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Agriculture
Canada

Research Direction generate
Branch de la recherche

Technical Bulletin 1989-5E
AGRICULTURE CANADA
OPE 8<V03/3C NO.

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LinPATY/PIRLTCTriEQUE OT^'A K1A CC5

A review oi trie wheat blossom midge,
Skodiplosis mosellana (Gehin)

(Diptera: Cecidomyiidae)

in Canada

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Canada

Digitized by the Internet Archive
in 2013

http://archive.org/details/reviewofwheatblo19895bork

A review of the wheat blossom midge,
Sitodiplosis mosellana (Gehin)

(Diptera: Cecidomyiidae)

in Canada

A. BORKENT

Biosystematics Research Centre
Ottawa, Ontario

Technical Bulletin 1989-5E

Research Branch
Agriculture Canada
1989

Copies of ilus publication are available from

Director

Biosystematics Research Centre

Research Branch, Agriculture Canada

Ottawa, Ontario

K1A 0C6

Produced bv Research Program Service

f Minister of Supplv and Services Canada 1989
Cm No A34-8 1989-5E
ISBN 0-662-16765-1

Cover illustration

The Hois on the map represent

Agriculture Canada research

establishments.

CONTENTS

Abstract /Resume 2

Material and methods 3

History in Canada 3

Way of life 4

Damage and control 7

Description 9

Acknowledgments 13

Figure captions 14

References 15

- 2 -
Abstract

The literature dealing with the morphology, biology and control of the
wheat blossom midge, Sitodlplosis mosellana (Gehin) is summarized. Each life
stage is diagnosed to allow easy recognition.

Resume

On a resume la litterature concemant la morphologie, la biologie et
la lutte contre la cecidomyie du ble (Sitodiplosis mosellana (Gehin)).
On a fait la diagnose de son cycle vital pour faciliter 1, identification.

- 3 -

In recent years the wheat blossom midge, Sitodiplosis mosellana (Gehin),
has regained notoriety as a serious pest of Canadian wheat. In 1983 some
areas in northeastern Saskatchewan had an average kernal infestation of 50%,
with losses estimated at over 30 million dollars due to the damage caused by
this insect (Olfert et al . , 1985). In that year over two million acres of
cropland were infested and this had risen to over nine million acres in 1984
(Harvey, 1986). In 1984 some areas in northwestern Manitoba reported grain
losses as high as 26% (Barker, 1984a, 1984b). In spite of this level of
impact, the wheat blossom midge is a relatively poorly known pest of wheat.
Virtually no studies have been completed in North America and we presently
depend nearly entirely on European investigations of its biology, morphology
and control. Even its Nearctic distribution has been poorly investigated and
its widespread occurrence has escaped notice by many farmers and
entomologists. Shrivelled kernels of wheat that are passed off as "seconds"
by producers, are often caused by this little fly.

This paper reviews our present knowledge of Sitodiplosis mosellana.
Hopefully, this contribution will provide impetus for further research on this
serious pest by allowing for easy recognition of each of its stages in both
the field and laboratory and by summarizing what we know about the biology of
the midge.

Material and Methods:

Specimens were examined from a variety of localities in Canada including
samples from B.C., Saskatchewan, Manitoba, Ontario, and Quebec. In addition,
Palearctic specimens from Finland, England, Germany, Switzerland and Japan
were also examined.

Specimens were collected directly from wheat heads (larvae) and by
sweeping (adults). Some adults were reared in the laboratory from larval
cocoons collected in the field. Cocoons may be extracted from soil using the
method described by Doane et al. (1987).

Specimens were collected into 70% alcohol and mounted in Canada Balsam
using the technique described by Saether (1969) and developed by Leo Forster
of our centre. Larvae are best cleared if a small puncture is made in the
body wall.

To find larvae attacking wheat heads, the kernels need to be separated
from the surrounding glume and lemma. Larvae are located either on the kernel
itself, at the base of the kernel, or on the inner surface of the glume. Once
located, the larvae may be recognized by their small size of up to 3.1 mm in
length and distinctive orange or yellowish-orange color.

History in Canada

The wheat blossom midge was first reported in Canada from near Quebec City
in 1819 (Sanderson, 1915). Fitch (1865) stated that the midge was first
noticed in the U.S.A. in 1820 in northwestern Vermont where by 1828 it had
become a serious pest. He further documented its spread through the
northeastern United States. By 1834 the wheat blossom midge was causing
severe damage in the Montreal area and by 1849 it had reached the north shore
of Lake Ontario. Its rate of spread was estimated at about 14 km per year.
Gorham (1927), in discussing the history of the spread of the midge in New
Brunswick, found the first records to be in 1841 or 1842. He claimed that it
was first noticed in Kings County and spread from there across the province.
In 1854 there were marked losses in Nova Scotia and the total losses in Canada
in 1856 were estimated at $2.5 million. By that time the midge had extended
its range to west of Toronto.

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Since Fitch's report (1865), there have been few reports of s. mosellana
from within in Canada. Ross reported it from southern Ontario in 1919.
Barker (1984a, 1984b) states that S. mosellana was first reported in
Manitoba in 1954. S. mosellana was first reported from the Fraser Valley of
British Columbia in 1904 (Fletcher, 1905). It apparently was stopped from
spreading eastward by the Cascade Mountains (Reeher, 1945) but did spread
south in Washington State. This pattern suggests that the wheat midge may
have been introduced independently into western and eastern North America.

In spite of these early reports that the pest could cause serious damage
($15 million worth of damage was reported in New York in 1954), very little
research on the wheat blossom midge was done in Canada between the years
1958- 1982. In 1983 and subsequent years, the losses incured in Saskatchewan
and Manitoba resulted in estimates of damage by J.F. Doane, P.S. Barker and
others. Discovery of widespread damage by the midge in Quebec by C. Ritchot
led to investigations of its distribution in that province.

Our records (Fig. 1) show that S. mosellana is now broadly distributed
in southern Canada. The lack of records in Alberta may be due to lack of
collecting. A record from near Pincher Creek, Alberta was given on a map in
the Canadian Insect Pest Review (Vol. 2) as well as several further records
from interior B.C., but I have been unable to confirm these. The localities
from interior B.C. that are included on my map are taken from Arrand (1959).
I have examined material from Kersley, B.C. collected by D.A. Arnott.

S. mosellana is known elsewhere from the USA, Europe, northern Africa,
Israel and Japan.

Way of Life

Sitodiplosis mosellana larvae have been recorded feeding on a variety of
grasses, including wheat, barley, rye, couch grass and other members of the
grass family. Barnes (1956) found S. mosellana females ovipositing on
Slender Foxtail grass (Alopecurus myosuroides) in England. Yukawa (1971)
reported the midge on Triticum aestivum L., Hordeum vulgare L., Agropyron
ciliare, and Agropyron tsukushiense , in Japan. Keller and Wilding (1985)
found S. mosellana feeding on Phalaris arundinacea L. in Switzerland.

Adult S. mosellana emerge in May, June or July, probably depending on
locality and weather conditions (Barnes, 1956), when the wheat ears are about
to, or are flowering. The length of the emergence period in Canada has been
estimated to be about three weeks. Katayama et al. (1987) reported an
emergence period of about 18 days in Kyoto Prefecture, Japan. Barnes (1956),
on the basis of a 26 year study, showed that the emergence period in
Harpenden, England, varied from 14-42 days. Basedow and Schutte (1982)
repotted an emergence period of up to seven weeks in West Germany. Because
larvae can diapause for a number of years, any year's adult emergence may be
composed of a number of generations.

Reeher (1945) reported that caged adults in British Columbia mated the
same day as they emerged. He also suggested that, after mating, the females
disperse while the males remain near the sight of emergence. Hence,
populations in fields that are newly infested by adults are almost all
females. Otherwise, nothing is known about mating in S. mosellana.

Sex pheromones have been described for only two species of Cecidomyiidae.
McKay and Hatchett (1984) have shown that females of the Hessian fly,
Mayetiola destructor (Say), release a sex pheromone to attract the male.
Lee and Lee (1985) described a similar phereomone for Thecodiplosis
japonensis Uchida and Inouye. As female pheromones have also been identified
in the sister group of the Cecidomyiidae, the Sciaridae (Girard et al. 1974)

- 5 -

as well as in many other Diptera, it seems likely that S. mosellana females
also use pheromones as sex attractants.

Basedow and Schutte (1982) reported a sex ratio of 1.2-1.5 females per
male in a laboratory population in West Germany. Emergence cages in the
field, however, gave a sex ratio of 1:1.

The female, using her highly extendable ovipositor, lays about 30-133
eggs. Basedow and Schutte (1982) found an average of 39-40 eggs per female.
These eggs are laid in clutches of 1-10 underneath the glume. Basedow and
Schutte (1973) showed that the eggs are laid on wheat ears which are free on
both sides from the boot (= uppermost leaf sheath of the comb) but which have
not yet blossomed. Although Barnes (1956), working in England, reported eggs
right on the young kernel and Fitch (1865), in New York State, observed
females laying eggs after the anthers had withered, Basedow and Schutte (1973)
state that such eggs do not complete development to the pupal stage.

More than one female may lay eggs on a given glume, which results in some
cases, in higher numbers of larvae per kernel. Females generally do not lay
during the day but rest near the base of the wheat stem with their heads
directed upwards. Beginning in late afternoon females fly amongst the wheat
heads and deposit their eggs. Wagner (1866) found adults flying throughout
the night. Fitch (1865) reported that the females will also lay during damp,
cloudy days and suggested that the midges are susceptible to dessication on
dry, sunny days and therefore avoid oviposition at that time. He found that
wheat in the shady areas of fields were also susceptible to ovipositing
females even on sunny days. This may explain the higher levels of infestation
in such areas reported by several workers (Felt, 1921; Fitch, 1865).

Basedow (1977a) found that females disperse downwind above the crop, and
upwind below the top of the crop level. Windy conditions are an impediment to
oviposition. Oakley (1981) noted that females do not fly at wind speeds of
over 10 km per hour or when temperatures fall below 15°C. Adults live 1-3
days under field condition (Basedow and Schutte, 1982).

The eggs hatch after about one week and the first instar larvae move to
the surface of the kernel and feed on sap. Oakley (1981) states that eggs
require more than 10 days at 15°C and less than four days at 20°C to
hatch. Some larvae feed on sap from the plant tissue supporting the seed.
Fitch (1865) described how more mature larvae can move from one kernel to
another, especially during wet, warm and calm weather conditions.

Larvae mature in 2-3 weeks and the fourth (and last) larval instar then
encase themselves in the third instar larval exuvia. Fitch (1865) suggested
that this response is facultative and that larvae may not form a case under
wet conditions but this has not been confirmed by any other worker. This
encasement protects the larva from dessication and they remain in this state
till wet weather conditions (either rain or heavy dew), generally about mid
August (Basedow, 1977c; Katayama et al., 1987). The larvae then escape
their case and drop to the soil. Fitch (1865) states that the larvae get to
the soil surface in two different ways. The first is by "skipping", a
behaviour peculiar to some cecids in which the larva bends its body in a U and
with a sudden motion, flips into the air. The second route to the ground was
by wriggling in moisture on the plant stem during wet weather and thereby
working its way down to the surface. Barnes (1956) suggested that the larvae
of S. mosellana were not able to skip. Wallengren (1937) stated that larvae
require 1.5-3 hours to penetrate into loose soil. Basedow (1973), however,
found that soil conditions were a determining factor in the time the larvae
took. He found that after a strong rain, larvae needed only 10 minutes to
penetrate the soil. After a light rain, an average of 3 hours was required.
Soft soil clearly helped the larvae in their migration from the wheat head to
below the soil surface.

- 6 -

The last instars form a globular, whitish-gray or pale yellow cocoon below
the soil surface and overwinter in diapause. Golightly (1952) found that
about 80% of cocoons were in the top three inches of soil but some were
present to a depth of six inches. The larvae in their cocoons are very
resistant to dessication (Wallengren, 1937). Basedow (1980) in Germany, found
that winter mortality ranged from 0-81% in Germany. In spring, if soil
conditions are wet enough, the larvae leave their cocoons through a little
hemispherical lid and pupate near the soil surface. According to Waede
(1959), larvae may build another cocoon in which to pupate but not all larvae
did so. The pupal period is about two weeks long and, as shown by Katayama
et al. (1987), the length of the pupal period is directly related to
temperature.

If spring conditions are not favorable, the larva may remain in diapause.
Barnes (1956) recorded larvae remaining in such a state for as long as 13
years before successfully reaching adulthood. Golightly (1952) extracted
cocoons from soil in England and found large numbers of cocoons in fields
seven years after they had last been planted to wheat. Basedow and Schutte
(1982) found that 7-57% of the larval population of S. mosellana may remain
in extended diapause. Basedow and Gillich (1982) found up to 96% of the
larval population may experience extended diapause. However, Wallengren
(1937) found substantial mortality (22-44%) of larvae in the first winter of
diapause. Basedow and Schutte (1971) found 28% mortality.

Basedow (1977c), in a detailed study, found that diapause is terminated by
temperature change of warm to cold to warm and subsequent high soil moisture.
Soil temperature must be below 10°C for at least 120 days followed by
warming and by moist soil conditions in the spring.

Wellso and Freed (1982) suggested an association between Sitodiplosis
mosellana larvae and a fungus called glume blotch, Septocia nodorum (Berk.)
Berk.. Their conclusion was based on the discovery that larvae were more
common in wheat heads also infected with the glume blotch. That the larvae
may have been feeding on the fungus is unlikely and it may well be that the
damage done by the larvae merely allowed the glume blotch to infect the
plant. Doeksen (1938), in the Netherlands, also reported heavy fungal and
bacterial infections after attack by wheat midge larvae.

Until recently, little was known about the population dynamics of S.
mosellana. Basedow and Schutte (1982) provided an extensive report on their
studies of two populations of S. mosellana in West Germany. Populations
were negatively affected by four main factors: parasites, low precipitation
during larval migration from the wheat to the soil, length of larval diapause,
and lack of synchrony between adult emergence and the susceptible stage of the
wheat. There was no evidence for intra or interspecific competition.
Finally, they found that extended larval diapause had a levelling effect on
the total population size. Barnes (1956) reported, on the basis of 28 years
of observation in England, that the midge populations have a five year
periodicity, with some anomalies in the data apparently attributable to
peculiar weather conditions. In Germany Basedow (1980) found that the
population could increase as much as 6 fold in one year. Basedow (1977a)
showed that the edges of wheat fields were more heavily infested than was the
centre. Felt (1921) found that populations of the wheat midge were higher in
lowlands and river flats, where there was greater moisture than on dry
uplands. Basedow and Gillich (1982) could find no means of predicting of the
number of adult midges in a large area, although it might be possible at the
local level.

- 7 -
Damage and Control

Most reports of damage by S. mosellana concern wheat, but Felt (1921)
reported extensive damage on rye in Albany Co., New York. Wright and Doane
(1987) showed that in NE Saskatchewan, annual canarygrass and oats were
infestation free, six row barley was only slightly infested, while triticale,
rye, wheat and durum wheat were all heavily infested.

The most immediate effect of S. mosellana infestation is the shrivelling
of the wheat kernel. In severe damage, the deformed kernel exhibits
longitudinal grooves in the pericarp and loss of both the endosperm and germ.
Generally, infestation leads to only moderate shrivelling, still making the
product unacceptable for human consumption.

Dexter et al. (1987) reported the details of the loss of quality and
corresponding changes in biochemical and commercial properties in kernels of
Canadian hard red spring wheat infested by wheat midge larvae.

Apparently, the infestation of a given kernel in a wheat ear has no impact
on the rest of that ear. Barnes (1956) described the relationship between the
number of larvae per ear and the ultimate impact on the kernels. Because
larvae attack individual kernels, infestation should not be described as
larvae per ear but rather as the percentage of kernels which are infested
(Lubke and Wetzel, 1984). For example, Barnes (1956) shows how analysis of 19
samples with 100% ear infestation had kernel losses ranging only from
8.5-17%. Olfert et al . (1985), in Saskatchewan, described a relationship
between average number of larvae per kernel and level of infestation. An
average of 1,2,3, and 4 larvae per kernel corresponded to infestation levels
of 38, 58, 78, and 96% respectively. In addition, infestation levels were
directly correlated with amount of yield.

Both Reeher (1945) and Olfert et al. (1985) found that one larva per
kernel resulted in a shrivelled grain and 3-4 larvae destroyed the kernel.
Barker (1986), in studies completed in Manitoba, suggested that at least 447
heads of wheat be sampled to estimate within 10% accuracy, the losses caused
by infestation.

Basedow (1980) found that the economic threshold in the German fields he
studied was 6 larvae/100 cm^ of soil, when sampled in April. Basedow and
Schutte (1973) pointed out discrepancies in previous reports on the level of
the economic threshold. They considered that losses of 200 kg per hectare to
be the economic threshold. Arnott and Arrand (1964) suggest that 30 larvae
per ear resulted in losses of 300 kg per hectare (as calculated by Basedow and
Schutte, 1973). Buhl and Schutte (1971) claim 15 larvae resulted in losses of
4 dt/ha. Basedow and Schutte (1973) claim that 13 larvae per ear is the level
where control becomes profitable. The uncertainty about the economic theshold
indicates that more studies are needed.

Levels of infestation can differ widely from area to area. Barker (1984a,
1984b) found S. mosellana to be absent in some fields in Manitoba but in
others (The Pas) to have as high as 26% grain losses. In Saskatchewan in 1983
some districts showed a 50% kernel infestation. Andersson and Nilson (1980)
reported losses averaging 500 kg per hectare in Finland. Helenius et al .
found about 20% of all ears infected with a 1.7% loss of kernels. Other
recent reports of damage from Europe are by Nijveldt and Bokhorst (1973) for
the Netherlands and Golebiowska (1980) for Poland.

There have not been any formal attempts to develop cultural methods which
would be effective against the wheat blossom midge (Barnes, 1956). European
workers have suggested that both deep plowing or burning of stubble might be
used to control the midge but their effectiveness has never been
demonstrated. Oakley (1981) pointed out that regular rotation of the wheat

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crop will keep the infestation of the wheat midge at low level- When a severe
infestation occurs, that field and adjoining fields should not be planted to
wheat or rye the next year. The basis of such recommendations, however, has
not been thoroughly tested.

Felt (1921) studied wheat fields in New York and found that there was no
apparent difference in levels of infestation between early or late plantings
of winter wheat. He also found that fields of wheat which were isolated from
other fields were equally susceptible to attack. In addition, fields in
lowland areas and river flats tended to be more heavily infested than were
those in dry uplands. In the Fraser Valley of British Columbia, Reeher (1945)
found that winter wheat and spring wheat seeded by the first week of April
escaped serious infestation. These data suggest that cultural practices may
be effective in some areas but not in others.

Felt (1921) pointed out that the long flight period and broad distribution
of the midge may make it impossible to employ cultural methods successfully
but this is at least partially contradicted by the results reported by Reeher
(1945), above.

Fitch (1865) described how in Seneca Co., New York no wheat was planted in
1858 due to the impact of the wheat midge in previous years. When in 1859 and
1860 wheat was once again planted, the wheat midge was absent from the crops.
However in 1861, the wheat was once again heavily attacked. This suggests
that failure to plant wheat in 1858 may have markedly reduced the population
levels of S. mosellana.

Different varieties of wheat show different levels of suceptibility to
attack by the wheat midge. In Finland Helenius et al . (1984), found variety
Jo 8187 was particular susceptible. Westdal (1957) also found that of the
wheat varieties he investigated in Manitoba, only Frontana was midge free,
because this wheat variety is a late bloomer and probably escapes the
oviposition period of the midge (Barker, 1984b). This characteristic may,
however, make this variety unacceptable as a means of controlling infestation
levels. Basedow (1977b), in West Germany, suggested likewise, that the wheat
varieties Kolibri and Kleiber may be less susceptible to attack. Wright and
Doane (1987) found little significant difference in the wheat cultivars they
tested in Saskatchewan, which included Kolibri. Basedow and Schutte (1974)
suggested that the varieties Farino, Florian and Starke showed some resistance
to infestation by S. mosellana. These varieties may have avoided
infestation because the correct stage of ear development didn't coincide with
adult oviposition. Merely planting wheat varieties which flower at different
times than the emergence of adult S. mosellana would probably provide only
temporary relief. Basedow (1972) discusses how early flowering rye in West
Germany has selected for an early flying population of S. mosellana.

Information on parasites and diseases is rather limited. Wellso and Freed
(1982) reported two species of Platygaster (Platygasteridae) were present in
some of the larvae they examined from Michigan. Affolter (1986), in a study
to uncover possible European biological controls to introduce into Canada,
found several parasites in a Swiss population of S. mosellana. These were:
Platygaster tuberosula Kieffer, Platygaster sp., Isostasius inserens
Kirby (Platygasteridae), Piestopleura thomsoni Kieffer (Platygasteridae),
Leptacis tipulae (Kirby) (Platygasteridae), and Pirene penetrans (Kirby)
(as Macroglenes penetrans (Kirby) (Pteromalidae) . P. thomsoni is a
hyperparasite of P. penetrans . Doeksen (1938) also reported exactly these
same species as parasitic on S. mosellana in the Netherlands. Affolter
(1986) reported 18-53% death of the midge hosts. Basedow and Schutte (1982),
in West Germany, found the following parasites: Pirene penetrans (Kirby),
Platygaster tuberosula Kieffer, Euxestonotus error (Fitch)

- 9 -

(Platygasteridae) , Leptacis tipulae (Kirby) and Isostasius punctiger
(Ness.) (Platygasteridae). They found 9-74% of the population parasitized and
a strong relationship between parasite and host population levels. Fitch
(1865) lists the following parasites from France: Isostasius inserens (as
Inostemma inserens) (egg), Leptacis tipulae (Kirby) (egg), and Pirene
penetrans (as Maccoglenes penetrans) (larva?). Euxestonotus error (as
Platygaster error) was recorded from the USA. Reeher (1945) found
Inostemma horni Ashm. ovipositing on the eggs of S. mose liana in
northwestern Washington state.

In England Golightly (1952) found that parasitism of larvae in wheat ears
was 25-58%.

Reeher (1945) found the mite Atomus pilosus Banks ate many eggs of the
wheat midge in B.C.

Doeksen (1938) found a polyhedral disease on larvae of S. mosellana in
the Netherlands, which has not been recorded in the New World. Keller and
Wilding (1985) discovered a fungal pathogen infesting adults of the wheat
midge in Switzerland.

The impact of predatory insects is poorly known. Stark and Wetzel (1987)
reported on the significant impact of predation by empid flies of the genus
Platypalpus on a German population of S. mosellana. Basedow (1973) found
that ground dwelling predators in West Germany, mostly carabid beetles,
accounted for losses as high as 43% of larvae which were migrating to the soil
from the wheat heads. At times of heavy precipitation, the losses were much
less, probably because the larvae can enter the soil more quickly under such
conditions. In addition, carabids become inactive under regular or heavy
rains (Henri Goulet, pers. comm.).

It would be valuable to further investigate the enemies of the wheat midge
in Europe and North America to test for the possibility of introducing some of
these locally. Considering that S. mosellana is at levels below the
economic threshold in many places in Europe, it may be expected that
introduction of parasites, diseases, or predators from there may provide
partial control in some areas in Canada. In addition, it may be fruitful to
investigate populations of S. mosellana in the Middle East, if they occur
there, because this is the area of origin of wheat and possibly of the wheat
midge also.

Description

Barnes (1956) discussed the problems of correctly identifying the wheat
midge in North America. Another pest in Europe, Contarinia tritici (Kirby),
is very similar in habits to S. mosellana. Felt (1912) compared European
and Nearctic material and considered the North American wheat midge to be
conspecific with European S. mosellana. He also recognized two other
species as occurring on wheat in North America. I have examined larval and
adult material from Europe, Japan and numerous place in Canada and found no
morphological difference between any of these samples. All specimens
attacking wheat in Canada were identified as S. mosellana. C. tritici has
never been recorded in North America. I have not seen any material from wheat
referrable to the other two species recorded by Felt (1912), Contarinia
fitchii (Felt) and Itonida tritici Felt.

In Canada there is no way of recognizing any stage of S. mosellana with
the naked eye, other than by locating the little orange or yellowish- orange
larvae in the layer between the glume and the kernel of wheat. No other
species of cecidomyiid in North America has been discovered attacking wheat in
that way. Otherwise, certain identification can only be determined with the

- 10 -

examination of detailed morphological structures. This section provides such
diagnoses.

Morphological terms follows those given by Gagne (1981) with additional
details for larval structures as used by Mamaev and Krivosheina (1965). Cecid
workers have traditionally used the term papillae for body sensilla, but I
have followed the broader use of sensilla as used in other groups of Diptera.

The different larval instars differ in a number of important characters
and these are described by Borkent (in press). Otherwise a number of authors
have described aspects of this midge's morphology (Barnes, 1928; Basedow,
1971; Doeksen, 1938; Harris, 1966). Photographs of live eggs, larvae, larval
cocoons, pupae and adults are shown in Co-operative Agricultural Extension
Service Publication, Saskatchewan Agriculture, "Orange Wheat Blossom Midge
Sitodiplosis mosellana" . Borkent (in press) provided a key of the larval
instars of S. mosellana.

The following provides a diagnosis of each stage, which will allow
recognition as compared to any other cecidomyiid. However, because of a lack
of comparative data the egg and pupa cannot be fully characterized at this
time.

It is important to note that fourth instar larvae may be found encased in
a whole or part of a third instar exuvia. This requires special care when
identifying distinguishing characters of a given individual.

Egg: (Based on Fitch, 1865). 6 times long as wide; long oval form, nearly
cylindrical for most of its length, rounded apices; surface smooth, shiny;
colorless to a faint tinge of pale red.

First instar larva: Total body length 0.39-1.59 mm; cuticle of dorsal body
surface with contiguous scales (Fig. 3A); head capsule with minute dorsal egg
tooth, with well developed cephalic rods which are about as long as the basal
width of the head capsule; antenna relatively thick and short, narrowed and
rounded only apically; spatula (not evident in some specimens) with 2 teeth;
sublateral sensilla of meso and metathorax simple (not setiform); prothoracic
spiracles located dorsolaterally; dorsal sensilla well developed setae on
thoracic segments and abdominal segments 1-8; abdominal tergites 1-7 with
total of 6 dorsal setae, lateral and medial pair larger than remaining two;
eighth abdominal segment with 2 dorsal setae between spiracles; abdominal
segments 1-9 with elongate lateral seta, about 2.8-4.8 times longer than
spiracle width; posteroventral sensilla of abdominal segments 1-7 simple, not
setiform; terminal segment (Fig. 2A) with 8 setiform sensilla on tubercles,
terminal 4 setae squat, blunt and of nearly equal length, lateral pair
slightly smaller than terminal 4; ventral pair of setae elongate (length/ head
capsule width = 1.3-2.0).

Second instar larva: Total body length 1.32-1.67 mm; cuticle of dorsal body
surface with contiguous scales (Fig. 3B); head capsule with well developed
cephalic rods which are about as long as the basal width of the head capsule;
antenna relatively thick and short, narrowed and rounded only apically;
spatula with 2 teeth; sublateral sensilla of meso and metathorax simple (not
setiform); prothoracic spiracles located dorsolaterally; dorsal sensilla well
developed setae on thoracic segments and abdominal segments 1-8; abdominal
tergites 1-7 with total of 6 setae, lateral and medial pair larger than
remaining two; eighth abdominal segment with 2 dorsal setae between spiracles;
abdominal segments 1-9 with lateral seta about 3.7-4.8 times longer than
spiracle width; posteroventral sensilla of abdominal segments 1-7 simple, not
setiform; terminal segment (Fig. 2B) with 8 setiform sensilla on tubercles,

- 11 -

terminal 4 setae with inner pair elongate and thin, lateral pair much smaller
than outer terminal pair, ventral pair of setae elongate (length/ head capsule
width = 1.2-1.7).

Third instar larva: Total body length 1.94-3.08 mm; cuticle of dorsal body
surface with fine, discrete spicules (Fig. 3C); head capsule with well
developed cephalic rods which are about as long as the basal width of the head
capsule; antenna relatively thick and short, narrowed and rounded only
apically; spatula with 2 teeth; sublateral sensilla of meso and metathorax
simple (not setiform); prothoracic spiracles located dorsolaterally ; dorsal
sensilla well developed setae on thoracic segments and abdominal segments 18;
abdominal tergites 1-7 with total of 6 setae, lateral and medial pair larger
than remaining two; eighth abdominal segment with 2 dorsal setae between
spiracles; abdominal segments 1-9 with lateral seta about 1.0 1.7 times longer
than spiracle width; posteroventral sensilla of abdominal segments 17 simple,
not setiform; terminal segment (Fig. 2C) with 8 setiform sensilla on
tubercles, terminal 4 setae with inner pair squat, blunt but smaller than
outer pair; lateral pair much smaller than outer terminal pair; ventral pair
short (length/ head capsule width - 0.37-0.69).

Fourth instar larva: Total body length 1.58-3.07 mm; cuticle of dorsal body
surface with large, contiguous scales, especially laterally (Fig. 3D); head
capsule with well developed cephalic rods which are about as long as the basal
width of the head capsule; antenna relatively thick and short, narrowed and
rounded only apically; spatula with 2 teeth; sublateral sensilla of meso and
metathorax simple (not setiform); prothoracic spiracles located
dorsolaterally; dorsal sensilla well developed setae on thoracic segments and
abdominal segments 1-8; abdominal tergites 17 with total of 6 setae, lateral
and medial pair larger than remaining two; eighth abdominal segment with 2
dorsal setae between spiracles; abdominal segments 1-9 with lateral seta about
0.6- 1.5 times longer than spiracle width; posteroventral sensilla of abdominal
segments 1-7 simple, not setiform; terminal segment (Fig. 2D) with 8 setiform
sensilla on tubercles, terminal 4 setae squat, blunt and of nearly equal
length, lateral pair much smaller than outer terminal pair, ventral pair short
(length/ head capsule width = 0.35-0.68).

Pupa: Antennal horn rounded, with low, posteriorly directed projection;
frons with elongate seta and reduced sensilla on tubercle; 2 setae at base of
clypeus; 4 reduced sensilla near anterior margin of wing base; 1 elongate and
1 short seta subapically on palp; patch of 3 sensilla near base of palp;
respiratory organ (Fig. 4A) elongate (0.24- 0.32 mm in length), narrrow, with
slight curve, 1 reduced sensilla near base; post no turn with patch of 3 reduced
sensilla dorsolaterally; tergites 2-8 (Fig. 4B) with patch of thick, stout
spicules anteriorly, patch of fine spicules posteriorly.

Male: Third flagellomere binodal, bifilar, with long circumfila loops (Fig.
5A); postvertical peak present (Fig. 5C); wing with Rt, joining C beyond apex
of wing (Fig. 5E); claws simple, bent beyond midlength, slightly longer than
empodium (Fig. 5D); genitalia (Fig. 6A, B): cerci and hypoproct bilobed,
aedeagus same color as rest of genitalia (not darkened) and extending beyond
hypoproct, gonocoxite with mediobasal lobe, gonostylus of nearly equal
diameter but tapering slightly on apical 1/4.

Female: Third flagellomere slightly binodal, bifilar, with closely appressed
circumfili (Fig. 5B); postvertical peak present (as in Fig. 5C); wing with

- 12 -

R5 joining C beyond apex of wing (as in Fig. 5E); claws simple, bent beyond
midlength, slightly longer than empodium (as in Fig. 5D); genitalia (Fig. 6C):
with somewhat protrusible ovipositor, sternite 10 small, cerci separate,
large, elongate.

- 13 -
Acknowledgements

I appreciate the assistance of several curators and colleagues who sent
material of Sitodiplosis mosellana for me to examine: Dr. P.S. Barker
(Manitoba), Mr. C. Ritchot (Quebec), Dr. K.M. Harris (Britain), Mr. B.C.
Townsend (BMNH, London), Mr. B.J.H. Brugge (Instituut voor Taxonomische
Zoologie, Amsterdam, Nederland), Dr. J. Yukawa (Kagoshima University, Japan).

I also appreciate the use of the distributional data freely provided by
Mr. C. Ritchot for Quebec.

Without the support of the staff of the BRC this work could not have been
completed in the time it was. I am grateful for the following assistance.
Mrs. Barb Bissett did an SEM study of larval structures, measured larval and
pupal material and completed the distribution map. Mr. Leo Forster completed
the mounting of specimens on microscope slides with his usual skill and
dexterity. I also thank Mr. Barry Flahey for providing the line drawings in
this paper. Dr. Lubomir Masner kindly checked on the proper names for the
Hymenoptera parasites.

Finally, I am grateful for the comments and criticisms of this paper
provided by Drs. Jeff Cumming and Henri Goulet.

- 14 -

Figures

1. Distribution map of Sitodiplosis mosellana in Canada.
. 2. Terminal segment (9) of larvae of Sitodiplosis mosellana. (A) first
instar; (B) second instar; (C) third instar; (D) fourth instar.
. 3. Photographs of cuticle of lateral portion of tergite seven of larvae
of Sitodiplosis mosellana. (A) first instar; (B) second instar; (C)
third instar; (D) fourth instar.

. 4. Pupal structures of Sitodiplosis mosellana. (A) respiratory
organ; (B) tergite seven.
Fig. 5. Structures of adult Sitodiplosis mosellana. (A) male third

f lagellomere; (B) female third flagellomere; (C) male head capsule; (D)
male fifth tarsomere of foreleg; (E) male wing.
Fig. 6. Structures of adult Sitodiplosis mosellana. (A, B) male genitalia;
(C) female genitalia.

Fig.
Fig.

Fig.

Fig,

- 15 -
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