| dc.description.abstract | The genus Dysdercus Boisduval is widespread throughout
the tropics and like the other Pyrrhocoridae, it associates
primanly with plants of the order Malvales, with both the
immature and adult stages feeding on the seeds. This genus
includes a number of species that are pests of cotton. Owing
to its economic potential, a number of workers have therefore
carried out field studies to show the relationship of the cotton stainers to their host plants. However, comparatively little work has been done on the reproductive biology of the insects,
particularly in relation to the development of the oocytes, oviposition
and fecundity in females kept under controlled laboratory
conditions.
A brief histological study of the ovary indicated that
as in most other hernipterans, the ovary is made up of seven
telotrophic ovarioles; the developing oocytes are nourished by
long nutritive cords that originate from the trophic core of the
gerrnartum,
Some aspects of the reproductive biology of the females
were carried out on three closely related species, Dysdercus
fasciatus Sign., D. nigrofasciatus St~l, D. cardinalis Gerst.
thus allowing comparisons of the reproductive patterns as a
function of variations in both the bionomics and the flight
activities of the species. Of the three species, D. fasciatus
was found to have the shortest pre-copulation period and the
earliest mean age at oviposition. Since it is restricted to a
relatively few host plants which have a distinct fruiting season
and provide a large supply of food during a given period, it is
likely that maximum advantage of the current favourable conditions
is taken by reducing both the pre-copulation and preoviposition
period. D. nigrofasciatus on the other hand, inhabits
a greater variety of alternative host plants and hence less
premium 1s placed on reproduction. Laboratory studies have
indicated that the pre-oviposition period of D. nigrofasciatus
is significantly longer (p < O. 02) compared to D. fasciatus.
The bionomics of D. cardinalis is little known but reproductive
studies, have indicated that as in D. nigrofasciatus , both the
pre-oviposition and pre-copulation period is longer compared
to D. fasciatus.
Of the several factors that control reproduction in
insects, emphasis here has been placed on only two of these, namely,
mating and nutrition. Observations on mated and virgin females of
Dysdercus indicated that in both..!2:....fasciatus and D. nigrofasciatus
there were no significant differences in the adult longevity, the rate
of oocyte development, pre-oviposition period, inter-oviposition period
and the total number of eggs laid during the adult life. In D. cardinalis
however, mating had a profound effect on the rate of development of
the oocytes and hence oviposition. The other factors concerned with
reproduction, as in the other two species were not influenced by mating.
The various species of Dysdercu& "hence show a varying degree of dependenee
on mating, but the latter did not have such a profound effect
on egg production and oviposition as was expected. This study indicated
that although a characteristic feature of the reproductive biology
of the cotton stainers is their long and continuous copulation during
the pre- and inter-oviposition period, this sustained copulation is not
necessary for egg production and hence needs further investigation.
In contrast to mating, nutrition as in most other insects
had a profound effect on the development of the oocytes. Although
the nymphal stages were fed throughout their period of development,
the newly emerged females had to be fed for a minimum
pericxi of 3 - 4 days before the first batch of eggs could be
developed and oviposited. Following each ovtpositiorr , a
new focxi intake is necessary for continued egg prcxiuction.
Even under poor nutritional conditions, there were no signs
of oocyte degeneration indicating that oosorption is not of
any importance in the regulation of egg prcxiuction. In
absence of feeding, the females could maintain life on water
but oocyte development in all three species occurred up to
stage where yolk deposition normally occurs. Henceforth, there
was no further development- and' the oocytes remained small and
transparent. Mating was drastically reduced in starved bugs but
of the few pairs that copulated, matingdid not have any effect
on the development of the oocytes, even in D. cardinalis, indicating
that both nutritional and mating stimuli act independently of each
other. Detailed studies would be necessary to indicate whether
the developing oocytes are dependent on an active neurosecretory
system or available focxi reserves or both.
Quantitative analysis of the haemolyrnph proteins in
D. fasciatus females has indicated that some of these proteins
are utilized for oocyte development. The haemolymph
protein concentration of newly emerged adults was high at
emergence and increased further within the next 24 hours
of emergence. Following this increase, there was a continual
decrease from day 1 to day 7 accompanied by a simultaneous
development and oviposition of the first batch of
oocytes indicating that the decline has most likely occurred
as a result of transfer of some of these proteins into the
developing oocytes. Some of these proteins may also be
utilized for the somatic growth which occurs during the first
few days of the adult life.
Simple flight tests carried out in the laboratory on
three species of Dysdercus have indicated that there is a
relationship between reproduction, flight activity and bionomics
of the species concerned. Males began to fly 2 .. 3
days following emergence as adults and most individuals
retained their flight capability throughout life. It was in the
females however, that differences in the flight pattern became
evident, although there was an underlying similarity. All
three species seem to have modified a basic reproductive
flight pattern. D. fasciatus females unlike the males were
incapable of undertaking any flight. Within 2 - 3 days of
emergence, the bugs entered a more or less continuous
copulation (which would certainly prevent flight) followed by
oogenesis and flight muscle histolysis. Since D. fasciatus
females feed on a limited number of host plants which
provide an abundant supply of food during certain times of the
year, the females seem to take maximum advantage of these
current favourable conditions by preventing flight and reducing
both the pre-copulation and pre-oviposition period. This allows
rapid population growth and colonization of the habitat. Starved
females, unlike the fed females, undertook some flight. The
mating frequency was reduced in starved pairs and there was no
oocyte development or flight muscle histolysis indicating that
under unfavourable conditions, the females are capable of undertaking
flight in order to find a suitable habitat.
Both D. nigrofasciatus and D. cardinalis females
however differed from D. fasciatus females in that fed females
of the two former species flew for a brief period followed by
oogenesis and flight muscle histolysis. Oviposition was
comparatively delayed in these species thus allowing the
females to undertake sufficient flight. Maximum flight activity
occurred at the time when the mating frequency was not high.
There is therefore a clear separation of flight activity from
reproductive activity. Since D. nigrofasciatus feed on a variety
of herbaceous host plants that are apt to be scattered, the females
need to undertake some flight to cover the available habitat.
The bionomics of D. cardinalis is not well known but it resembles
D. nigrofasciatus in both the reproductive and flight activities.
The flight activity' in these species was further enhanced
under starvation; there was no oogensis or wing muscle histolysis.
Reduced mating frequency in starved pairs permitted more flight,
the latter ceased only when feeding occurred. Since all these
species face unfavourable conditions in the field, this enhanced
flight activity and reduction in reproductive activity is of an ecological
significance since it enables the bugs to use up the
available reserves for migrating to biotopes where conditions for
reproduction are favourable. | |
