Mother Monarch Butterflies Medicate Their Offspring

An infected monarch butterfly struggles to emerge from it's chrysalis

While humans may rely on mom’s chicken soup to fight off infection, mother monarch butterflies lay eggs on medicinal plants to protect their offspring from disease. According to a new study by Emory professor Jaap De Roode and his research team, female monarch butterflies that are infected with parasites lay eggs on anti-parasitic milkweed species to protect their newly hatched caterpillars  from infection.

Parasite spores on monarch wing scales, photographed under a microscope

Monarch butterflies often carry a protozoan parasite similar to malaria, which can have detrimental effects on butterfly development. Infected butterflies have difficulty emerging from their chrysalis, fly poorly, and die young. The parasite is transmitted between adults when mating, or from mother to offspring through the surface of  their eggs (caterpillars often eat their egg shells after hatching for extra nutrition). However, monarch caterpillars can fight infection by ingesting certain species of milkweeds that contain cardenolides, toxic steroids produced by plants.  The toxins prevent parasite spores from establishing in the caterpillar’s gut which can prevent infection, or at least reduce the number of parasites the caterpillar caries.  Cardenolides can cause side effects in healthy caterpillars, like a slightly shorter life span, but these risks are minor compared to the damage caused by infection.

DeRoode and his research team tested how monarchs use the plants to prevent infection both as caterpillars and as adults. They found that caterpillars were unable to detect the presence of parasites and could not distinguiqh between medicinal and ordinary species of milkweed. However, mother monarchs could tell the difference. Infected mothers laid the majority of their eggs on medicinal plants, while unifected mothers showed no preference.

Caterpillars of other species can self medicate, so why haven’t monarch caterpillars caught up with the trend? DeRoode thinks this is partly because the medicinal plants only prevent infection, but can’t cure the parasites once the caterpillar has caught them.  By the time a caterpillar is a few days old, it’s fate is already sealed.  It makes more sense for female butterflies to start caterpillars on a protective diet from the moment they hatch, even if there are some costs to early medication.  Also monarchs aren’t as mobile as some other caterpillar species and don’t stray far from the plant that they hatched on. Even if infected caterpillars could benefit from medicinal plants , it’s unlikely that they would venture off in search of treatment.  Species of caterpillars that successfully self-medicate are those that frequently switch plants and are infected by curable parasites.

If  you’d like to read the original paper, see the link below.  If you’re interested in learning more about monarchs and their diseases, or you want to get involved in monarch research, the Monarch Health project  is looking for volunteers to help track monarch pathogens.

Behavioural resistance against a protozoan parasite in the monarch butterfly
Lefèvre T, Chiang A, Kelavkar M, Li H, Li J, de Castillejo CL, Oliver L, Potini Y, Hunter MD, & de Roode JC (2012). Behavioural resistance against a protozoan parasite in the monarch butterfly. The Journal of animal ecology, 81 (1), 70-9 PMID: 21939438

Zombie Caterpillars Lurch Through Forest Canopy, Infecting Their Brethren

gypsy moth caterpillar "face"

The zombie caterpillar apocalypse has begun!   A recent study published in Science by Dr. Kelli Hoover and her research group at Penn State showed that a virus infecting gypsy moth caterpillars  causes them to become like the living dead.  Infected caterpillars crawl to the treetops where they die and their putrefying corpses shower virus particles over the terrified survivors.

An infected caterpillar hangs decomposing from a branch

Gypsy moth caterpillars are normally nocturnal and only forage in the the tree tops at night when they can avoid predators.   During the day they return to their hiding places  in the forest understory.  However, caterpillars infected with the virus climb to the top of the tree during the day and feed continuously.  They also stop normal development toward adulthood but become ravenously hungry,  growing larger and larger and providing the virus with more flesh to feed on.  Eventually, they die and their corpses liquefy and drip infectious material over the remaining caterpillars.

The coolest part?  The behavior is caused by a single gene in the virus.    This baculovirus has a gene that codes for an enzyme called  EGT, which  inactivates caterpillar molting hormone.  Caterpillars have to molt to grow and develop, so when they become  infected,  caterpillars are stuck in perpetual childhood, eating, growing bigger but never developing to adulthood.  The EGT enzyme also causes the climbing behavior, because without the urge to molt, caterpillars are driven to eat without stopping, and so they feed continuously day and night without ever coming down to rest.

Dr. Hoover and her team showed that EGT was causing the climbing behavior with a simple experiment.  They took some natural  strains of the virus with the EGT gene and two strains where they had artificially  inactivated the EGT gene.  Then they took Gypsy moth caterpillars, infected some with each virus  and placed them in soda bottles with holes.    The caterpillars with the natural virus climbed to the top of the soda bottle and died, just like they did in the wild.  Those that received the virus with the disabled gene died on the bottom of their cages.  In the wild, caterpillars that die in the understory or forest floor wouldn’t be able to infect many others, so the EGT gene is very beneficial for the virus becaus it allows it to spread rapidly.  Lots of parasites use this kind of mind control to force their hosts to spread them , but this is an exciting result, because it’s one of the first times science has shown that a single gene in a parasite can alter host behavior.

Of course, no zombie story is complete without some heavy-handed social commentary.  The gypsy moth is an invasive species introduced to North America in the 1860s and it spread quickly through America’s hardwood forests.   It’s since become a huge pest, and it’s boom and bust population growth cycles can cause massive defoliation in forests.  Scientists are actually using simlar viruses to control gyspy moth populations in areas that are too sensitive to use pesticides.  So the virus may actually provide forestry researchers with a new tool to control the gypsy moth menace.   As always, it’s human nature not the zombie hoard we should truly fear.