of Rhode Island GreenShare Factsheets
Pieris rapae (L.)
The imported cabbageworm adult is one of the most common
butterflies in the Northeast. The velvety-green caterpillar of
this butterfly feeds on the leaves of cabbage, cauliflower, broccoli,
Brussels sprouts, turnips, radishes, kale, lettuce and weeds
of the mustard family. The imported cabbageworm is one of the
most damaging and destructive enemies of these plants. The caterpillars
chew irregular holes in the leaves and usually eat their way
into cabbage heads from the bottom. The fecal pellets of the
caterpillars will also stain cauliflower. These caterpillars
can be controlled with microbial insecticides based on strains
of Bacillus thuringiensis effective against caterpillars,
or row covers can be used to keep the adults from laying eggs
on plants in the garden.
The first sign of the imported cabbageworm is the 40 mm
(1.5 inch) long white butterfly, first seen in early spring.
The butterfly has black-tipped front wings with one or two black
dots and one black dot on each hind wing. The females fly during
daylight hours looking for suitable plants on which to lay their
yellowish, elongated eggs, singly, on the underside of leaves.
The velvety-green caterpillar has a faint yellow stripe down
its back and a row of faint yellow spots on each side. When fully
grown, the caterpillar is slightly more than 25 mm (1 inch) long.
The eggs laid by the butterfly on the underside of leaves
hatch in five to seven days. There can be as many as three generations
of the imported cabbageworm in the Northeast. Adults may be seen
almost anytime during the summer.
The pupa, or chrysalis, of the imported cabbageworm is green
or tan, 20 mm (3/4 inch) long, and attached to the plant or other
surface by silken threads. The imported cabbageworm overwinters
as a pupa, attached to any convenient stem, branch, fencepost,
or other hard surface in the area.
The eggs, larvae, and pupae are found on plants in the cabbage
family, including broccoli, cauliflower, collards, kale, Chinese
cabbage, and Brussels sprouts, as well as cabbage. The larvae
also feed occasionally on turnips, radish, mustard, and even
lettuce. When large larvae are feeding, you will find wet masses
of dark green excrement dropped on the surfaces of leaves below.
It is common for high larval populations in July and August to
damage plants so severely that they die or become unmarketable.
The larvae eat large holes in the leaves. As they feed, they
frequently move toward the center of the plant in order to feed
directly on cabbage heads and on leaves shading cauliflowers
from the sun. They are also often found inside broccoli heads
when they are cooked.
Natural: Controlling weeds around the garden, especially
plants of the mustard family, should help decrease the numbers
of this pest. Destroying and removing the remains of plants in
the fall, as well as fall tillage, reduces the number of over-wintering
pupae. Hand-picking the larvae may be labor intensive but can
significantly reduce the numbers and keep damage down. Early
Globe, Red Acre and Round Dutch cabbage have shown some resistance
to cabbageworm. Several plant breeding programs are working on
developing glossy varieties of cauliflower, broccoli and other
crops that will be resistant. Barriers such as row covers or
a nylon stocking stretched over a cabbage head can keep the butterflies
from laying eggs on the plants. The imported cabbageworm has
many insect natural enemies, including several parasitic wasps
and predators of eggs and young larvae. You may find dead caterpillars
with a large mass of white or yellow cocoons nearby. The cocoons
are from a parasitic wasp introduced to this country specifically
as an enemy of imported cabbageworm.
Insecticides: B.t. (Bacillus thuringiensis),
an insecticide derived from a bacterium and sold as Dipel, Bactur,
Sok-Bt or Thuricide, is the preferred control for the imported
cabbageworm. B.t. is less effective under cooler conditions.
Regardless of what is used, the smaller the caterpillars are,
the easier they are to kill. Insecticides must be used in late
afternoon or early evening to protect bees. This is also the
time when the caterpillars feed most actively, making the insecticide
most effective. Follow instructions on the label.
Adapted from the University
of Maine Cooperative Extension and the Connecticut Agricultural
Experiment Station, 1999
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
of Rhode Island Cooperative Extension provides equal program