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What happens when a monarch skips a meal?

Monarch caterpillar eating milkweed
Credit: Insectarium de Montréal (André Sarrazin)
Chenille de monarque qui mange de l’asclépiade
  • Chenille de monarque qui mange de l’asclépiade
  • Chenille de monarque qui mange de l’asclépiade
What happens when a monarch skips a meal?

The monarch butterfly caterpillar has a voracious appetite. It feeds exclusively on its host plant, milkweed, which it devours at an astonishing clip. In less than two weeks, its weight increases 2,700-fold! But what happens when it doesn’t get enough to eat?

The importance of eating at the larval stage

For a number of insects, feeding during the larval stage is essential to proper development in adult life. It’s effectively during this stage that the organism acquires the food resources needed to form adult tissues.1 In the monarch caterpillar, an insufficient quantity of food may affect aspects of the butterfly’s shape.

The impacts of a reduced diet

With the aim of understanding the effects of lack of food on the butterfly, a group of researchers subjected some monarch caterpillars to food stress. The ones deprived of food for 48 hours, once they became adults, had smaller wings and were smaller in size than the individuals that had enough to eat.1 Even though the monarch can complete its development cycle following a reduced diet, the effects felt in the adult stage are not negligible, especially when migration time comes around.

Wing size and migration

Monarchs that are part of the migrating generation – in Québec, those that are born late in August – have to travel thousands of kilometers to reach their overwintering grounds in Mexico. For that generation, the development of characteristics allowing for optimal flight abilities is crucial. The researchers observed that the monarch late arrivals – that is, the ones that made their way more slowly along their migration corridor – presented smaller wings.1 Thus, monarchs that miss out on resources during the larval stage seem to progress more slowly towards their wintering area.

How many milkweeds are needed to feed a monarch caterpillar?

According to the scientific literature, a caterpillar will visit on average three to four milkweed stems during its development.2 On those stems it prefers to consume the young leaves, because these have more interesting nutritional qualities.2,3 It’s therefore important for a caterpillar to have access to many milkweed plants if it’s to enjoy a sufficient amount of quality food.

To prepare for the arrival of these insects and give them every chance to fully develop, it’s important to incorporate the monarch’s host plant and other nectar-producing plants in your garden. Don’t forget to report all your monarch and milkweed observations on the Mission Monarch website. That way you’ll be contributing to the conservation of this emblematic species!

References

  1. Johnson, H. et al. (2014). Effects of Larval Food Stress on Wing Morphology and Color in Monarch Butterflies. PlosOne, 9(4), 1-9.
  2. Fisher, K.E. et al. (2020). Estimates of common milkweed (Asclepias syriaca) utilization by monarch larvae (Danaus plexippus) and the significance of larval movement. Journal of Insect Conservation, 24 (2), 237-307.
  3. Lavoie, B. and Oberhauser, K.S. Compensatory Feeding in Danaus plexippus (Lepidoptera: Nymphalidae) in response to variation in host plant quality, Environmental Entomology, 33 (4), 1062-1069.

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1 Comment(s)
Doug Taylor's picture
Doug Taylor

Hi, two things
In mid June this summer, we photoed some Monarch caterpillars feeding on Asclepias ovalifolia in a public garden in Red Deer, Alberta. The 4th and 5th stage caterpillars preferred the seed pods to the leaves.
There is a noticeable difference in air density and oxygen partial pressure between the butterfly reserves in Mexico (8,600 feet asl) and the central plains and lower elevations where the butterflies migrate to. Perhaps the butterflies don't fly past Mexico because they can't fly over it.
Doug

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