Oak Woodland Invertebrates
The Little Things Count
Richard Little, Ted J. Swiecki, and William Tietje
University of California
Agriculture and Natural Resources
Publication 21598
The printing of this publication was funded in part by the Renewable Extension Act—USDA.
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Credits: Jack Kelly Clark: inset p. 3, background pp. 4, 7, 8, 9, 10, inset bottom right p. 8;
Ron Mumme: inset top left on cover, inset p. 11; Richard Little: inset bottom left on cover,
inset top left p. 8; Edward Ross: inset right center on cover, inset p. 4, inset p. 5, inset p. 9,
inset p. 10, inset p. 12; Ted Swiecki: background on cover and pp. i, 1, inset top right p. 8,
inset bottom left p. 8.
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Oak Woodland Invertebrates
The Little Things Count
Richard Little
Agriculture Inspector, Biologist, and Staff Entomologist
San Luis Obispo County, California
Ted J. Swiecki
Plant Pathologist
Phytosphere Research
Vacaville, California
William Tietje
Natural Resource Specialist
Integrated Hardwood and Range Management Program
Department of Environmental Science, Policy, and Management
University of California, Berkeley
University of California Cooperative Extension, San Luis Obispo County
Contents
Zooming in on a Hidden World . . . . . . . . . . . . . . . . . . . . . . 2
Filbert Weevils and Filbert Worms . . . . . . . . . . . . . . 2
Oaks for Dinner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Who Is Eating Whom? Oak Enemies’ Natural Enemies . . . . 4
Tarantula Hawk Wasps . . . . . . . . . . . . . . . . . . . . . . . 5
California Oak Worm . . . . . . . . . . . . . . . . . . . . . . . . 6
Human Impacts: Usually More Harm Than Good . . . . . . . . 6
Oak Galls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
What’s in the Soil? Invertebrates Down Below . . . . . . . . . . 10
The Big Picture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
“When we try to pick out anything
by itself, we find it hitched to
everything else in the universe .”
This well-known quote from the pioneering con-servationist John Muir serves as both a starting
point and a framework for our exploration of a largely
unnoticed universe. This universe exists within one of
California’s most important and environmentally impact-
ed ecosystems: the oak woodlands. The universe we
explore here is the realm of the invertebrates, minute
animals whose lives are “hitched” to almost every part
of the oak woodland ecosystem.
Zooming in on a
Hidden World
California oaks (Quercus spp.) are the dominant tree spe-cies in the coastal ranges and the foothills surrounding
the Central Valley. Oak trees cover about 10 million acres,
roughly 10 percent of California’s land area. Many woodlands
in California include several different types of oaks. More than
25 oak species, natural hybrids and varieties, are native to
California, and nine of these occur only in California. Some
oak woodlands contain other native trees, including gray pine
(Pinus sabiniana) and California buckeye (Aesculus californica),
and a variety of shrub species. The ground below the oaks is
typically carpeted with non-native grasses introduced from the
Mediterranean region. However, native bunch grasses and a
variety of forbs occur in many woodlands, and springtime dis-
plays of native wildflowers often brighten the understory.
Oak woodlands teem with a wide variety of animal
life. Over 300 species of vertebrates (animals with backbones)
use California’s oak woodlands, including 170 bird species, 80
mammal species, and 60 species of amphibians and reptiles.
Although oak trees provide valuable habitat elements (nesting
sites, cover, thermal protection, food) for many different ver-
tebrates, relatively few species make direct use of oaks as a
food source. For example, less than 20 percent of the bird and
mammal species in oak woodlands eat acorns, and the per-
centage of mammals that browse directly on oak leaves, twigs,
and roots is even smaller. But the myriad of invertebrates in
oak woodlands converts the sunlight, carbon dioxide, and
mineral nutrients absorbed by oak trees into food for many
birds, reptiles, amphibians, and mammals.
When we look closely at individual oak trees, we
begin to glimpse the universe of the invertebrates. They
2
Filbert Weevils
and Filbert
Worms
Whether you are collecting acorns for eating, as
California’s Native American pop-
ulation did, or for planting,
wormy acorns can be a problem.
Insect larvae can destroy much of
an acorn’s insides and may intro-
duce fungi and bacteria that will
decay what is not eaten. A large
percentage of the acorn crop from
a given tree may be destroyed by
these insects, but damage levels
can vary widely from tree to tree
and year to year. Although acorn
feeders do not harm mature
oak trees, they can reduce oak
regeneration.
The two different types of
insect larvae that are responsible
for most wormy acorns are the fil-
bert worm (Cydia latiferreana) and
filbert weevil (Curculio spp.) lar-
vae. Although both types of larvae
are whitish, they can be distin-
guished in several ways. The fil-
bert worm is the larva, or caterpil-
lar, of a brownish, nondescript
moth. Filbert worm larvae are rel-
atively active and have true legs.
When disturbed, a filbert worm
larva may drop down on a silk
strand as a spider does.
Larvae of the filbert weevil do
not make silk. They are legless,
relatively inactive, and tend to
curl up in a C shape. Filbert wee-
vil adults are brown beetles with
extremely long, thin snouts.
Typically, only a single filbert
worm larva is found in an acorn,
but up to eight filbert weevil lar-
vae can be found in one acorn.
buzz and drift through the air; climb along and tunnel
through oak leaves, stems, and roots; dig through the soil;
and swim in watercourses and seasonal ponds. Inver-tebrate
organisms vastly outnumber their vertebrate counterparts in
the oak woodlands: 10 to 100 million individual inverte-
brates may be present within a single acre of oak
woodland.
Invertebrates lack not only backbones, as their name
implies, they have no internal skeleton at all. This lack of
internal support limits the size that these animals can attain.
In our oak woodlands, most invertebrates are less than 1 inch
(2.5 cm) long; some are microscopically small. Nearly all of
the invertebrates in the oak woodlands fall within a large tax-
onomic group or phylum called the Arthropoda: literally,
“joint-footed” invertebrates. The arthropods are further divid-
ed into classes that include the Insecta (insects), Arachnida
(spiders, ticks, mites, and scorpions), Crustacea (pill bugs
and crabs), Chilopoda (centipedes), and Diplopoda (milli-
pedes). All of these classes are represented in the oak wood-
lands, but insects and arachnids are by far the most numer-
ous. More than 5,000 species of insects and arachnids may be
found in California’s oak woodlands.
Oaks for Dinner
Virtually every part of an oak tree—roots, trunk, bark, branches, leaves, and acorns—serves as food for a
number of invertebrates (see Filbert Weevils and Filbert
Worms on page 2). Invertebrates, especially insects, may
be categorized as oak pests if they feed on living plant tis-
sues. However, only a handful of the more than 800 species
of insects that feed directly on the tissues of living oaks can
cause enough damage to be considered significant pests.
3
Insect-infested acorns typi-
cally fall earlier than intact
acorns. This observation may
have led to the Native American
practice of burning the oak
understory to control these pests.
Properly-timed burning would
destroy wormy acorns from the
current season and reduce pest
populations in the following
year.
Present-day acorn planters
generally use water rather than
fire to help separate good acorns
from bad. Feeding by filbert
worms and filbert weevils creates
air-filled pockets in the acorn,
which cause acorns to float in
water. Pitching out the “floaters”
leaves only the viable acorns for
planting. Alternatively, discard-
ing acorns with one or more exit
holes—small holes produced by
mature larvae as they leave the
acorn to pupate—also eliminates
many nonviable acorns from a
seed lot.
Most of the insects that feed on oaks are specialists
that can tolerate the high levels of tannins in oak tissues.
Tannins are bitter-tasting chemicals produced by plants and
can discourage feeding by both vertebrates and inverte-
brates. As California’s oak-feeding arthropods have evolved
along with their host plants over the past 15 million years,
the oaks have waged chemical warfare by varying their tan-
nin content and composition to discourage would-be oak
feeders. This strategy has been only partly successful. As
they coevolved with the oaks, oak-feeding insects accumu-
lated adaptations that allow them to tolerate or even utilize
the tannins in oak tissues. In the most extreme example of
coevolution, cynipid wasps and other arthropods that pro-
duce oak galls (see Oak Galls on page 8) actually put oaks
to work for them by manipulating oak biochemistry. Many
oak-feeding arthropods subsist exclusively on oaks.
Who Is Eating Whom?
Oak Enemies’ Natural Enemies
In the ongoing battle against hungry invertebrates, oaks have many allies, including other hungry
invertebrates. That is, many of the native oak-feed-
ing arthropods that exploit oaks for food are
themselves food for other arthropods. Many
general predators, such as spiders, dragonflies,
mantids, and lacewing larvae, feed on almost
any arthropod they can subdue, including other
predators. Some other predatory arthropods are
4
Cocoons of a braconid parasitoid wasp (Apanteles sp.) are attached to
a caterpillar feeding on a valley oak leaf. Braconid wasps are internal
parasites (parasitoids) of various insect groups, and many form silken
cocoons on the outside of the host insect’s body. Parasitic insects like braco-
nid wasps help reduce populations of leaf-eating insects under natural condi-
tions, thereby minimizing the amount of insect-related leaf damage on oaks.
more specialized. For example, lady beetles generally feed on
only aphids, scale insects, and mealybugs.
Parasitic insects (technically, parasitoids) also eat
other insects, but do so while living either inside or on the
host insect’s body. Most parasitic insects eventually kill
their hosts. Many families of wasps and several families of
flies parasitize arthropod eggs, larvae, nymphs, pupae, or
adults (see Tarantula Hawk Wasps at right). Even
wood-boring insects tunneling within oak branches are not
safe from parasites. Some parasitic ichneumonid wasps use
their slender “stingers,” or ovipositors, to penetrate overly-
ing wood. This allows them to lay their eggs on or inside
wood-boring insects tunneling beneath the bark. Indeed,
even parasites are not safe from hyperparasitic species, that
is, insects that parasitize other parasitic
insects.
The intricate system of
interactions between oak feed-
ers and their arthropod
predators and parasites
helps to limit populations
of destructive insects,
restricting the amount of
damage that they cause.
Populations of oak feeders
sometimes surge when con-
ditions are unfavorable for
their predators and parasites,
leading to extensive defoliation
5
Tarantula Hawk
Wasps
When walking through the oak woodlands during spring and
summer, you may hear, then see, the
tarantula hawk wasp (Pepsis sp.). The
buzzing produced by this wasp as it
flies can be loud enough that people
sometimes think it is a black hum-
mingbird with orange wings. The
tarantula hawk is the largest wasp in
California. Specimens with wingspans
as large as 31/2 inches (9 cm) and
bodies exceeding 11/2 inches (4 cm)
have been found. The body is an iri-
descent, steel blue, sometimes with a
greenish tint.
As its name implies, females of
this wasp species prey on tarantulas,
which they hunt by flying low over
the ground. The female tarantula
hawk wasp is usually successful in her
attack on the spider, often biting
off one of the spider’s legs and
then feeding on its body fluids.
After stinging and paralyzing
the spider, the wasp drags
the spider up to 100 yards
(91 m) to an already-pre-
pared hole. The wasp plac-
es the tarantula in the hole
and lays an egg on its abdo-
men. A larva emerges from
the egg and eats the paralyzed
spider. Once the spider is con-
sumed, the larva pupates and
emerges the following spring as an
adult. Interestingly, the size of the spi-
der used to provision the larva deter-
mines the size of the adult wasp that
emerges.
Tarantula hawk wasp adults sip
nectar from milkweed blossoms and
mate on or near milkweeds. As with
all wasps, only the female can sting.
Only a few humans have been stung
by tarantula hawks. While not consid-
ered dangerous, the sting is said to be
indescribably painful.
A tarantula hawk wasp (Pepsis sp.) paralyzes a tarantula by stinging it,
sometimes following an intense struggle. The tarantula hawk drags the
paralyzed spider off to its nest in the ground where it lays a single egg on
the outside of the tarantula. The emerged larva of the tarantula hawk
wasp then consumes the tarantula.
(see California Oak Worm at left). However, damage is
typically limited either by the insect’s own life cycle or by
a resurgence in populations of natural enemies. A mature
oak tree has considerable energy reserves and is therefore
not threatened by an occasional bout of defoliation. For a
healthy oak, a new flush of leaves normally replaces the
damaged set—and, life goes on. However, if an oak is
weakened by disease, construction damage, or chronic
stress, a serious defoliation episode can push a tree into a
decline that may result in its death.
Human Impacts: Usually More
Harm Than Good
Humans often disrupt the stability of the interactions between oaks, their arthropod residents, and natural
enemies of oak pests, generally to the detriment of the oak.
Although various methods can be used to reduce populations
of injurious insects, in most cases human actions result in
unnaturally high levels of insect damage.
Negative human impacts are especially common
among oaks that are incorporated into urban landscapes.
Fertilization can increase plant tissue succulence that
favors insect damage. Summer irrigation of oaks can lead
to unseasonal growth flushes that are highly susceptible to
insect attack. Summer irrigation also increases the risk of
various debilitating root diseases. These diseases can stress
trees, making them more attractive to insects that special-
ize in attacking weakened trees.
To counter pest problems brought on by improper
cultural conditions, some oak owners resort to applications
of broad-spectrum insecticides. Unfortunately, such prod-
ucts usually do more harm to populations of predatory and
6
California Oak
Worm
Because of the severe defoliation it can cause during major out-
breaks, the California oak worm or
oak moth (Phryganidia californica)
is one of the best known and most
important native insect pests of
oaks. Young oak worm larvae
skeletonize the lower leaf surface,
whereas larger larvae chew through
the entire leaf blade. Two genera-
tions of the oak worm are typically
produced in Northern California,
but a third generation can occur in
the south, or in the north following
warm, dry winters. The oak worm
occurs along most coastal areas in
California and can be found on
both live oaks and deciduous oaks.
Deciduous oaks are less likely