Spiders
INTRODUCTION. There are over 38,000 species of spiders
worldwide, with about 3,500 occurring in North America.
Many species of spiders are household pests. Wherever
their food is available; spiders are likely to be
found. All spiders are predators, feeding mainly on
insects and other small arthropods.
In the United States, three spider groups are considered
medically important: the black widow, brown recluse, and
Hobo spiders.
RECOGNITION. Non-insect arthropods with 2 body regions,
cephalothorax (head and thorax) and abdomen connected by
a tiny waist (pedicel). Antennae absent. Usually 8 simple
eyes, occasionally 6. Below eyes are 2 chelicerae or jaws
which end in a hollow fang, connected internally to poison
glands. With 1 pedipalp (palp) between chelicerae and 1st
pair of legs on each side. Legs slender, 4 pairs. Abdomen
unsegmented, with a group of fingerlike spinnerets
(produce silk) on posterior or rear end. Males usually
smaller than females and with terminal pedipalp segment
greatly swollen, may also differ in coloration. The
immature stages are egg and spiderlings, the latter
closely resemble the adults except for size and sometimes
coloration.
IDENTIFICATION. Since black widow, brown recluse, and
Hobo spiders are of primary importance in human
environments, their identification is essential.
Several different species may be involved and which
can only be identified by experts. However, group
recognition alone is sufficient for the purposes here.
Black Widow. Adult female body length about ½" including
an almost spherical abdomen about ¼-3/8" in diameter, with
overall length including legs of about 1 ½ - 1 3/3"; males
about half female size. Color typically black, abdomen on
underside with 2 reddish triangular markings usually
joined to form a reddish hourglass-shaped marking but
sometimes separated (“split hourglass”) or only a single
mark; usually with red markings above spinnerets.
Brown Recluse. Adults usually about ¼ - ½" in body
length. Color tan to dark brown with a darker
fiddle-shaped marking on top of cephalothroax whose
neck points towards abdomen. Uniquely, with 6 eyes
arranged in 3 groups of 2 (diads) in a semicircle.
Found primarily in southern states.
Hobo Spider. Adult female body length about 7/16 - 5/8"
including oblong abdomen, male body length about slightly
smaller. Total size including legs about 1-1 ¾" in
diameter. Color variable with carapace
(top of cephalothorax) light to medium brown, with dark
stripe to either side of lighter midline stripe, dark
stripe with lateral extensions in posterior half; abdomen
with interrupted light midline stripe with about 5 light
colored triangular shaped loops on each side bordered by
a dark background. Last 2 or 3 loops chevron-shaped.
Found only in the northwest United States.
BIOLOGY. After mating, females produce egg sacs.
Depending on the species, one or more sacs may be made.
Early instars (spiderlings) spin a silk thread into the
air and float out on the breeze like kites. This is
called ballooning and provides for general dispersal of
the species. Spiderlings go through 4 - 12 molts before
maturity. Most spiders live for 1 to 2 years. Spiders are
predators, paralyzing or killing their pry with venom.
They typically feed by injecting a predigestive fluid
into the body of their pry and then suck in the digested
liquid food. Spiders can survive without food for several
weeks to a few months. Most spiders are nocturnal and hunt
or capture prey primarily at night. During the day, they
hide and remain inactive in cracks and crevices. Spiders
rarely bite people and only do so as a means of defense.
CONTROL. Spider control is a multi-step process.
1. Inspection. A thorough inspection of the building is
essential and many have to be made at night because most
spiders are nocturnal.
2. Identification. Accurate identification is important
for both pest management and medical reasons.
3. Prevention. This consists of making sure that the
building is in good physical condition, and properly
screened and sealed to reduce entry. Also, changing the
lighting to off-building locations, from mercury vapor
to sodium vapor lamps, or in the case of homes,
changing the bulbs near the entrances to yellow bulbs,
may be of help in reducing attractiveness to insects
and hence spiders.
4. Sanitation. Such practices consist of keeping the
premises free of debris such as boxes, papers, clothing,
lumber piles, etc; it is wise to wear protective gloves
and clothing when cleaning out such accumulations of
clutter. A thorough housekeeping should be done twice
each year. Outside, remove debris, firewood and lumber
piles, landscape timbers, stones, etc. Keep the grass
mowed to 3” or less and trim back any vegetation in
contact with the structure.
5. Mechanical measures. The key to control is the
timely mechanical removal of spiders and webs, but
especially the egg sacs with a vacuum, both inside and
outside; seal and dispose of the bad immediately. If a
broom is used for the removal, then it is suggested
that if a spider is present on the web that an
appropriately labeled insecticide be used for a quick
contact kill before removal of the web.
6. Pesticide application. If desired, chemical control
consists of the application of appropriately labeled
dusts, wettable powders, microencapsulated, or lacquer
pesticides in typical spider harborages. Perimeter
treatments are helpful. If web-building spiders are the
problem, lightly dusting the web with a non-repellent
dust is very effective. Control of spider insect food is
desirable but will only help in long term as spiders can
go for weeks or months without food.
