NDSU Crop and Pest Report

Entomology


ISSUE 8   June 24, 2004

SUGARBEET ROOT MAGGOT: COOL WEATHER DELAYING ACTIVITY

Weather impacts on flies: below-normal temperatures have persisted during the past couple of weeks, and the extended forecast suggests that a few more cool days are ahead. Cool and breezy conditions create unfavorable flying weather for sugarbeet root maggot flies. Movement of flies into sugarbeet fields will continue at a slow pace until better flying weather returns.

Fly activity: although a large amount of flies began infesting beet fields June 13-15, massive numbers have since been observed in sheltered areas (farmsteads, shelterbelts, road ditches, smallgrain fields and down-wind sides of utility poles). Those flies will continue to invade beet fields during the next week or so, with Sunday through Wednesday being the best days for activity.

Control strategies: growers that applied a liquid insecticide (Lorsban 4E, Nufos 4E, or Pilot 4E) during the past week should ready themselves to apply a second treatment early next week if high fly activity returns. This long fly emergence period will make 2004 a good year for using 2 split applications of a liquid insecticide for maggot control. Postemergence granules applied during the past 2-3 weeks should perform well if they were incorporated and if at least 1/4 inch of rain was received afterward. Always read, understand, and follow all pesticide labeling instructions and precautions - itís the law.

Mark Boetel
Research & Extension Entomologist
mboetel@ndsuext.nodak.edu

 

BUG BRIEFS

Aphids in the wheat are still very low in number. NDSU crop scouts have found a few more fields with aphids this week, but infested stem levels are below 25%. Treatments should be considered when aphid infestations reach a level when 85% or more of the stems have at least one aphid present prior to being fully headed.

As alfalfa is cut for hay, growers should be reminded to survey fields for the presence of alfalfa weevil. If weevil larvae are present, it is important to determine larval survival beneath the swath. If 8 or more larvae per square foot are present, they should be treated to avoid significant injury to regrowth alfalfa. Weevils have only been a concern in south central and south western counties.

There continue to be some scattered reports of crop damage due to cutworm feeding. With slow growth of plants and slow growth of the caterpillars, some of this feeding damage has lingered on for a while. In many cases, pupa are beginning to be found in these fields, indicating the end of feeding, and various cutworm moths are beginning to emerge and fly at night.

 

SOYBEAN APHID FIELD SCOUTING

First of all, there have been no reports of soybean aphid soybeans in our region yet. The latest report out of

Minnesota is the same, no aphids yet. There have been reports from Iowa of some isolated infestations in vegetative stage soybeans that did result in some very early spraying.

As we get closer to the time for monitoring soybean aphid in our fields, there is a new development in the area of scouting and decision making that should prove very useful.

When using the treatment threshold of 250 aphids per plant and 80% of the plants with these levels, the response from people who will do the field scouting is less than enthusiastic. No one wants to count all those aphids. Probably what will happen is that to streamline the process, people will do some counting and eventually "train" themselves to recognize what 250 soybean aphids per plant looks like.

This approach is used in some of our insect scouting programs already. In wheat, we recommend treatment when 85+% of the stems have at least one aphid present. In this case, it has been demonstrated mathematically that under those conditions, if you were to count aphids, you would find an average of 13 aphids per stem; the treatment threshold. These sampling methods are referred to as binomial sequential sampling. The binomial refers to two outcomes or classifications. The sequential sampling aspect refers to the process of keeping track of what is and isnít infested and then being able to conclude, often times quickly, that a number of plants needed to make a decision with high levels of reliability has been reached.

So, now we have a proposed protocol for a binomial sequential sampling plan for soybean aphid from researchers at the University of Minnesota. A discussion can be found at:

http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm

The description for this method is:

1. Do not treat that field
2. Treat that field, and
3. Resample that field in 3-4 days

An example of the worksheet which accompanies the internet discussion above is at:

http://www.soybeans.umn.edu/pdfs/2004aphidsampling/speed%20sampling.pdf

In addition, there are examples worked out on the internet of how the procedure will look under the three different outcomes.

This procedure has been worked out by Erin Hodgson, a student working with Dr. Dave Ragsdale and his soybean aphid research project.

 

WHEAT MIDGE UPDATE

The degree day accumulations for the start of wheat midge emergence are approaching. Wheat fields in the southern areas of the state that are heading this week will be in good shape. Here are the key degree day accumulations for important emergence events:

DD (40EF)

Wheat Midge Biological Event

1300

10% of the females have emerged

1475

peak emergence with 50% of females

1600

90% of the females have emerged

Currently we are at 1000+ DD in many of the southern counties, and about 900+DD in the east central areas. Certainly the cool temperatures are delaying midge development, maybe more so than the progress of the wheat. At the current rate of accumulating degree days towards midge emergence, we may still be about 9 days from emergence in the southeast.

Remember, overwintering midge numbers were low throughout the surveyed areas of the state and risk of infestation is not expected to be very high. However, if emergence is underway and the wheat crop is heading, and therefore at risk to infestation, scout the field to determine if adult midge numbers warrant treatment.

Deciding to treat

Wheat is ONLY susceptible to midge attack between heading and flowering. The decision to treat wheat for midge should be determined by growth stage, type of wheat being grown, and the number of adult midge found on wheat heads during the evening scouting activities.

Hard Red Spring Wheat

Treatments are warranted when 1 or more midge are observed for every 4 or 5 heads.

Durum Wheat

Treatments are warranted when 1 or more midge are observed for every 7 or 8 wheat heads.

Treatments after 50% of the first heads have flowered are not recommended due to reduced levels of efficacy and for the protection of a parasitic wasp that attacks the midge eggs.

Phillip Glogoza
Extension Entomologist
pglogoza@ndsuext.nodak.edu


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