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Indirect interactions in agricultural milkweed habitat: the effects of root damage on monarch (Danaus plexippus) success
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Monarch butterfly populations are declining independently on both sides of the Rockies, but little research to date has investigated the causes of this decline in the West. Monarch caterpillars sequester some defensive compounds from their larval host plants (milkweeds), which protect the butterflies against their predators. Milkweeds are found in agricultural ditches throughout the arid Western areas, where the plants can be attacked by both leaf and root herbivores. Little research to date has focused on how root damage affects host plant defensive compounds and consequently the monarchs. I conducted a greenhouse experiment in which I subjected half the plants to a root-damage treatment to determine how damage would directly affect plant production of defensive compounds and indirectly affect monarch caterpillar development relative to controls. Results showed that damaging the roots increased defensive compounds in the milkweed aboveground plant parts (shoots and leaves). Monarch caterpillars that ate root-damaged plants were smaller and remained in chrysalides for less time compared to caterpillars that ate control, undamaged plants. Morphometric measurements of the butterfly wings further showed that butterflies feeding on root-damaged plants had smaller wing thoraxes, radii, areas, and perimeters than monarchs feeding on control plants. These results show that damage to the milkweed roots, which is probably very common in the nematode rich soil adjacent to agricultural systems, is detrimental to monarch success across multiple life stages. To the extent that monarchs rely on milkweeds from agricultural systems, I conclude that the reduced performance of monarch larvae and adults caused by root-damage could have long-term negative effects on the population dynamics of the monarch butterfly.