STUDIES ON NATURAL ENEMY COMPLEX OF APHIDS INFESTING OILSEED BRASSICAS
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Date
2020-01-30
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CSKHPKV. Palampur
Abstract
Studies on natural enemy complex of aphids infesting oilseed Brassicas were carried
out to find the distribution and abundance of aphid species and associated natural enemies in
different parts of the state. The field experiments were undertaken during the rabi season of
2015-16, 2016-17 and 2017-18 at the Experimental Farm of the Department of Entomology,
CSK Himachal Pradesh Krishi Vishvavidyalaya, Palampur on mustard variety RCC-4 and the
data on the population build-up of aphids and their associated natural enemies were recorded
at weekly interval. At other locations of the state, the observations were recorded at
fortnightly interval during rabi 2016-17 and 2017-18. The crop was found to be infested with
all the three aphid species viz. mustard aphid, Lipaphis erysimi (Kaltenbach), cabbage aphid,
Brevicoryne brassicae (Linnaeus) and green peach aphid, Myzus persicae (Sulzer). L. erysimi
appeared early in the season and remained active for a longer duration. B. brassicae activity
started late in the season and reached the peak towards the maturity of the crop. M. persicae
appeared for a shorter duration and its population was scarce at all the locations. Three natural
enemies i.e. coccinellids, syrphids and one parasitoid, Diaeretiella rapae M‟Intosh were
found to be associated with the aphid species. They appeared late in the season and their peak
population did not synchronize with peaks of aphid population and thus were unable to check
the aphid population. Correlation coefficients indicated that the temperature favoured the
build-up of aphid population and their natural enemies while rainfall exerted negative impact.
The population of coccinellids was negatively correlated with the population of L. erysimi and
M. persicae while, positive correlation with B. brassicae was observed. On the other hand,
population of syrphids was found to be positively correlated with aphid population.
Regression analysis revealed that none of the weather parameter alone was responsible for the
population build-up of aphids and associated natural enemies.
In order to know the biological control potential of the aphid parasitoid D. rapae,
biological and behavioural characteristics were studied in the laboratory using M. persicae, B.
brassicae and L. erysimi as host. The parasitoid successfully completed its life cycle on
different instars in all the aphid hosts. It was longer in early instars compared to that in later
instars. Oviposition period, post-oviposition period, fecundity (number of eggs
laid/female/day) and total fecundity (total number of eggs laid/female) of D. rapae
parasitizing nymphs of different age groups (1-2 days and 4-5 days old) of M. persicae, B.
brassicae and L. erysimi were studied and the results showed that age of the host at the time
of oviposition by the parasitoid significantly affected the duration of oviposition and
fecundity of the parasitoid. Older nymphs (4-5 days old) of the aphids were able to defend
themselves against ovipositing females of the parasitoid more effectively than the younger
nymphs (1-2 days old). Immature survival of the parasitoid, D. rapae was found to be highestxii
in the third instar (85.81+10.31%) of M. persicae, B. brassicae (83.70+5.62) and fourth instar
(87.67+14.48) of L. erysimi. Highest survival was observed in older instars than in younger
instars. A female-biased sex ratio was found for the parasitoid with mean values of 79.99,
73.38 and 79.32 per cent for M. persicae, B. brassicae and L. erysimi, respectively. Adult
longevity of D. rapae was found to be significantly longer for females in comparison to that
of males in all the aphid hosts. Mean longevity of adult females of D. rapae was significantly
longer while parasitizing nymphs of younger age groups (1-2 days) in comparison to older
age groups (4-5 days). Among different host plants tested, M. persicae reared on capsicum, B.
brassicae reared on cabbage and L. erysimi reared on mustard was found to be the best host
for D. rapae on the basis of high mean fecundity, high parasitization (%), female biased sex
ratio and the shortest time required by the parasitoid for completion of total developmental
period. Host and parasitoid sizes were determined based on the length of their hind tibia and it
was longest in M. persicae, followed by B. brassicae and L. erysimi. The adults of D. rapae
emerged from M. persicae were the largest (males= 0.626 mm and females= 0.689 mm),
followed by adults emerged from B. brassicae (males= 0.539 mm and females= 0.565 mm)
and L. erysimi (males= 0.516 mm and females= 0.521 mm), respectively. Host stage
preference of D. rapae on different instars of three aphids was studied in choice and nochoice preference tests. Second instar of M. persicae was the most preferred in both choice
and no-choice tests. The parasitoid showed higher preference towards third instar nymphs of
B. brassicae and second instar of L. erysimi in both choice and no-choice tests, respectively.
D. rapae exhibited type II functional response to all the three aphids. The per capita
parasitization and search efficiency of the parasitoid were observed to decrease with increase
in parasitoid density. Similarly, the search efficiency of the parasitoid gradually decreased
with the increase in the parasitoid density.
It was concluded from the study that D. rapae is effective biological control agent
which can be mass produced on aphids infesting oilseed Brassicas for utilization in
augmentative biological control. The study can be helpful in further standardizing the
protocols for mass rearing of the parasitoid.