Never seen one. I think Monarch population is probably not in decline. Monarch butterflies are colourful and popularly raised by hobbyists. It is possible to mail order for the pupae within USA. Sightings will include specimens raised by hobbyists. The marking of butterflies as cultivated or wild is not clear in iNat. I didn’t really mark that because the butterflies came from eggs obtained from a wild source over here. But in USA, there are farms supplying the pupae. These are for educational purposes. What I’ve learned about my local butterflies species is they are more common at some times of the year. It may be rain cycle. and butterflies are often parasitised by wasps. Some wasps may hunt the caterpillars and some deposit the eggs in them. It is during the rain season they are more common, perhaps it is due to the weather may have made the parasitic wasps less active, and also the plants grow more new leaves in the rain season. In places with distinct four seasons, naturally the population is very low at some times of the year.
I read about the bacteria infecting butterflies, and they blame on tropical milkweed. I don’t know if I’m qualified to comment because this is really very complex when it involve microorganisms. We have had deformed butterflies too in the tropics. No idea why. Probably due to cultivation, sometimes the food supply ran out suddenly due to keeping too many caterpillars, some didn’t develop well ,and sometimes the population inside the container simply crashed. Butterflies are kind of delicate creatures. In nature, the caterpillars will be highly controlled by predators, but they have cleaner environment to pupate as they tend to seek a place further from their original host plants. In nature, the population are more diversified in the genes. My local populations have seldom reached the density of migration types.
Humans will have changed the landscape within 200 years. With artificially created reservoirs and parks, large scale monoculture and pesticides.
The nature of insects is that the populations aren’t always constant. Like locust populations aren’t constant. In some years, they grow into big numbers.
The local groves, where monarchs overwinter, have seen a HUGE declines since the 1960s in the Santa Cruz area. From groves that had thousands upon thousands to ones that had a few hundred. I eye-witnessed this decline over the last several decades.
I supposed the monarchs could have moved on to other groves, although they were known to go back to the same groves year after year.
Despite the apparent decline locally, I would be very happy to know if monarchs have a generally thriving population.
The Monarch population as a whole is probably not declining, I agree. But the North American migratory populations definitely are.
The studies I cited are specifically about the North American migratory population. Survey data, which is much less biased than iNat data, and genetic data independently point to the same fact—the effective breeding population of Danaus plexippus plexippus has not declined. Also, the genetic data show that monarchs increased in population dramatically about 200 years ago. There are probably more migratory monarchs in North America now than ever before in history. Even if there were declines, they would be returns to pre-colonial levels.
The situation in California is definitely different than the situation in the rest of the North American population. Both the western and eastern populations are genetically indistinguishable, but the western population is a very small percentage of the overall population—something like two percent or less. This does leave the western population more vulnerable to extinction, but it also means that even if all the western monarchs disappeared it wouldn’t put a dent in the overall population. The western population has proved to be extremely resilient and quickly bounces back after bad years.
I think an important point to consider in this discussion is that monarchs exploded in population around 200 years ago due to agricultural practices that allowed milkweed to thrive, and any current declines (if they are indeed declines) are likely just bringing monarchs back to a pre-Colombian level. I cited this research in my initial comment if you want to read the study.
Good point about monarch rearing in northern areas—tropical milkweed likely isn’t as popular in Wisconsin as it is in Florida. It is grown as far north as Winnipeg though. However, the biggest benefit of tropical milkweed according to its proponents is that it is incredibly easy to grow in gardens and indoors as food for captive-reared monarchs. Asclepias syriaca is by no means hard to grow, but it’s not something that most gardeners want to grow, especially not indoors.
I’ve never seen anyone in my area try to grow Tropical Milkweed in any setting other than a greenhouse. People here do internationally grow Common Milkweed in gardens all the time for this purpose (they don’t grow it indoors, they make cuttings from plants outdoors, either in their garden or growing wild).
But given that we aren’t returning to pre-Columbian ecosystems, that may still indicate something wrong.
We are returning to pre-Columbian ecosystems in one sense. We’re reforesting (and newly foresting) a lot of land, and the amount of forest in the US is relatively similar to the estimated pre-Columbian forest cover. Forests of the United States - Wikipedia. This reforestation is not necessarily a good thing, as the forests are nowhere near the same composition as pre-Columbian forests. However, this is still significant because loss of forest for agricultural land use is the hypothetical cause of the monarch population bloom (that data is in the genetic study I cited). The monarchs don’t care whether the forests are high quality old growth or if they’re pine plantations, they can’t use them either way.
There is a lot to be debated here–how many of the pre-Columbian forests were closed canopy vs savannah type? I would appreciate any insight into that, but I think the point still stands: the apparent modern declines in monarchs, which probably aren’t even declines, aren’t a sign that monarchs are going extinct.
You say: " monarch summer populations (the actual population) [has not declined for as long as we’ve been monitoring it]. And you also say: "captive rearing monarchs and planting tons of tropical milkweed, has now led to [75%+ OE levels as far north as Chicago ].
Can both statements be true; i.e. can it be true that despite the alleged soaring OE levels in the summer breeding monarchs in the northern USA, those populations “have not declined for as long as we’ve been monitoring them”?
In 2019 Andy Davis’s wife co-authored this graph that shows OE levels in the migratory overwintering site monarchs in both Mexico and California have been low (averaging single digit percentages) during the 2010’s. 2020’s collected data that I am familiar with will be published this coming Spring and will also show single digit percentages.
I agree that both statements are true. I think your graph below shows a very interesting phenomenon–migratory success seems to be lower as a result of OE/Tropical Milkweed. However, the recent genetic study I cited found no decline in effective population size over that time, so I suspect that monarchs (so far) are able to bounce back from low winter numbers. I am definitely concerned that widespread use of Tropical Milkweed will lead to a decrease in migratory success and might even spell the end for the migration. However, the effective population size (the breeding population size) will likely remain relatively stable even if the migration ceases. The migration has many long-term benefits, and if monarchs stop migrating forever they’re going to eventually have serious consequences from it. So we should try to preserve the migration, but we should also push back against the narrative that monarchs are declining.
With regard to the notion that migratory success might be substantially declining, Dr. Chip Taylor and colleagues have shown, using extensive tag-recapture data, that speculation regarding that hypothesis is unsupported: https://news.ku.edu/2020/08/18/new-findings-debunk-idea-migration-mortality-major-reason-declining-population-monarch.
The graph below, published in 2023 by the World Wildlife Fund-Mexico, shows that for the past 15 years the overwintering population in Mexico has been stable overall rather than decreasing. Thus we are left with no substantial evidence supporting the speculation that migratory success might be substantially declining or that the size of migratory/overwintering population is in substantial decline.
Thanks for sharing the data. I misread your initial graph as showing migratory success when it really shows OE infection rates.
Interestingly, it seems to show declining OE infection in the western population concurrent with the home rearing fad.
Yes, that is interesting. Technically it’s the percent of heavily infected overwintering adults that is declining, so it might be that OE is more prevalent and less severe, or it might be that the most heavily infected monarchs aren’t making it to the overwintering sites. Or it might just be that OE levels are declining, for whatever reason. I’m open to any of those possibilities.
Really interesting discussion that has been started here!
As someone who is not from the USA but from South Africa with a bit of knowledge of our local butterflies here, is there any credence to the theory that any observed decline in adult butterfly numbers could entail a later increase due to cues in changing weather and climate? I know that larva of many species can enter stasis before they hatch out if the conditions aren’t right, something I’m sure applies to northern hemisphere regions as well with their dramatic shifts in season
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