Nebraska's Forest Health Report May 2024

Nebraska's Forest Health Report May 2024
Nebraska's Forest Health Report May 2024
Drought Continues in South Eastern Nebraska

Drought conditions still persist in the SE central region of Nebraska and soil moisture is very low around the Lincoln and surrounding areas. Bouts of severe weather that included tornados and derechos this month caused intermittent flooding, hail and strong winds.  Stem diseases, like cystospora canker, diplodia tip blight and fire blight can become a problem following these types of events if susceptible trees have sustained damage.  Trees, whose roots have been underwater for extended periods of time, can develop root rot and root stability may be in question after flooding.  A great article written by Graham Herbst, Eastern Community Forester, explains how to exam and care for trees after flooding.  You can review Graham's article here:
Ash Sawfly Larva on Ash Leaf, Lincoln NE
Photo Courtesy of Laurie Stepanek

Sawflies are quite numerous with roughly over 1,000 species represented in 10 families.  The insect gets its name by its saw-like ovipositor, an organ shaped tube(s) used for egg laying. Adult sawflies can look like flies but are more closely related to bees and wasps.  Most sawflies have one generation per year consisting of egg, larva, pupa and an adult stage.  Though the larva looks similar to caterpillars, several differences can be noted for identification.  Sawfly larvae have 6 or more pairs of prolegs without gripping hooks behind 3 traditional pairs of legs, while moth and butterfly larvae have 2-5 pairs of prolegs with gripping hooks.  Sawfly larvae typically are spineless, hairless and consistently have a head segment distinct from the body.  
Commonly in Nebraska we find brown headed & black headed ash sawfly, European pine sawfly, and yellow headed spruce sawfly, oak sawfly, among others.  Defoliators of deciduous trees are more of a nuisance pest in our state.  The ash sawfly for instance, in a high population year, can inundate ash trees, dropping fecal matter in abundance and alarming homeowners during the invasion.

Brown headed ash sawfly larva crawling around and up an ash tree
Brown headed ash sawfly adult 
Courtesy of
European Pine Sawfly Larva
Gyorgy Csoka, Hungary Forest Research Institute,

European pine sawfly larva that feed on evergreen needles can congregate in high numbers, becoming destructive, and defoliating entire branches of Austrian, Scotch, mugo, jack or ponderosa pine trees.  
Photo courtesy of Eric Berg
European sawfly larva group on pine branch, notice the needles have been eaten.
Control of sawflies is usually not necessary if populations are in small numbers.  Healthy trees, especially deciduous trees, can handle slight defoliation and a few larvae can be picked off a small tree/shrub easily.  If high numbers are present and feeding is expected to be severe, treating with an insecticide may be warranted.  Insecticides will work best when the larvae are small.  Several products on the market will control sawflies.  Look for active ingredients of permethrin, bifenthrin, acephate or insecticidal soap/oils.  
Elm Zig Zag Sawfly - Is It On It's Way?
The elm zig zag sawfly (EZS) is a non-native, invasive pest from Asia that was discovered in Virginia in 2021.  Since that time, it has been found in 7 additional states - Pennsylvania, North Carolina, Maryland, New York, Vermont, Massachusetts, & Ohio.  
This sawfly can produce up to 6 generations in a year and can defoliate elm trees rather quickly.  Slippery elm, Siberian elm, American elm, Chinese elm and elm hybrids are all susceptible to infestations.  
This sawfly is parthenogenetic, meaning that no males have ever been recorded to exist.  Each generation is female, produced by asexual reproduction.  The adults can fly up to roughly 55 miles per year.
Photo by Matt Bertone, NC State University
Pictured above is an EZS larva.  Notice the lime green coloration with dark banding on the head.  The 2nd and 3rd leg have noticable "T" shaped dark markings. 
The larvae can produce one of two types of cocoons before pupation. One that looks like spun gold sugar with webbing attaching it to the leaf (upper left).  This cocoon holds a larva that will pupate in the summer months.  The other cocoon is a spun shell (upper right) and more so found after defoliation has occurred in the late summer months and found down on the ground or on other surfaces - cars, homes etc.  This second type of cocoon house larva that will pupate the next year.
Photos by Matt Bertone, NC State University
The adult (above left) is black in color with 3 pairs of light-colored legs.  The EZS gets its name by the pattern left after larva feed in-between leaf veins on the elm leaf (above right).  Feeding can by quite devastating with multiple generations in a year. 
EZS has not been found in Nebraska at this time.  Contact FH for more information or if you suspect you have found EZS.
Adult Photo by Gyorgy Csoka, Hungary Forest Research Institute,
Leaf Pattern Photo by Tom Macy, Ohio DNR Division of Forestry,
Gray’s Creek CA, 13 August 2023. Owen Clarkin photo
All leaf tissue except the midrib and some vein tissues have been consumed by EZS on this elm branch.
Venturia Leaf Spot & Shoot Blight of Aspen
This fungal disease infects emerging leaves and shoots of aspen by windblown spores in the spring.  Once infection occurs, the leaves develop blackened and distorted leaf tissue.  The fungus moves to expanding shoots where cankers form, causing the shoots to discolor and curl. These infected shoots are a source for spore development the following year.  Continued infection on aspen may cause stunted growth, giving the tree a shrub like appearance and also weaken the tree.
Wet springs followed by wet summers increase infection rates.  Pruning out infected shoots and thinning dense groves will help.  If fungicide applications are necessary, spray fungicides labeled for Venturia sp. in the spring as new leaves/shoots emerge to help protect the shoots from infection. Look for fungicides with an active ingredient of myclobutanil, mancozeb, or copper among others.

Photo submitted by Terry Morris
Cedar Apple Rust
Rust projections on infected apples
Photo by Rebekah D. Wallace University of Georgia,

Cedar-apple rust and Cedar hawthorn rust diseases require 2 hosts to complete their life cycle. The fungus overwinters on juniper or eastern redcedar trees in the winter. The spores are released in the spring from gelatinous orange galls on junipers or cedars and land on apple, crabapple and other susceptible deciduous trees. After infecting their growing season hosts, spores are released in late summer to infect their Juniperus host, completing the life cycle. If severe defoliation is occurring each year, an application of fungicide labeled for rust at bud break and 2 additional applications 7-14 days apart will help to lower the effects of the disease.

Rust Fruiting Bodies on Cedar
Photo by Terry S. Price, Georgia Forestry Commission,
Pine Needle Scale
Pine needle scale infestation on pine sp.
Photo by USDA Forest Service - Coer d'Alene Field Office,

Scale insects are a diverse and numerous group of insects. Scales have sucking mouthparts that suck nutrients out of tree tissue. In some cases, damage is minimal and chemical treatments are not necessary. Often natural predators and parasites control the populations of the insects but sometimes scales can heavily infest a tree to the point of dieback, creating the need for more aggressive control.
Scales are classified into two categories - soft scales and armored scales.  Soft scales feed from the phloem tissue of plants, causing them to excrete honeydew, a waste product highly sweet that attracts other insects and mold. Soft scales also can develop a waxy layer over their body protecting them from predation.  Armored scales feed from mesophyll cells of plants, do not excrete honeydew and develop a hard, detachable outer shell for protection.
Pine needle scale is an armored scale that can infest may species of evergreens.  In Nebraska it commonly infests pines, namely white, mugo and Austrian.  They also can infest spruce and fir.  
Adult pine needle scale
Photo by Robert J. Bauenfeind, Kansas State University,

Pine needle scale has a first-generation spring hatch roughly around the time of lilac bloom, where immature scales are called the "crawler" stage. The crawlers emerge from eggs that have been protected by the adult female scale covering over the winter.  Armored scale crawlers are only mobile until their first molt.  They lose their legs and become immobile.  This generation develops into adults, mate in July, and lay eggs for the second generation.  

Immature crawlers are small, oval shaped and range from rusty red to purple in color
Photo by Howard Russell, MSU Diagnostic Services
If high numbers of scale are present, needle browning, needle loss and branch dieback can occur.  Oils or traditional insecticides can be applied at the crawler stage to lower populations in either the 1st or 2nd generation.  Traditional insecticides as overhead sprays during the adult stage are not as effective due to the protection of the adult scales outer shell.   
Compiled by Jennifer Morris, Forest Health Specialist
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