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Oak Woodland Invertebrates

The Little Things Count


Richard Little, Ted J. Swiecki, and William Tietje


University of California

Agriculture and Natural Resources

Publication 21598


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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

x

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