Thursday, 27 December 2018

Everybody’s Talking About American Monarch Butterflies

American Monarchs are in the news

Male Monarch Butterfly, Female Monarch (Photos: Steve Andrews)

Every time the Monarch Butterfly (Danaus plexippus) makes the news it is the latest on the iconic insect’s struggles in America, where it has migrated in billions from Canada and the northern states down to California and Mexico each autumn. Over the past decade there have been reports of the butterfly’s alarming decline, due to habitat loss, pesticides, herbicides, disease and climate change. Modern farming using the herbicide Roundup (Glyphosate) on maize and soya-bean crops, is eliminating the once common milkweed species that grew in farmlands throughout the United States.  Legal and illegal felling of trees in Mexico in areas where the monarch overwintered is another big problem.

A disease known as “OE,” which is the abbreviation for Ophryocystis elektroscirrha, a protozoan parasite, takes its toll on the butterflies causing severe weakness, and cripples with deformed wings that do not live long. Some butterflies are so weak they fail to be able to emerge from their chrysalises, others that do die shortly after. It appears that OE is widely distributed among all Monarch populations. The microscopic spores are spread from infected adult butterflies onto the leaves of milkweed plants. Caterpillars that eat them become infected. OE is not a threat to other types of butterfly apart from other species in the Danaus genus.

It is very distressing to see how badly Monarch butterflies have been doing in recent years, and there is even talk of the butterfly being threatened with extinction.

Saving the Monarch

Conservationists and concerned citizens of America have been doing all they can to halt the decline in Monarchs and to help the butterfly survive in great numbers again. One of the main methods being used is the cultivation of milkweed (Asclepias) species. Many people have taken to growing these plants and rearing the butterflies in captivity by keeping, eggs, caterpillars and chrysalises indoors or in protective enclosures. The idea is to keep them safe from predators, such as wasps.

Tropical Milkweed (Photo: Steve Andrews)

The problem with these methods is that one of the most commonly grown milkweeds known as Tropical Milkweed (A. curassavica) is a non-native species that is suited to tropical and subtropical areas. With warmer weather due to climate change it is being grown in many places where once it would not have survived. It can cause a very real problem because it can build up large numbers of OE spores and become a source of infection. This happens in places like Florida where it is warm enough for the plant to grow all year around. This also means that Monarchs can continue to breed in such areas and will not have any need to fly elsewhere. Many scientists and Monarch conservation and research groups, such as the Xerces Society, are recommending that only native milkweeds should be grown. This is good advice. There are many species of Asclepias that grow in all zones of North America and right up into Canada. These plants die down for the winter and resume growth again the following year. These native species do not allow the build-up of OE on them. These endemic milkweeds encourage the Monarchs to migrate because it means that late in the year there are no food-plants available for the females to lay their eggs on. It was all working really well until humans interfered by destroying milkweeds with herbicide and then, in an effort to help, by growing a plant not native to the north American states. The Tropical Milkweed is getting a bad name but the reason is due to the very real problems it can cause in America. However, in some places there is no other choice!

Elsewhere in the World

Female Monarch on Balloonplant (Photo: Steve Andrews)

Monarchs live in non-migratory populations in many other parts of the world, including the Canary Islands, Portugal, Spain, New Zealand and Australia. In these countries there are no native milkweeds, and indeed, the butterflies have only been able to colonise large areas due to the prevalence of the introduced Tropical Milkweed and two other plants that were formerly in the Asclepias genus. The Balloonplant (Gomphocarpus physocarpus) and the Swan Milkweed (G. fruticosus) are naturalised in all the places named above and are both used as Monarch caterpillar food-plants.

Female Monarch returns to my garden to lay eggs on Tropical Milkweed

I lived in Tenerife in the Canary islands for nine years, and it was there that I started successfully rearing Monarchs. I fed the caterpillars on Tropical Milkweed, which is the only milkweed found on the island, apart from very occasional specimens of the Gomphocarpus species. It is because the Tropical Milkweed was brought to the island as an ornamental garden flower that Monarchs were able to colonise Tenerife, where it is said they were first seen back in 1887. How they reached the islands is uncertain. Now that Monarchs are on Tenerife, they have no need for migration because, although it gets plenty of snow and ice up on Mt Teide, around the coasts and in the south, the temperatures remain warm enough for Tropical Milkweed to keep growing and the butterflies to keep breeding. The situation is similar in parts of mainland Spain and Portugal where this non-native milkweed has been grown in gardens and where the Gomphocarpus species have become naturalised.

Monarch Butterflies in Portugal
Recently eclosed Monarchs (Photo: Steve Andrews)

I have lived in Portugal for the past four years, and this year I was successful in rearing four generations of Monarchs with the caterpillars feeding on plants of Tropical Milkweed and Balloonplant I managed to grow enough of in the garden here. I had at least 30 butterflies each time. The last lot of adult butterflies emerged in late November but where they went I have no way of knowing. I have cut what was left of the milkweeds down to short stalks so if any females came back they would have found nowhere to lay their eggs. I originally obtained eggs from someone I know who has a butterfly farm up in Aveiro further north. He tells me there are no Monarchs there in winter and he gets his eggs sent up each year from the Algarve. According to As Borboletas De Portugal, a Portuguese butterfly book I have, along with the Algarve, the northern coastal city of Aveiro, is one of the only places that Monarchs can be found in the country. Is this because of the butterfly farmer I know there, or are there naturally occurring wild ones, and how did they get there? I have been told that the butterflies can sometimes be seen in the Lisbon area too. Where did they come from? Did they migrate from the south? I often wonder what happens in winter. Do any Monarchs go south here to the Algarve where there are non-migratory populations or do all the butterflies in central and northern parts, such as the ones I released, simply live short lives and fail to breed due to the cold weather and lack of food-plants. There is very little information available online or in books about the behaviour of Monarchs in Portugal and Spain, and indeed in Europe. Is anyone else studying these butterflies here apart from me? Google for information on Monarchs and most of the results are about those in America.

I have had the idea that the non-migratory resident Monarch populations, wherever they are worldwide, are forming a genetic reservoir of the species. If its migratory populations ever do become extinct, the species could be reintroduced by moving some from the resident colonies elsewhere.

The Wanderer

I have wondered whether as climate change causes milder winters would the butterflies colonise other more northerly countries if the non-native food-plants grew in sufficient numbers there? Would any Monarchs eventually evolve into a migratory form like their American cousins? A very small number of Monarchs reach the UK some years but are unable to breed due to lack of any type of milkweed growing there and are unable to survive the cold winters. But the fact that they got there at all reveals their wandering nature, which is why an alternative name for the Monarch Butterfly is the Wanderer.