Root fungi make or break monarchs’ chances against parasite

monarch caterpillar

Monarch caterpillars take in chemicals in their milkweed diet that help them fight of parasitic infections. Fungi around the plant's roots shape the levels of those chemicals that specific milkweed species produce. 

Austin Thomason/Michigan Photography

Milkweed plant fungi call the shots in monarch butterflies’ fight against the spores of a protozoan parasite, researchers report October 13 in Proceedings of the Royal Society B.

The parasite Ophryocystis elektroscirrha infects monarchs (Danaus plexippus) as caterpillars, but chemicals from their milkweed diet help fight off the disease. Milkweed plants produce varying levels of these chemicals in their leaves, and researchers wanted to figure out what makes one plant more potent than another. In greenhouse studies, researchers fed monarch caterpillars different milkweed species grown with different levels of fungi that live within the plants’ roots. Separately, nutritious phosphorus and toxic steroids produced in leaves gave infected butterflies a better shot at survival. 

But, fungi’s influence on milkweed plants was highly individual. In some species, lots of root fungi translated to more toxins, but in a different species, lots of root fungi had the opposite effect. Fungal tweaks to milkweed chemistry can help or hinder the health of an infected monarch. 

Helen Thompson is the multimedia editor. She has undergraduate degrees in biology and English from Trinity University and a master’s degree in science writing from Johns Hopkins University.

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