The ichneumonid Diadegma semiclausum Hellen has been recorded in many
parts of the world as an important parasitoid of diamondback moth, Plutella xylostella
(Linnaeus), a serious pest of brassica vegetable crops worldwide. Some aspects of
reproductive biology were studied in controlled laboratory conditions of 21±1°C, 16:8 h
(light:dark) and 50-60% RH. Diadegma semiclausum adults emerge only during the
photophase. It has a protandrous emergence pattern because the male developmental
time is shorter than the female. Most males emerge in the first half of the photophase
whereas females emerge during the second half. Both males and females become
sexually mature in <12 h after emergence. When paired with 3-d-old virgin mates, more
newly emerged females (<12-h-old) mate, compared to newly emerged males. Females,
immediately after eclosion (<1-h-old), do not carry mature eggs in their ovaries, and
hence this is a strong synovigenic species. Maternal age affects the egg load, which
reaches the maximum by 8 d after emergence. Egg resorption occurs in host deprived
females and the number of mature eggs declines with age when >20 d. Females can
mature eggs without a food supply, suggesting that it is an autogenous species. Host and
plant cues do not enhance the initiation of the egg maturation process in newly emerged
females. The longevity of adults D. semiclausum is immensely affected by food
availability. Sugar fed individuals live 15-20 fold longer than those not provided with
food or water after emergence. Females live longer than males. Body size does not
affect the longevity of males, but large females live longer than small females, in the
absence of food. A single mating does not affect the longevity of either males or
females. Egg laying reduces the longevity of the females. Males show active courtship
behaviour. Mating success increases with an increasing mate age. Female age is more
important than male age, for mating success. Body size does not affect mating success.
Neither age nor body size affects the premating and mating periods. Adults mainly mate
during the photophase. Repeated mating occurs in continuously paired males and
females. A single male can repeatedly mate the same female up to 5 times during 16 h
of pairing. Mating duration and mating intervals decrease non-linearly in subsequent
matings, after the first mating.
Females oviposit during the photophase. More eggs are laid in the first few
oviposition bouts. More fertilised eggs are deposited in the first two oviposition bouts.
Host stage affects body size and developmental time of offspring. The developmental
period of the parasitoid is the longest if the host is parasitied at the second instar. Males
developing in the third and fourth instars and females in the third instar DBM larvae
have the largest developmental periods. The host stage does not affect the sex ratio,
parasitism rate and emergence rate. The sex ratio is highly male-biased. Parasitoids
produce more female offspring, at higher host density conditions. Parasitism rate is
inversely related to the host density. Fecundity is greater at a higher host density. At the
density of 30 hosts, a D. semiclausum female can parasitise > 600 larvae in her lifetime.
Longevity, host larvae mortality, parasitoid emergence rate and egg load at death, are
not affected by host density. The parasitism rate reduces with the increase of the
female’s age. The sex ratio becomes increasingly male-biased, with the increase of age
in parasitisng females.
