For more information and for pictures of the bags and the caterpillars go to http://landscapeipm.tamu.edu/files/2012/09/BagwormsPub.pdf.
What is the stripped caterpillar with yellow dots that is eating my fennel and parsley?
Black Swallowtail caterpillars hatch from eggs that are laid on fennel, parsley, dill and rue. These caterpillars may grow to 3-4 inches before they begin metamorphosis. They are easily removed by hand if you do not want them eating your herbs.