Western tiger swallowtail (Papilio rutulus)
Pest description and damage Tiger swallowtail larvae feed on leaves of cherry family plants, particularly native cherry, (Prunus emarginata), willows, cottonwood and aspen, ash and a few other species. Generally limited to isolated numbers on single trees. Larvae spin a silken pad on which they rest. Initially larvae look like small bird droppings molting into bigger black and white bird droppings. The last molt is a thick green larva with black and yellow bands and conspicuous yellow eyespots on the thorax. The head is tucked under the body of the caterpillar. When disturbed the larvae may evert orange "horns" with a peculiar smell to deter predators. The chrysalis is a 1 inch long tan and marbled capsule with visible outlines of legs, antennae, eyes and proboscis. The joy of the large yellow and black butterfly in the garden outweighs the minimal damage inflicted by the caterpillar.
Biology and life history Adult butterflies emerge in June, mate and lay a single egg on a leaf of host plants, then move on to other leaves. Eggs hatch and the caterpillar spends summer feeding on host leaves. In fall the larvae crawl away to pupate. They spend winter in the chrysalis. There is a single generation in a year.
Pest monitoring Watch for damage in spring. Look for a shiny patch of silk on leaves, leaves that have been chewed from the tip in a squared pattern along the vein or from tip to base.
Relocate larvae onto less visible hosts or ignore these insects. Significant damage is rare.
These insects are heavily parasitized and rarely become major pests.
See "Caterpillar" in:
For more information
Butterflies and Moths of North America. http://www.butterfliesandmoths.org/species/Papilio-rutulus