ECONOMIC CONSIDERATIONS

No.i:uo.

NORTH AMERICAN TlIY,SANOrTERA—

HINDS.

117

thoughit isalsopossiblethat

more

tendertissues

may

beanattraction.

Such

species avoid the lightand, if aleaf be turned over, the insects will

move

aroundto the under side again.

The

constant su(^king of myriadsof larv» andadults soon causes thefeeding

ground

to wither, the leaf becomes encrusted with dead cellsand dark colored spotsof excrement and^itisnotlong})('fore its deathresults. Unless disturbed, mostspecies do not travel

much,

and thusin timethere appears to be something of a colony feeding around the place

where

the

mother

has fedanddeposited her eg*gs.

Though many

plants thus suffer

from

the destruction of their leaves, theonion seems to be

most

severel}^

afflicted. (See Thrips tahaci, p. 183.)

Grasses and cereals

may

be included in a third class in which the nature of the injury is

somewhat

different. Besides the abstraction of sap

from

theleaves of these plants, Thrips cause a greater injury

by

attacking the tenderaxial stems, thus cuttingoft'directly thesupply of saptothe head,w^hich thereforefails to bearfruit and

may

beentirel}^

killed. This is the waj^ in w^hich "Silver

Top"

^is caused,

and

it is

impossibleto estimate with any degree of accuracy the

damage

which results to the

hay

crop. Besides

working

in this way, Thrips are charged withattackingdirectlythe

growing

kernels of cereals. In the case of wheat, rye, oats, etc., they suck the nutritious milk directly fj'om the

growing

kernels inthe ear

and

producean abortive condition of much, if notall, of thehead, which is thencalled "pungled.'"

Greenhouse speciesappeartobel)ecoming

more numerous

and

more

injurious each year.

The

principalinjury here isdone tothe leaves, andnearlyallkindsofgreenhouseplants are subjectto attack. Thrij^s tahaci^ which has recently

come

into prominence,especially in

cucum-

berand carnation houses, hasanunusually wide range of food plants.

It has alreadyproved to bea serious pest, capable of the complete destruction of a crop, and is exceedingly difficultto control.

BENEFICIAL FORMS.

PredaceousThrijys.

— ^The

^lateB.D.

Walsh

once expressed the opin- ion that Thrips "are generally, if not universally, insectivorous, and that those that occur on the ears of the wheat, both in the United States and in Europe,are preying there

upon

the eggs or larvse ofthe

Wheat Midge

{Diplosls tritici)^ and are consequently not the foes, as has been generally imagined, but thefriends, of the farmer" (127 and

132).

Such

an opinion

from

so eminent an entomologist is likeh^ to have

some

basis in fact, thoughw^e question whether his conclusionis

even usually correct. Thrips have beenfrequently found in the galls caused

by

other insects, eitherwith the

makers

of the gallsoralone, and the conclusion has been drawn, though frequently,

we

suspect, without adirectobservation to that effect, that the Thrips were prey- ing

upon

the

makers

of the galls.

Walsh

also wa-ites that he has

118 PROCEEDINGS OF THE NATIONAL MUSEUM.

^vol.xxvi.'

"found

Thrips prejang

upon

the gall-making larv;^ of

more

than twenty differentgalls, so thatthereis

now

no

manner

^ofdoubt in

my

mind

that Thrips is a true cannibal insect"' (132). All recorded obseryations which I haye seen

seem

to agree that such gall-fre- quenting forms belong to the Phloeothripidse, and in yery

many

of the cases noted it is said that they are in thepupal stage (123).

It seems to

me

entirely possible that in

many

cases their presence in the gall

may

be incidental, they haying entered it for protection.

It is impossible forThrips to

make

for themselves an entrance into

any

closedgall, and

when

present in suchit can onl}' beafter the exit ofthegall

maker

or

some

parasite

upon

it, so here certainl}-^theTlwips

is not predaceous. Furthermore, it does not

seem

improl)a])le

from what we know

of the food ^ha])its of theTubulifera, whichfeedmainly

upon

leayes or decaying

wood

orfungi, thatthey

may

livepeacefullyin

company

with the true

maker

of an open gall which they can readilj enter, hnding there the

same

favorable conditions for abundant foo|

and

a secure retreat as does the gall maker. Ph3dloxera galls ha^

often been found to contain Thrips, l)ut the

same

doubt exists ast^

the real purposeof theirbeing there. ^\'^alshstates that he hasfounc six or seven i"ed Thrips pupa^ in nearly every gall of PhylJoxeTG caryae-foJliv. Thiso))servationshows-plainlyoneobject forwhichthese]

insects seek out and enter galls, as a safe refuge during pupation,]

and

this will accountfor the frequent presence of larvje andadults ir^

bothinhabited open galls, as those of Phylloxera,

and

deserted close]

galls, as those of Cynipida?. It

may

be true thatThrips prey

upon

thi gall makers, but further observations

upon

this point are desirable before

we

can fully accept that conclusion.

'"''

Thrips

fhyUoxer

^•a''''' of Riley's manuscript (one of the Phloeothri- pidee) is said

hj him

to

"do more

than any otherspecies to keepthe leaf-inhabiting grape Phylloxerawithin

bounds"

(165).

A

^species

Phlwothrips has been observed destro3^ing eggs of the

Gypsy Mot|

(353).

Some

species of Thripidro have been observed feeding

upon

othei insects

and

are undoubtedly beneficial. Thi'lps6-maeulatus has beei repeatedly observed feeding

upon "mites"

or

"red

spiders," an]

other species have been said todo thesame. Rileyobserveda Thripl larvafeeding

upon

theeggsof theCurculioin Missouri(143a

and

114) Thripstrifasciatus

Ashmead

isapparentlypredaceous

and was

observe]

feeding on the cotton Aleurodes {Alewrodes gossypii) in Mississipj (386). I have occasionally noticed that under the influence of cor finement withoutplant food Anapliotltripsstriatns^ which iscertain]

normally her})ivorous,

becomes

cannibalisticandwill feed

upon

itsowi species.

FJoicerfertilizers.

—

Itisvery probable that afew flowers, of whic!

the

"wild pansy"

is one, are fertilized

by

Thrips, although such a"

relation

must

^be exceptional.

Few

flowers are adapted to thus profit

No.isio.

NORTH AMERICAN THYSANOPTERA—

HINDS.

119

by the presence of Thrips, as their action

would

tend almost entirely to self-fertilization of the flowers, which Nature does not generally approve. Therefore I believe that their value in this wa}^

must

be very limited.

NATURAL CHECKS.

Insects

and

Acari^ etc.

— The most

important insect

enemy

appears

to be Trl'plilepsinsidiosus Say, which is very

common

on flowers and ma}^ often be found with a Thrips impaled

upon

its rostrum and heldin theair while the captor sucks the juices

from

the l)ody of its victim.

The

eggsof TripJdeps are laid inasimilar

manner

tothose of Thrips

and

the larva? of theformer also prey

upon

the larvte of the latter.

The

length of the lifecycle of Trlphleps isabout the

same

as that of Thrips. Megilla macidata also devours Thrips in great

num-

bers

when

both arc aliundant. Chrysopa and

Sipphus

larva? have been found feeding

upon

the larva? of Thrips taljaci.

Heeger

has recorded

Scymnus

^{ater^} GyrojpTmeim manca^

and some

fly lar^'te as preying

upon

them,

and

Uzel has found TrlpTdeps inimdnalso.

1 have frequently found Anaphotlirlpsstriatus bearing one or

more

small, scarletAcari(probably thelarva? of a

Tromhidiwn)

attachedto

some membranous

area of the body.

Both I'^zel and Quaintance have found the eggs and adults of

Nem-

atode

worms

in the Iwdies of Thrips, Uzel recording over ²⁽⁾⁰

worms from

one specimen.

riantpximsltes.

—

^{Thaxter(297) has taken}

^Empusa

{Entomojyhthora) spliaerosperma Fries

from

a speciesof Thrips

which

it

was

destroying in larval, pupal, and adult stages. Pettit has found in Michigan another parasite

which

he thinks willprove to be aGregarinid (464).

It

was most

abundantin the moist breeding ^cages, causing the insects todie and turn black. I have rarely found a fungus

growing

in a dead specimen which appears to bea species of2facrosporlum, but it

was

not possible to tell whether the fungus caused the death of the insector

came m

^later.

Rahi.

— Of

all the natural checks, none can

compare

in efiiciencv with a hard dashing rain. It has been noted that Thrips tdbaci and Anaphothripsstriatus, which

become

^extremel}' abundant during hot, dry weather, disappear almost entirely as soon as theheavy showers of

midsummer

begin,

and

as long assuch showerscontinue atfrequent intervals the Thrips do not again

become

abundant.

The same

result will probablybe found true in

most

outdoor leaf-inhabiting species.

ARTIFICIAL CHECKS.

These fall naturally into

two

groups, insecticides and cultural methods.

Insecticides.

— ^So

^{far as}

we know,

no attempts to controlThrips

by

means

of insecticides have been

made

outside theUnited States.

Here