Forest Health Protection Rocky Mountain Region • 2011

Introduction to Sap-Sucking Insects, Gall Formers, and Mites

Honeydew, spittle masses, growth deformities, or dried out foliage

Sap-sucking insects feed on the sugary sap produced in foliage and transported in the soft phloem tissue beneath the bark. Some insects’ feeding affects plant growth hormones, causing distinctive foliage or shoot deformities called galls. Gall formers feed protected within the gall. They may suck sap, as do gall-forming adelgids, or pierce and scrape the surface, as do some gall-forming midges. Mites (Acarina) are tiny arthropods that are more closely related to spiders and ticks than to insects. The class Acari contains 45,000 described species, but this is possibly only a portion of the total number of species yet to be described. Spider mites puncture individual cell walls on the outside of foliage and suck out the contents. This leads to a characteristic mottling of the foliage.

There are other types of mites that will be seen and noticed in a forested setting, but most are quite inconspicu-ous. One group of mites that may catch the observer’s eye is the mites that are found associated with bark beetles. Opening up a bark beetle gallery or looking at an individual bark beetle may reveal the presence of mites. These bark beetle associates fulfill a wide array of functions, including being predators of bark beetle eggs and larvae, grazers of associated fungi, and even predators of other mites. Because mites are wingless, they must hitchhike to new bark beetle galleries on their bark beetle hosts. This pattern of movement is referred to as phoresy. These mites are an integral part of the bark beetle system.

General Features—

• Hosts and symptoms of sap feeding insects, gall formers, and mites are described in table 1 (see figs. 1-4).

• Sap-sucking insects like aphids and scales feed on foliage, twigs, branches, and, occasionally, on the trunks of trees. These insects often produce large amounts of sweet, sticky honeydew that can sometimes be seen as a fine mist coming from the trees. Honeydew can coat branches and objects below the trees, which can subsequently be covered by black sooty mold.

• The majority of sap-sucking insects belong to the orders Hemiptera (true bugs) and Homoptera (aphids, leafhoppers, scales). Common gall formers belong to the insect orders Homoptera (adelgids), Hymenoptera (cynipid gall wasps), and Diptera (gall midges) and to the mite order Acari, which includes gall-forming mites (eriophyid mites) and spider mites.

Figure 1. The boxelder bug is a seed-feeding insect that may overwinter in and around buildings. Photo: William M. Ciesla, Forest Health Management International, Bugwood.org.

Figure 3. Frothy mass covers spittlebug on juniper. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org.

Figure 2. Pinyon spindle gall. Photo: Brian Howell, USDA Forest Service.

Figure 4. The western conifer seed bug may seek overwintering sites around buildings in the fall. Photo: Whitney Cranshaw, Colorado State University, Bugwood.org.

Forest Health Protection Rocky Mountain Region • 2011

Tabl

e 1. C

ommo

n sap

-suck

ing an

d gall

form

ing in

sects

and m

ites i

n the

Roc

ky M

ounta

in Re

gion.

Inse

ct/m

ite

Host

Sy

mpt

om

Blac

k pine

need

le sc

alea (N

uclas

pis ca

liforn

ica)

Pond

eros

a and

Aus

trian p

ine

Tiny,

black

disc

s on n

eedle

sBo

xelde

r bug

a (fig.

1) (L

epto

coris

trivi

ttatu

s) Se

eds o

f box

elder

and o

ther m

aples

No

table

home

inva

der in

fall

Coole

y spr

uce g

all ad

elgid

(Ade

lges c

ooley

i) Sp

ruce

and D

ougla

s-fir

Galls

on sp

ruce

; woo

lly m

ateria

l on D

ougla

s-fir n

eedle

s and

twigs

Eriop

hyid

mites

(man

y spe

cies)

Aspe

n, oa

k, ma

ples,

and o

thers

Area

s of d

eform

ed fo

liage

, ofte

n red

or pi

nk-co

lored

Euro

pean

elm

scale

(Gos

sypa

ria sp

uria)

El

ms, p

rimar

ily A

meric

an el

m Ho

neyd

ew, b

lack s

ooty

mold,

and s

cales

on br

anch

esGi

ant c

onife

r aph

ids(C

inara

spp.)

Ma

ny co

nifer

s Ho

neyd

ew an

d aph

id co

lonies

Hack

berry

budg

all ps

yllida

Hack

berry

En

large

d, sp

heric

al-sh

aped

kille

d bud

s

(Pac

hyps

ylla

celtid

isgem

ma)

Hack

berry

nipp

legall

mak

era

Hack

berry

Ni

pple-

shap

ed sw

elling

s on l

eave

s

(Pac

hyps

ylla

celtid

ismam

ma)

Oyste

rshell

scale

(Lep

idosa

phes

ulm

i) Ma

ny ha

rdwo

ods

Grey

mod

eling

on ba

rkPe

tiole

gall a

phid

(Pem

phigu

s spp

.) As

pen,

cotto

nwoo

d, an

d othe

r pop

lars

Marb

le-siz

ed ga

lls on

leaf

petio

lePi

ne ne

edle

scale

(Chio

nasp

is pin

ifolia

e)

Pine

s, sp

ruce

, and

occa

siona

lly ot

her c

onife

rs Tin

y, wh

ite, te

ardr

op-sh

aped

scale

s on n

eedle

sPi

nyon

need

le sc

ale (M

atsu

cocc

us a

calyp

tus)

Piny

on pi

ne

Tiny,

black

, bea

n-sh

aped

bump

s on y

ear-o

ld ne

edles

Piny

on sp

indle

gall m

idgea

Piny

on pi

ne

Footb

all-sh

aped

swell

ing jo

ining

need

les w

ith th

e fas

cicle

(P

inyon

ia ed

ulico

la) (fi

g. 2)

Pitch

or R

esin

midg

ea (Cec

idom

yia sp

p.)

Pond

eros

a pine

, prim

arily

Re

sin pi

ts wi

th ma

ny tin

y, br

ight r

ed m

idge l

arva

eSp

ider m

ites (

Oligo

nych

us sp

p.,

Spru

ce, ju

niper

s, an

d othe

rs Dr

ied fo

liage

; dus

ty ap

pear

ance

Tetra

nych

us sp

p. an

d othe

rs)Sp

ittleb

ugsa (A

phro

phor

a spp

.) (fig

. 3)

Prim

arily

junip

ers,

oaks

, and

herb

aceo

us un

derst

ory

Froth

y mas

ses o

n twi

gs an

d foli

age

p

lants;

also

in so

uthwe

stern

dwar

f mist

letoe

Wes

tern c

onife

r see

d bug

a Se

eds o

f pine

s and

Dou

glas-fi

r Da

mage

d see

d; ca

n inv

ade h

omes

in th

e fall

(L

epto

gloss

us o

ccide

ntali

s) (fig

. 4)

a Not

discu

ssed

in th

is gu

ide.

Introduction to Sap-Sucking Insects, Gall Formers, and Mites - page 2

Forest Health Protection Rocky Mountain Region • 2011

• Spider mite injury is usually related to drought conditions.• Spider mites produce very fine webbing that causes a distinctive dirty appearance due to the accumulation of

dust, cast mite skins, and eggs.• There are many natural enemies that help control sap-sucking insects and mites. Insecticide use can some-

times prolong infestations by killing natural enemies of these insects.• There are a number of non-native sap-sucking insects that have been introduced to other parts of the country

that have become serious forest pests. The central Rocky Mountain Region does not have serious problems with non-native sap-sucking insects in its conifer forests, but several non-native species are found in decidu-ous landscape trees.

• Sap-sucking insects such as leafhoppers are known to vector certain plant diseases (e.g., X-disease on chokecherry).

Introduction to Sap-Sucking Insects, Gall Formers, and Mites - page 3