Venables, E. P.

English

Journal of the Entomological Society of British Columbia

28 B. C. E NTO MOLOGICAL SOCIETY --------~-----------Observations on the Woolly Aphis of the Apple Eriosoma lanigerum (Hausm) By E. P. VENABLES, Dominion Elltomological Laboratory, Ver1lon, B . C. Q vVING to the prevalence of th e \\'oolly aphis of the apple through-out the orchard sections o f the Okanagan valley in Brit ish Columbia, the Dominion Entomological Branch undertook a special study of the insect in order that the control mig ht be placed on a more sat isfactory basis. The need for thi s study became increasingly important, owing t o the conclusion arr ived at by Mr. C. L. McLarty, P lant Pathologist at the Summ erland Experimental Station, namely, that this aphi s was closely associated w ith the spr ead o f th e perennial canker of the apple (Gloeosporium perennans Child s & Zeller), a serious disease w hich had recently made it s appearance in som e Okanagan orchards. McLarty has secured good evidence that the spread of the disease, in individual cankers, is due to th e feeding of the woolly apple aphis upon th e tender healing ti ssue around the canker m argin. From his experim ents, exclusion o f th e aphi s re sulted in the healing of the ca nkers. From a s tudy of the lit eratur e. it was apparent that the investiga-tion would be a complicat ed one. Point s in the li fe cycle known to occur in one loca lity might not holel good in a different area. In parts of East ern Canada and the lJ nited Stat es the migrati on of thi s aphi s from elm to apple is an important factor in the life cycle, w hich is furth er complicated by th e presence o f th e roo t form on apple . and abo that feeding and coloni zat ion may take place upon th orn (Crata eglt s) and m ount a in as h (Sorbus.) In Briti sh Co lul11bi a li ttle detail ed info rm at ion exi sts regarding thi s aphi s. It ha s bee n de finit ely sho \\'n in our studies that the elm plays no pa r t in th e life cycle. th e roo t forl11 occurs at Vancou ve r but is apparent ly abse nt in the int eri o r , th orn and m ountain ash are pl entiful but are no t attacked. In o rder that th e life hi story o f th e w ool ly apple aphi s might be clearly under stood, it was eyi dent that it wou ld be necessa ry to study, t o som e exte nt. the allied species o f Eriosoma occurring in the di strict. PROCEEDINGS; 1929 29 It was particularly important t o ascertain which species . adopted the elm as a winter , host. As a result of our studies at Vernon , B. c., 111 the Okanagan va ll ey, the following species of the genl.\s have been found and th eir winter and summer host plants determined. Eriosoma ulmi Linnaeus. Summer hos t , roots of currant and g oose-berry; winter host, elm. Eriosoma americana Riley. Summ er host, roo ts of June berry (Amelanchier spp.) ; wi nter host, elm . Eriosoma lanigera Hausmann . Summer and winter hos t, apple. Another species has been found upon elm in smaJI numbers, but its identity is at present in doubt. Life-History That the life-history of th e " 'oolly aphis of th e a pple is a compli-cated problem is evident t o anyone who ha s studi ed the litera ture. Various forms of the in sec t hav e been described, a nd it s migration to certain other hos ts ha s been shown to be a norm al procedure in .pa rts of North America and elsewher e. Our observations, commenced in 1928, were based on the a ssump-tion tha t the life-hi story \\'ould co nform more or less cl osely t o that 111 other parts of the continent . We have found the winged fo rm to be prese nt only in th e autumn at' Vernon, Oyama and Ke IO\\·na. At Pe nticton , some 80 miles south 'of Vernon, t\\' o \Vinged individual s were ta k en in a colony in early June, and thi s is the only indication that we have which suggest s any mig ra -ti on by flight in ea d y summer, a lthough ve ry careful \\'atch \\"as k ept at all times . The winged femal es of th e fa ll gen eration appeared in late August on caged trees. and by early Septemher ,,"ere extrem ely numerous, both in the cages and in the fi eld. So far as our observat ions have gone, we have been unabl e t o trace th e migrati on of thi s aphi s to any other pl a nt . None have been tak en on elm trees a t any tim e, although a fair number of elm s occur nea r som e o rchard s. A point o f great interest in connection with t he se a late individua ls is that , a lthough they contain embryon ic young, both male s and female s, these are ne ve r produced . Careful tests \Vith numbers o f thes e potentiall y r eproduct ive fall migrants fail ed to indicate tha t their progeny are deposi ted either on apple or elm. In the caged tree, th ey clustered by hundreds on t!l <.' cotton walls and died. When th e door \\"as left open those upon ~ h : cotton flew off rapidly . No ne of the true sexual fo rms were found upon apple trees , a lthough winged females were numerous upon them. That 30 B. C. ENTOMOLOGICAL SOCIETY thes sexual forms are deposited upon elm and, also, to some extent, on apple, has been clearly demonstrated in sections of the eastern United States, and is referred to as the normal procedure in the literature. Davidson, in Science Progress No. 84, 1927, in dealing with the illigration of the woolly aphis of the apple, quotes the ob~ervations of two investigators in Switzerland who have established the fact that in that country the alate females give birth to sexual forms which are inoperative and die without laying eggs, and that they have, in that country, apparently lost the habit of completing the bisexual cycle owing to the absence of the American elm in Europe, and even where this alternate host is found, sexual reproduction does not occur. In France, Marchal, in 1919-1924, was able to trace the fall migrants from apple to elm where these alate females produced the sexual forms , which, however , were inoperative, so that there was no return from elm to apple in the spring. Theobald, in England, states that the true sexual brood is extremely rare, and that it has occurred only twice in twelve years on an apple tree kept under observation. From these facts it appears that in the woolly apple aphis we have an illustration of progressive suppression of sexual reproduction, pos-sibly owing to the absence of the primary host , elm, in the early days when apple trees were first introduced. In our experiments, the only way in which we were able to secure normal sexual forms from the winged migrants was by enclosing a number of them in a tightly corked vial. These females died in a short time and a few young would be found crawling on the glass. Placed in cylinders over apple twigs, the winged migrant s refused to reproduce and no sexual forms were encountered upon apple. Caging individuals over elm gave no results; in fact, it looks as though the suppression of the sexual forms, as noted in the quotations above, is progressing to include the alate progenitors of these forms, and to render them also inoperative. It is hoped that more extended observations will be carried out in this connection. Injuries As stated before, one of the main reasons for this investigation was on account of the strong probability that the woolly aphis of the apple is the main cause of the spread of perennial canker. Whilst this point is acceded to by tho se who have studied the disease, there is no informa-tion a s yet to show the exact status of the insect in this connection. In order t o gain information on the problem of twig and wood injury caused by the feeding of the aphis, genera\1y referred to as "gall-PROCEEDINGS, 1929 31 ing," we commenced a study of the galls themselves, and of the mouth parts of the aphids. The injury to branches and twigs of apple by the aphis takes the form of nodular swellings or galls at the point of feeding. These galls may be one-half inch to two or three inches in length, depending on the severity of the infestation. After a time the bark splits and an over-growth of the surrounding tissues commences as a result of the tree attempting to heal the wound. This offers an ideal situation for the colony of aphids. When settling down in pruning wounds the aphids congregate at the edge of the cut and attack the newly formed tissue, causing a somewhat similar condition to that on the twigs, large warty growths and nodules appearing beneath the colonies. In the case of twig galls, it will be found, if they are cut through horizontally, that the wood is, in all cases, blackened and dead, and that the pith itself is frequently severely injured. The attempt by the tree to heal this area results in large callus growths around the colony of aphids. The question as t o why a few individuals of the woolly aphis can cause such peculiar gall formation on apple twigs, while an infinitely greater number of the green apple aphis produce no such result, is of interest, and led us t o make a series of comparative measurements of the feeding setae of the two specie s. As a result of this it was found that the setae of the woolly aphis are approximately one-third longer than are those of the green apple aphis (Aphis mali.) Comparison with the setae of E. americana, the leaf form from elm, showed the same condition to exist. From these observations it would appear that the formation of twig galls may be partly due to the fact that the woolly aphis is able t o penetra te through the cambium and reach the xylem or wood beyond. The data supporting these points is based upon measure-ments of the feeding setae of numerous individuals. Microscopical preparations o f wood sections made with the microtome, and stained with Flemming's triple stain, have clearly demonstrated the path fol-lowed by the setae. and also that the cambium is the point at which feeding takes place. Cases have also been noted where setae paths occurred in the xylem or wood by way of the medullary rays . The question of gall formation is of great interest, and the foregoing points are offered as being :;uggestive and of possible value in connection with our work on the transmis sion of perennial canker. The cause underlying gall formation by various insects has been the subject of critical enquiry by several workers. B. W. Wells, in the A.merican Journal of Botany, Volume 7, 1920, in a careful examination of data on the subject, mentions that Rosen's contention that continuous sucking at one point for a period of 15 days is the initial cause of gall formation, cannot be conceded in view of the fact that numerous 32 B. C. E W rOM OLOG I' CAL SOC1ETY herrri'pterous ins~ct s pn'jduce' 'no g alling, a lthough th ey may rem ain fixed at one point fo r long period, a s is the ca se with scale 'in sect s, and many aphids,. . Wells' summary of tl}e findings of P rillieu x, R osen, Petri and others goes ' to show that a true int erpl:etation of the l1atu~ e ,of gall 'formation is lacking, and that the statem ent that som e specific stimulus is inje ~ted b)' the insect into the plant s has no t bee n proved, althoilgh, a.s" he remark s, the ve ry minut e quantity inj ect ed wo uld ma k e it s detection diffi cult. The galls l.:i r oduced by the w oo lly a phi s on th e roo t s o f a pple are of a diffe rent cha racter to those found on the ae rial portions of th e tree, being more gl obular, in outlin e and o ft en ucc urrin g in clu ster s. Natural Enemies , The cag ed tree a t Vern on has been a n exce ll ent demo nst ration o f th e part played by the " a i' ious in sect enemi es of th e woolly a phi s, the cage serving t o shut off th ese natural en emi es a nd a ll owing th e in fest a tion to, develop uncheck eci. Thi s tree was <1.\)out eig ht fee t in heig ht , a nd \\'as , a t th e tim e o f caging, ca r efully exam in ed for s ig n s of infes t a ti on. No a phids wer e found either upo n the limb s o r t runk, and th e tre e \\'a s considered " clean ." The cage \\'as con stru cted o f hea\')' co tton, and \\'as appare ntly insect ,proof. ' Two week s a fter caging . a sm a ll colony of the woll y a phi s of the appl e, consisting o f eig h t o r t en indi\'idual s. \\'as found t o have devel-oped in a knot ho le n ea r th e ground, a nd th e pr esence of , thi s colony rendered artificial infes tation unn ecessa ry . By th e m onth of Jul y thi s co lo ny ha d developed to snch a n ext ent that the whole o f the youn g \\' oocl a nd older t w igs ,,'e r e coated \\'ith masse s of wool, a nd hy Aug n st th e \\' hole tree \\'as ve ry h eav il y in-fe sted. In our experi ence, sy rphid fli es a r e on e of th e m a1l1 fact or s in the co ntro l o f the \\' oo ll y a phi s under na tura l condition s. Sy q ,hids becam e very num erous during Aug ust and Sept ember. hv \\' hi ch tim e the \\' oolly a phi s r eaches it s m aximum in the int eri o r o f l.)riti sh Columbia. \ Ve have succeeded in r ea ring thr ee species of sy rphid s w hich a ppear to be of importance in co ntro lling th e \\'0(11 )' aphi s, one o f these being new to science , namely, Syrphus venablesi Curra n, Syrphus opin-ator O. S., and Syrphus meadii J ones . F rom our observa ti ons , these three species occur in numbe r s and a re o f m a jo r importa nce . Lacewing larvae a ppear to prefe r t o feed upon th e green aphi s on the twigs, and find the \\' oo l1 y coloni es diffi cult t o deal w ith , a lthough PROCEEDI N GS, 1929 33 scattered migrating individua ls a re picked up , It is interesting to note that syrphid eggs "are g enera ll y ' laiEf in close proximitydo, colonies of woolly aphid s, and a re fr ec1uent'ly buried beneath the wool befor e they hatch, owing to th e increase in the s ize of th e colony subsequent t o th e laying of the eggs, \\'hereas the lace wing ' flie s lay their eggs on the leaves infest ,ecl " ' ith the gree n aphi s and a re seldom found deposited near colonies of woo lly aphid s, Coccinellids, o r lady-bird bee tl es; are appa rently m ore a t hom e feed-in g .on green aphids o\\' ing to th e difficulty of dealing \\'i th the woolly ma sses, but they undoubt edly pick up ma ny exposed individuals which ~ir e wandering' about on the limb s, We ha ve di scove red no sig n of a ny hy m enopterous paras ites attack-ing the \\' oo ll y aphi s of the appl e in th e Oka nagan , In eas t ern Ca nada a nd ' th e U nit ed St a t es one of the mos t important checks is exe rcised by a l11inute Chalcid fly , Aphelinus mali DeGeer , which deve lops within the bodi es o f th e in sec t s and produces severa l ge neration s in a season, V'ie have, during th e pa st sea so n. for variou s r easons, broug ht in coloni es o f aphid s from eas t ern Canada a nd ha ve found the Aphelinus t o be quite num erous am ongs t th em, T he ad visability of importing thi s pa ras ite I S being con sider ed, In Ne,,' Zeala nd and ;\u stralia th e wooll y aphi s of the appl e is a pes t o f a ve ry se rious nature a nd th e g overnments of thos e countri es ha ve . through th eir entom ological o ffi cers, imported th e Aphelinus w ith m os t remarkable success , A n inter es ting point regarding thi s parasite in 0:'e \\' Zealand is th e fact that it ex ist ed in the island s pri o r t o it s intro-duction from A m eri ca, but \\'as a ttacking a phids o th er than the woolly aphi s, T he impo rt ed parasites, ho\\'ever , at once comm enced to de stroy thi s a phi s a nd a re doing exce ll ent \york, The foregoing r emark s a re off ered m er ely a s the outcom e of a season' s \\'ork. and much r ema in s t o be clone t o develop certain phases o f the problem , 

Observations on the Woolly Aphis of the Apple

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Observations on the Woolly Aphis of the Apple

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Journal of the Entomological Society of British Columbia