New evidence emerges on decline of monarch butterflies

Dramatic fall in monarch numbers has been widely blamed on Roundup weedkiller

Monarch butterflies make Herculean migratory trips each autumn, travelling more than 3,500km from eastern Canada and the northeastern US to Mexico.

Monarchs are spectacularly beautiful butterflies, beloved by the public, but their numbers have been dropping alarmingly since 2000. The herbicide Roundup was quickly identified as the culprit behind their decline but new suspects have now emerged to share the stage with Roundup. The story is told by science writer Gabriel Popkin in the March edition of Scientific American.

The monarch is "king of the butterflies", hence the name monarch. They are native to both North and South America but no longer found in South America. Monarch populations are also found in Hawaii, Spain, Portugal, Australia and New Zealand.

The monarch is very striking looking – orange wings laced with black lines and bordered with white dots and a wingspan of about 10cm. Monarchs can tolerate toxins ingested from milkweed, toxins that kill other species. The distinctive monarch colouring warns potential predators – “I am poisonous, stay away “. Any predator that eats a monarch becomes very ill.

American monarchs are famous for their seasonal migration. Western monarchs breed west of the Rockies and fly south to overwinter in California. Eastern monarchs breed on the Great Plains and Canada and fly south, covering up to 160km a day, to overwinter in Mexico.


The monarch life cycle is as follows: the female lays eggs (up to 500) exclusively on the leaves of milkweed plants. The eggs develop into caterpillars (larvae) after three to five days. The caterpillars spend nine to 14 days eating the leaves. The caterpillar next cocoons itself inside a chrysalis (pupa) where it undergoes metamorphosis into a butterfly over eight to 13 days. This butterfly emerges from the chrysalis, finds a mate and can lay more eggs after five days.

Each February/March overwintering monarchs wake up, find mates and migrate north and east to lay eggs. This is the first generation of the year. The monarchs repeat this cycle three more times as they migrate northwards. A monarch generation lasts about one month except for the fourth generation, the overwintering generation, that lives for up to eight months.

The numbers of monarchs overwintering in Mexico have been in serious decline over the past three decades – a 66 per cent reduction between 1997 and 2007. This time frame coincides with the introduction (in 1996) and large-scale deployment of Roundup, the Monsanto herbicide based on the chemical glyphosate. Monsanto also genetically engineered Roundup-resistant agricultural plants, including corn, canola, soya and cotton. Growing crops are sprayed with Roundup which kills all the other "weed" plants without harming the main crop.

Farmers took to Roundup like bees to honey. Roundup was the most commonly used herbicide in the United States in 2007. In 2015, 89 per cent of corn, 94 per cent of soya beans and 80 per cent cotton in the US were Roundup-resistant strains.

Unfortunately for the monarch, Roundup kills milkweed plants – the midwestern milkweed population declined by 58 per cent between 1999 and 2010 and overwintering monarch numbers in Mexico declined steeply. And so the hypothesis went forth in 2012 that the Roundup-induced loss of midwestern milkweed is killing the monarchs. This hypothesis was reasonable and widely accepted. It was especially welcomed in eco-warrior circles where Monsanto is demonised as a big environmental “baddy”.

New evidence

However, Popkin cites new evidence indicating factors other than Roundup contribute to the monarchs’ decline. For example, monarch numbers in the US, both during and after the summer breeding season, have shown no steady decline although Mexican monarch numbers fell steeply.

It was also found that only 38 per cent of monarchs migrating to Mexico depart from midwestern corn fields, so how could milkweed loss there account for such big losses in Mexico? And many Mexican monarchs come from US areas with relatively few Roundup-treated crop fields.

Roundup seems to play a role in the decline of the monarchs, but it may be a minor one. It seems that many monarchs are dying as they migrate south in the fourth generation. Possible reasons for this include increased virulence of a protozoan parasite that infects monarchs and/or increasing incidence of drought that reduces plant food that sustain the monarchs on their long journey.

Popkin reports that, disturbingly, some scientists who favoured the milkweed hypothesis initially advised against publicising the new findings listed above. But the bottom line is that monarchs can be helped to recover only when the exact causes of their decline are discovered.

William Reville is an emeritus professor of biochemistry at UCC