Participants at Monday's Greater Cincinnati BYGLive! Diagnostic Walk-About observed Elongate Hemlock Scale (EHS) (Fiorinia externa) on its namesake host. This non-native armored scale has a wide conifer host range beyond hemlocks. It may be found on firs, Douglas-fir, spruces, cedars, and occasionally pines and yews. I first came across EHS in southwest Ohio in 2010 infesting a Cilician fir (Abies cilicica) in a high-profile landscape.
EHS is sometimes called “Fiornia scale." It's native to Japan and was first found in New York, NY, in 1908. Since that time, it has spread into Connecticut, Delaware, Maine, Maryland, Massachusetts, Michigan, New Hampshire, New Jersey, Nevada, North Carolina, Pennsylvania, Rhode Island, Tennessee, Virginia, and of course, Ohio.
EHS occurs on the underside of needles and on cones. The common name is descriptive. Mature females are protected by an elongate, waxy, slightly translucent covering that ranges in color from yellowish-brown to brownish-orange. Mature males have a smaller elongate white covering.
The mobile first instar nymphs (crawlers) are soft-bodied and lemon yellow. Second instar nymphs (settled crawlers) attach themselves to the needles and enclose themselves in an oval, amber-colored covering. They also secrete waxy white filaments that commonly form a tangled mass on the underside of infested needles to create an unsightly mess.
EHS spends the winter as both fertilized females and eggs. Overwintered eggs hatch in early spring, but overwintered females continue to produce eggs throughout the spring. This means crawlers are found over an extended period of time.
The crawlers settle, feed, and eventually develop into mature males and females. The mature scales mate to produce a second generation and eggs produced by the second generation females give rise to individuals that overwinter.
Like all armored scales, the elongate hemlock scale feeds by inserting their piercing-sucking mouthparts into needles to withdraw nutrients from mesophyll cells. Unlike "soft scales," armored scales don't suck juices from phloem vessels so they don't need to jettison excess sap in the form of sugary, sticky "honeydew." Sooty molds are not a collateral issue with EHS or any other armored scale.
EHS damage symptoms include chlorotic spots or banding on the needles, overall needle yellowing, and needle loss. Severe damage can produce branch dieback and heavy infestations may weaken trees making them susceptible to other pests or succumbing to drought.
While there is no doubt that EHS can cause injury to native hemlocks, heavily infested trees may occasionally show few noticeable symptoms making them a possible point source for EHS. I only discovered that the Cilician fir mentioned earlier was heavily infested when I started taking pictures to use with fir identification; there were no discernable symptoms.
Likewise, the heavily infested hemlock viewed by the Walk-About participants showed few obvious symptoms. There was no twig or branch dieback and annual growth rates were acceptable. Needle chlorosis was only evident through a close examination. Both the fir and hemlock have been infested for years.
Be aware that Spruce Spider Mites (Oligonychus ununguis) commonly infest hemlocks although they seldom cause significant injury. They use their fang-like mouthparts (chelicerae) to rupture plant cells so they can feed on the contents. The resulting small chlorotic spots, known as stippling, may present symptoms that confuse an EHS diagnosis or vice versa.
The Cilician fir that I've been monitoring since 2010 has never been treated for EHS, yet scale populations have risen and fallen dramatically. EHS has a number of insect enemies drawn from native predators and parasitoids that target other armored scales.
Whether the population cycles are associated with natural enemies, some inherent host-related defense, or a combination of both can only be speculated.
However, a number of university-based resources note that unless EHS suppression exceeds 90%, the scale populations will rapidly rebound. This is true of many non-native insects feeding on native plants. Their lifestyle is so rewarding the non-natives continually outpace native enemies. This means insecticides may need to be employed to reduce the impact on the overall health of infested trees.
Effective insecticides against EHS include the neonicotinoid, dinotefuran (e.g. Safari, Zylam, etc.), and the insect growth regulator buprofezin (e.g. Talus). Dinotefuran can be applied as a trunk or foliar application from mid-May through the fall. Buprofezin is only effective against crawlers and should be applied when crawlers are at their maximum numbers from mid-May into early July.
An Insect-Killing Fungus
Last year, Tom Macy (Forest Health Program Administrator, Ohio Department of Natural Resources) sent out e-mail messages asking for help with locating EHS infestations in Ohio as well as EHS that are infected by an entomopathogenic fungus, Colletotrichum fioriniae (formerly C. acutatum var. fioriniae). The request was part of a program monitoring for the fungus that was being done by Bill Laubscher and Tim Tomon with the Pennsylvania Bureau of Forestry.
The tell-tale sign that EHS is infected is the occurrence of blackened accretions on top of the scale indicating the fungus has sprung forth in the form of black mycelia after killing its scale host. Mycopathogens that kill insects is nothing new. Some can produce spectacular declines in their insect hosts which are known as epizootic events and are analogous to epidemics in human populations.
The potential significance of C. fioriniae as a biocontrol agent first came to light in 2002 with a widespread elongate hemlock scale epizootic event across several states including Pennsylvania, New York, New Jersey, and Connecticut. Subsequent bioassay research published in 2009 showed this fungus is capable of producing mortality rates greater than 90%. You can read the paper published in the Journal of Insect Science that documents this event by clicking on this hotlink: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3011895/
What's also fascinating about C. fioriniae is the genus is closely related genetically to a group of fungi that are almost exclusively plant pathogens. This includes plant-nasties such as C. acutatum which is responsible for various anthracnose plant diseases.
We often stress that plant pathogens don't infect animals which remains true. I've never known of anyone losing fingers to the fire blight bacterium. However, C. fioriniae doesn’t behave like many other animal pathogens by simply jumping from one animal host to another.
Apparently, it hides out within hemlock needles as an endophytic fungus. The prefix "endo" means internal, or within, and the suffix "phytic" means plant. This fungus has an intimate relationship with hemlocks and remains poised within the plant to spring forth and infect elongate hemlock scale. Apparently, it's a case of mutualism with the fungus acting as natural protection against the scale.
It's easy to imagine how this fungus may have evolved to infect a sedentary sucking insect. The scale is a stationary target. Once it settles and inserts its piercing-sucking mouthparts into the needles, it's a sitting duck for fungal infection. Perhaps the fungus is even sucked-up by the scale.
The ultimate ability for C. fioriniae to provide widespread relief from EHS has yet to be demonstrated. However, the 2002 epizootic event provides a tantalizing glimpse of the potential.