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Attracting Monarch Butterflies: Or, Which Milkweed is the Right Choice?

By: Liz Clift

Milkweed is sometimes considered a noxious weed, because its propagates readily – each seedpod contains hundreds of seeds. As such, it has been discouraged from growing on a variety of landscapes. However, monarch butterflies (Danaus plexippus) rely on milkweed (Asclepias spp.) to reproduce – milkweed is the sole source of food for their larvae (adults can sip nectar from a variety of wildflowers). In recent decades, there has been a lot of campaigning to encourage people to plant milkweed in an attempt to aid monarch recovery.

And based on acres occupied in the Mexican forest where monarchs overwinter, their numbers are on the rise. This past winter, they occupied 10 acres of forest, compared to their record low number of 1.66 acres in 2013.

However, monarchs still face problems, and one of those problems has to do with the type of milkweed people have been planting. Until fairly recently, the primary milkweed commercially available in the United States was tropical milkweed (Asclepias curassavica), which in warmer southern climates doesn’t die back during the winter.

Monarch Butterfly on Tropical Milkweed. Photo Credit: Tony Fischer Photography from Flickr Media Commons.

Since tropical milkweed doesn’t die off, some monarchs aren’t completing their migration, preferring instead to stay in the southern areas of some southern states, where tropical milkweed can bloom year-round.

So, some monarchs aren’t flying as far. No big deal, right?

If only that was the case.

Unfortunately, milkweed can also host a protozoan parasite Ophryocystis elektroscirrha (OE) which infects monarchs and queen butterflies (Danaus gilippus). The long and the short of it goes like this:

An adult monarch (or queen butterfly) carrying OE spores lays its eggs on a milkweed plant and in the process scatters those dormant spores on the eggs and the leaves of the milkweed plant. Larva consume their egg casing as they hatch, and may pick up OE that way, or through consuming the infected milkweed plant.

Once the dormant spores are in the monarch larva’s digestive tract, enzymes break the spores open and release the parasite. The parasites move into the intestinal walls and begin to reproduce asexually – and each OE parent cell can reproduce many times.

The majority of the damage to the butterfly happens during its time in the chrysalis. About three days before the adult emerges, OE spores begin to form and show up as dark patches that can be seen from the outside layer of the chrysalis. The infected adult butterfly may be too weak to emerge from the chrysalis, or to cling to the chrysalis while their wings fully expand. Those that survive are often smaller than healthy monarchs, and have shorter forewings, and they carry the spores on their abdomens. The cycle repeats.

OE also damages the outer layer of a monarch’s abdomen, which causes the butterflies to dry out and lose weight faster than normal. This is especially problematic for the butterflies when there are shortages of water or nectar.

The cyclical nature of this parasitic infection is exacerbated by the fact that when a milkweed plant doesn’t die off (or get regularly trimmed down) in the winter, OE can continue to survive on it. If a milkweed plant is killed off over the winter, the returning monarchs (at least those that are uninfected) have an improved chance of being able to produce eggs that will grow into healthy adults.

In recent years, more milkweed plants have become commercially available, and many organizations dedicated to saving the monarch provide resources for people to help them select milkweeds that are regionally appropriate and also appropriate for a particular set of growing conditions. This effort will, hopefully, curb OE infections in monarchs.

Swamp Milkweed (Asclepias incarnata). Photo from Tom Potterfield, Flickr Creative Commons.

Aside from planting regionally appropriate species of milkweed, how else can people help save the monarch?

There are also opportunities for citizen scientists to help collect data on infection rates in monarchs. Testing for OE spores is conducted by gently pressing a piece of clear tape to the abdomen of a monarch. The tape sample is then sent to a lab, where it is evaluated for spores (and total spore count) under light or electron microscopes.

This research helps scientists better understand not only what percentage of the monarch population is infected with OE, but how OE spreads through a population, or a region. This may lead to advances in how OE is treated—or how the message about the importance of planting native milkweed is spread.


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