Fungus Kills Caterpillars to Feed Its Plant Host

An insect larva mummified by the Metarhizium fungus

Plants and fungi look like peaceful woodland scenery, but underneath the soil some plant-fungi pairs act as a deadly team mummifying insects and draining them of their nutrients, according to a new paper published  in Science.  The study by Canadian grad student Scott Behie and his research group, shows that the soil fungus Metarhizium can parasitize soil insects and deliver their nitrogen to it’s symbiotic plant partners.  While soil bacteria often break down dead insects, this is one of the first examples of a fungus specifically targeting living insects for their nutrients.

Plants and fungi have a long history of cooperation; plants need nitrogen, but only certain types of nitrogen-based molecules will do.  Plants often get around this problem by forming symbiotic relationships with fungi that can convert nitrogen into biologically useful forms, sometimes even allowing the fungi to live inside their root cells.   In return, fungi get some of the sugars that the plants produce through photosynthesis. However,  Metarhizium and it’s plant partners take things a step further:  in addition to converting nitrogen into plant friendly forms, Metarhizium is a also deadly insect pathogen.  When an insect is infected with Metarhizium the fungus quickly consumes the insect’s tissues from the inside out, mummifying it within a few days.  It now seems that Metarhizium is sharing it’s insect prey with it’s plant host.

Behie and his research team wanted to find out whether the fungus was specifically targeting insects to share with it’s host plant, or whether the insect infection was unrelated.  To test this, Behie injected waxmoth larvae with a special type of traceable nitrogen, called Nitrogen-15 .  He then infected some of the caterpillars with Metarhizium and left some healthy.  He then put some healthy and infected insects in pots with plants (common beans and switchgrass) that have symbiotic relationships with Metarhizium .  After a month, he tested the plant leaves for the Nitrogen-15 from the caterpillars. He found that in plants that had infected caterpillars had 40 to 50% more Nitrogen-15 than those with healthy caterpillars.  The Nitrogen-15 injected into the caterpillars had found it’s way into the leaves in just a few weeks.

However, it is possible that the Metarhizium–infected caterpillars died more quickly than the healthy caterpillars, and simply released their Nitrogen-15 as they decomposed.  To confirm that the Metarhizium was actually transferring Nitrogen directly from the insects to the host plant, Behie did a second experiment with another insect pathogen (Aspergillus flavus) that did not have a symbiotic relationship to plants.  Plants with the non-symbiotic fungs had only tiny increases in Nitrogen-15 , no different from the increases found in plants without fungus at all.  Only plants that had Metarhizium were able to gain Nitrogen-15 from the insects.

The discovery that Metarhizium can both kill insects and aid plants is exciting to agricultural scientists.  Metarhizium is used as a natural pesticide to protect crops from insect pests.  However, it now seems like it might also be natural fertilizer too, converting once deadly pests into a useful source of nutrition for plants.   Since Metarhizium is an incredibly common fungus, found all over the world, it’s likely that there may be some underground mummifcation going on in your back yard.

SW. Behie, P. M. Zelisko, M. J. Bidochka1 (2012). Endophytic Insect-Parasitic Fungi
Translocate Nitrogen Directly
from Insects to Plants Science DOI: 10.1126/science.1222289

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

The incredible shrinking butterfly?

Climate change is causing plankton to shrink... are butterflies next?

Is climate change causing cold blooded organisms to shrink?  That’s the finding of a new paper by Dr. Andrew Hirst and his research group at the Queen Mary’s School of Biological and Chemical Sciences.  The researchers collected data from 40 years of experiments on copepods, a type of plankton.  They took the growth and feeding data on several species  and fed it into a model that also included predictions about climate change.  Their results show that cold blooded critters will grow up to be smaller if the climate warms up, and that this could have a big impact on ecosystems.

Most invertebrates (butterflies included) have the same response to temperature.   In warm climates, they grow quickly and reach maturity faster (this is called the temperature size rule, and about 80% of cold blooded animals follow it).  We often think of growth and development as the same thing, but development is how fast an individual matures and reaches adult hood, and growth is how fast it packs on weight.   So an individual can mature quickly, but still be small.  Anyone who remembers high school knows that chronological age, size, and maturity rate are three different things).

Hirst’s paper is important because it shows that temperature affects development more strongly than growth.   Individuals are growing quickly in warm environments, but they’re developing even faster, meaning they reach adulthood and stop growing while they’re still tiny.  Being small is a big deal when you’re a copepod or an insect because small females can lay fewer eggs, smaller males can’t compete as well for mates, and small individuals are less able to survive droughts and famines.   It’s also important because predators rely on insects and copepods as a food source; they’re the cornerstone of a lot of ecosystems.

We’re excited about this paper here at the Butterflies and Science blog because Jess also studies whether climate change is causing insect shrinkage.  Back in the 80s a few scientists did experiments on the effect of temperature on butterfly body size and development using the sulphur butterfly in Colorado.   Jess spent her summer re-doing some of those experiments on the same  populations.  The rocky mountains have been warming more quickly than other parts of the country, so there’s good reason to think that butterflies might be evolving to be smaller too.   Right now Jess finishing up her experiments, but pretty soon we should know if climate change is making butterflies smaller too.

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.

There’s a new butterfly in town

From time to time here at Butterflies and Science we’ll pick out a fun scientific paper about butterflies and highlight why we think it’s cool and important. This week has been a good one for butterflies with two cool news stories about them! The first one is about new species being formed as a hybrid of two others!

Move over Eastern tiger and Canadian tiger swallowtail there is a fancy new hybrid butterfly in town, the Appalachian tiger swallowtail!

One of my new favorite hybrids: A male Appalachian Tiger Swallowtail

Scientists at the University of Texas-Austin and Harvard University have discovered this new species of swallowtail living in the Appalachian Mountains. The Appalachian tiger swallowtail has evolved as a hybrid of the Eastern tiger and Canadian tiger swallowtail. The Canadian tiger swallowtail is found in the northern US and is more adapted to colder climates than the Eastern tiger swallowtail which is found well, in the Eastern US. The Eastern tiger swallowtail is unique in the ability to have two forms (colorations). One of these forms is the usual yellow wings with black stripes, but the other is an all black form that mimics the more poisonous Pipevine swallowtail.

The yellow striped form of the Eastern tiger swallowtail

The black form of the Eastern tiger swallowtail

What’s so cool about this hybridization is that the Appalachian tiger swallowtail inherited some of its cold tolerant genes from the Canadian tiger swallowtail and the genes to have the mimic form from the Eastern tiger swallowtail. It is a true hybrid of its parents both inside and out! Now the Appalachian tiger swallowtail is its own species that rarely mates with either the Canadian or Eastern tiger swallowtail.

So how do these sorts of hybrids arise? Glad you asked!

Generally when we think of new species arising it happens when one species splits into two and becomes isolated over time. In the rare case of hybrids two different, yet related species are able to mate with each other to create viable and fertile offspring. This happens a lot in plants, but is pretty rare in animals. The key is that the Eastern tiger and the Canadian swallowtail have only been unique species for about 600,000 years, before that they were the same. That may seem like a long time, but in evolutionary time that’s barely a blip! Because they were so closely related they were still able to mate with each other and have healthy, fertile offspring. Those offspring then diverged from both parents and have now become their own unique species. That is what makes this study so unique and exciting! It’s not often that conditions for this kind of speciation are right.

If you want to read more about this you can check out the scientific paper in PLoS Genetics here or you can read a less technical and shorter version here from ScienceDaily.