The Monarch Monitoring Project is a long-term study on monarch migration through Cape May, NJ. It is a part of the New Jersey Audubon Research Department, and closely affiliated with the Cape May Bird Observatory.

Wednesday, September 16, 2015

Question of the Week: Why Are Monarch Caterpillars Eating my Parsley?

A few people have approached us this week with questions about the little critters that they observe in their gardens, most notably the fat, green caterpillars that can be found munching on plants in the carrot family, such as parsley. Many people have mistaken these crawlers for their cousin, the monarch, while in actuality they have been observing black swallowtail larvae.

A black swallowtail caterpillar on parsley

Some distinguishing features of monarch caterpillars are the black, yellow and white stripes, and two sets of black tentacles on the thorax and abdomen. Black swallowtail caterpillars are green and black with yellow spots. They also have an orange, forked gland called an osmeterium that will emerge when the caterpillar is feeling threatened.

A monarch caterpillar on milkweed

Monarch caterpillars feed exclusively on milkweed, which is why milkweed availability is so important to monarch population health. We frequently recommend that people plant milkweed in their gardens, in addition to other nectar-rich “butterfly-friendly” flowers, which provide crucial food sources for adult butterflies. These flowers include aster, seaside goldenrod, and zinnia. 

An adult monarch (left) and black swallowtail (right)

In many pollinator-friendly gardens, gardeners will plant two sets of parsley (one for the caterpillars, and one for the cupboard), in addition to milkweed to encourage a greater diversity of winged visitors. Both of these caterpillars metamorphose into glorious butterflies who are essential pollinators which will make your flowers bloom brightly. Consider both species welcome guests in your garden!

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