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WALNUT APHID Chromaphis juglandicola Kaltenbach -- Homoptera, Aphididae (Contacts) ----- CLICK on Photo to enlarge &
search for Subject Matter with Ctrl/F. GO TO ALL: Bio-Control Cases 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. |