In North America, the monarch butterfly, Danaus plexippus (Linnaeus) (Fig. 1), is special as it is a classic example of a great migration. They travel on air currents and cross > 3000 miles in winter to reach the overwintering sites. Broadly, there are two populations of monarch butterflies based on their migration patterns, and they are eastern and western populations. The eastern population breeds in the east and overwinters in central Mexico (Fig. 2). The western population breeds primarily in the west of the Rocky Mountains and overwinters in coastal California. Recent studies suggest that these two populations are interbreeding, and the Rocky Mountains are not necessarily a barrier separating these two populations. The overwintering sites of monarchs were not revealed to the world until the 1970s (Fig. 3). They overwinter on fir trees in oyamel fir forests of highlands of central Mexico (Figs. 4-6). The western population migrates to western states, returning to overwinter on forest grooves of Coastal California after several generations.
Why is the monarch butterfly important?
Unlike bees which are excellent pollinators of many plants, the monarch butterfly is not an excellent pollinator. It is not good prey for other insects or birds, either. The monarch butterfly emerged as a symbol of nature conservation or environmental health and is now a conservation icon. They are regarded as the favorite native insect of North America. It is the state butterfly in seven states in the USA.
Life cycle and migration
The adult butterfly lays eggs on milkweed (Asclepias spp.), and eggs hatch in 3-5 days at 72-82 °F (22-28 °C). The larvae consume milkweed foliage and develop through five larval stages in ~11-18 days in summer. The fifth instar forms a chrysalis (pupa) on the plant or any structure they can find, such as a building or fence. The adult monarch butterfly emerges from the pupa in 8-14 days in the summer (Fig. 7). The eastern population of monarch butterflies overwinters from October to March in Central Mexico. The overwintering adults fly north by the end of April. They become reproductively mature, breed, lay eggs in the southern USA, and go through the first generation in May (Fig. 8). The second and third generations move further North. The larvae of the fourth generation develop in the North during August, but their adults migrate to Mexico for overwintering. This means adults of one generation fly South to overwintering sites, whereas adults of up to three generations fly North.
The eastern population migration flights are along “monarch highways,” referred to as “flyways” (red lines in Fig. 2). Depending on where they originate in northern America, there could be many flyways, and these flyways unite to one single super flyway in central Texas. The generation of monarchs returning to overwintering sites in Central Mexico is flying there for the first time. They fly during the daytime and roost on pine, fir, and cedar trees at night.
The larval stages of monarchs are host-plant specialists. The larvae feed and develop on milkweed and other closely related species to Asclepias spp. This means they must feed on milkweed to develop and will not survive on other host plants. Milkweed has a network of pressurized latex canals, and the latex oozes out when cut. It contains cardenolides, which are toxic to insects. The latex is also a glue that sets quickly once exposed to air. When insects feed on milkweed, the latex often intoxicates the insect, or they get stuck to the glue. Younger monarch caterpillars cut out a trench nipping the veins before feeding on the leaf tissue within the island. The late instar larvae cut the veins at either mid-vein or the petiole and let the latex ooze out before feeding. They sequester the toxic chemicals in their body and are brightly orange-colored to warn predators that they carry poison.
Adult monarch butterflies forage on any flowers they can find on their path toward North or South. They need nectar as a sugar source and other nutrients to sustain flight. When they start flight North in the spring, they must feed on nectar to reach Texas. Similarly, in the mid to late summer, on their journey toward the South, they need to consume nectar from flowers to reach Mexico. What they feed on their way southward helps them sustain their return flight and overwinter in Mexico. Adult monarchs do not feed at all during the winter. Along the migration journey (both northward and southward), adults feed from patches of flowering plants, which appear as islands or stopping-off points to refuel their energy needs. Thus, planting various flower gardens with good nectar resources is critical, especially in urban areas.
Monarch butterfly populations are showing a declining trend. As the overwintering habitat in Mexico is destroyed by logging or deforestation, they have fewer places to overwinter. The migratory monarch is now listed in the red list of “endangered species” by the International Union for the Conservation of Nature, an authority on global biodiversity. Other factors are rising, such as diseases, pesticide use, and loss of larval hosts (milkweed). The larvae need milkweed to develop, and the adults need nectar for migration and day-to-day survival. Thus, more nectar sources and milkweed plants are needed to sustain the migration pattern. More flowering plants and milkweeds can help conserve the migrating monarchs, especially by planting plants along farm-reserved lands and urban gardens.
Popular programs can help build these habitats, such as the monarch waystation (https://www.monarchwatch.org/waystations/), growing gardens million pollinator garden challenge (http://millionpollinatorgardens.org/), etc.
A few characteristics for building a monarch garden
- Structured gardens with mulch harbor more monarch caterpillars than non-structured open gardens with much vegetation at various stories.
- Gardens with unimpeded North-South access recruit more monarchs than impeded access gardens. If planted close to the structure (home or barn, or shed), ensure the garden has North-South access; thus, choose the side of the structure facing east or west.
- More monarch caterpillars are found on taller milkweed species (common milkweed), such as on swamp milkweed (A. incarnata) and common milkweed (A. syriaca), than on butterfly milkweed(A. tuberosa) (Fig. 9).
- Plant milkweeds on the garden’s perimeter (along the mulch) for more eggs and larvae. If the milkweeds are hidden among other plant species, it is difficult for the adults to find the milkweed. Accessibility to plants is critical to recruiting more eggs and larvae.
- Tall milkweed species, such as swamp milkweed, common milkweed, and showy milkweed (A. speciosa)recruit 2-3 times more eggs and larvae than shorter Asclepias spp. These tall milkweeds attract larger bees, such as bumble bees and honey bees. For smaller native bees, butterfly milkweed (A. tuberosa) and whorl milkweed (A. verticillate) are the best (Fig. 9).
- Swamp, butterfly, and green milkweeds are less likely to spread from where they are originally planted. In contrast, common, showy, and narrow-leaf milkweeds spread around with aggressive rhizomes (Fig. 9). They are good for large areas, such as golf course roughs, conservation areas, etc.
- Many nativars (cultivars of native milkweed) are available in the market, and they are as good as wild or straight milkweed species in recruiting monarch larvae.
- Avoid developing your garden into an ecological trap where the recruited monarchs die due to other factors, such as a garden planted near high road traffic. Avoid opportunities for invaders, such as invasive European paper wasp (Fig. 10) and tropical milkweed (A. curassavica) (Fig. 9D) as discussed below.
- Butterfly boxes are ideal conditions for European paper wasp (Fig. 10) to thrive. Remove them!
- Rearing monarchs in captive conditions for conservation is not recommended, and it reduces or dilutes genetic diversity, fitness, and migratory success.
European paper wasps [Polistes dominula (Christ)]
Native to Mediterranean countries, European paper wasps(Fig. 10) are invasive wasp species in North America. These wasps are widespread throughout the continental USA — from California in the West to Maine and Kentucky in the East. It is an urban pest that is adapted to thrive in all sorts of human dwelling structures, such as a home, garage, under the patio, shed, etc. These wasps feed on caterpillars, including monarch larvae (young and late stages), which they chop up to feed the young wasp larvae. Young caterpillars are particularly vulnerable to attack and get killed; late-stage caterpillars drop to the ground when attacked.
Tropical milkweed (Fig. 9D) is a perennial and will not die during winter. The monarch butterflies perceive the lack of senescence as a signal to continue laying eggs and not leave the host. This phenomenon affects the migration of monarch butterflies, and they stay on the host for an extended period. Dieback of the host in winter signals the monarchs to return to overwintering sites in Mexico. In addition, they are susceptible to diseases, such as one caused by the pathogen Ophryocystis elektroscirrha as they spend more time on the tropical milkweed. The butterflies emerging from infected pupae are often deformed and cannot properly fly. At high temperatures, tropical milkweed can have toxic levels of cardenolides and kill the developing monarch larvae.
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