Joint Statement Regarding Captive Breeding and Releasing of Monarchs
This document is available for download as a PDF here.
Are we helping or hurting monarchs by releasing large numbers of captive-reared individuals? Across the
country, people purchase monarchs for release at weddings, funerals, and other celebrations; and to raise in
classrooms and other educational settings (1). Following news of the dramatic decline in monarch numbers,
some people are rearing large numbers of monarchs in backyard operations or obtaining them from
commercial breeders or other organizations and releasing them with the goal of supplementing local
populations. While raising and releasing small numbers of monarchs can offer important scientific and
educational opportunities and foster a connection to nature, we believe that releasing commercially produced
and continuously mass-reared individuals is unlikely to benefit monarchs, and could actually hurt them, as a
result of mass rearing conditions that promote crowding and disease spread, or cause the loss of genetic
diversity or adaptation to captive rearing conditions. Large-scale captive rearing and subsequent release can
also limit the ability of monitoring programs to understand natural population dynamics. For these reasons, we
recommend against large-scale captive rearing of monarchs for release into the wild, and we summarize the
potential impacts below.
Negative effects of mass rearing conditions
Mass production of monarchs makes it easy to transmit disease. Monarchs are susceptible to diseases that can
be transmitted among larvae (2). There are no requirements that commercial breeders or others who raise
captive monarchs follow specific disease-prevention protocols, nor are there agencies that routinely test
captive stock for diseases (3). In our research labs, we raise several hundred or more monarchs per year for
scientific purposes, house them singly or at low densities in hospital-like sterile conditions, and shut down our rearing practices annually for deep cleaning. Even under these conditions, it is hard to keep pathogens in
check, and we periodically experience disease outbreaks. It is relatively easy to screen for the most common
monarch pathogen, Ophyrocystis elektroscirrha (OE), but doing so requires constant vigilance. A decade ago (4), and more recently in 2013, several of the authors of this document found that many purchased monarchs from commercial growers were heavily infected with OE.
Additionally, unknown pathogens occasionally affect monarchs in our labs (suspected Serratia, Nosema, and nuclear and cytoplasmic polyhedrosis viruses); these pathogens often kill 30% or more of our monarchs. Three of us have been rearing monarchs from wild material for over two decades, and these die-offs in our study animals only started in 2004–05. Because it began independently in our laboratories at the same time that we noticed increased releases of commercially bred and other captive-reared monarchs, we wonder if the causative pathogens could be traced to these releases, as has been suggested in bumble bees. There is evidence that the dissemination of commercially bought, captive-reared bumble bees used to pollinate glasshouse tomatoes and open-field crops has helped spread the pathogen Nosema bombi into wild bumble bee populations in North America, and this appears to have caused catastrophic declines in four native bumble bees (Bombus terricola, B. affinis, B. franklini, and B. occidentalis) in the wild (5,6).
Although the impact of releasing commercially bred monarchs into the environment has not been well studied,
the potential exists for population declines of wild monarchs through the following mechanisms: 1) a build-up
of higher levels of pathogens in commercial breeding facilities that are then spread to wild monarchs; 2) more
virulent strains of pathogens evolving in commercial breeding facilities that infect wild monarchs; or 3) the
introduction of novel strains of pathogens to regions where they would not naturally occur, due to the
shipment of monarchs. These potential risks have been considered for bumble bees (7), and it is reasonable to
assume that the same risks exist between commercially bred monarch butterflies and their wild counterparts.
There is also concern that monarchs reared in captivity might be less fit than those that grow outside in a
natural environment. A recent study found that reared monarchs were significantly smaller and less likely to be
recovered in the Mexican overwintering grounds than their wild counterparts 8. Smaller monarchs live shorter
lives as adults and smaller females lay fewer eggs. In addition, if monarchs are reared at high densities they can become stressed and injured (9,10).
Genetic consequences: Adaptation to captive conditions and loss of genetic diversity
We also are concerned with the potential genetic consequences of breeding and releasing monarchs; breeding
closely related individuals can lead to the negative fitness effects of inbreeding (11). While we don’t know if
breeders work to maintain genetic diversity, if commercial breeders or other monarch growers use small
numbers of monarchs in their initial captive population, and if they continue to breed related individuals over
successive generations, then many of the monarchs they release would be closely related. Furthermore,
breeders often share monarch lineages (often referred to as “stock”) with each other, meaning that even the
monarchs coming from different facilities can be related.
Studies in species ranging from fruit flies to fish show that animals can genetically adapt to captive conditions in as short as one or two generations (12). Conditions experienced in captivity can differ greatly from those in the wild in terms of temperature, moisture, light intensity, day length, food sources, density, presence vs. absence of predators, and more. When this happens, researchers see a high frequency of genetic traits that would be harmful or result in a lower survival rate for these individuals and their offspring if they are released into the wild. The more generations that are continuously raised in captivity, the more extreme this effect becomes.
Negative effects on scientific research
To better understand and protect the monarch, researchers and citizen scientists carefully track the size,
migration, and geographic range of the monarch population. When captive-reared monarchs are released in
places and times when they are rare or not naturally present, it confounds our ability to document the real
state of the wild monarch population for several reasons.
- 1) Large-scale releases in places where wild monarchs are already present can bias monitoring and make
the population seem artificially large.
- 2) Releases at times and locations where monarchs are not present lead to false measures of monarchs’
“occupancy” of the landscape. Because we know that releases occur, this could alternatively lead scientists
to disregard observations that are actually real. For example, a monarch observed and photographed in
April 2015 in Minnesota was reported to Journey North. This observation was subsequently removed from
the Journey North database because of its extreme earliness at the location, and the chance that it could
have been captive-reared and released. However, it may have been a very early migrant, and could thus
help us understand impacts of weather and climate on monarch migration.
- 3) Collection of monarchs from one area, subsequent mass breeding, and translocation to other areas could
distort estimates of genetic diversity and gene flow in wild monarchs (3,13). Releases that occur during times of the year when natural monarch abundance is low (such as in early spring), have unusually large influences
on monarch ecology and genetics. The study of monarch biogeography, especially in western North
America, is still in its infancy, and many important questions critical to monarch conservation are still largely unanswered. Releasing monarchs into the landscape without marking or monitoring interferes with our
ability to answer those questions.
For all of these reasons, we do not support the release of purchased or mass-reared monarchs. It is our collective judgment that purchasing and releasing monarchs, or rearing and releasing excessive numbers, could do more harm than good.
We realize that rearing on small scales can offer benefits for education, outreach, connecting with nature, and
citizen science. Members of the public who choose to bring in small numbers of wild-collected monarch eggs
and larvae for personal enjoyment, outreach, or to participate in citizen science projects are encouraged to
follow protocols for safe rearing and collect data on their reared monarchs for programs such as the Monarch Larva Monitoring Project and Monarch Health.
All rearing of monarchs should be undertaken with extreme care, restricted to a single generation annually if the butterflies will be released, and carried out using safe rearing practices and vigilant monitoring for health and disease.
Individuals who care about monarchs and who wish to contribute to monarch conservation are encouraged to participate in citizen science monitoring and habitat restoration projects, and to support natural habitats with diverse ecological communities where monarch populations can be self-sustaining.
Monarch Health, Odum School of Ecology, University of Georgia
Department of Biology, Sweet Briar College
Department of Entomology, Washington State University
The Xerces Society for Invertebrate Conservation
Conservation Biology Graduate Program, University of Minnesota
Southwest Monarch Study
Conservation Biology Graduate Program, University of Minnesota
Monarch Larva Monitoring Project, Department of Fisheries, Wildlife and Conservation Biology, University of Minnesota
Northwest Lepidoptera Society
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