Untimely Management
Wise management practices are essential in providing long-term habitat for monarchs and other wildlife, but if appropriate guidelines are not followed, monarchs and other organisms using the site could be harmed. Untimely or too frequent management, like mowing, causes additional concern for an already vulnerable monarch population. Eggs, larvae, pupae, and even adults may be killed directly by the mower, and mowing may also destroy critical host or nectar plants. To limit mortality to monarchs and other pollinators, follow the guidelines recommended in MJV’s Mowing: Best Practices for Monarchs Fact Sheet.
Vehicle Collisions
While little research has been conducted to quantify the direct impacts of vehicle collisions on the monarch population, mortality along our roadways does occur, particularly during the fall migration. Despite threats that come with restoring roadside habitat for wildlife, the benefits of increasing habitat availability and connectivity are likely to outweigh the costs to monarchs. With the development and continuing land conversion, wildlife habitat continues to decline. Roadsides provide an opportunity to replace some of the habitat that has been lost from the landscape to benefit monarchs, but additional research is needed to determine what characteristics may make the roadside habitat more or less effective. For additional information, please see our Monarchs and Roadsides: Frequently Asked Questions resource, as well as the Xerces Society's literature review: Pollinator Habitat Enhancement and Best Management Practices in Highway Rights-of-Way.
Spread of Introduced Species
In parts of the southern U.S. and California, the year-round persistence of non-native tropical milkweed (Asclepias curassavica) allows monarchs to breed throughout the winter. These year-round tropical milkweed patches foster higher transmission rates of the protozoan Ophryocystis elektroscirrha (OE), increasing the likelihood that monarchs become infected with the debilitating parasite. Therefore, it is recommended that tropical milkweed be cut back in the fall and winter months in the southern U.S. and California and should be gradually replaced with native milkweeds as they become available.
Another alien invader is jeopardizing the process of monarch reproduction by confounding female monarchs during the egg-laying process. Black swallow-wort (Cynanchum louisea, formerly C. nigrum or Vincetoxicum nigrum) and pale swallow-wort (Cynanchum rossicum, formerly Vincetoxicum rossicum) are members of the milkweed family (Asclepiadacea) and are native to Europe. Female monarchs will lay eggs on black swallow-wort, even when it is growing in the same field as common milkweed (Casagrande and Dacey 2007), even though it is not a viable food source for monarch caterpillars. Thus, swallow-worts may act as a “sink” for monarchs; caterpillars from eggs laid on these invasive plants will not survive. In addition, swallow-worts (like other invasive species) can crowd out native milkweed and nectar sources, decreasing the availability of appropriate food sources for monarch caterpillars and adults. Black and pale swallow-worts should not be confused with Cynanchum laeve (common names sandvine, honeyvine, bluevine milkweed, and smooth swallow-wort), a similar plant that is native to eastern and central U.S. states and Ontario. C. laeve is an appropriate monarch host and is found in many of the same states as the invasive species. Sandvine has heart-shaped leaves and white flowers and is native to North America. Read more about these invasive species on our MJV Invasive Species Alert: Black swallow-wort (Cynanchum louisea) and pale swallow-wort (Cynanchum rossicum) handout.
References:
Casagrande, R.A. and J.E. Dacey. (2007). Monarch Oviposition on Swallowworts (Vincetoxicum spp.). Environ. Entomol. 36(3): 631-636. doi.org/10.1603/0046-225X(2007)36[631:MBOOSV]2.0.CO;2
Majewska, A. A. & Altizer, S. (2019). Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies? Insects 10(8): 253. doi.org/10.3390/insects10080253
Majewska, A. A., Sims, S., Wenger, S. J., Sneider, A., Altizer, S., and Hall, R.J. (2019). Multiple Transmission Routes Sustain High Prevalence of a Virulent Parasite in a Butterfly Host? Proc. R. Soc. B. 286, 20191630. http://dx.doi.org/10.1098/rspb...