. Give them a chance to work at low populations; looks like they're doing a great job here....

Ian MacRae, University of Minnesota

Janet Knodel
Extension Entomologist
Janet.Knodel@ndsu.edu


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