Spider mites are microscopic or nearly
microscopic animals with two body regions and four pairs of legs.
They do not have antennae. Some common species are the two-spotted
or red spider mite. These mites have two spots on the back and
have tail-end spots in some species. They may be clear, orange, or
reddish. The clover mite is reddish brown or grayish, and are flat
with long front legs.
Some mites are beneficial because they feed on plant
mites. Predatory mites occur on infested plants, but are larger
than their prey and move more quickly. Some may be purchased for
Spiders resemble spider mites, except they are
larger and have two clearly distinct body regions with a thin
waist between. Most spiders are predators of insects, and thus are
One common poisonous species is the black widow spider.
Black widows are shy and like dark places. They spin a
characteristically messy web. They are shiny black, moderately
sized spiders with a reddish or orange hourglass marking on the
underside of the abdomen. Males and immature females can have
stripes of red, yellow, and black on the abdomen.
Ticks are larger than mites and are important
because they are parasites of man and animals. The most prevalent
tick in Arizona is the brown dog tick. It is specific to dogs and
can develop infestations in kennels, backyards, and inside homes.
This tick is not known to transmit human diseases.
Scorpions have two body regions, the
cephalothorax and the abdomen. The abdomen is elongated into a "tail"
which ends in a bulbous segment called a telson. The telson
contains the stinger. Scorpions are active at night, feeding on
insects such as cockroaches or house crickets.
Millipedes (Class Diplopoda) are elongate
invertebrates. They are generally round in cross section and, with
the exception of the first four or five, all of the body segments
possess two pairs of legs. They are relatively slow moving, and
feed on fungus and decaying plant material. At times, they can be
a pest to vegetables or other plants in greenhouses.
Centipedes (Class Chilopoda) strongly resemble
millipedes. Their antennae are longer, they are more flattened in
cross section, they have one pair of legs per body segment and
they can move rapidly. They are beneficial in that they are
predators of other arthropods. Centipedes are nocturnal and
actively seek dark shelter if exposed. The first pair of legs
possess venom glands, which they use to subdue their prey.
Sowbugs and pillbugs (Class Crustacea) are oval
with a hard, convex outer shell made up of a number of plates.
Sowbugs are highly dependent on moisture, which accounts for their
common association with damp habitats. Generally, they feed on
decaying plant material. They will occasionally feed on young
plants in greenhouses and gardens but impact in these situations
Garden centipedes or symphylans (Class
Symphyla) look like tiny centipedes, except they have 10-12 pairs
of legs. They are not common and live under stones, in damp soils
rich in organic matter, and in rotting wood.
Phylum Mollusca - Slugs and Snails Slugs and
snails may also feed on plants and cause damage similar to that of
insects. These creatures are found in moist conditions so they
aren't common in Arizona, but can build up in places that are
watered frequently such as greenhouses and nurseries. Slugs and
snails leave a shiny trail of mucus behind themselves, which is a
sure sign that insects are not the culprit.