Woody plant development and timely topics for early February

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By John Ball

SDSU Extension Forestry Specialist

The winter weather has returned to the sub-zero temperatures we all dislike as we walk out in the morning. Fortunately, January and early February is when trees are able to tolerate their coldest temperatures so the -20s are not a concern to them. Most of our “winter” cold injury actually occurs during unusual temperature fluctuations in fall and spring when trees are either going into or coming out of dormancy and are sensitive to cold temperatures.

Timely topics

Green ash is one of the most common trees in South Dakota and considering the number of ash tree in windbreaks, along urban streets, and in yards already, it is a surprise anyone would think we need more. In addition to the numerous pest problems we already have with this species, the anticipated arrival of the emerald ash borer should make us a little more cautious about planting this tree. Surprisingly, there are still some nurseries in the state that are planting and promoting it.

We also need to be a more vigilant in examining dying ash. Emerald ash borer is presently as close as Omaha/Council Bluffs to the south, Welcome, Minnesota to the east and (oddly enough) Winnipeg to the north. All these infestations were detected in trees or panel traps before the borer became well established in the communities, perhaps only present for a few years. This is in contrast with the Michigan experience where the insect went undetected for a decade or more. Recent studies indicate that year 8 is the tipping point, if not detected by this point, the borer population and tree mortality quickly expands. So the quicker it is identified and managed, the slower the spread.

Unfortunately, an emerald ash borer infested tree is not as easy to detect out here on the Northern Plains as it was out East. Many of our ash already have extensive dieback just due to the harsh growing conditions. There are lots of other insects that like to feed on ash. It is important to be able to determine whether what you see on a dying ash is one of our native pests or the emerald ash borer.

The most common ash borer in South Dakota is our native clearwing ash borer (Podosesia syringae). This insect makes an exit hole about the size of a pencil (1/4-inch) and usually the ground beneath the holes is covered with powdery sawdust. The galleries are often found deep within the wood, rather than just beneath the bark, and are usually clean of material.

Carpenterworm (Prionoxystus robiniae) is another common boring insect of ash trees. This insect creates an exit hole about 1/3-inch in diameter, slightly larger than a pencil. There will often be sawdust around the hole and on the ground beneath the tree. Sap may also be oozing from the holes and sometimes the empty pupal case left by the emerging adult insect can be found attached to the bark surrounding the exit holes. The galleries may be 5/8-inch wide, often empty of sawdust, and extend deep into the tree. Trees infested by carpenterworms often have their branches weakened by the extensive tunneling and affected branches often break off in high winds.

It is possible to find both these insects tunneling in the same tree as the picture on the left shows. The clearwing ash borer larva, the one to the left, is about 1-inch long (at maturity) and is creamy white with a shiny brown head. The carpenterworm larvae, the one on the right, can become almost 3 inches long at maturity and is pinkish-white with a dark head. The clearwing ash borer has a one-year life cycle so you will typically only find the larvae in a tree from June to the following May. Carpenterworm may have a three-year life cycle so it is common to find larvae of various sizes at almost any month of the year.

Other common borers of dead or dying ash trees are the redheaded ash borer (Neoclytus acuminatus) and the banded ash borer (N. carprea). These insects create an oval exit hole, almost “fuzzy” D-shaped in appearance and are about 1/4-inch diameter. The galleries beneath the bark are winding, often follow the grain of the wood and are packed with sawdust-like material. They are similar to those created by the emerald ash borer. The primary difference will be that these galleries may extend deeper into the wood than those created by the emerald ash borer. If you cut into the wood beneath the bark and galleries are still present, the most likely cause is the redheaded or banded ash borer, not the emerald ash borer.

Another group of insects that can be found boring into dying ash are the ash bark beetles (Hylesinus). There are at least two species in South Dakota, the eastern ash bark beetle (H. aculeatus) and the western ash bark beetle (H. californicus). These insects create a round exit hole 1/16-inch diameter, about the size of a BB, and often these holes will encircle a shoot. The galleries beneath the bark consist of a main tunnel with numerous smaller tunnels that run off from it, following the wood grain.

The emerald ash borer creates a crisp D-shape hole (1/8-inch) as it exits the tree. The larvae as they tunnel form galleries just beneath the bark. These tunnels form a serpentine pattern and are filled with a sawdust-like material. There is no powdery sawdust on the trunk adjacent to the holes or on the ground beneath them. This is just a brief description of the common borers and bark beetles found in ash trees. If you are not sure if you are looking at symptoms and signs of the emerald ash borer or one of our native insects, please contact me or any of the Department of Agriculture foresters to have the pest identified. Better to know for sure it isn’t emerald ash borer then possibly miss an infestation.

A thorny problem

I saw this tree down in a woody area of Overland Park, Kan., recently. This is the common honeylocust (Gleditsia triacanthos), a tree native to much of the East and even into southeastern South Dakota. The common honeylocust was not a widely used ornamental until the 1940s and 50s. The reason for this hesitation was the long – 2 to 3 inches – branched thorns that protrude from the lower trunk and branches. The thorns are believed to be a defense against large herbivores that roamed the continent thousands of years ago. While they have died off, the honeylocust apparently did not get the message and continued to produce large thorns. As least most of them anyway as some are thornless.

Most of the trees we planted in our communities and windbreaks are from the subspecies, the thornless honeylocust (G. triacanthos var. inermis). Sometimes considered a forma instead (G. triacanthos f. inermis). These are plants that have the thornless characteristic differing from the species, and it comes true from seed, the offspring of thornless honeylocusts are also usually thornless. Thank goodness for that, I would not want to work around a honeylocust with thorns!


While we focused a lot of attention on mountain pine beetle and pine engraver beetles in the Black Hills until recently, the most common pine borer throughout the state is the Zimmerman pine moth (Dioryctria). This is not a single insect species, but three closely related moths with slightly different life cycles. The tree injury is the same, however, with infested pines often with distorted trunks and broken branches. Large pitch masses usually occur in association with branch whorls. A common control procedure is to treat infested trees with a trunk spray of an insecticide labeled for Zimmerman pine moth and containing permethrin as the active ingredient. Make the application in early May and with sufficient hose pressure to thoroughly wet the trunk, particularly at the whorls.

Q Does this tree have pine wilt? (See photo.)

A Pine wilt disease is caused by the nematode Bursaphelenchus xylophilus, which colonizes the sapwood of living pines though it can also survive in dead trees and even logs for a short time. The nematodes are introduced into a tree by longhorned beetles. The beetles can carry thousands of nemtodes from an infected tree or log as the beetles emerge. The microscopic nematodes then leave their beetle host when it is feeding on another pine tree. Once in this tree, the nematode population quickly increases as they feed in the resin ducts and xylem of the tree. The nematodes, and their associated bacteria, also introduce a toxin which hasten the death of these cells. The infested tree’s foliage yellows by mid to late summer and then wilts but remains attached to the shoots. The disease causes symptoms only in exotic pines so we see it mostly on Scotch pine (Pinus sylvestris) and Austrian pine (P. nigra) though even mugo pine (P. mugo) can be killed.

While a rapidly wilting Scotch or Austrian pine is the first clue that the tree may have the nematode, another symptom is the presence of blue stain in the wood. The blue staining is from a fungus and it is also carried to the tree by the longhorned beetles. The blue stain fungus is also a food source for the pine wood nematode and usually when you find the fungus in a rapidly wilting pine, you find the nematode. But not always. The pictures of these blue stained “cookies”, trunk cross-section take at a lower whorl of branches, were quickly followed up with them after arriving at our lab. When we opened the box and lifted the cookies, we thought they were not infected. Usually trees infected by the nematode have such dry wood that a piece has almost no weight. These cookies were still moist and sappy. As expected, we were not able to extract the nematodes from the wood so the tree did not die of pine wilt.

While we have found pine wilt as far north as Highway 212, so far we have not been able to find the nematode in pine trees farther north. However, we did not find pine wilt as far north as Watertown until a few years ago and it was only found as far north as the Nebraska border twenty years ago so perhaps it is a matter of time before we find it in Aberdeen too.

Gardening in the Black Hills

Gardening in the Black Hills Classes will take place Feb. 27 through April 3, Tuesday evenings 6:30-9:00 p.m. Pennington County Master Gardeners and the South Dakota State University Extension Service are offering a series of classes entitled “Gardening in the Black Hills.” The series will cover many gardening topics ranging from the basics to more advanced gardening skills. Each subject is taught by a certified SDSU Extension master gardener or Extension personnel. Preregistration is required by February 23rd, $35, for information call: 605-394-1722.

Spring Fever 2018 Gardening Event

Spring Fever 2018 Gardening Event will be held in the Rushmore Room at Ramkota Hotel in Rapid City on March 3, from 8:00 a.m. – 4:00 p.m.This year’s conference features two presentations by Dr. Linda Chalker-Scott of Washington State University, speaking on “Evidence Based Gardening Information” and “The Root of the Problem, When Plants Don’t Thrive.” Dr. John Ball, SDSU professor and Extension forestry specialist will present “Selecting Trees for Western South Dakota.” Short “table talks” topics include: “Growing Vegetables in Containers on Your Deck or Patio”, “Caring for House Plants”, “Conserving Water in Your Garden”, and “Using Compost and Mulch in Your Garden.” Lunch catered by Minerva’s is included in the $35 registration fee. The event includes a Silent Auction, Door Prizes and a Free Table. Registration limited to 200, please register before February 27. Visit www.blackhillsgarden.com under the “Welcome” tab for registration blanks and more information.

South Dakota State Horticulture Society 2018 Spring Workshop

The South Dakota State Horticulture Society 2018 Spring Workshop is April 21 at the Plains Dining and Recreation Center, located at 960 4th St. NE in Huron, S.D. Registration begins at 9:30 a.m. with the program starting at 10 a.m. We are extremely fortunate to have Mr. Brent Heath, of Brent and Becky’s bulbs as our speaker for the workshop. Brent is a noted authority on bulbs who has spoken all over the country and world, educating and promoting the use of flowering bulbs of all sorts. He will speak on four different topics: “Bulbs as Companion Plants”; “Pest Resistant Bulbs”; “Tantalizing Tulips”; and “Lovely Long Lasting Lilies and Awesome Alliums.” Cost for the day-long workshop, including lunch is $30 for members; $35 for non-members. Send payment to Glenda Oakley, treasurer by April 4. For mailing address, email her at goakleyhuron@yahoo.com. Mr. Brent Heath is co-owner with his wife Becky of Brent and Becky’s Bulbs. Brent and Becky, along with their family has been growing bulbs since 1900 when Brent’s grandfather moved to Virginia after falling in love with the daffodil. Today, they still love the Daffodil, but offer so much more. Bulbs, seeds, perennials, plants, supplements, books, gifts — they’ve really evolved. They live and work on their 28-acre farm and gardens in Gloucester, Va. They are well-known flower bulb suppliers, garden writers, photographers, lecturers, consultants and educators as they developed 8-plus acres of educational demonstration gardens. You can view their website, catalog, learn more about Brent and their company and read more about the topics on which he is speaking at www.brentandbeckysbulbs.com.

Pine infested with Zimmerman pine moth. iGrow photo
Honeylocust with thorny trunk. iGrow photo
Blue stanined scotch pine. iGrow photo
Pine potentially infested with pine wilt nematodes. iGrow photo
Clearwing ash borer and carpenter worm lavae. iGrow photo
Ash bark beetle damage. iGrow photo