Northern Prairie Wildlife Research Center
Caterpillars of Pacific Northwest
Forests and Woodlands
Numerous techniques may be used for collecting caterpillars. One of the basic
approaches is to visually search plants, in particular where feeding damage
and perhaps feces (frass) can be seen. Another method of search involves clipping
foliage into a bag and then inspecting the foliage indoors where a microscope
may prove helpful. Tools and techniques used for the collection of caterpillars
include a sweep net, beating sheet, burlap skirts, funnel traps, and sifting
soil. The sweep net is similar to an aerial net but used to brush over vegetation
in a manner that dislodges caterpillars from their host plant. The beating sheet
is held under a plant while the foliage is shaken by beating branches with a
stout pole. Caterpillars fall off the foliage and onto the sheet. Burlap skirts
can be tied around tree trunks to trap caterpillars while they are enroute between
feeding sites and resting or hiding places. Funnel traps can be set under plants
to collect caterpillars as they drop from the foliage. Sifting soil will result
in the capture of root feeders and caterpillars preparing to pupate in the soil.
An excellent means of acquiring caterpillars is to capture live adult females
and rear the caterpillars from her eggs. In some species the female must be
coaxed into depositing eggs by providing proper conditions of light, temperature,
humidity, flying space, and a substrate for oviposition. The caterpillar of
Egira februalis was obtained from eggs deposited
from a live trapped female.
Careful collecting can conserve habitat where caterpillars and other organisms
live. Avoid trampling plants and disturbing unstable soils. Try to grow plants
for the food your caterpillars require or at least prune wild plants with
care and an understanding that you may need more foliage later. Collect only
as many caterpillars as you can raise, and only raise as many as can be supported
without harm to local host plants. Once your observations are completed release
nonvoucher specimens back into the environment from which they were acquired.
Do not introduce exotic species and be aware of legal obligations regarding
collecting on private and public lands. In particular, heed the importance
of protecting rare and endangered species.
The rearing of caterpillars is helpful in associating field-collected larvae
with the adult, testing food plants for suitability, or associating parasitoids
and diseases with the caterpillar stage of respective species. Caterpillars
can be observed in the field, or reared in cages in the field or indoors.
Rearing indoors has numerous advantages and disadvantages. One advantage
is that the specimen is less likely to be lost. Another advantage is that
the caterpillar will likely grow faster indoors because of warmer temperatures.
Faster growth allows the observer to see size and color patterns for each
instar ahead of the presence of the same instar in the field; however, indoor
rearing requires that food must be provided from potted plants, clipped foliage
from the field, or artificial diets. Unsuitable rearing conditions will result
in high mortality. Temperature control, dehydration, fungal growth, starvation,
cannibalism, and overcrowding are common problems. Additionally, the use of
closed containers may cause problems due to excessive condensation and poor
sanitation. The severity of the impact of cannibalism and disease may be reduced
by raising the caterpillars individually. The presence of slightly moistened
peat moss is helpful at the time of pupation. The peat moss provides a medium
within which the caterpillar can bury itself, and the moisture helps to prevent
dessication which can be a major obstacle when rearing in dry indoor conditions.
A color slide or print will provide a record of the caterpillar at various times
of development. An excellent photograph can be acquired by using all of the
following: (1) A 35 mm, single lens reflex camera with exchangeable lenses.
Instamatic type cameras will not allow the photographer to get close to the
subject or to fill the frame with the subject. The photographs presented here
were taken with a 50 mm macro lens mounted on a 25 mm extension tube. (2) Film
with a low ASA rating. The photographs here were taken with color slide film
ASA 25. This film speed provides superior quality in grain but requires more
light than faster films. (3) A flash system, either a bracket or a ring flash.
I use a bracket system which consists of two flash units that are mounted on
opposite sides of the camera. The lens, film, and flash units allow shooting
pictures at f/16 and f/22 at a distance of about 20 mm from camera lens to caterpillar.
Photographs can be taken in the field but shadows, wind, cluttered backgrounds,
and other unwanted features (like other insects) may interfere with obtaining
the best picture. Most of the caterpillars illustrated in this booklet were
field-collected but photographed in a staged indoor setting.
Caterpillars should be preserved as voucher specimens for eventual study of
traits that photographs do not reveal. Improperly preserved caterpillars will
rot and turn black. The simplest method for preservation is to heat water to
about 180°C. If you don't have a thermometer you can obtain an appropriate
temperature by bringing the water to a boil and then letting it sit off the
burner for a couple of minutes before putting the caterpillar into the water.
Extremely hot water may cause the caterpillar to burst. After it has been in
the hot water for 3 or 4 minutes, transfer the caterpillar to 70% ethyl alcohol
(isopropyl alcohol is less desirable) for permanent storage. Although this technique
will provide a properly inflated specimen, an unfortunate side effect is that
the caterpillar will lose most or all of its color. See Peterson (1962) and
Stehr (1987) for additional ideas on preserving caterpillars.
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