More to the monarch than meets the eye

Author: Natalie Steiner

The abundance of invertebrates in the world is staggering – arthropods (the group that includes arachnids, insects and crustaceans) represent 83% of all known living animal species [1]. Within that group, the order of Lepidoptera alone (butterflies & moths) has 180,000 individual species which equates to 10% of all living creatures on earth! Butterflies are wide spread across nearly every type of ecosystem from deserts to mountains and are a great example of why sometimes the world’s smallest animals are some of the most interesting. Here we discuss the life cycle, predation avoidance and mimicry of the African monarch as well as ways in which YOU can help butterflies worldwide.

During their Askari stay, many volunteers will see the African monarch butterfly Danaus chrysippus (also known as the Plain Tiger). This species is commonly widespread throughout southern Europe, Africa and Asia. It is a member of the milkweed butterflies (sub-family Danainae) so called as the larva/caterpillar predominantly feeds on milkweed plants (Asclepias).

 

Life Cycle

Like other butterfly species, the African monarch life cycle consists of 4 stages of complete metamorphosis – egg, larva/caterpillar, chrysalis/pupa & adult as per Fig. 1 below. The entire lifecycle typically takes less than 8 weeks from egg to adult, with each caterpillar moulting up to 5 times as it grows larger, before spinning a silk pad and shedding its skin to form an exoskeleton in which the adult butterfly develops and eventually emerges from.

Predation Avoidance & Mimicry

Milkweed plants are an important food source to this species whilst in the larva/caterpillar stage. The caterpillar absorbs the toxic cardenolides (heart depressants) from the sap within the leaves, making it distasteful to predators such as birds [3]. These compounds are retained in the body and wings of the adult butterflies providing a suitable defence against predators. Aposematic colouration is practised by both caterpillars and butterflies, the use of bright contrasting colours such as red, orange and yellow to warn predators of their potential toxicity. However, some caterpillars feed on different, non-toxic plants; as a result some adults are toxic, whilst others are harmless and edible. To help mitigate predation of palatable adults, the adult male butterfly also absorbs other toxins from various plants. This allows him to create pheromones to attract females then, during mating, the male transfers these chemicals to the females making all adults unpalatable to predators.

So what is mimicry? For any bird that predates on an African monarch butterfly, the disagreeable experience will make it unlikely to attack any similar coloured butterflies. Over time this has led to the evolution of mimics in other butterfly species as a way of avoiding predation.

Some non-toxic butterflies mimic the colouration of the African monarch  to trick predators into thinking they are also toxic, this is known as Batesian mimicry. The palatable female Common diadem (Hypolimnas misippus) for example, is very similar in colour to the African monarch. This form of mimicry only benefits the mimic and works most effectively where the mimic is less prevalent than the model/toxic butterfly, otherwise predators may not have sufficient negative experiences and will start to learn to distinguish between the model and the mimic.

Mullerian mimicry occurs when another toxic species also evolves with similar colours to the model – both model and mimic benefit from this with increased numbers of similar looking toxic individuals. The White-barred acraea (Acraea encedon) and its similarity to the African Monarch is an example of this.

Female common diadem ©Muhammad Mahdi Karim

White-barred acraea ©Kate Braun

 

 

 

 

 

 

 

Migration

Whilst the African monarch butterfly is a common sight on the open savannah and grasslands at Askari, sadly on a global scale, not all butterflies are faring quite so well. Unlike the African monarch which does not undertake any significant seasonal migration in Africa, it’s North American cousin, the monarch butterfly (Danaus plexippus) migrates up to 4,000kms each year across 4 separate generations to avoid the harsh American winter climate and to find larval food plants which do not grow in its overwintering areas [7]. It is the only insect to travel such great distances to warmer climates.

In 2015, the US Fish and Wildlife Service released a report estimating that the monarch population had decreased by 96.5%, that’s a loss of 970 million butterflies! [7] What is responsible for the drastic population loss of monarchs in North America?

 

Conservation Issues

As with many other species, the monarch butterfly is subject to a number of factors that are contributing to the population decline:

  1. Habitat Loss

Native grasslands are being converted for residential, commercial and agricultural development reducing the available areas in which native milkweed can grow and which monarchs can use to lay their eggs upon. It is estimated that 167 million acres of monarch habitat has been lost in the mid-western USA since 1996 [9]. This loss of grasslands also results in a decrease in the availability of nectaring plants for food on the migratory route.

In Mexico, the forests used for over-wintering have been subject to deforestation and over-logging by both local people for firewood and large-scale timber companies. Given that the butterflies return to the same trees year after year, this habitat loss disrupts the natural migratory cycle with less trees available for diapause/hibernation.

2. Use of herbicides

The increased use of herbicides and genetically engineered seeds for cultivation which are pre-treated to prevent the growth of weeds and shrubs close to agricultural land has contributed to a significant loss of native milkweed.

3. Non-native plants

The planting of tropical milkweed, albeit with good intentions to increase food plants for monarchs, has had the opposite effect. The plant contains a parasite (Ophryocystis Elektroscirrha) which the monarchs ingest through consumption of the leaves. The result is an infection of spores causing the adult butterflies to be weak and fragile with a severely reduced lifespan.

4. Climate Change

Extreme weather events such as severe droughts in California and other parts of the USA coupled with colder winters in Mexico adversely affect the survival rates of the monarch at all stages of its life cycle.

 

What can you do to help?

Whilst the fate of a butterfly in North America may not seem important when it’s cousin in Africa is thriving, it is an indicator of the health of our planet and a reminder that all ecosystems are fluid, dynamic and linked in some way, regardless of their location. A seemingly small change in one can have huge knock on effects in another.

 

There are some very easy steps to help protect butterflies wherever in the world you may live:

  • Avoid using herbicides and pesticides in the garden, try natural methods instead – planting mint, dandelions and fennel helps to attract natural predators such as ladybirds, lacewings and praying mantis.
  • Plant ‘butterfly-friendly’ shrubs and flowers to provide habitat, nectar and host plants on which eggs can be laid by native butterflies. If all gardens contain suitable habitat it helps to create way stations and habitat corridors to allow butterflies to move between locations.
  • Where possible, don’t buy genetically engineered food or seeds. The use of these seeds which contain herbicides prevents any weeds from growing with the crops but also prevents any native weeds or grasses surviving in the margins at the edge of the fields.
  • Get involved in protecting and saving native grasslands and areas of important habitat in your local area.
  • Buy Forest Stewardship Council approved wood to be sure that the timber products you buy are being sourced in a sustainable and environmentally sensitive way, which benefits not only butterflies but all other wildlife too.
  • Be climate change aware. Reducing your own individual energy consumption and carbon emissions does have an impact – plus it could save you money!

 

Globally, butterflies are vital pollinators of plants regardless of the ecosystem they inhabit. A worldwide reduction in butterflies will have a significant impact on the yield of fruits and vegetables that humans use for food as well as affecting birds, animals and other insects that use trees, flowers and fruiting plants for food, shelter and survival. Ultimately, protecting butterflies and their habitats is good for all living creatures that inhabit the planet….and that includes you.

 

References

  1. Featured image ©Natalie Stenier
  2. Anna Thanukos, The Arthropod StoryUniversity of California, Berkeley, as at 23 January 2017
  3. Fig 1. ArtsEdge Kennedy Center – http://artsedge.kennedy-center.org/~/media/ArtsEdge/Images/LessonArt/grade-k-2/activities/butterfly-dance/ButterflyCycle_4-3.ashx
  4. http://www.learnaboutbutterflies.com/Africa%20-%20Danaus%20chrysippus.htm
  5. BBC Wilddlife magazine January 2017 edition
  6. Taylor, Chip (January 29, 2014). “Monarch Population Status”. Monarch Watch.
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