Monarchs in decline: a collateral landscape‐level effect of modern agriculture

Abstract

We review the postulated threatening processes that may have affected the decline in the eastern population of the monarch butterfly, Danaus plexippus L. (Lepidoptera: Nymphalidae), in North America. Although there are likely multiple contributing factors, such as climate and resource-related effects on breeding, migrating, and overwintering populations, the key landscape-level change appears to be associated with the widespread use of genetically modified herbicide resistant crops that have rapidly come to dominate the extensive core summer breeding range. We dismiss misinterpretations of the apparent lack of population change in summer adult count data as logically flawed. Glyphosate-tolerant soybean and maize have enabled the extensive use of this herbicide, generating widespread losses of milkweed (Asclepias spp.), the only host plants for monarch larvae. Modeling studies that simulate lifetime realized fecundity at a landscape scale, direct counts of milkweeds, and extensive citizen science data across the breeding range suggest that a herbicide-induced, landscape-level reduction in milkweed has precipitated the decline in monarchs. A recovery will likely require a monumental effort for the re-establishment of milkweed resources at a commensurate landscape scale.

Effects of phenological synchronization on caterpillar early-instar survival under a changing climate

“…Summer breeding success does not appear to be affected by changing climate (Stenoien et al 2016), but monarch populations could be impacted by climate change via increases in extreme weather events during overwintering or reduced rainfall and concomitant decrease in milkweed abundance in the southern spring breeding grounds (Oberhauser et al 2017). Conservation efforts must focus on the life history stages that affect population dynamics to be effective; for a long-ranging migratory species like the monarch, these can occur thousands of kilometres apart across major biomes and jurisdictions, greatly increasing the challenges of conservation.…”

“…The main threats causing monarch population declines appear linked to the disappearance of milkweed plants linked to intensive agricultural practices, especially in the US corn belt (Flockhart et al 2015;Stenoien et al 2016), although research also suggests declines occurring during migration or on overwintering grounds (Inamine et al 2016). Summer breeding success does not appear to be affected by changing climate (Stenoien et al 2016), but monarch populations could be impacted by climate change via increases in extreme weather events during overwintering or reduced rainfall and concomitant decrease in milkweed abundance in the southern spring breeding grounds (Oberhauser et al 2017).…”

Limits of Concern: suggestions for the operationalisation of a concept to determine the relevance of adverse effects in the ERA of GMOs

“…Over a time period of 11 years, host plants in agricultural fields were considerably reduced as well as the egg production of the US Midwest Monarch butterfly population [84]. It generally shows that indirect effects through agricultural practices on individual species, either alone or in combination with other adverse effects, can be severe [85]. …”

Complex Outcomes from Insect and Weed Control with Transgenic Plants: Ecological Surprises?

“…One of the most fascinating insect migrations is performed by the North American monarch butterfly, Danaus plexippus (Lepidoptera: Danaidae), that migrates from its summer distribution area, covering a large part of North America, to winter in a small part of central Mexico, in spectacular masses (Stenoien et al, 2016). For this reason, as well as for its spectacular coloration, the monarch has become a symbol of beauty and freedom-one of the conservation icons of North America.…”

“…Glyphosate is now also implicated in the decline of the monarch butterfly (Stenoien et al, 2016), further illustrating the various kinds of environmental damage that reliance on a few plant protection chemicals may bring. However, the monarch may not be the only species at risk for similar reasons-a total of 39 protected European lepidopteran species have maize weeds in their host plant range (Lövei et al, 2016).…”