Oystershell scale – also called mussel scale (OSS; Lepidosaphes ulmi) is a sap-sucking armored scale insect in the order Hemiptera, family Diaspididae. It inserts its stylet through the thin bark of woody host tissue to feed on fluids of non-vascular cells.
Its native region is unknown. Oystershell scale has apparently been in North America since the early 1700s. It is now present throughout the United States and much of Canada as a common pest of many deciduous tree species in urban settings. More than 100 hosts have been identified. These include several genera of native tree species: poplar and cottonwood (Populus), willow (Salix), maple (Acer), plum and cherry (Prunus), birch (Betula), beech (Fagus), ash (Fraxinus), alder (Alnus), and walnut (Juglans). Known hosts also include some forest understory plants, i.e., snowberry (Symphoricarpos spp.), ceanothus (Ceanothus spp.), and lupine (Lupinus spp.). Recent outbreaks of OSS on quaking aspen (Populus tremuloides) in the southwestern US have raised concerns about OSS’s potential as a high impact invasive insect.
Quaking aspen is the most widely distributed tree species in North America. As a deciduous species in the conifer-dominated western forests, aspen makes a disproportionately large contribution to maintaining biological diversity. Aspen stands also make significant contributions to carbon sequestration, nitrogen mineralization, water yield potential, and revenue from hunting, and ‘‘leaf peeping’’ tourism.
During the past two decades aspen stands across the western United States have experienced both acute mortality events and chronic decline. These stresses are attributed to a combination of abiotic events, biotic agents, and chronic ungulate herbivory. This situation has stimulated significant conservation efforts.
The Arizona population of quaking aspen is at the southwestern edge of the tree’s range. Aspen there is limited by the annual balance of temperature and precipitation and generally is not found below 2200 meters elevation. At lower elevation aspen tends to occur in small stands within the ponderosa pine (Pinus ponderosa) forest type. At higher elevations, aspen stands become larger and more abundant, growing in the mixed-conifer and spruce-fir forest types. Aspen in Arizona is increasingly stressed by the warming climate.
Oystershell scale has been present in natural aspen (Populus tremuloides) stands in northern Arizona since at least 1991. The first report was on the Prescott National Forest. In 2009 and 2010, the scale was detected in natural aspen stands on the Kaibab National Forest. Although at that time, OSS populations were relatively small and were not causing significant aspen dieback or mortality, by 2017 a study documented outbreaks of OSS-caused dieback and mortality in the same exclosures.
As of 2021 Crouch et al. report that oystershell scale is widespread in lower elevation aspen stands (below 2550 m) in northern Arizona. It is absent at higher elevations, probably because of an elevational or climatic constraint.
OSS is particularly pervasive in exclosures created at these lower elevations to protect aspen from browsing by elk, deer, and cattle. The highest levels of infestation and mortality are found on recruiting stems in these exclosures. Because recruiting stems are critical for maintaining and promoting aspen stands, ongoing OSS outbreaks threaten the long-term success of aspen conservation efforts in Arizona and potentially throughout the Southwest.
Biology of Oystershell Scale
The oystershell scale’s life cycle is completed entirely on the host’s woody tissue (i.e., the surface of the bark on stems and branches). Reproduction can be either bisexual (the result of mating) or parthenogenetic. Scientists do not know which method is used by the Arizona population.
OSS overwinters as eggs beneath the protective waxy shells, or tests, of dead females. The eggs hatch in late spring or early summer depending upon the location. First instar nymphs (called crawlers) disperse along the host’s stem or branch to establish a feeding site. This dispersal stage lasts only a few days. While they usually crawl only a short distance, larvae might also be dispersed longer distances by wind or animals. Once the larvae finds a suitable site and begins to feed, it develops a waxy outer shell, or test, for protection. Individuals remain at their feeding site through adulthood. In areas with bisexual populations, males shed their tests and fly to locate females during the fall. Females can lay 50–100 eggs under their test, after which the female dies and shrivels. The dead female’s test protects the eggs over the winter until the eggs hatch and crawlers emerge the following spring or summer.
Oystershell scale infestations initially affect only a few branches or small areas of the stem, but their
significant reproductive capacity means they can rapidly increase and encrust entire branches, leading to branch dieback and eventually death of the host plant. On many plants, including aspen, OSS blends in well with the underlying bark, so extensive colonies and injury symptoms may develop before the infestation is observed. OSS damage might also increase host susceptibility to secondary pathogens by weakening host defenses or creating infection courts.
Crouch et al. studied two OSS-infested sites in northern Arizona. Both sites were in ungulate exclosures at low elevations, in aspen stands within ponderosa forest type. On average, OSS was present on 79.6% of aspen stems sampled. The infestation was more severe at the lower elevation site: 57.6% of stems at this site were severely infested compared to 16.3% at the higher elevation site. Crouch et al. were unable to determine whether the difference in infestation between the two study sites was caused by the slightly warmer climate associated with the lower elevation or by the nearly decade longer time since OSS establishment in the lower site.
While OSS was present on all size trees, scale numbers and impact were greater on the “tall regeneration” class, which represents recruiting stems. On these trees, 89.7% of stems supported OSS; the impact on 51.2% of stems was rated as severe. Regarding mortality, half of tall regeneration stems were dead whereas only 5.1% of overstory trees, 27.5% of saplings, and 36.3% of short regeneration stems were dead. While Crouch et al. could not determine with certainty that OSS killed these trees, they believed it to be a factor given the intense severity of infestation.
OSS is particularly pervasive in ungulate exclosures. Scale was present in 63.9% of the exclosures sampled. This contrasts with OSS presence in only 12.9% of the 124 permanent aspen monitoring plots in this area.
Beyond northern Arizona, OSS has been documented in natural forest settings in Idaho, Utah, South Dakota, Nebraska, Nevada and New Mexico (not always on aspen). OSS has also been documented on aspen in urban settings in Fort Collins & Durango, Colorado; Rock Springs, Wyoming and Tahoe City, California. Crouch et al. conclude that OSS is likely widespread in the interior West.
Management strategies have been developed for armored scales, including OSS, in orchards or urban areas. These are pruning affected branches, scrubbing to remove the scale, horticultural oils, and insecticides (contact and systematic). Crouch et al. note that management strategies have not been developed for use in forest settings because the scale has not been considered a problem in forests. They say the priority should be on development of rapid response strategies. They think it is worthwhile to explore possible natural enemies, including entomophagous fungi.
Some silvicultural treatments might be effective. One approach is “clearfelling,” in which all mature aspen are removed from the stand in hopes that the scale will die out before stump sprouts reach sufficient size. This might be effective in isolated aspen stands. However, the presence of OSS on understory plants or aspen regeneration would make clear-felling alone insufficient for eradication. Sanitation thinning removes heavily infested trees with the goal of improving the vigor of residual trees. However, it is not known whether OSS prefers weakened or vigorous hosts. Forest managers in northern Arizona are testing both clearfell and sanitation thinning treatments.
Prescribed fire seems more promising since it can kill most aspen and other host plants as well as presumably the scales. Aspen trees naturally regenerate from the roots following fire. Prescribed fire is also relatively inexpensive to apply over large areas and is a natural component of aspen stands.
Crouch et al. note several challenges that complicate management of OSS in natural aspen stands. Many are linked to the insect’s small size and associated difficulty of detection. These characteristics mean that OSS is likely to go undetected until outbreaks become severe; strategies that depend on detection (e.g., scrubbing, pruning, or targeted spraying of oils and insecticides) are unlikely to be initiated early; and the scales can easily be transported to new areas inadvertently. A fourth, unrelated, challenge stems from the scales’ wide host range. Silvicultural strategies don’t affect the understory hosts.
Implications for potential future spread
Crouch et al. attribute OSS’ impact to three interacting factors: (1) its hypothesized role as a sleeper species, (2) potential interactions between OSS and climate change, and (3) the species’ polyphagous nature.
Sleeper species are non-native species that remain at low population levels for long periods of time before an environmental trigger awakens sleeper populations, causing them to experience rapid population growth. By the time sleeper species awaken, their populations are large and already at the spread stage of the invasion process. They note that OSS has been in North America for three centuries, but only recently has demonstrated a threat in natural forest settings. It is unclear which factor(s) led to this development, but Crouch et al. think the increasingly warm climate and stressed hosts are likely explanations.
Aspen in northern Arizona are under increasing environmental stress, which might increase the trees’ susceptibility to OSS. On the other hand, a warmer climate might directly improve the insect’s fitness and abundance. Future warming at higher elevations and latitudes may promote spread of OSS into areas that are apparently not suitable now.
The scale’s wide host range means trees in urban settings provide reservoirs of established OSS that can spread into the forest. Crouch et al. note that most riparian vegetation is deciduous so potentially a host of OSS. They do not discuss the potential role of anthropogenic factors, such as planting of host species in the wildland-urban interface.
Research and monitoring needs
Crouch et al. call for increased monitoring to determine where OSS is established in western North America. They concede that determining priority areas for monitoring is difficult to specify since the host range is so broad. They note a role for citizen scientists in identifying and reporting presence of OSS in both urban and natural forested areas. Crouch et al. have developed an OSS rating system that they hope other scientists and managers will use in their monitoring efforts.
Crouch et al. call attention to the need for improved understanding of OSS’ biology in natural systems, including quantifying its rate of intensification and spread; patterns and timing of mortality; dispersal distances in natural forest settings; ability to survive on dead or cut hosts; temperature and humidity requirements for various OSS life stages; the relationship between OSS and host tree vigor; and the role of native insects as natural control agents. Scientists also do not yet understand OSS’s elevational and climatic limitations – key information for identifying aspen stands at greatest risk. Another issue is whether OSS infestations facilitate infection by fungal pathogens.
Finally, there is an urgent need to develop management strategies that are affordable, efficient, and effective at the landscape scale. Research on silvicultural strategies to manage OSS is already under way in northern Arizona. However, there is a need for research on potential systemic insecticides, biological control agents, and prescribed fire.
Crouch, C.D., A.M. Grady, N.P. Wilhelmi, R.W. Hofstetter, D.E. DePinte, K.M. Waring. 2021. Oystershell scale: an emerging invasive threat to aspen in the southwestern US. Biol Invasions (2021) 23:2893–2912 https://doi.org/10.1007/s10530-021-02545-0