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Chromaphis juglandicola Kaltenbach -- Aphididae




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Walnut aphid is native to the Old World and invaded California at the beginning of the 20th century.  It soon covered all the walnut growing areas of the State.  In spite of attacks by several native natural enemies, the aphid frequently covered the trees with exuded honeydew and the resultant black sooty mold fungus that grows with it (DeBach 1974).  Routine insecticide treatments were required, beginning with nicotine sulfate and progressing to a variety of synthetic organophosphates and chlorinated hydrocarbons.  These caused the usual problems of upsets, resurgences, development of resistance and drift of toxic materials outside the area being treated. 


Biological control was not originally considered because it was thought that aphids are not particularly amenable to such control as they can multiply so rapidly in the spring that damage is done before their natural enemies can exert a significant influence.  Dr. R. van den Bosch, in charge of the first biological control project launched against this aphid, suspected that this applied principally to the less effective, high density, omnivorous feeding natural enemies, but that perhaps not to the more host specific kinds.  Therefore, he concentrated his search on the latter and in southern France found a parasitoid Trioxys pallidus Haliday in 1959 which showed a high degree of host specificity.  It was successfully imported to California and became effective against the aphid in the coastal plain of southern California, spreading rapidly and destroying a high percentage of the host population.  But the problem was not completely abated because this parasitoid was not well adapted to conditions in northern and especially central California where, in spite of large scale liberations, it never became permanently established.  It was evident that the French strain of T. pallidus lacked genetic characteristics enabling it to reproduce and survive in areas of extreme summer heat and low RH (DeBach 1974).


Trioxys pallidus had also been observed by van den Bosch on the hot, dry central plateau of Iran during 1960, and he subsequently obtained live specimens from there in 1968.  They were cultured and colonized in central California with spectacular results.  Small releases made in June and July gave rise to abundant parasitoid populations which dispersed significantly by fall.  Further releases were made in the Central Valley in 1969 from insectary cultures.  Surveys of release plots in October 1969 showed the parasitoid to be established in all new release sites and to have spread to a distance of at least eight miles.


The impact of the Iranian T. pallidus was closely evaluated during the 1969-1970 seasons.  It was shown that starting with a low initial population early in the year, the parasitoid could increase rapidly and greatly reduce aphid populations by summer or autumn.  The extent of control was such that very few aphids escaped parasitism to reproduce.  As a result very few individuals were present to start the next year and of these over 90% were parasitized by parasitoids that had survived the winter.  Population indices showed an average reduction of from 2,550 aphids per sample in mid-May 1969 to only eight per sample in mid-May 1970.  During 1970 the parasitoid was found to be present and often abundant in most sites sampled, even though some were miles from the nearest known areas of previous establishment.  By 1971-2 biological control of walnut aphid was complete throughout California, except where upsets occurred due to the adverse effects of insecticides used against the codling moth or the walnut huskfly and to a lesser extent from the Argentine ant which interfered with parasitoid activity (van den Bosch et al. 1970).


It is questionable whether or not the French and Iranian strains of T. pallidus are merely ecotypes or distinct species.  Messenger & van den Bosch (1969) stated that M. J. P. Mackauer believed that he could not detect even the most minor morphological differences, and found only some differences in size and coloration.  However, laboratory hybridization tests showed that the two stocks do not mate, and therefore no hybrid offspring result.  Please also see the following for additional details on biological control effort and biology of host and natural enemies (Schlinger et al. 1960, Sluss 1967, Messenger 1970, van den Bosch 1971, Clausen 1978).



REFERENCES:          [ Additional references may be found at:   MELVYL Library ]



Clausen, C. P.  1978.  Aphididae.  In:  C. P. Clausen (ed.), Introduced Parasites and Predators of Arthropod Pests and Weeds:  A World Review.  U. S. Dept. Agric., Agric. Handbk. No. 480.  545 p.


DeBach, P.  1974.  Biological Control by Natural Enemies.  Cambridge University Press, London & New York.  323 p.


Messenger, P. S.  1970.  Bioclimatic inputs to biological control and pest management programs, p. 84-102.  In:  R. L. Rabb & F. E. Guthrie (eds.), Concepts of Pest Management.  No. Carolina State Univ., Raleigh, North Carolina.


Messenger, P S. & R. van den Bosch.  1969.  The adaptability of introduced biological control agents.  In:  C. B. Huffaker (ed.), Biological Control.  Plenum / Rosetta Press, New York.  511 p.


Schlinger, E. I., K. S. Hagen & R. van den Bosch.  1960.  Imported French parasite of walnut aphid established in California.  Calif. Agric. 14:  3-4.


Sluss, R. R.  1967.  Population dynamics of the walnut aphid, Chromaphis juglandicola (Kalt.) in northern California.  Ecology 48:  41-58.


van den Bosch, R.  1971.  Biological control of insects.  Annu. Rev. Ecol. System. 2:  45-66.


van den Bosch, R., E. I. Schlinger & K. S. Hagen.  1962.  Initial field observations in California on Trioxys pallidus (Haliday), a recently introduced parasite of the walnut aphid.  J. Econ. Ent. 55:  857-62.


van den Bosch, R., B. D. Frazer, C. S. Davis, P. S. Messenger & R. Hom.  1970.  Trioxys pallidus--An effective new walnut aphid parasite from Iran.  Calif. Agric. 24(11):  8-10.