host selection by the pine engraver

7/28/2019 Host Selection by the Pine Engraver

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Host Selection by the Pine Engraver

ROGFJR F. ANDERSON,' U.S.V.A., Agr. Rea. Adm., BUTeau of Entomology alul Plant Quarantine

An important problem in forest en-tomology is the determination of factorswhich influence tree selection by insects.This is especially true in the study ofthose insects which spend part of theirlifetime in the subcortical region of hosttrees. Attacks by many of these inner-

bark feeders are frequently predisposedby a decreasing resistance of the treescaused primarily by other unfavorableintrinsic or extrinsic factors. One of thesefactors which has been associated withseveral bark beetle epidemics is deficient

rainfall (Felt (1914), Blackman (19~4),Craighead (19~5a),S1.George (19~9),andEaton (1941). Craighead (19~5b) alsorecognized that depredations of thewestern pine beetle, Dendroctonus brevi-c07nis Lec., were connected with hostdecadence.

These findings, which emphasize theimportance of host vigor, suggested theneed for a study of the way bark beetlesselect the weakened trees most suitablefor attack; therefore, the work reportedhere was undertaken. The bark beetleused for this study was the pine engraver,Tps pini (Say). All the trees and logs were

jack pine, Pinus banksiana Lamb. Thefieldwork was carried out in north-centralMinnesota during the summer months of1940 and 1941.

Ips pini is typical of the bark beetlesin that the adults bore into the inner

bark where they carryon their reproduc-tive functions. The males make the initial

attack by boring into a tree or log wherethey excavate a small nuptial chamber inthe inner bark. Soon each male is joined

by one or more females which excavatetunnels radiating out from the centralnuptial chamber. Along the sides of thesegalleries the females cut niches into whichthe eggs are deposited. After the larvaeare hatched, they tunnel away from theegg galleries and feed on the fresh, un-discolored parts of the inner bark. When

the larvae have transformed into adults,the young beetles continue to bore andfeed in the inner bark for a short time

IThe writer is very grateful to F. c.'Craighead and H. J..MaeAloney, both of the Bureau of Entomology and Plant Quar.antine, and A. C. Hodson, of the University of .Minnesota, forhelping to make this study possible.

before they emerge. They then flyawayand attack other trees or logs. A more de-tailed account ofthe biologyof this speciesis given by Clemens (1916).

CONCENTRATION OF ATTACK.-Many

subcortical feeding insects frequentlyconcentrate their attacks on a few treesrather than infest lightly Itlarge number.Such a response is especially favorable tothe attacking insects whenever the resist-ance of the host has to be overcome be-fore the attack can succeed. The writerhas observed this gregarious characteris-

tic by many species of bark beetles in-cluding Dendroctonus brevicomis I.ec.,D. frontalis Zimm., D. monticolae Hopk.,and Ips pini (Say), and by the flatheaded

borers Melanophila californica Van. D.and Agrilus anxius Gory, and round-headed borers of the genus llfonochamus.

When Ips infestations were studied itwas noted that the initial entrances weremade after the logs had been exposed formore than a week. The number of attacksthen increased rapidly. The long preinfes-tation period probably resulted becausethe logs were exposed in a rather openarea where other breeding material wasabsent. This condition necessitated anemigration of the attacking beetles froman adjacent jack pine stand where theywere reared.

The rapid rate of increase of infesta-tion following initial attacks suggestedthat the attacking beetles themselveswere conducted to determine whether

this inea was correct. The tests consistedof caging freshly attacked logs in areasremoved from the site of initial attacksand placing suitable uninfested logs out-side the the cages but adjacent to them.Shortly after the infested logs had beencaged-freq uently within an hour-beetlesbegan to congregate on the screen, at-tempting to reach the logs inside. Soonthey would move to the adjacent unin-fested logs and attack them. In 78 such

tests, 77 per cent of the logs outside thecages became infested on the first day, 9

per cent on the second day, 10 per centon the third day, and the remaining 4

per cent on the fourth day. ~Vhen thefirst attack was delayed longer than 1day the weather was usually rainy and

596


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Ai'WEHHON: HOST SELECTION BY PINE ENGRAVER .597

cool. or the 25 checks, 7 became infestedin an average of 14days, whereas the otherll'-ldid not become infested through the1 to 3 week exposure periods. Beetles

caged alone an'd an extract made frombeetles were both unattractive.

These observations indicate that theattraction of beetles is more dependent onlhe activity of pioneer att.acking beetlesthan on the attractiveness of the host ma-lt'rial itself. As would be expected fromthe previous observations, the strength ofallraction was found to be dependent onthe number of fresh attacks. Based on 16observations, where the new attacks at

the begilllling of the test averaged 0, 4,26, and l'-l6,the beetle entrances made dur-ing the following24-hour period averaged0, 2, 60, and I:J2, respectively. The corre-lation coefficient was +0.94, which indi-('Utesthat the relationship is highly sig-nificant.

This attraction, however, did not con-tinue very 101l~.Exposure of logs withattacks 4, 7, 14, and ~8 days old resultedill no new entrances being made duringthe following24-hour period. Similar logs

with attacks ollly I-day old averaged 13new entrances dming the next ~4-hours.In another obsel'\'ation newly attackedlogswere caged and the number of beetlesattracted to the cage were counted daily.During the first :Jdays there averaged 10to !l9 beetles per obseryatioll, the largest

llIuuber being observed on the sccondday. On the fourth day and t.hereafterthere was a marked decrease, so thatfrom the sixth to tenth days only 1 or 2beetlcs were observed per observation.The beetles were not removed from thecage when the observations were made.

l


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598 JOURNAL OF ECONOl\fiC ENTOMOLOGY Vol. 41, No.4

produced a rich, resinous, yeasty, acidodor, but no odor could he detected closeto the inoculated logs. None of the treatedlogs and none of the logs placed beside

the jars of mash were attacked soonerthan the checks.

The pleasant odor which could be de-tected close to freshly made entranceholes suggests that attraction may bedue to an odoriferous substance emitted

by the males.Whatever may be the nature of the at-

tractant, it undoubtedly is responsible forconcentrating the beetles in the most suit-able host material. It may also explain

the group killing of trees which is charac-teristic of attacks by the more aggressivespecies ofDendroctonus. When a large pop-ulation of beetles is produced, large num-

bers of adults are attracted to the few

scattered trees that have been locatedand are being attacked by the pioneer

beetles. As the infestation develops, the

attraction becomes stronger and the influxof beetles greater. Very soon all the barkspace in the trees originally attacked isfilled, and the excess beetles attack ad-

jacent trees. Even though the beetles maynot find these trees as desirable as those

initially attacked, they are able to over-

come quickly the resistance of more vigor-ous trees by attacking simultaneously inlarge numbers.

INITIAL krTRACTION TOHOST TREES.-

A few observations on how the first at-

.tacking beetles select the most desirabletrees appear pertinent. Person (1931)

theorizes with reference to the western

pine beetle that-"This initial attraction,[due to volatile odors of respiratory fer-

mentation] particularly in trees onlyslightly subnormal, is probably so weak as

to be detected only by beetles in the im-

mediate vicinity of the tree." The present

work also showed the initial beetle attrac-tion to uninfested suitable host material

to be weak.

When uninfested logs were exposedalone, it took the beetles 11 days or more

to find and attack them. During the same

time beetles were attacking logs from the

same trees used for other experiments in

the same general area. Obviously these

logs did not produce a strong attractant.The beetles could not readily detect

suitable host material even after they had

attacked. One series of logs was cut and

stored in a building for a year before

being exposed to beetle attack. Eventhough the inner bark of these logs was

brown at the time of exposure, 7 of the10 logs were subsequent.ly attacked (4 by

two entrances each, 2 by 10 each, and 1 by36. None of the attacks progressed veryfar, although some of the egg galleries

had been extended to a distance of 10 em.from the nuptial chamber. No larvae sur-vived. Attacks also occurred in water-soaked logs in which brood survival wasvery poor.

It was easy to induce Ips p ini to at-tempt an attack even on vigorous trees

by attracting the beetles to the trees with

newly infested logs. Some of the attractedbeetles attempted to attack the trees, butafter they reached the inner bark they all

stopped boring and vacated the tunnels,

None was trapped in the resin. Kincteenentrances were attempted in the two trees

tested.These observations suggest that suit-

able breeding material by itself did not

produce an attractant strong enough todraw I ps p ini beetles from a distance.Instead, they were induced to enter the

log or were repelled only after they wereclose to it. After a suitable tree was en-

countered, the attacking beetles producedthe attractant and thereby drew other

beetles to the tree. }'rom this time on inthe same manner more and more beetles

became concentrated in the part.icular

tree or log being attacked.The reason some beetles went to unin-

fested trees rather than to trees already

attacked is not known. Probably they

searched for new material only when theywere outside the effective range of an

attraction center, or the threshold of the

receptors to the attractant was so high in

some beetles that they were stimulated

only when they were extremely close to

an attraction center. It was observed,

however, that practically all of the beetles

attracted by the fresh attacks were light

brown, which indicated that they hademerged recently.

EFFECT OF INNlm-HARK MOISTURE

CONTENT o c .r HOST SELECTION.-Inept as

some beetles were in detecting suitable

host material, probably most beetles were

successful if suitable trees and logs wereavailable. An attempt was made, there-

fore, to determine the factors affecting

the beetles in the selection of suitable

host material. Because of its great impor-


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August 1948 ANDERSON: HOST SELECTION BY PINE ENGRAVER 599

Topped, no livebrnncl,es re-

maining SO~6 49

Table I.-Moisture content of the inner barkof damaged and undamaged trees. 1940.

Topped, 1 to 5 live branchesremaining 2155 18

Per cent NumberUndamaged 215S 27

NumberoEntrances

per Log

8.50.2

5.3~.6

DriedUntreatedSoaked 5 hoursSoaked 1~ hours

Further observations on this relation-

highly significant. A similar decrease wasalso noted in ~ completely storm-toppedsnags. In these the inner-bark moisturecontent averaged 281 per cent at a trunk

height of 2 feet, 273 at 6 feet, and 26~ at. 10 feet.

A difference in inner-bark moisturecontent was noted also in branchless snagsof red pine, Pinus resinosa Ait., as com-

pared to that in unbroken trees. Theaverages were 25915 and 20814 percent for 3 snags and 3 unbroken trees.

After the observations already de-scribed, suggesting that Ips attack wasassociated with moisture content, otherexperiments were conducted to learn moreabout this relationship. Log sections 1foot long were taken from the same tree toeliminate differences due to variations in

,different trees. The water content of theinner bark of these logs was then varied

by wetting some of the logs and by dry-ing others. To speed the drying, the logswere split and then air dried; to increasethe water content, the logs were placedupright in a shallow pan of water. Af-ter various periods, the treatments were

discontinued and the cut ends of the logswere paraffined. These logs from the sametree, but with different inner-bark mois-ture contents, were then exposed in alocation where beetles were known to be

present.In the first test the logs with various in-

ner-bark moisture contents were exposedtogether but were randomized. After a

period of 3 weeks the 10 drier logs withinner bark moisture contents of 13~to 300

per cent averaged 2.4 beetle entrancesper log, whereas for the 13 wetter logswith 311 to 448per cent inner bark mois-ture there were 4.3 entrances per log.

In a second test the treatments wereseparated from each other by about 10feet. Twelve logs were used for eachtreatment. The counts of beetle entranceswere made after 1 month of exposure, butmoisture determinations were not made.Preference for the dried and soaked logsis shown in the following tabulation:

MOISTURE TREES

CONTENT OBSERVEDTREE CONDITION

tance in determining the vigor of plants,moisture was the most obvious factor,and it was carefully studied.

The first obser\'ation on the effect of

moisture was made in July 1940on somepole-sized trees broken by an ice storm 3months previously. The 10- to I5-footsnags which lacked living branches wereattacked by Ips pim:, whereas snags hav-ing only a few short living branches (evenonly one 3-foot branch) were generallyavoided by the beetles. These two typesof snags were damaged by the storm to .almost the same degree; yet the beetleswere able to differentiate between them.Moisture determinations made of theinner bark in thcse trees, however, showedthat there was a considerable difference

between the two classes. The inner-barkof the snags attacked was almost 100 percent wetter than that of snags which thebeetles passed by. These data are sum-marized in table 1.

Each determination was made from asample of about 80 square centimeters of

bark. The inner succulent tissue was sep-arated from the dry outer bark and im-mediately placed in the drying container.The outer bark was discarded. Tempera-tures were kept at 100 to noo C. to drythe samples to constant weight. All moist-ure percentages are based on the dryweights of the samples.

It was found that the moisture contentdecreased somewhat as the height in-creased; therefore, the data in table I are

from samples collected at a trunk heightof about 5 feet. The moisture content ofundamaged trees averaged ~17per cent ata trunk height of 3 feet, ~II at 7 feet,!t05 at nfeet, !tl!t at 15 feet, !t01 at 19feet, and 186 at !t3 feet. Five trees weresampled at each of the heights listed. Thecorrelation coefficient is -0.47, which is


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60 0 JOURNAL OF ECONOYUCEKTOllIOLOGY Vul. 41 , Nu. 4

FACTOHS OTHER THAN l\fOISTUHE CON-

TENT ASSOCIATED ,"VITH HOST SELEC-

TION.-Differences other than those illmoisture content were also noted. Cam-

bial activity continued in the snags whichretained one or more live branches, forthe inner bark continued to separateeasily from the wood. In the branchlcsssnags, on the other hand, the inner bark

became adherent to the wood-an indica-tion that cambial cell division had ceased.Likewise, a difference in the activity ofthe resin-secreting cells was note(l in thetwo types of trees. On the branched snagsthe exposed wood areas, where hark

samples had been removed, becamc coatedwith resin, whereas on the branchlesssnags no resin flow occurred; conse-quently, a black mold soon covercd theexposed wood. .

As previously mentioned, the activityof microorganisms did not appear to at-tract Ips beetles from a distance. 1\ever-.theless, it may be that some type of de-composition is associated with the attrac-tion of certain host material for nearby

beetles. It was fOllnd that logs wet with

acetic acid, then dried, wcre morc attrac-tive to Ips beetles than were lllllrealedlogs. By the rapid attraction tcchnique

previously described, it was found in 12tests that the treated logs averaged 4.8entrances per log as compared to 1.8 forthe untreated after an exposure period of18 hours. The differencc ncccssary forsignificance at odds of 19 to 1 is 2.21.Buchanan (1941) reportcd a similar in-crease in the attractiveness of scctions

cut from trees injected with cthyl alcoholto an ambrosia beetle, Xylosandl'1ls ger-manus (Blfd.).

Person (1931) concludcd lhal ferment-ing inner bark was the substall('e mostattractive to the western pinc beetle. Themoisture content and acetic acid selectiontests already mentioned support Person'stheory that volatile decomposition prod-ucts may be associated with initial aLt.rac-tion when the beetlcs are in the immediale

vicinity of uninfested suitable hosts. }


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August 1948 ANDERSON:HOSTSELECTIONBY PINE ENGRAVER 601

separated (6 inches) from each other andwere not in contact with the cage contain-ing the attractant. In one test, however,

pairs of short log sections were tested by

standing one on the other. For half of the10 replicates the treated (acetic acid) logswere placed on top and for the remainderthe untreated logs were on top. After anexposure period of 18 hours the differencein the number of entrances in treated anduntreated sections was not significant.This observation indicates that the odorif-erous attraction was not discernible by the

beetles when the treated logs were in con-tact with those untreated.

The moisture content of the inner barkalso appeared to influence the selection ofhost material by species of.Monochaumus.In 13 logs which had inner-bark moisture

percentages of 316 to 448, there was atotal of ~1 egg niches cut by sawyers(Afonochamis), whereas there were nosawyer egg niches in the 10 logs withmoisture contents of 13~ to 300 per cent.Another series of 6 soaked logs had 33sawyer egg niches after having been ex-

posed for 15 da.}'s,whereas there was only

1 niche in the 6 dried logs and no attackson the 6 untreated logs.El


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60~ JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 41 , NO.4

LITERATURE CITED

Blackman, M. W. 1924. The effect of deficiency and excess in rainfall upon the hickory bark beetle.JOUR.EeON. ENT. 17(4): 460-70.

Buchanan, W. D. 1941. Experiments with an ambrosia beetle, Xylosandnls germantls (Blfd.). JOUR.

EeON. ENT. 34(8) 867-9.Caird, R. W. 1985. Physiology of pines infested with bark beetles. Bot. Gaz. 96(4): 709-88.Clemens, W. A. 1916. The pine bark beetle. Cornell Univ. Agr. Expt. Sta. Bul. 3R3, 287-98.Craighead, F. C. 1925a. Bark-beetle epidemics and rainfall deficiency. JOUR. EeON. ENT. 18(4): 577-86.Craighead, F. C. 1925b. The Dendroctonus problems. Jour. Forestry 23(4): 3tU-54.Eaton, C. B., 1941. Influence of the mountain pine beetle on the composition of mixcd pole stands of

ponderosa pine and white fir. Jour. Forestry 39(8): 710-8.Felt, E. P. 1914. Notes on forest insects. JOUR. EeON. ENT. 7(5): 373-5.Person, H. L. 1931. Theory in explanation of the selection of certain trees by the western pine beetle.

Jour. Forestry 29(5): 696-9.St. George, R. A. 1929. 'Weatber, a factor in outbreaks of the hickory bark beetle. JOUR. I~CON.ENT.

22(8): 578-80.

These were registered, but so far as I an aware,none of them attended the meetings. .

EIGHTH mTEH.NATIONAL CONGH.ESS OFENTOMOLOGY, STOCKHOLM

August 8-14, 1948

The total advance and first day registration atStockholm was 510-possibly 50 of these did notappear during the Congress. They represented 87countries, the largest delegations aside from Scan-dinavia, in which I include Finland, were from Eng-land, 63; France, 45 and the United States, 24 as

follows:

The opening session in the Music Hall was ad-dressed by the Prime Minister of Sweden, T. Er-lander, Dr. Trajurdh, the President, the Presidentdesignate, 1\1. Jcannel and Dr. Jordan, the Perman-ent Secretary. It was announced that the next meet-

ing would be held in Amsterdam in 1951. Prior to themeeting the invitational letter of Dr. S. A. Rohwerwas presented to the Executive Committee, a veryselect group which included Dr. Bradley, represent-ing Dr. Johannsen. A commitment had been made atBerlin in 1988 that precluded acceptance of theU.S.A. invitation.

The meetings were held in the Eastern HighSchool, an ancient building without our concept of

Annand, P. N.Bradley, G. H.Bradley, J. C.Bradley, Mrs. J. C.Butt,P.H.Butt, Mrs. F. H.Chapin, E. E.Clarke, J. T. GatesCory, E. N.Forbes, Will. T. 1\1.Haller, H. L.Hayes, W. P.

Hayes, Mrs. W. P.

Calvert, P. P.Curran, C. H.Johannsen, O. A.

Huckett, H. C.Kaan, Dr. H. W.Matthews, Ambassador

H. F.Metcalf, Z. P.Metcalf, Mrs. Z. P.Moorehouse, Col., USA

Nelson, F.Park, ThomasParker, H. L.Parker, Mrs. H. L.Russell, Louise

Usinger, T. L.

Dos Panos, C. F.Dos Panos, Mrs. C. F.Sherman, J. D.

modern and adequate facilities. There were elevensections, one devoted to arachnids. It was difficultto keep track of the pro~ress in thc sections in orderto hear the papers one wanted to hear, which in thecase of the writer was in most cases, those deliveredin English. Many addresses were in French and mostof the Scandinavians used English or German. Al-though Italian is an official language, ProfessorSilvestri spoke fluently in English.

The entertainment was outstanding in every way.Each day at noon a bountiful repast with aqua vita,

beer, wines and liqueurs was arranged at s,9mefamous restaurant (an international word appar-eRtly), museum or slot (castle). These affairslasted for two to two and one-half hours, after whichscientific sessions were resumed from g :30 to 5 P.M.and then similar affairs were schedulcd for the even-ing. Trips by bus, train and boat to points of inter-est took up a good dcal of the time and cnubled oneto become better acquainted with fellow members.On one occasion the cocktail party given by Am-bassador Matthews conflicted with the Congrcssand in an endeavor to muke an appt'arance atboth parties IIgroup went riding in a taxi on a wildgoose chase without catching up with the main

party which had gone to Drottningholm by boat.The trip to Uppsala and the home of Carl von

Linne occupied an entire day. Most of the membersof the congress went by train, but the writer wasfortunate in being the guest of Standard Oil throughthe courtesy of Franklin Nelson and Frederik Haag-

berg. After lunch at Flustret in Uppsala, a famousrestaurant, the name of which means "the cntranceto the beehive," we saw at our leisure Linne's home,attended another repast at the University of Upp-sala and returned with Captain Strid to his homewhere Annand, Haler and Professor and Mrs.Butovitch were entertained at a supper of manycourses and nearly as many kinds of drinks with

much "skolIing" that laster! from 6 to 1!l:80.As hosts, thc Swedes cannot be exceeded. They

did everything possihle to make the stay in Stock-holm exceedingly pleasant. The sessions were prof-itable, especially from the contacts made with for-eign entomologists and the opportunities to learnhow they work and to see their collections and labo-ratories.

Eo N. COln

host selection by the pine engraver

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