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 Metarhiziuminfected 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

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.