Page 138 - The Book of Caterpillars: A Life-Size Guide to Six Hundred Species From Around the World

BUTTERFLY CATERPILLARS

FAMILY Pieridae
DISTRIBUTION Western North America, from Alaska and British Columbia to
California and New Mexico
HABITAT Open areas in mountains and high plains
HOST PLANTS Mustard family, such as rockcress (Arabis spp.), tansymustard
(Descurainia spp.), whitlow grass (Draba spp.), and peppergrass
(Lepidium spp.)
NOTE Species whose life cycle may be accelerated at high altitudes
CONSERVATION STATUS Not evaluated, but usually common within its range

ADULT WINGSPAN
1½–2¹∕₁₆ in (38–53 mm)
CATERPILLAR LENGTH
1⅜ in (35 mm)
PONTIA OCCIDENTALIS
WESTERN WHITE
136
(REAKIRT, 1866)

Western White caterpillars hatch from single yellowish eggs
that turn orange; to ensure the offspring have enough to eat, the

adult female lays only on a plant where no other butterfly eggs

are present. The young caterpillars feed on the host plant leaves,
also ingesting the mustard oils they contain, which make the
larvae distasteful to birds. The caterpillars may have only four
instars and can reach pupation within a week as an adaptation to
the unpredictable and ephemeral climate of higher elevations.
The pupae hibernate attached to a twig by the abdomen tip
and a silken belt.

There is just one generation at high altitudes but several at
lower altitudes. Western White caterpillars eat dozens of plants
in the mustard family (Cruciferae) but prefer to eat flowers
and young fruits, so, unlike Cabbage White (Pieris rapae)
caterpillars, they are not pests on cabbage. Caterpillars of several
closely related species—especially P. protodice—are similar to
the Western White.

The Western White caterpillar is bluish gray
with vivid yellow and white bands, and is
covered with black spots and hairs, while the
head is mostly bluish gray with an extension
of a yellowish stripe. The setae are short and Actual size
arise from raised bumps giving the caterpillars
a spotted appearance.