2015 drought is responsible for current bark beetle issues


University of Idaho Extension, Idaho Department of Lands, and U.S. Forest Service offices in our region are currently getting inundated by calls about dead and dying conifers. Most of these trees are being killed by bark beetles, but the real culprit is stress from last summer’s drought and overstocking. If dry conditions continue this summer and fall, the number of trees killed by bark beetles could increase.

Bark beetles are a natural part of Idaho forests, and we have many bark beetle species. A University of Idaho publication titled “Field Guide to the Bark Beetles of Idaho and Adjacent Regions” catalogs over 100 species in Idaho. Different bark beetles attack different species and even different parts of conifers. For example, many of the browning Douglas-fir this year were attacked by beetles that specialize in small Douglas fir and Douglas fir tops. These are different species from the Douglas-fir beetle, which normally kills larger trees, as it did after the 1996 ice storms.

Other bark beetles are also on the upswing. Lodgepole pine and ponderosa pine are being attacked by pine engraver beetles (commonly referred to as by their genus name, “Ips”). We are also seeing an uptick in grand fir attacks by fir engraver beetles (“Scolytus”).

Why are they so active now?

All of these bark beetles are native insects which continually reside in Idaho forests. The primary reason they are more active now is the drought we had last summer. Drought stressed trees are more vulnerable to attack by bark beetles. The moisture stress trees experience as a result of drought is compounded when trees are overstocked (too many stems per acre) – the case in many Idaho forests.

To learn more about bark beetles and other forest insect and disease issues, consider attending the annual Idaho Panhandle Forest Insect and Disease Field Day, to be held in Bonners Ferry on Friday, July 15.  The program meets at 8:00 a.m. at the Boundary County Fairgrounds and will end by 5:00 p.m. Those wishing to participate should pre-register at the University of Idaho Extension Office in Boundary County by Friday, July 8. A $15.00 registration fee covers resource materials and refreshments. For registration questions, contact the University of Idaho Extension Office in Boundary County at (208) -267-3235. Registration forms can also be downloaded at http://tinyurl.com/2016ForestryFieldDay. The program is co-sponsored by University of Idaho Extension and the Idaho Department of Lands. Continuing education credits are available for pesticide recertification (6), Idaho Master Forest Stewards (7), Idaho Pro-Logger (7), and the Society of American Foresters (2).

What to do about it

Many of the trees dying now were attacked by bark beetles last year, but it typically takes several months after an attack for trees’ crowns to turn red. Generally, if you see brown trees in the summer, the bark beetles that caused that damage have already left that tree – cutting them will not affect bark beetle issues. Dead trees can be a safety hazard, so remove them from areas where that is a concern. Otherwise, standing dead trees (called snags) create habitat for a wide variety of birds, small mammals, and other wildlife species.

The exception would be Douglas-fir stems 12” or larger, or recently fallen Douglas-fir 8” or larger, which can harbor Douglas-fir beetles for up to a year after being killed. These trees should be removed from the site or debarked to prevent additional broods of beetles from attacking standing green Douglas-fir.

Prevention is the key

Insecticides or other direct controls are rarely used to manage bark beetles. The primary strategy with most forest insect and disease issues is to manage forests so they are naturally resilient to insects and diseases. Our soils are still relatively dry from last year’s drought. If we do not get additional rain this summer, current beetle populations could grow larger. It is almost never practical to irrigate forests. The main strategy in managing bark beetles is to reduce the number of trees competing for the same amount of moisture by thinning. Thinning can also be used to favor the most drought tolerant species for the site (usually pines and larch), which further aids forest resilience. For sapling trees, that means creating at least 12-15 foot spacing between tree stems (6 feet initially for lodgepole pine to avoid making them too bushy). For larger trees, spacing should be increased proportionally, up to 40 feet for trees with trunks 24 inches in diameter.

To avoid creating habitat for pine engraver beetles (the most common bark beetle to cause problems from downed green trees), avoid thinning lodgepole and ponderosa pines from November thru June, or remove, burn, or debark stemwood larger than 3” in diameter. For more information on treating slash from thinning while minimizing bark beetle risks, see “Managing Organic Debris for Forest Health” (PNW 609, available at http://www.cals.uidaho.edu/edComm/pdf/PNW/PNW0609.pdf ).

Bark beetles and other forest insects are a natural part of Idaho forests. The key to keeping them from killing more trees than you want them to is managing for the best species for the site, and keeping stand density low enough to avoid moisture stress on the trees.

By Chris Schnepf
UI Extension Forestry Educator

Guidelines for alfalfa pest control

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How do you determine if it is economical to spray for alfalfa pests? Use of the proper chemical for the pest in question, at the appropriate time, is critical to pest control. You may want to simply cut your hay early and then monitor closely for pests, rather than treat the stand with an insecticide then have to wait for the required interval before you can harvest. This article by Glenn Shewmaker, Extension Forage Specialist at the University of Idaho, outlines 7 points to consider in alfalfa pest control. His points include the following:

1. If you can see insect damage from your pickup, you are at least two weeks late in scouting for problems.
2. If you start spraying early in the season, you will probably need to continue throughout the season because you likely have taken out many of the beneficial insects that help reduce population surges of damaging insects.
3. Insect damage on alfalfa seedlings can reduce the stand for its life, so pay close attention to new stands.
4. It is usually best to cut hay early then monitor for resurgence of insect pests than to treat with an insecticide and wait for the preharvest interval.
5. An IPM strategy will optimize economic returns in alfalfa.
6. The more you use a specific insecticide, the less effective it may become because these applications can select for insects with resistant qualities to those insecticides.
7. Check extension information sources. Just because a crop adviser wants you to spray, it doesn’t mean it is economical. For example, there is no evidence that pesticide application for western flower thrips is warranted, despite the leaf distortion they cause.

Alfalfa weevil may be the first insect pest to appear in early spring. A common recommendation is to cut or spray when 40 percent of terminal buds show chewing damage.

Most thresholds are established based on sweep net data, so invest in one just as you would a soil probe for soil sampling. The standard sweep net is 15 inches in diameter with a 26-inch handle. The net should be swung in a 180-degree arc, such that the net rim strikes the top 6 inches of alfalfa growth. Sweep right to left, take a step and take a second sweep from left to right. Repeat until the desired number of sweeps is taken for the pest management guide you are using. After the last sweep, quickly pull up the net to force the insects into the bottom of the net bag and grasp the net bag about half way up. Slowly invert the bag to count insects. Some slow-moving insects, such as caterpillar larvae, can be emptied into a white bucket to be counted. Large numbers of active insects may require chilling the entire net bag to count highly active insects.

It is important that the proper chemical is used to target the proper insect and stage of development, and that the pesticide is registered for that use in your area. Pesticide registrations change frequently, so you must follow the label and beware of “internet solutions.” Most state extension services have web resources with reliable information. Industry representatives and websites are also excellent resources of information. Beware of “spray and pray” salesmen who think there is a silver bullet for every problem.

We must be good stewards of the technology for pest management, or it will become rapidly ineffective or illegal to use. Think of the long-term effect of applying pesticides. The most practical and economic strategy may be to cut early and monitor closely after harvest.