AgAlerts 2009 From Griggs County
By John Swenson, Griggs County Extension Agent

                                                                                                                        NDSU Crop Pest Reports

Issue # 7, June 16, 2009

• Alfalfa Harvest

• Bronze Birch Borers

• Wheat Stem Maggot Control

• Other Insects to Scout For

Alfalfa Harvest

    The stage in which alfalfa is determined has changed from 10% flowering to the PEAQ (Predication Equations for Alfalfa Quality) system.  This system uses height as the main determining factor when considering prime alfalfa harvest.  

The taller the stem, the earlier harvest must start in order to harvest prime hay.  For example, an alfalfa plant that is 20 inches tall can be harvested at late flower for the most mature stem and still make prime hay; whereas, a 30 inch tall alfalfa plant must be harvested by late vegetative stage in order to make prime hay. The following table show what growth stages are used in the PEAQ system.

To obtain greater than 151 Relative Feed Value (RFV) in the bale, harvest must begin when the PEAQ system estimates the forage quality at 175 to 180, since harvesting losses will reduce the quality of the final product.

Bronze Birch Borers 

    Are your birch trees starting to die from the top down?  If so you might have a problem with bronze birch bores.  Adult borers are slender, metallic-coppery beetles about 3/8 inch long and are rarely seen.  They are often termed “metallic wood boring beetles." Larvae occur underneath the bark and are white, segmented, legless grubs with an enlarged area behind the head (giving them the name “flatheaded borers”). They are about 1/2 inch long when mature.

Damage

    Initial damage is usually discovered when the top part of a birch tree suddenly wilts and dies.  Careful examination of branches and the trunk usually reveals raised ridges or bumps, commonly referred to as "gouting" of the branches. These are ridges formed when a larval tunnel is closed in by callous tissue of the tree.

    Often, D-shaped holes are found in the bark and these may be stained with rust colored sap. These holes are emergence holes made by adult beetles.  Heavy attacks and continued re-infestation result in most of the branches dying from the top down. Eventually the trunk is girdled and the entire tree dies.

Management

    Reducing problems with bronze birch borer starts with understanding your birch tree’s requirements. The following steps will help promote healthy, vigorous birch trees that are less susceptible to borer attacks than unhealthy trees. Bronze birch borers are not capable of successfully attacking healthy trees.

1. Select an appropriate location for your birch tree when planting.

    Ideal locations for birch trees are sites where the soil will remain cool and moist, but where the tree will receive full sunshine on its leaves most of the day. East and north sides of homes are generally best.

2. Select the most appropriate species or variety of birch when planting.

    The white-barked birch trees are more susceptible to bronze birch borer attacks than species without white bark (River Birch, Heritage River Birch, Yellow Birch, Sweet Birch).

3. Follow cultural practices that will create conditions conducive to tree survival.

•Sufficient watering is probably the most important factor in maintaining a healthy birch tree. During the growing season, a slow (2-3 hours), deep (8-18 inches) watering once per week is a general rule for maintaining adequate soil moisture.

•Mulching (2-4 inches deep) over the tree’s root system will help moderate soil temperatures and keep soils moist. Good materials are wood chips, shredded bark, or leaf compost.

•Pruning should not be done from May 1 to July 1. This is during the borer flight period and it has been shown that female borers are attracted to fresh pruning wounds. Wound dressings should not be used. Excessive pruning (more than 25% of the canopy) should be avoided as this increases light penetration to the root zone and can increase soil temperature and reduce moisture levels.  (article by Melissa Misek, Griggs County Extension Summer Intern)  

Wheat Stem Maggot Control

    NDSU has been receiving inquires about adding insecticides with herbicides at the 4-5 leaf stage to control wheat stem maggot.  At this time, there is little research on the  effectiveness or the economics of this practice.  The following is a summary of what research that has occurred to date.
    In 2008 emergence of the wheat stem maggot occurred at 1240 AGGD (Accumulated Growing Degree Days) with peak emergence at 1560 using the 32 degree wheat growing degree day model. Currently at the Dazey NDAWN station, 1297 AGGD have been recorded since April 1, 2009.  Entomologist in Fargo are beginning to collect wheat stem maggots in 4-6 leaf wheat using sweep nets so emergence is on schedule with 2008.  If temperatures remain in the 70-80 degree range we should see peak emergence in 10 to 15 days.
    To scout for wheat stem maggots a sweep net is needed.  Sweep areas of the field and look for the adult wheat stem maggot fly.  Adult flies are small (1/4 inch) long with a yellow-green thorax, black stripes on thorax and abdomen, and bright green eyes. The hind tibia is greatly enlarged.  There have been no economic thresholds developed at this time.  For a picture of the adult fly, click on the following link "Wheat Stem Maggot".
    The most effective control was at the 4-5 leaf stage prior to peak emergence.  In 2007, the populations of wheat stem maggot were higher than in 2008 which resulted in a yield response in 2007.  In 2008, there were no yield responses regardless of the timing of application.

Other Insects to Scout For

    Alfalfa Weevil has been found in southern North Dakota.  This insect has cause problems in past years.  Feeding injury will be on the top of the plant causing the leaves to have a white appearance.  Under heavy infestations, harvest should occur as soon as possible.

    Potato Leaf hopper can be a problem in newly seeded alfalfa.  Potato leafhoppers are not a problem in established stands.

    Forest Tent Caterpillar have been showing up in parts of the state.  These insects will form webs and will feed on ash, aspen, basswood, birch, cottonwood, elm, maple, oak, poplar, and other hardwoods.

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E-mail: john.swenson@ndsu.edu
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