Section 4 - Insect Management

Wheat Stem Sawfly

Wheat stem sawfly (Cephus cinctus) is native to North America, however it is still a very important insect pest, especially in western North Dakota and Montana. The sawfly infests spring, winter,

and durum wheat. It not only causes reduced yield and quality, but also lodging

which can make harvest difficult if not impossible.Female sawflies deposit their eggs directly into the wheat stem. The mature larvae cause damage by chewing a notch around the inside of the stem which often results in lodging.

Current management practices for the sawfly are limited. Insecticide treatments are often ineffective because the egg and larva are protected in the stem. It is also difficult to reduce the adult sawfly population due to its extended emergence. Because each female lays eggs in multiple stems, and multiple females lay eggs in a single stem the adult population must be reduced at least 90% before there is a reduction in infested stems. Solid stemmed varieties are less susceptible to sawfly infestation

and less likely to lodge. However, many solid stem varieties do not yield as well as the hollow stemmed varieties

(in the absence of sawfly).Research was initiated in western ND in 2008 to expedite development of higher yielding solid stem varieties and to further

investigate the potential of using an insecticide to control wheat stem sawfly. Research will be continued to determine

if there is an insecticide treatment that will effectively control wheat stem sawfly. Until an effective treatment is determined, insecticide use is discouraged to conserve beneficial insects that are parasitoids of the sawfly.

Hessian Fly

The Hessian fly overwinters as a maggot or pupa in winter wheat, volunteer grain, and wheat stubble. Overwintered maggots pupate and emerge as adults from April to May, infesting winter and spring planted wheat. By June, maggots

pupate, emerging as adults in August to lay eggs for the overwintering generation.

Thimet and Cruiser are registered as a planting time and a seed treatment for wheat, respectively, but populations of this pest rarely warrant the need for such treatments in North Dakota. Burying stubble and destroying volunteer grain after the first killing frost or early in the spring before fly emergence helps suppress adult populations.

Soybean aphid

Begin scouting fields at V3-V4. However, no treatment recommended at this time and is discouraged, so insecticides do not reduce the presence of predators and parasites of soybean aphids, such as the Multicolored Asian lady beetle. The critical growth stage for making most soybean aphid treatment decisions appears to be the late vegetative to early reproductive stages (Vn-R3). Assessing aphid populations at this time is critical. Research has found that the best results from an aphid treatment occurred from mid-July to early August. Treatment to manage soybean aphid would be recommended at growth stages R1-R4, when aphids are abundant on most plants. Treatment guideline: When aphids number 25 or more per sampled leaflet or 250 total aphids per plant.

The recommended economic threshold for R5 (seed fill) growth stage is increasing aphid populations and above 250 aphids per plants on 80% of the plants in field. This would require multiple scouting trips to determine a growing aphid populations and not static one. Research indicates little yield benefit to treating soybean aphid populations at the R6 (full seed) growth stage. Common sense should be used in determining treatments for soybean aphid in R5/R6 soybean including yield expectations, net returns, moisture stress, and increasing or decreasing aphid populations.With high temperatures (>90 F) the upper limit of any foliar insecticide is about 2 weeks, with one week to 10 days more realistic. On the positive side, high temperature above 90 F will also slow and eventually stop reproduction of soybean aphids and increase mortality rates (shorter life span of only 11 days compared to a typical 30-39 day life span).

Synthetic pyrethroid insecticides (e.g. Warrior, Asana XL, Decis 1.5 EC, Proaxis, Taiga Z) have been shown to generally provide longer suppression of aphid populations than most organophosphate insecticides (OPs, e.g. Lorsban 4E, Penncap-M, Orthene). However, the OPs have been observed to work more quickly, and are not affected by high temperatures of >90F like SPs are. Keep preharvest interval in mind, which can range as little as 14 days for Orthene 97 Pellets and Lannate LV to as much as 60 days for Pounce 3.2 EC and Arctic 3.2.

Soybean aphids have tended to initially colonize fields near shelterbelts, southern field edges, or perhaps some other protective structure. Particular attention should be given to smaller fields (less than 30 acres) that are surrounded by wooded areas. Check field edges first to detect the earliest colonization. Surveys have indicated that it takes about 3 to 4 weeks for aphids to be detected in other areas of the field. Eventually, the larger colonies develop where initial colonization occurs.

Another method of sampling developed at the University of Minnesota is a “speed scouting” sampling method recommended for use through the pod fill stage. If a plant has less than 40 aphids, consider it non-infested; however if the plant has 40 or more aphids (counting additional aphids is not necessary after 40), consider the plant infested. Based on the speed scouting sampling plan, three treatment decisions are possible: 1. Do not treat the field; 2. Treat the field; or 3. Resample the field in 3-4 days. More information about the speed sampling method on the Internet: www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm.An NDSU Bulletin on managing soybean aphids can be found online at www.ag.ndsu.edu/pubs/plantsci/rowcrops/e1232w.htm.

European Corn borer

Managing corn borer is a challenge due to the lengthy emergence interval of the moths from overwintering. In N.D., borers have the potential

for one or two generations during the season. The two generation borers are present in the southern region of the state. They begin emerging in early June and represent the first flush of larval feeding. The single generation borer is presMore

than 100 soybean aphids on the underside of a soybean leaf. Pest feeding can inhibit the plant’s ability to make grain, or kill it outright. (Photo John Obermeyer, Purdue Extension entomologist)ent throughout N.D., emerging from mid-June to August. The challenge is to distinguish

when egg laying and larval populations can be tolerated or if they need to be controlled. Corn should be monitored weekly for at 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 N.D. developed in mid-to-late July and August as a result of the late emergence of the numerous single generation type borers. In other years, the two generation borers emerging first may contribute more to significant infestations.

Degree day models havebeen used to predict the flight occurrence of corn borer moths, and can help identify priority times for field scouting.

Degree day information for corn borer can be found online at www.soils.wisc.edu/wimnext/corn/euroborer.html and http://ndawn.ndsu.nodak.edu/index.html. Field scouting worksheets and economic threshold guidelines can be found in the 2009 N.D. Field Crop Insect Management Guide, online at www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm. Scroll down and click on “Corn Insects.”Worksheets and more information for treatment decisions can also be found online at www.nwroc.umn.edu/Cropping_issues/2008/Issue9/ 07_15_08_no2.htm.

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