Forest Insect and Disease Newsletter

Seasonal observations

It is just a theory, but I have come to believe that after a long cold winter and prolonged captivity indoors, Minnesotan's five senses are most assuredly heightened. As we stop to smell the Trillium, listen to a chorus of tree frogs, or try to catch a glance of that elusive warbler singing in the tree tops, it is inevitable that some of us will encounter less familiar springtime sights, sounds, and smells in our backyards and forests. These oddities often generate concern, confusion, or best of all... a knowing smile when these ephemeral natural "weirdos" are spotted. Here are just a few of the interesting observations being reported this spring:

Where did all these eggs come from, and what are they?

It started out innocently enough in early May. Homeowners in Sandstone and Two Harbors wondered what these were and were they detrimental. Insect eggs, yes, but what laid them? Then more reports of eggs laid on house siding, windows, and even laundry drying on the line from locations like Grand Marais, Side Lake and Duluth. Lately, reports of hundreds of egg masses found on houses came from Cook County to Itasca County. Homeowners were scraping or hosing them off, some even using power washers. We'd never seen anything like them before and neither had forest health specialists from northern Wisconsin, who were being flooded with questions, too. Extension entomologist to the rescue: Phil Pelliteri, University of Wisconsin Extension, identified them as eggs of the variegated cutworm, Peridroma saucia.

Apparently, variegated cutworm moths from outbreaks in southern states were blown north over a period of a few weeks in May and eggs were laid at night on emerging crops, gardens, fence posts, buildings, trees and almost anything that holds still long enough. Moths were still flying around in Duluth this week.

All these eggs will hatch, yielding tiny, translucent caterpillars with a few dark hairs and over-sized, black heads. They will be hungry. And, they have an extremely wide host range: most vegetable garden plants, fruit trees, rhubarb, raspberries, strawberries, ornamental plants, shrubs, hostas, canola, wheat, corn, sunflower, clover and other row crops. Cutworms feed at night and rest during the day so it is hard to find them, but the damage will be easy to spot: seedlings nipped off at ground level and large holes eaten out of leaves all season long. The variegated cutworm is also called the climbing cutworm because mature caterpillar stages will spend both days and nights on their host plants in contrast to most other species of cutworm. After feeding, they'll pupate in the soil, emerge as moths and start another generation, likely in July. Adults created from this generation will migrate south in October to other states in order to overwinter.

We really don't know if this invasion of cutworms will result in damage to gardens, ornamentals or crops, so monitoring your plants over the next few weeks would be a good idea. If they are causing a problem, hand-picking in the late evening is suggested and that is the same time pesticide applications should be done. Apparently Btk isn't effective on them, but Neem products and chemical pesticides such as carbaryl are effective. Only use products labeled for the plant species you are growing and read the label for re-entry and safety, especially for food crops.


More information can be found on these websites:

Photo credits:

Photo of eggs is from The Backyard Arthropod Project

Photo of cutworm identification is from Department of Entomology, University of Nebraska-Lincoln

Eriophyid mite galls

Eriophyid mite gallsThey may look like colorful bumps, spindles, balloons, velvety carpet, or even flowers, and many are caused by a group of arachnids called eriophyid mites. There are many species of these mites; each has a preference for a specific host tree species, such as apple, maple, cherry, ash, birch, and each causes an abnormal- looking growth called a "gall" on leaves or flowers. Eriophyid mites overwinter on their host tree in bark crevices and other protected places, and become active just as buds begin to expand in the spring. While feeding on young, expanding tissues, the mites release growth-regulating chemicals that trigger rapid cell division and unusual growth patterns in the surrounding tissue. The mites continue feeding inside the gall and eventually reproduce. As summer progresses these galls dry up and many new mites emerge through a tiny exit hole in search of overwintering sites. The galls, while sometimes unsightly, rarely cause any significant harm to affected trees, so no chemical treatment is recommended.

Sawyer beetle larvae

Sawyer beetle larveWhile relaxing at a campsite, walking past a pile of firewood, or hiking through a pine stand, you may pause to ask your companions, "Do you hear that squeaking noise?" The larvae of several wood-boring beetle species, such as the white-spotted pine sawyer (Monochamus scutellatus), become so large and feed so voraciously that they can be heard chewing their way through the sapwood of dead or dying pines. Armed with a pair of stout mandibles, these legless larvae can grow to over two inches long and will feed up to six inches below the bark surface for two years before emerging as adult longhorned beetles in late spring. Feeding activity resumes after winter with the first warm weather, and on a quiet evening their squeaky crunching can be heard from over 30 feet away. These insects play an important role in nutrient cycling in our forests, but if they are keeping you awake at night simply knock on the tree; their fear of woodpeckers will silence them quickly!

Slime flux

Slime FluxAlthough not a serious disease of trees, slime flux is commonly observed in the spring on maple, oak, willow, and especially elm. The condition is caused by pockets of anaerobic bacteria and fungi that invade pockets of suitable sapwood such as "wet wood" (a water-soaked condition common in elms) or wood in sealed cracks, cavities, or old wounds. Feeding by these microorganisms results in the production of metabolic byproducts such as ethanol and carbon dioxide in an oxygen-free environment. Occasionally these gasses and fluids build up to excessive pressures sufficient to escape to the bark surface and ooze down the stem. When exposed to oxygen, the fluid is rapidly invaded by aerobic microorganisms, turns dark brown or black, and takes on a foul odor and slimy consistency. This slime flux is highly attractive to a variety of insects in including many species of bees, wasps, ants, butterflies, and moths. It also will stain or bleach the contacted bark with prolonged exposure. There is little evidence to suggest that wet wood or slime flux significantly degrade wood strength and rarely do these conditions have any negative impact on tree health. However the sour-smelling ooze and invasion of insects are often seen as a nuisance to homeowners. Drilling into slime flux-producing wood to relieve pressure and drain the infected wood is no longer recommended; the addition of oxygen into this tissue can result in the introduction of wood decay fungi and create a hazardous situation. The best advice is to wash the flux away with a mild soapy water solution.

Hypoxylon canker

Hypoxylon cankerAs oaks and other hardwoods suffer through prolonged drought conditions and eventually succumb to the cumulative stress of water deficits, secondary insects such as the two-lined chestnut borer and diseases such as Armillaria root rot, you may observe the unveiling of the hypoxylon canker fungus. Caused by several Hypoxylon species, hypoxylon canker is a secondary disease that affects extremely stressed trees that have little or no ability to mount a defense against invading insects and diseases. Hypoxylon sp. actually play a useful role in our forest ecosystems: they are wood decay fungi that break down woody debris on the forest floor; rarely do they attack living tissue. However, it is thought that many trees become infected with these fungi at a very early age through wounds or natural openings. During the life of the tree, this fungus simply survives in small colonies beneath the bark that have little or no impact on tree health. When the host becomes significantly stressed, however, the fungus begins to attack the tree as if it were already dead, and rapidly invades the nutrient-conducting tissue and water-depleted sapwood beneath the bark. Eventually the bark falls off the tree, exposing vast, sporulating fungal mats, which may be brown, black or dark gray. As spores are released, the mats change color to a light gray. Trees that are still alive at this point quickly wilt and die. In truth, the hypoxylon canker is not to blame. Rather, trees affected by this canker were killed by a variety of other stress agents and this fungus delivers the final blow. Hypoxylon may also be observed on isolated dead branches but is not a threat to the tree's overall health. However, if the canker is observed on the main stem it is likely that the tree will soon die. Because hypoxylon causes extensive wood decay, these trees should be removed quickly if they present a hazard.

Cordyceps - parasitic fungus of insects

Cordyceps- parasitic fungus of insectsThe picture shows two fruiting bodies of a species of Cordyceps growing out of the head of an insect larva. The black fruiting bodies that make the larva look as if it has horns are each about one-and-five- eighth inches long. The larva was brought into my office in Grand Rapids this spring. There are about 400 species of Cordyceps and all of them are parasitic, mostly on insects. Besides killing the insects, some of them even change the behavior of the insect, such as the one that causes infected ants to climb to the top of a plant before they die. This presumably helps with the dispersal of the spores of the fungus.

Watch very good video about Cordyseps narrated by Sir David Attenborough.


Some species of Cordyceps have a long history of medicinal uses in Tibet, Nepal and China. A Modern Marvels show on the History Channel reported that mushroom hunters in Nepal can earn $900 per ounce of Cordyceps. Capsules and tablets made from powdered Cordyceps fruiting bodies, mycelium and spores can be purchased on the internet. They are claimed to increase energy and endurance, boost the immune system, and possibly even cure cancer. Of course most or all of these products lack good clinical trials. I am not recommending any of these supplements and if anyone is considering trying any of them, I suggest you watch the video listed above showing Cordyceps growing out of a variety of insects. That should make most people have second thoughts about trying any.

There has, however, been important medical research on chemicals produced by Cordyceps species. One of the drugs produced through this research is called cyclosporine. It was derived from the asexual reproductive stage of Cordyceps subsessilis. I am particularly fond of this prescription drug because it has kept me alive for almost 20 years now. This drug suppresses the immune system and was first used in a successful liver transplant by Dr.Thomas Starzl in the early 1980's. This was the first drug to make organ transplantation possible.

Rough bullet galls on bur oak

Rough bullet galls on bur oaksLandowners in Polk County have noticed clusters of galls on twigs and branches of recently-planted bur oaks. At first glance, these galls appear to be wrinkly acorns crowded on the stems, but they are galls caused by a cynipid wasp. Galls do not kill the twig or branch, but can cause reduction in the growth rate of the infested oak.

At this time of year, four stages of galls can be found: new ones that are developing (bumps on the stem), last year's galls with emergence holes (10 to 15 mm), last year's galls under nine millimeters without emergence holes, and all older galls. These older galls can remain attached for up to five years but will not produce an insect again, so the tree usually looks worse than it really is.

Rough bullet galls first appear as small, green bumps on the twig near a bud. As the gall grows in size, its color changes to red then to dark brown. Completely formed galls are round with a point at the tip and are 8 to 15 millimeters in diameter. They are commonly found in clusters along the twig and only rarely cover the entire length of the twig. A single adult cynipid wasp emerges from each gall in September, and females lay eggs in dormant buds at the tip of the twig.

The smaller galls (8-9 mm) are parasitized and contain the natural enemies of this gall maker. These insects emerge the following spring and will parasitize any new galls formed on the same tree or nearby bur oaks. This observation sets the direction for management strategies; chemical control is not recommended, in order to preserve the natural enemy population so it can build up and control the gall insect infestation.