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DIPTERA, Agromyzidae (Fallen 1810) -- <Images> & <Juveniles> Description & Statistics
Commenting on host preferences, Clausen
(1940/1962) stated that the hosts are all monophlebine Coccidae with the
exception of several questionable records from Dactylopius. Larvae and
puparia of the species occurring on Drosicha
corpulenta Kuw. in Japan, which had
previously been recorded as C. grandicorne Rond., were found by
Thorpe (1931) to be distinct from any that had been described. This species, recorded on Drosicha and Icerya seychellarum
Westw. is an effective parasitoid of the latter but does not attack I. purchasi
Mask. (Kuwana 1922). It is probable
that two species were involved and that the form from Icerya may prove to be one of those recorded on that genus in
other parts of the world. The
Australian C. iceryae Will, which attacks I.
purchasi, is the best known of the
genus. It was established in
California in 1888 from material shipped by Alfred Koebele, and proved very
effective in certain areas. Its
status as a biological control had been somewhat obscured by the more
conspicuous Rodolia cardinalis Muls., which was introduced
at the same time [please refer to Case History section for cottony-cushion
scale work]. Agromyzidae is a smaller
cosmopolitan family with about 1,010 species known by the year 2000. They are most numerous in the Palearctic. Important characters include a costa which
is broken at the end of S-c (or at or near end of R-1, if S-c vestigial or
fused with R-1). The first M-2 cell
is usually present, but cross-vein-like M-3 is often close to wing base;
femora often with conspicuous bristles.
The abdomen is often depressed, and the female ovipositor is sometimes
long and well sclerotized. Most species of Agromyzidae are
phytophagous, usually as leaf or stem miners of broad-leafed plants. All entomophagous species are primary,
solitary or gregarious endoparasitoids of nymphal and adult monophlebid scale
insects. One species, Cryptochaetum iceryae, has been widely used with considerable success in the
biological control of cottony cushion scale, being the dominant natural enemy
in coastal California (Quezada & DeBach 1973).
The family Agromyzidae is commonly referred to as the leaf-miner
flies, for the feeding habit of larvae, most of which are leaf miners on
various plants.
A worldwide family of approximately 2,500 species.The species are
small, some with wing length of 1 mm. The maximum size is 6.5 mm. Most
species are in the range of 2 to 3 mm.
Adult agromyzids can be recognized by the distinctive sclerotization
of the head. The upper part of the frons, above the ptilinal suture (known as
the frontal vitta) is lightly sclerotized and lacks setae, while the lower
part of the frons and the dorsal area of the head tends to be much more
heavily sclerotized and setaceous. Thus the frontal vitta often forms a
distinctive patch on the head, different in colour and texture to the rest of
the head. The compound eyes are usually oval and fairly small although in
some species they are larger and more circular.
The wings are usually hyaline although those of a few tropical species
have darker markings. A few species, including all Agromyza spp, are
capable of stridulation, possessing a "file" on the first abdominal
segment and a "scraper" on the hind femur.
Agromyzidae larvae are phytophagous, feeding as leaf miners, less
frequently as stem miners or stem borers. A few live on developing seeds, or
produce galls. The biology of many species is as yet unknown. There is a high
degree of host specificity, an example being Phytomyza ilicis, the
Holly leaf miner that feeds on no other species. A number of species attack plants of
agricultural or ornamental value, and are therefore considered pests. Biology & Behavior
Smith & Compere (1916) gave an early
account of the biology and behavior of Cryptochaetum
iceryae, which they recorded as Lestophonus monophlebi Skuse; and later in greater detail by Thorpe
(1931). Adults are sluggish and feed
mostly on honeydew. Freshly emerged
female flies have well developed eggs in the oviduct, and mating and
oviposition occurs within a short interval.
They prefer 2nd instar hosts and at oviposition the fly stands on the
host, inserting the ovipositor by a downward thrust. The 0.19 X 0.08 mm. egg is slightly kidney
shaped, and bears a minute funnel shaped micropyle at the larger anterior
end. The egg increases considerably in size during
incubation. The first instar larva is
caudate, although it is often referred to in this family as embryonic, for
the body is a transparent cylindrical sac 0.3-0.4 mm. long and slightly
curved, with little indication of segmentation and with the caudal segment
bifurcate and the lobes finger-like, with tips broadly rounded. There is no trace of a tracheal system or
of heart or sensory organs. There are
apparently no distinguishable mouth parts.
The 2nd instar larva is a bit cylindrical and has 10 distinct
segments. The tails are of markedly
variable diameter, a bit shorter than the body and they terminate
bluntly. Several short pointed
cuticular spines occur in a transverse row on the dorsum of the 2nd abdominal
segment, and similar spines completely encircle the following 5 segments,
almost completely covering the 6th and 7th abdominal segments. A simple closed tracheal system is
present, and the longitudinal trunks, with little evidence of branching,
extend from the first thoracic to the posterior margin of the 7th abdominal
segment. During the latter part of the
stage, usually 6 fine branches are developed in the expanded bases of the
tails. There is very little growth in
this stage, and no blood flow can be detected. The 3rd instar larva is similar in form to that
of the 2nd, the greatest difference being in the tails, which are now 1.5-2.0
times as long as the body. The bases
of the tails are greatly expanded, being as wide as the preceding body
segments, and the fragile filaments beyond the bulbs are of uniform
diameter. The tail's hypodermis in
this and preceding instars, consists of a single layer of cells with enormous
nuclei. The lumen of the tail is
filled with blood, although no circulation can be observed. The tracheal system is still closed, and
there is a dense network of fine branches just beneath the epidermis, several
of which extend into the tails for ca. 2/3rds of their length. Two transverse commissures occur in the
anterior part of the body and one in the last abdominal segment (Clausen
1940/62). The 4th larval instar is quite different from
preceding instars. It is very robust
and each of the 10 distinct body segments bears a band of minute setae. The tails are exceedingly long, being 3-4
times the body length. Except for the
basal bulb, the tails are very slender, kinked and irregular. The tracheal system is complete, with
spiracles on the anterior margin of the 1st thoracic segment and on the
dorsum of the 7th abdominal segment.
The anterior spiracles are pointed, dart-like structures, heavily
sclerotized and set in pits, while the posterior pair are distinctly dorsal
and are in the form of very heavy, dark hooks directed toward the head. The latter are apparently completely
closed, and the anterior pair is not open until late in the stage (Clausen
1940/62). At first the puparium is a pale yellow, but
finally becomes black. There are 10
distinct segments, and the dorsally located operculum extends to the
posterior margin of the 3rd segment.
The anterior spiracles are terminal in position when fully
extended. The tails remain attached
to the puparium, but they are shrunken and brittle. The prothoracic spiracles of the pupa are internal and do not
protrude through the integument. First instar larvae derive their food directly
from the host's body fluids, which are absorbed through a delicate
integument. The third stage feeds on
the fat body, and gross feeding occurs in the final stage. The larva is virtually incapable of
movement prior to the 3rd molt.
Before pupation, when the host's body contents have been largely
consumed, the anterior spiracles are extruded to their full extent and forced
through the host integument, usually at the lateral margin. The host integument then dries and closely
envelops the puparium. Because of
this, the skin is broken at the time the puparial operculum is raised,
allowing the adult fly to escape. There are 1-6 individual flies able to attain
maturity in each host. The life cycle
takes ca. one month, and 5-6 generations occur per year in California. Vayssiere (1926) and Thorpe (1934) studied
another species, Cryptochaetum grandicorne Rond. This is a solitary internal parasitoid of Guerinia serratulae F. in Europe.
There are several differences in the morphology of the immature stages
and in the manner of development when compared to C. iceryae. There are only 3 larval instars rather
than 4. The life history is well
adapted to the cycle of the host, and the pupal period of 6 months or more
covers the time in which the host is in the dormant phase. Oviposition occurs only in 1st instar
hosts after they have become fixed on the food plant (Vayssiere 1926, Thorpe
1934). The egg is longer and more curved and the
anterior end is relatively wider than in C.
iceryae. The increase in size during incubation is much less, and the
1st instar "embryo" larva is more elongate and may be distinguished
by a pair of unpigmented mandibles projecting from the open mouth. The 2nd instar larva has 11 body segments
and bears a transverse row of digitate spines on the dorsum and sides of the
3rd thoracic segment and 4 rows, completely encircling the body, on each of
the following 8 segments. The tubular
tails increase much in length during the 2nd stage. The tracheal system is similar to that for 3rd instar C. iceryae. This instar may persist for 3-4
months. The 3rd instar larva has the
tail filaments shrunken and often broken off, so that in the latter part of
the period they may be shorter than the body. The anterior spiracles are palmate and lightly pigmented, while
the posterior pair show an opening near the base of the spine. Just before pupation, both pairs of
spiracles are thrust through the host integument. Vayssiere (1926) found that the hook-like posterior spiracles
of the 3rd instar larva are fixed in one of the large tracheae of the host,
but the occurrence of this habit was not found by Thorpe (1934). The biology and behavior of Cryptochaetum sp. parasitic in Drosicha corpulenta and others of that genus in Japan differ in some
respects from the two species just discussed (Clausen 1940/62). Adult flies feed mainly on honeydew
secreted by Kermes miyasakii Kuw., which occurs on the
same trees as the host insect. There
are two generation per year, and the summer brood of females oviposit in the
young scales, passing the winter as young larvae within the living hosts. In this generation both male and female
scales are parasitized, with development being completed in springtime. These parasitized scales usually do not
leave their hibernating sites in crevices in the trunk of trees. Male hosts are in the prepupal stage at
the time of death, and emergence of the spring brood of Cryptochaetum coincides with that of male hosts, which is usually
10-25 May in central Japan (Clausen 1940/62). Females of the spring brood oviposit in Drosicha females, which at this time
have just completed their final molt.
Female parasitoids make a deliberate examination of host scales and,
when satisfied, stand with their fore- and middle legs on the lateral margin
of the body, bringing the ovipositor forward and inserting it just beneath
the margin. They seem to prefer the
thorax for oviposition. The hosts are very large, and even those from
which the overwintering brood emerges are large enough to provide sufficient
food for several parasitoids. However,
only one develops to maturity in each host, and the puparia are always
oriented with the anterior end toward the host's head. Cryptochaetum sp. eggs in Japan measure 0.35 X 0.08 mm., and
are thus much larger and more elongate than the eggs of either C. iceryae
or C. grandicorne. The mature
larva is 3.2-4.0 mm. long, with the caudal filaments measuring 4.0-5.5
mm. The latter are uniformly thick
for their entire length, except of the basal portions, which are
bulbous. The puparium is a deep red, which
shows through the host's integument.
The tails remain unbroken nd turgid even after emergence of the adult
fly from the puparium. Clausen (1940) noted that the life cycle given
above, in which the parasitoid passes through two generations annually on the
same host generation, is much different from that given by Vayssiere and
Thorpe for C. grandicorne, which has only a single generation annually and
which provides for the long summer period when hosts are not available, by undergoing
a prolonged pupal diapause. The reproductive capacity of C. iceryae
and C. grandicorne is about 200 eggs for both species. C.
grandicorne and the Cryptochaetum sp. from Japan are
solitary, while 1-6 C. iceryae develop in each female of Icerya purchasi. In Australia
the remains of a single female Monophlebus
showed 62 emergence holes of an undetermined Cryptochaetum (probably C.
monophlebi Skuse). The parasitized coccids were found
underneath the bark of eucalyptus trees and in the soil to a depth of 8
cm. Adult parasitoids extended their
emergence over 4 months (Clausen 1940/62). References: Please refer
to <biology.ref.htm>, [Additional references may be found at: MELVYL Library] Clausen, C.
P. 1940.
Entomophagous Insects.
McGraw-Hill Book Co., NY. 688 p. Cole, F.
R. 1969.
The Flies of Western North America.
Univ. Calif. Press, Berkeley & Los Angeles. 693 p. Frick, K. E.
1952. Univ. Calif. Publ. Ent.
8: 339-452. Frick, K. E.
1959.
Proc. U. S. Nat. Mus. 108:
347-465. Quezada, J.
R. & P. DeBach. 1973. Bioecological and population studies of
the cottony cushion scale, Icerya purchasi Mask., and its natural
enemies, Rodolia cardinalis Muls., and Cryptochaetum iceryae Will., in southern California. Hilgardia 41(2):
631-88. Smith, H. S. & H. Compere. 1916.
Calif. Comm. Hort. Mon. Bull. 5:
384-90. |