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Killer Found- Mystery Solved:
Thousand Cankers Disease on Black Walnut
by Yvonne Barkley

This story begins in the summer of 2005. A landowner in Emmett, ID reported black walnut (Juglans nigra) trees dying. The tree would look fine until the hot weather hit, and then the foliage would quickly wilt and the tree would die. The symptoms progressed rapidly, with some trees dying in as little as one month. Another call came in that same year from Meridan, ID – same thing. The next year, the same patterns of symptoms began to be reported in Boise. Initially, the common thought was that trees had suffered many years of drought stress and had finally succumbed to the lack of sufficient moisture. But as landowners and resource managers began to look more closely at the affected trees they found that there were large numbers of very small holes in the bark, which were initially thought to be ambrosia beetles.

Figure 1. Foliage rapidly wilting and dying on mature black walnut. (Photo courtesy of Whitney Cranshaw from forestryimages.org)

Samples were collected and brought back to the University of Idaho for identification of the insect involved. Dr. Stephen Cook, University of Idaho Insect Ecologist, and Frank Merikel, Manager of the University of Idaho William Barr Insect Museum, concurred that the insect involved was NOT ambrosia beetles, but instead was the walnut twig beetle (Pityophthorus juglandis), a native bark beetle on black walnut in New Mexico and Arizona.

While searching for information on this beetle, I came across an article describing a problem on black walnut from Boulder, CO, and the description and photos matched what we were seeing here exactly. I contacted Dr. Ned Tisserat Plant Pathologist at Colorado State University, and the story he told me was the same as ours.

In March of 2008 I went to Boise to meet with the folks at Boise Park and Recreation and Jim Hoffman, Forest Pathologist, USDA Forest Service. Samples were collected and sent to Ned in Colorado. We were all pretty sure by now that there was a fungal associate that produced the wilt-like symptoms and rapid death. By June, 2008 Ned had our results – thousand cankers disease of black walnut.

The insect - walnut twig beetle. Native to North America, the walnut twig beetle’s (Pityophthorus juglandis) primary range is New Mexico, Arizona, and Chihuahua, Mexico, and coincides with the natural distribution of Arizona walnut (Juglans major), a likely native host.

Attacks by adult walnut twig beetles primarily occur on branches greater than 2 cm diameter. Very large branches and even the trunk can be colonized during advanced stages of thousand cankers disease. Adults lay eggs in the summer and larvae tunnel under the bark feeding on the phloem as they grow. Larvae feed for 4-6 weeks in meandering tunnels that run perpendicular to the egg gallery and pupate at the end of the tunnel. The life cycle is complete by late fall and adults (mostly) spend the winter under the bark in the galleries made by the larvae. The following spring (late-April), the first adults emerge and fly to near-by branches to mate and lay eggs. More adults emerge to produce a second generation in early summer. Peak flight activity of adults occurs from mid-July through late August and declines by early fall as the beetles enter hibernation sites. A small number of beetles produced from eggs laid late in the season may not complete development until November.     

The pathogens – Geosmithia morbid and Fusarium solani. Ned Tisserat and Whitney Cranshaw of Colorado State University have observed two different types of cankers on dying black walnut trees. The first pathogen identified is an unnamed fungus in the genus Geosmithia (a currently proposed species name for this fungus is Geosmithia morbid). This fungus is introduced to the tree as beetles are tunneling and grows in advance of the bark beetle. The branch cankers are not visible until the bark is peeled back –

Figure 2. Meandering galleries under the bark of a large branch. (Photo courtesy of Whitney Cranshaw from forestryimages.org)

Figure 3. Walnut twig beetles with the associated canker caused by Geosmithia. (Photo courtesy of Whitney Cranshaw from forestryimages.org)

The second pathogen associated with this disease is the fungus Fusarium solani. In association with the walnut twig beetle and Geosmithia fungus, these cankers can commonly be two meters or more in length and can wrap around more than half the circumference of the trunk. These Fusarium cankers are subtle and can be identified by cracks in the bark or dark brown to black stains. When the bark is peeled back, the cankered area (the inner bark and cambium) is water-soaked and dark-brown to black.  The importance of the Fusarium fungus in the development of these large cankers is still being studied.

Diagnosis and management. We have a name and know what it is, but unfortunately we have not identified any effective controls for thousand canker disease. Because of this, stopping the spread of the disease is vital. In Idaho, black walnut is the only reported susceptible species of tree. The disease currently ranges from Emmett east to the Boise area and up into the Boise foothills. Bark beetles are traditionally not very strong flyers, and so the spread of the insects, and the associated fungi, is partially dependent on prevailing winds.

Thousand cankers disease most likely spreads through the transportation of infested wood material (logs) and perhaps thorough nursery stock. Logs should not be moved from infested areas unless they have been dried for at least three years, kiln-dried or all of the bark has been removed from the log.

For more information, or if you think you have black walnut that has been infected with thousand cankers disease, please contact Yvonne Barkley at the UI Extension Forestry office by phone (208) 885-7718 or e-mail yvonnec@uidaho.edu.

Parts of this article have been excerpted from “Walnut Twig Beetle and Thousand Cankers Disease of Black Walnut” by Ned Tisserat and Whitney Cranshaw, Colorado State University, Boulder, CO.

Balancing Biomass
by Chris Schnepf

There is a growing discussion about forest biomass (also known as forest organic debris) in Idaho. Forest organic debris includes tree limbs, boles (trunks), needles, leaves, snags, and other dead organic materials. Common reasons for removing organic debris include reducing bark beetle hazard, preparing a site for tree planting, harvesting forest biomass for energy, reducing fire risk, and aesthetics.

All these issues are important. But forest organic debris left on site is not necessarily wasted. Organic debris protects soil from excessive moisture loss, recycles nutrients for trees and other forest plants, adds structure and organic matter to the soil, reduces soil erosion, and provides food and habitat for a wide variety of wildlife. Many forest owners are unclear on how to reconcile the potentially conflicting objectives related to forest organic debris on their site.

A new UI Extension publication titled Managing Organic Debris for Forest Health - Reconciling fire Hazard, Bark Beetles, Wildlife, and Forest Nutrition Needs (PNW 609) is designed to help forest owners think about a strategy that fits their unique forest and site conditions and values and goals.

This publication is 60-pages, and spirally bound for field use. It includes over 70 color illustrations that outline the role of forest organic debris in Inland Northwest forests and provides general management strategies to help forest owners and those who work with them ask better questions to plan the best treatment strategy for each site in order to keep forests and wildlife more healthy and sustainable, while keeping risk from fire and insects within acceptable limits.

The publication is available through local University of Idaho Extension Offices and can also be downloaded as a PDF file at http://info.ag.uidaho.edu/pdf/PNW/PNW0609.pdf

Don’t Let Water Quality Bug You Out
by Randy Brooks

Using insects to monitor water quality may sound like something from a Far Side cartoon, but in reality, bugs are a “quick and dirty” method of assessing water quality. The traditional water quality monitoring approach has been to collect stream water samples and have them analyzed for physical and chemical contaminants. Since water sampling and analysis is expensive, insect monitoring is a more economical and quicker method of determining water quality. In the U.S., much as canaries are used in mineshafts, the use of stream organisms as biological indicators of water quality has become widespread over the past few decades.

Biological monitoring is used to assess a water body’s environmental conditions. One type of biological monitoring is a biological survey or biosurvey (also called stream insect survey if only monitoring aquatic insects). This involves collecting and analyzing aquatic organisms (fish, bugs, and algae) to determine health of an aquatic biological community. 

Aquatic insects are termed benthic macroinvertebrates (BMI’s). Benthic means bottom dwellers, and macroinvertebrates are organisms that are large (macro) enough to be seen with the naked eye and lack a backbone (invertebrate). BMI’s inhabit all types of running waters, from fast-flowing mountain streams to slow-moving muddy rivers. Examples of aquatic macroinvertebrates include insects (in their larval or nymph form), crayfish, clams, snails, and worms. Most live part or nearly all of their life cycle attached to submerged rocks, logs, and vegetation.

There are three groups (taxa) of BMI’s:

• Group one BMI’s are pollution sensitive and found in good quality water. This group includes stonefly larvae, caddisfly larvae, water pennies, riffle beetles, mayfly larvae, gilled snails, and dobsonfly larvae.

• Group Two BMI’s are somewhat pollution tolerant organisms that can be found in good or fair quality water. This group includes crayfish, sowbugs, scuds, alderfly larvae, fishfly larvae, damselfly larvae, watersnipe fly larvae, crane flies, beetle larvae, dragon fly larvae, and clams.

• Group Three BMI’s are pollution tolerant organisms that can be found in any quality water (from good to poor). This group includes aquatic worms, midge fly larvae, black fly larvae, leeches, pouch snails, pond snails, and other snails.

 Aquatic BMI’s are good indicators of stream water quality because:

• They are affected by the physical, chemical, and biological conditions of the stream.

• They cannot escape pollution and show the effects of short-and long-term pollution events.

• They may show the cumulative impacts of pollution.

• They may show the impacts from habitat loss not detected by traditional water quality assessments.

• They are a critical part of the stream's food web.

• Some are very intolerant of pollution.

• They are relatively easy to sample and identify.

The basic principle behind surveying BMI’s is that some are more sensitive to pollution than others. If a stream site is inhabited by organisms that can tolerate pollution (those from Group Three) - and the more pollution-sensitive organisms are missing (those from Group One) - a pollution problem is likely.       

For example, stonefly nymphs (see Figure 1) are very sensitive to most pollutants and cannot survive if a stream's dissolved oxygen falls below a certain level. If a biosurvey finds no stoneflies present in a stream that used to support them, a conclusion might be that dissolved oxygen has fallen below the point that keeps stoneflies from reproducing - or has killed them outright.       

In this example, the absence of stoneflies might indeed be due to low dissolved oxygen. But is the stream under-oxygenated because it flows too slow or because pollutants in the stream are damaging water quality by using up the oxygen? The absence of stoneflies might also be caused by pollutants discharged by factories or running off the watershed, high water temperatures, habitat degradation such as excess sand or silt on the stream bottom that has ruined stonefly sheltering areas, or other conditions. When changes are noticed, a biosurvey is best accompanied by an assessment of habitat and water quality conditions (such as a stream reach inventory) in order to help explain biosurvey results.       

Because BMI’s are stationary and sensitive to different degrees of pollution, changes in their abundance and variety illustrate pollution’s impact on the stream. Loss of BMI’s in a stream, or better yet, loss of trees along a stream bank, are environmental impacts that society can relate to. Similarly, when a pollution control activity takes place - say, a fence is built to keep livestock out of the stream-a biosurvey may show that the sensitive BMI’s have returned and a habitat assessment might find that the formerly eroded stream banks have recovered and trees now shade shading the stream.

BMI’s are quantified by species richness (number of unique types of invertebrates found in a sample), abundance (total number of invertebrates in a sample), relative abundance (number of invertebrates in a sample from one species relative to another), and species diversity (distribution of total individuals across species in the sample). Once counted, the invertebrates can be compared to samples taken in the same stream at earlier times, such as before and after a suspected pollutant has entered the stream. One popular index for monitoring species richness is the “EPT index”. This measures the total number of species within the three most pollution sensitive aquatic insect orders: Ephemeroptera (mayflies, Figure 2), Plecoptera (stoneflies, Figure 1), and Trichoptera (caddisflies, Figure 3). This index assumes that streams showing high EPT richness/numbers are less likely to be polluted than streams showing relatively low EPT richness in the same region. If a stream has few EPT’s (compared to other streams in the region), water quality has likely been impacted. This will warrant further investigation however, to discern the reasons for the lack of EPT’s.

Figure 2. Mayfly. Note the three tails.

Figure 3. Caddisfly (aka periwinkles)

In summary, BMI’s are a useful indicator of water quality in northwest streams. Most all of Idaho’s streams have good water quality conditions and support healthy populations of BMI’s and EPT’s. Next time you’re along or in a stream, pick up a rock off the bottom and see what’s crawling on it. You might be surprised to find several of these beneficial, indicator species.

Proper protocol exists for sampling aquatic BMI’s in Idaho, and can be found at http://www.deq.idaho.gov/water/data_reports/surface_water/monitoring/burp_field_manual_2007_entire.pdf