Degree of Threat: High - medium
Comments: Threats include invasive herbs, grasses, and shrubs at many sites, inappropriate fires at some. Other threats applicable at some sites include human disturbance associated with roads, recreation and development, grazing, herbicides and pesticides. Se USFWS (1998, 2010).
The 1998 USFWS review suggests exotic plants as probably the greatest overall threat. Threat from prescribed fires seems to be mainly in the Puget prairies and might not be applicable to current practices. Threats from exotic plants are probably a concern at most or all occurrences but are especially severe in the Puget Prairie portion of the range where this threat must be considered operational now. Grazing is a threat in the Southern Cascades and elsewhere, but the potential severity of this impact is uncertain and some level of grazing is probably compatible.
Use of BTK against forest pests has been suggested as a threat (USFWS, 1998 and 2003) and between Asian Gypsy Moth and the Spruce Budworm and other native conifer pests this is a potentially widespread but not universal concern. However there does not appear to be any actual evidence whether or not BTK poses any risk to Mardon Skipper. None of the major scientific studies on BTK impacts are referenced by USFWS (2003) or in the Center for Biological Diversity listing petition, and no relevant evidence is given. It is simply not true that BTK is lethal or even toxic to all caterpillars. There are extremely few data on BTK sensitivity of skippers and sensitivity can vary greatly among species in the same genus in several other families. So it is quite possible but undocumented and very far from certain that Mardon Skipper larvae would have high mortality from BTK spraying. According to USFWS (2003) this skipper hibernates as a pupa. Only actively feeding caterpillars can be harmed by BTK. If this species hibernates as a pupae or as a fully fed larva, then the risk from BTK could be substantial or nonexistent depending on when applications occur relative to when eggs hatch. Among Lepidoptera in general mortality ranges from zero to 100% depending on species and instar in the lab and slightly less in the field; with virtually the full range of sensitivity found even within genera and sometimes even within instars of the same species (e.g. Peacock et al., 1998). However almost all species tested are sensitive as first or second instars and about half are highly so. With few exceptions the lethal period of BTK residue to Lepidoptera is about 10 days or less although for a few species it can remain lethal for a month. Both sunlight and rain usually rapidly affect BTK residue. Therefore if BTK is applied before the adult flight season, the risk should be minimal. Few or no larvae would likely be exposed. If it is applied during or within a week after the flight season most or all first instars could be exposed--depending on where exactly they feed. If it is applied substantially after the adult flight season then risk is unknown (possible range: zero to nearly 100% mortality) but should be assumed to be very high until actual data are available for the instars that would be exposed. Data for other skippers, including congenerics, might not be a reliable indicator of sensitivity.
Even ignoring the uncertainty related to BTK it appears that between exotics, livestock grazing, herbicides, and direct human use and in some places fire most populations are under some degree of threat. Some may be too reduced now to have much chance of persistence. Threats appear very widespread but variable, less in the southern Cascades than elsewhere.